Chapter 1: General Physiology

Medical physiology is concerned with how a state of health and wellness is maintained in a person and, therefore, it takes a global view of how the body systems function and how they are controlled. There are 10 body systems, each with unique contributions to body function (Table 1-1). However, it is the integration of the body systems that allows the creation of a stable internal environment in which cells are able to function. For example, the maintenance of normal blood pressure requires the integration of several organ systems:

• The major determinants of blood pressure in the cardiovascular system are the rate of blood flow (cardiac output) and the vascular resistance.

• The volume of blood is a key determinant of blood pressure and is controlled by a balance between fluid and salt intake, via the gastrointestinal system, and their excretion via the renal system.

• Appetite and thirst are controlled by the nervous system, which, together with the endocrine system, integrates the body systems.

Such ability to maintain a stable internal environment is a central concept in physiology and is referred to as homeostasis.

Negative Feedback Control

The stability of the body's internal environment is defined by the maintenance of several physiologic controlled variables within narrow normal ranges (Table 1-2). The characteristic of minimal variation in a controlled variable is explained by the presence of negative feedback control mechanisms. Negative feedback is the initiation of responses that counter deviations of a controlled variable from its normal range and is the major control process used to maintain a stable internal environment. A negative feedback control system contains the following elements (Figure 1-1):

• A set point value, which is at the center of the normal range and is treated by the control system as the target value.

• Sensors that continuously monitor the controlled variable.

• A comparator, which interprets input from the sensors to determine when deviations from the set point have occurred. The comparator initiates a counter response.

• Effectors are the mechanisms that restore the set point to its normal level.

In the control of blood pressure, the controlled variable is mean arterial blood pressure (MAP). The normal set point for MAP is approximately 95 mm Hg. Pressure sensors are located in the carotid sinus and relay information to a comparator located in the central nervous system. If MAP suddenly changes, the activity of effectors (e.g., cardiac contractility, vascular tone, and urinary fluid excretion) is altered to restore normal blood pressure.

Pathologic states occur when homeostasis is not maintained. For example, diabetes mellitus is a disease in which the blood glucose concentration (a controlled variable) is pathologically elevated. Diabetes is caused by dysregulation of the hormone insulin, which is the major effector in the control of blood glucose concentration. Virtually all hormones affect homeostasis and can therefore contribute to pathologic conditions when dysregulation occurs.

The Internal Environment

The goal of homeostasis is to provide an optimal fluid environment for cellular function. The body fluids are divided into two major functional compartments (Figure 1-2):

1. The fluid inside cells, taken collectively, is the intracellular fluid (ICF) compartment.

2. The fluid outside cells is the extracellular fluid (ECF) compartment, which is subdivided into the interstitial fluid and the blood plasma.

The concept of an internal environment in the body correlates with the interstitial fluid bathing cells. There is free exchange of water and small solutes in the ECF between interstitial fluid and plasma across the blood capillaries. In contrast, the exchange of most substances between interstitial fluid and intracellular fluid is highly regulated and occurs across plasma cell membranes.

The volume of total body water is approximately 60% of body weight in men and 50% in women. About 60% of total body water is ICF and 40% is ECF. Approximately 80% of ECF is interstitial fluid and the remaining 20% is blood plasma, which is contained inside the vascular system. Figure 1-2 compares the basic compositions of body fluids. ECF is high in NaCl and low in K⁺, whereas ICF is high in K⁺ and low in NaCl. Interstitial fluid is similar in composition to plasma, except that interstitial fluid has almost no protein. Osmolarity is the same in all compartments.

A large interstitial fluid volume is protective in the setting of hemorrhage. The capillary membrane that separates the intravascular space from the interstitial space is completely permeable to water, which allows the interstitial fluid to freely move into the intravascular space and helps to maintain adequate blood volume in a patient who is hemorrhaging. Because approximately 80% of the ECF is interstitial fluid and 20% is blood plasma, a hemorrhaging patient must lose about 5 L of ECF before the volume of the plasma volume is decreased by 1 L. The reverse is also true; to replace 1 L of plasma volume, approximately 5 L of intravascular isotonic saline must be infused.

Indicator Dilution Principle

The volume of a body fluid compartment can be determined from the volume of distribution of an indicator molecule using the following equation:

To use the indicator dilution principle, the marker molecule must only be distributed in the specific body fluid compartment to be measured. Marker molecules that can be used to determine total body water, ECF, and plasma volumes are listed in Table 1-3. Although there are no selective markers for the ICF or interstitial fluid volumes, these volumes can be derived as follows:

• The ICF volume is the difference between the volume of total body water and the volume of ECF.

• The interstitial fluid volume is the difference between the volume of ECF and the volume of plasma.

Example

After a patient is infused with 400 mg of the ECF marker inulin over a 1-hour period, a stable plasma inulin concentration of 1 mg/dL is measured. During the infusion, 200 mg of inulin was excreted in the urine. Using Equation 1-1, the ECF volume is calculated as follows:

V = (Q −q) ÷ C

V = (400 mg − 200 mg) ÷ 1 mg/dL

V = 200 dL = 20 L

Overview of Solute Transport

Cells constantly exchange solutes (e.g., nutrients, wastes, and respiratory gases) with the interstitial fluid. The transport of solutes across cell membranes is fundamental to the survival of all cells, and the transport mechanisms are therefore present in all cells. Specializations in membrane transport mechanisms often underlie tissue function. For example, excitable tissues express many voltage-sensitive membrane transport systems that account for the ability to generate and propagate electrical signals.

The plasma membranes that separate the cytosol from the ECF are formed from phospholipids and are effective barriers against the free movement of most water-soluble solutes. Most biologically important substances require a protein-mediated pathway to cross cell membranes. Solute transport can be categorized based on the use of cellular energy (Figure 1-3).

• Active transport requires adenosine triphosphate (ATP) hydrolysis.

  • Primary active transport occurs via membrane proteins that directly couple ATP hydrolysis to solute movement.

  • Secondary active transport couples the transport of two or more solutes together. In secondary active transport, energy is used to develop a favorable electrochemical driving force for one solute, which is then used to power the transport of other solutes (e.g., the inwardly directed Na⁺ gradient is used to drive glucose uptake from the intestine).

• Passive transport does not require ATP hydrolysis or coupling to another solute.

An alternative classification of solute transport is based on the transport pathway (Figure 1-3).

• Diffusion of lipid-soluble substances (e.g., gases) may occur directly through the plasma membrane.

• Transport of ions and small molecules more frequently occurs via membrane-spanning proteins, which serve as pores or carriers through the lipid bilayer.

• Movement of macromolecules occurs in membrane-limited vesicles; macromolecules enter cells by endocytosis and exit cells by exocytosis.

Similar to physiologic solutes, drugs also utilize the same fundamental transport processes to reach their target site. For example, anesthetic gases are lipid soluble, allowing diffusion through the lipid membranes of the brain. In contrast, most diuretics are secreted by membrane transport proteins into the lumen of the nephron to reach their site of action in the kidney.

Primary Active Transport

Membrane proteins that directly couple ATP hydrolysis to solute transport are called ATPases. There are relatively few examples of primary active transport systems, some of which are illustrated in Figure 1-4A and discussed below:

• The Na⁺/K⁺-ATPase (known as the “sodium pump”) is present in all cells and transports 3Na⁺ out of a cell in exchange for 2K⁺, using one ATP molecule in each transport cycle. The action of sodium pumps accounts for high Na⁺ concentration in ECF and high K⁺ concentration in ICF.

• Ca²⁺-ATPases are located in the plasma membrane and endoplasmic reticulum membrane and function to maintain very low intracellular [Ca²⁺].

• H⁺/K⁺-ATPases pump H⁺ out of cells in exchange for K⁺ and are present in several epithelia. H⁺/K⁺-ATPase is responsible for the secretion of acidic gastric juice in the stomach.

• H⁺-ATPases are expressed inside cells, including the vacuolar H⁺-ATPase, which acidifies lysosomes; ATP synthase is a form of H⁺-ATPase, which operates in reverse to synthesize ATP in mitochondria.

• The multidrug resistance (MDR) transporters are ATPases that extrude a wide variety of organic molecules from cells. MDRs are physiologically expressed in the liver, kidney, and blood-brain barrier.

The expression of MDR transporters (e.g., P-glycoprotein) is one mechanism by which bacteria and cancer cells can become drug resistant. The effectiveness of a drug will be reduced if it is transported out of the target cell by MDR transporters. Inhibition of the P-glycoprotein in cancer cells as a potential method of restoring the sensitivity of tumor cells to chemotherapeutic agents has become an intense area of research.

Secondary Active Transport

There are many examples of secondary active transporters, some of which are shown in Figure 1-4B and discussed below:

• Cotransporters (symporters) couple the movement of two or more solutes in the same direction. Examples of Na⁺-driven cotransporters include Na⁺/glucose uptake in the intestine and diuretic-sensitive Na⁺/K⁺/Cl⁻ and Na⁺/Cl⁻ uptake in the kidney. H⁺/peptide cotransport in the intestine is an example of Na⁺-independent cotransport.

• Exchangers (antiporters) couple the movement of two solutes in the opposite direction. Na⁺-driven exchangers include Na⁺/Ca²⁺ and Na⁺/H⁺ exchange, which are important for maintaining low intracellular [Ca²⁺] and [H⁺], respectively. Cl⁻/HCO₃⁻ exchange is an example of an anion exchanger. It is widely expressed, for example, in red blood cells, where it assists in HCO₃⁻ transport into and out of the cell as part of the blood-CO₂ transport system.

Passive Transport

Passive solute transport can only occur along a favorable electrochemical gradient. Simple passive transport is characterized by a linear relationship between the transport rate and the electrochemical driving force. Pathways for simple passive transport include diffusion through the lipid bilayer or via pores or channels in the membrane (Figure 1-5A). Fick's law of diffusion describes the simple diffusion of an uncharged solute (s):

Permeability is a single coefficient relating the driving force for diffusion to net flux. The membrane permeability to a solute is proportional to the lipid solubility of the solute and inversely proportional to its molecular size. Gases are an example of molecules that are able to move through the lipid bilayer of cell membranes by simple diffusion because they are small and lipid soluble.

The transport of ions and other small water-soluble molecules across a cell membrane requires transport proteins that span the membrane. There are many examples of passive transport through membrane pores and carriers, the most numerous of which are ion channels. Ion channels have the following general components (see Figure 1-5B):

1. A pore region, through which ions diffuse.

2. A selectivity filter within the pore, causing the channel to be highly selective for a particular ion (e.g., Na⁺ channels).

3. A gating mechanism that opens and closes the channel. In the closed state, no ions flow through a channel but the channel is available for activation. In the open state, ions flow down their electrochemical gradient. Some channels can also enter an inactivated state, where the gating structure is open but other parts of the channel protein block the pore. Recovery from inactivation and passage through the closed state are needed before the inactivated channels can reopen. Channel gates may be controlled by one of the following mechanisms:

   • Membrane voltage (voltage-gated channels).

   • Chemicals (ligand-gated channels).

   • Mechanical forces in the membrane (e.g., stretch-activated channels).

Passive transport can also occur via carrier proteins called uniporters, which selectively bind a single solute at one side of the membrane and undergo a conformational change to deliver it to the other side. Solute transport via uniporters is called facilitated diffusion because it is faster than simple diffusion. A characteristic feature of facilitated diffusion is the saturation of the transport rate at high solute concentrations (see Figure 1-5C). The GLUT family of related genes encodes uniporters for glucose transport; GLUT transporters are a common example of facilitated diffusion, which mediates glucose uptake in many tissues.

Vesicular Transport

Macromolecules are transported between the ICF and the ECF using membrane-limited vesicles. Endocytosis describes the ingestion of extracellular material to form endocytic vesicles inside a cell. There are three types of endocytosis:

1. Pinocytosis is the ingestion of small particles and ECF that occurs constitutively in most cells.

2. Phagocytosis is the uptake of large particles (e.g., microorganisms) that occurs in specialized immune cells.

3. Receptor-mediated endocytosis allows uptake of specific molecules and occurs at specialized areas of membrane called clathrin-coated pits (e.g., uptake of cholesterol bound to low-density lipoproteins).

Exocytosis is the export of soluble proteins into the extracellular space. Such proteins are synthesized in the cell and packaged into intracellular vesicles. When vesicles fuse with the plasma membrane, the soluble proteins are secreted and the vesicle membrane is incorporated in the plasma membrane. There are two pathways for exocytosis:

1. The constitutive pathway is present in most cells and is used to export extracellular matrix proteins.

2. The regulated pathway is present in cells that are specialized for the secretion of proteins such as hormones, neurotransmitters, and digestive enzymes. An increase in the intracellular Ca²⁺ concentration is a key event that triggers regulated exocytosis.

Lambert-Eaton syndrome is a neurologic condition resulting from autoantibodies that bind to and block Ca²⁺ channels on the presynaptic motor nerve terminals. By blocking the Ca²⁺ channels, the Ca²⁺-dependent exocytosis of vesicles filled with acetylcholine (a neurotransmitter needed for muscle contraction) is inhibited, resulting in muscle weakness.

Osmosis

In clinical medicine, the infusion of intravenous fluids requires an understanding of the forces that move water within and between body fluid compartments. Water transport across a barrier is always passive, driven either by a diffusion gradient or by a hydrostatic pressure gradient. Osmosis is water movement that is driven by a water concentration gradient across a membrane (Figure 1-6A). Water concentration is expressed in terms of total solute concentration; the more dilute a solution, the lower its solute concentration and the higher its water concentration. When two solutions are separated by a semipermeable membrane (i.e., one that allows the transport of water but not solutes), water moves by osmosis away from the more dilute solution.

Osmolarity

Osmolarity is an expression of the osmotic strength of a solution and is the total solute concentration. The osmolarity contributed by each solute present is the product of the molar solute concentration and the number of particles that the solute dissociates into when dissolved. For example, 1 mol of glucose dissolved in 1 L of water produces a solution of 1 Osm/L; 1 mol of NaCl dissolved in 1 L of water produces a solution of approximately 2 Osm/L. Two solutions of the same osmolarity are termed isosmotic. A solution with a greater osmolarity than a reference solution is said to be hyperosmotic, and a solution of lower osmolarity is described as hyposmotic.

Osmolarity can be converted into units of pressure, which allow osmotic and hydrostatic pressure gradients to be mathematically combined; for example, when considering fluid filtration across capillary walls (see Chapter 4). The concept of osmotic pressure (π) is illustrated in Figure 1-6B and is calculated by the van't Hoff law:

Clinical Estimation of Plasma Osmolarity

Although blood plasma contains many solutes, a reasonable clinical estimate of plasma osmolarity can be obtained by considering only the Na⁺, glucose, and urea concentrations:

The measured plasma osmolarity, which is determined in the laboratory, can sometimes be greater than the estimate of plasma osmolarity, calculated using Equation 1-4. This difference between measured and estimated plasma osmolarity is called the osmolar gap, and is caused by the presence of additional solutes in plasma. For example, patients with alcohol intoxication or ethylene glycol poisoning have an increased osmolar gap.

Effective Osmolarity

In many biologic situations, membranes are not completely semipermeable, allowing solutes the ability to permeate the membrane. When the membrane is permeated, the observed osmotic pressure gradient is reduced. The concept of effective osmolarity (or tonicity) incorporates the effect of solute permeation. The tendency of a solute to permeate a membrane, and therefore to reduce the effective osmotic gradient, is expressed using the reflection coefficient (σ), which is the fraction of the measured osmolarity actually exerted. For solutes that do not permeate a membrane, σ = 1.0; when the membrane is freely permeable to the solute, σ = 0 (see Figure 1-6C). The effective osmolarity of a solution is calculated as the product of the osmolarity and the solute reflection coefficient (e.g., if σ = 0.5, the effective osmolarity exerted is only 50% of the measured osmolarity).

The terms isotonic, hypotonic, and isotonic are used to describe the effective osmolarity of a solution relative to a cell.

• An isotonic solution has the some effective osmolarity as the cells and causes no net water movement.

• A hypotonic solution has a smaller effective osmolarity than the cells and causes cells to swell.

• A hypertonic solution has a larger effective osmolarity than the cells and causes cells to shrink.

Because water moves freely across most cell membranes, a steady state condition occurs in which the ECF is isotonic with respect to the ICF. If the steady state is temporarily disrupted, osmotic water movement between ICF and ECF must occur until the tonicity again becomes equal in each compartment. For example, if a patient is intravenously infused with a hypotonic solution, the ECF tonicity is initially decreased and water moves into the ICF by osmosis (cells swell). Conversely, if a hypertonic solution is infused, the ECF tonicity initially increases and water moves out of the ICF (cells shrink).

The combined action of membrane transport systems in a resting cell results in a steady state condition in which stable ionic concentration gradients exist across the membrane, and the osmolarity remains constant to prevent changes in cellular volume. Ionic gradients are responsible for the generation of a resting membrane potential difference across the cell membrane of all cells in which the inside of the cell is electrically negative with respect to the outside (Figure 1-7A). The presence of membrane voltages is fundamental to the function of excitable tissues (e.g., nerve and muscle), which are able to generate and propagate electrical signals in the form of action potentials. Nonexcitable cells are also affected by their membrane voltage because it contributes to the overall electrochemical driving force for ion transport.

Ionic Basis of Membrane Potentials

Membrane potentials arise because cell membranes contain ion channels that provide selective ion permeability and because there are stable ion diffusion gradients across the membrane. The following steps are involved in the development of a diffusion potential (see Figure 1-7B):

1. In the example shown in the figure, two potassium chloride (KCl) solutions are separated by a membrane that is permeable to K⁺ but not to Cl⁻.

2. K⁺ diffuses from the upper to the lower compartment, down its concentration gradient. In this case, Cl⁻ cannot follow.

3. A voltage difference develops as the K⁺ ions leave the upper compartment, leaving a net negative charge behind. A very slight separation of KCl ion pairs is enough to generate physiologic voltages.

4. The negative potential in the upper compartment attracts K⁺ ions and opposes the K⁺ concentration gradient. An equilibrium potential is established when the voltage difference and the concentration gradient are equal but opposite driving forces. At the equilibrium potential, there is no net movement of K⁺.

This simulated example is analogous to most resting cells, which contain a high [K⁺] and have numerous open K⁺ channels at rest (note, however, that the major intracellular anion in cells is protein, not Cl⁻).

The equilibrium potential is a function of the size of the ion concentration gradient and is calculated using the Nernst equation:

Example

The only ion channels open in a resting cell are K⁺ channels. If the intracellular [K⁺] = 155 mM/L and the ECF [K⁺] = 4.5 mM/L, predict the resting membrane potential.

The magnitude of the K⁺ diffusion potential that develops is calculated using Equation 1-5:

Table 1-4 lists typical ion concentration gradients and equilibrium potentials for the major ions in excitable and nonexcitable cells.

Resting Membrane Potential

The measured membrane potential (V_m) will usually be a composite of several diffusion potentials because the membrane is usually permeable to more than one ion. Ion permeability is best expressed in terms of electrical conductance to reflect ion movement through channels. V_m can then be expressed as the weighted average of ion equilibrium potentials for permeable ions. For example, in the case of a cell with permeability to K⁺, Na⁺, and Cl⁻:

Example

To estimate the membrane potential that will arise, using data in Table 1-4 for a nonexcitable cell and assuming that 80% of total membrane conductance is due to K⁺ channels, 5% is due to Na⁺ channels, and 15% is due to Cl⁻ channels:

In most cells, V_m is primarily a function of ECF [K⁺] because K⁺ conductance predominates in most cells at rest. It should be noted that although the Na⁺/K⁺-ATPase is electrogenic (3Na⁺ pumped out of the cell for every 2K⁺ pumped into the cell), its direct contribution to the membrane potential is very small. The importance of the Na⁺/K⁺-ATPase in the development of resting membrane potentials is to maintain resting ion concentration gradients.

Equation 1-6 illustrates the general prediction that changes in V_m will only occur if equilibrium potentials are disturbed (i.e., if ion concentration gradients change), or if the membrane conductance to an ion changes because ion channels open or close. The following terms are used to describe changes in the membrane potential:

• Depolarization is a change to a less negative membrane potential (membrane potential difference is decreased).

• Hyperpolarization occurs when the membrane potential becomes more negative (membrane potential difference is increased).

• Repolarization is the return of the membrane potential toward V_m following either depolarization or hyperpolarization.

Hyperkalemia is a potentially fatal condition in which the serum [K⁺] is increased. An increase in the serum [K⁺] depolarizes V_m, which can cause fatal cardiac arrhythmias. Using the following example, consider the effects on the heart when a normal serum [K⁺] of 4.5 mM is doubled to 9.0 mM.

Part 1

Calculate the expected resting membrane potential for cardiac cells using the data in Table 1-4 for excitable cells (note [K⁺]₀ = 4.5 mM) and assuming the following fractional membrane conductances values: g_K/g_m = 0.90, g_Na/g_m = 0.05, g_Cl/g_m = 0.05.

Part 2

Calculate the expected change in the resting membrane potential when the serum [K⁺] is doubled to 9.0 mM. Assume all other variables are unchanged.

The first step is to calculate the new equilibrium potential based on Equation 1-5 (the Nernst equation):

Recalculate the resting membrane potential using the new value for E_K:

Depolarizing excitable cardiac myocytes from a membrane potential of −87 mV to −69.5 mV may be enough to trigger extracardiac action potentials, which may lead to a fatal arrhythmia!

Electrochemical Gradient

The electrochemical gradient for an ion is the net driving force for ion flux, which is a combination of the membrane voltage and the ion concentration gradient. An equilibrium potential calculated using the Nernst equation is an expression of the ion concentration gradient in mV. The equilibrium potential can therefore be directly compared to the measured membrane voltage. By convention, the electrochemical gradient is defined as (V_m − E_x) (see Figure 1-7C).

• For cations, a positive value for the calculated electrochemical gradient drives flux out of a cell; a negative value represents a net driving force for inward cation flux.

• For anions, a positive value for the electrochemical gradient describes a net driving force for inward anion flux, whereas a negative value indicates an outwardly directed driving force.

Action Potential

Excitable tissues (i.e., neurons and muscle) are characterized by their ability to respond to a stimulus by rapidly generating and propagating electrical signals. The signal assumes the form of an action potential, which is a constant electrical signal that can be propagated over long distances without decay. An action potential is an all-or-none impulse that occurs when an excitable cell membrane is depolarized beyond a threshold voltage (Figure 1-8A). Once the threshold has been exceeded, there is a phase of rapid depolarization, which ends abruptly at a peak voltage greater than 0 mV; the overshoot is the amount that the peak voltage exceeds 0 mV. A slower repolarizing phase returns membrane potential toward V_m. An afterhyperpolarization (undershoot) is observed in nerves (but not in muscle), in which the membrane potential is transiently more negative than the resting membrane potential.

The characteristic phases of an action potential are explained below by specific changes in membrane Na⁺ and K⁺ conductance with time (Figure 1-8A):

1. Rapid depolarization after threshold voltage is exceeded is due to the opening of voltage-gated Na⁺ channels.

2. The peak voltage where rapid depolarization abruptly ends and the membrane enters the repolarizing phase has two components:

   • Closure of inactivation gates on Na⁺ channels.

   • Opening of voltage-gated K⁺ channels.

3. Repolarization of the membrane potential progresses due to the decreasing Na⁺ conductance and the increasing K⁺ conductance.

4. Afterhyperpolarization occurs because K⁺ conductance exceeds that at rest, causing V_m to approach E_K.

Refractory Periods

In the nervous system, it is necessary to encode the intensity of a stimulus rather than merely indicating whether or not a stimulus is present (e.g., how loud is a sound?). Because individual action potentials all have the same amplitude and action potentials never summate, the stimulus intensity must be encoded by action potential frequency. The maximum action potential frequency is limited because a finite period of time must elapse after one action potential before a second one can be triggered.

• The absolute refractory period is the time from the beginning of one action potential when it is impossible to stimulate another action potential. The absolute refractory period results from closure of inactivation gates in Na⁺ channels; inactivated channels must close and the gates must be reset before channels can be reopened.

• The relative refractory period is the time after the absolute refractory period when another impulse can be evoked, but only if a stronger stimulus is applied. A stronger stimulus is needed because a small number of Na⁺ channels have now recovered from inactivation and the membrane is less excitable due to high K⁺ conductance.

Action Potential Propagation

Action potentials are only propagated in one direction along a nerve axon or muscle fiber (see Figure 1-8B). The impulse in one area causes local current flow, which depolarizes the adjacent area to threshold, generating a new action potential downstream. This process is repeated to propagate the action potential signal; conduction is unidirectional because the upstream region is in its refractory period.

The speed of action potential conduction is faster in larger diameter fibers because they have lower electrical resistance than small diameter fibers. Conduction speed is also increased by the myelination of nerve axons. Myelin consists of glial cell plasma membrane, concentrically wrapped around the nerve. In the peripheral nerves, the myelin sheath is interrupted at regular intervals by uncovered nodes of Ranvier. Action potentials are propagated from node-to-node rather than conducting along the whole nerve membrane because voltage-gated Na⁺ channels are only expressed at nodes of Ranvier. This process, called saltatory conduction, is illustrated in Figure 1-8B.

Diseases that result in demyelination of either the central nervous system (e.g., multiple sclerosis) or the peripheral nervous system (e.g., Guillain-Barré syndrome) will significantly impede nerve conduction speed, impairing the function of the nervous system.

Synapses are specialized cell-to-cell contacts that allow the information encoded by action potentials to pass to another cell. Synapses occur at the junction between the processes of two neurons or between a neuron and an effector cell (e.g., a muscle or gland). There are two types of synapses:

1. Electrical synapses occur where two cells are joined by gap junctions, which conduct current from cell-to-cell via nonselective pores. Cardiac muscle is an example of cells that are electrically coupled via gap junctions.

2. Chemical synapses involve the release of a chemical transmitter by one cell that acts upon another cell (Figure 1-9). Action potentials in a presynaptic cell cause the release of the chemical transmitter, which crosses a narrow cleft to interact with specific receptors on a postsynaptic cell. Excitatory neurotransmitters depolarize the postsynaptic membrane, producing an excitatory postsynaptic potential. Inhibitory neurotransmitters hyperpolarize the postsynaptic membrane, producing an inhibitory postsynaptic potential. Chemical synapses have the following functional characteristics:

   • Presynaptic terminals contain neurotransmitter chemicals stored in vesicles. Action potentials in a presynaptic terminal cause Ca²⁺ entry through voltage-gated Ca²⁺ channels, triggering the release of neurotransmitter by exocytosis.

   • There is always a delay between the arrival of an action potential in the presynaptic terminal and the onset of a response in the postsynaptic cell. The delay is short (<1 msec) when the postsynaptic receptor is a ligand-gated ion channel (ionotropic receptor), but long (>100 msec) if the receptor is linked to an intracellular second messenger system (metabotropic receptor).

   • Transmitter action is rapidly terminated. One of three processes can remove transmitter molecules from the synaptic cleft: diffusion, enzymatic degradation by extracellular enzyme (in the case of acetylcholine), or uptake of transmitter into the nerve ending or other cell (usually most important).

The basic function of all types of muscle is to generate force or movement. Electrical or chemical stimuli are transduced into a mechanical response. The three anatomic types of muscle are skeletal, cardiac, and smooth. Both skeletal and cardiac muscle are classified microscopically as striated muscle. Skeletal muscle is also referred to as voluntary because it remains relaxed in the absence of nerve stimulation. Cardiac and smooth muscle can function without nerve input and are referred to as involuntary.

Neuromuscular Junction

Skeletal muscle does not contract until stimulated by action potentials arriving from a motor neuron (Figure 1-10A). The synapse between a motor neuron and a skeletal muscle cell is called a neuromuscular junction or end plate (see Figure 1-10B). Every skeletal muscle cell (fiber) has only one neuromuscular junction, near its midpoint. An individual neuromuscular junction consists of a small, branched patch of bulb-shaped nerve endings, called terminal boutons. Motor neurons branch to activate a group of muscle fibers, known collectively as a motor unit. Muscles that are subject to fine control (e.g., muscles of the hand) have many small motor units.

Motor neurons release acetylcholine as their neurotransmitter. Acetylcholine is synthesized in the cytoplasm of presynaptic terminals from acetyl coenzyme A and choline, via the enzyme choline acetyltransferase, and is stored in vesicles within the nerve terminal. The amount of acetylcholine within a single presynaptic vesicle is called a quantum.

The postsynaptic muscle cell membrane immediately opposite the presynaptic terminals has a high density of nicotinic acetylcholine receptors. Nicotinic receptors are ionotropic receptors that function as nonselective Na⁺ and K⁺ channels. When opened, nicotinic receptors cause skeletal muscle membrane potential to depolarize because the combined equilibrium potential for Na⁺ and K⁺ (E_cation) is approximately 0 mV. Each quantum of acetylcholine produces a small depolarization of the muscle membrane, called a miniature end-plate potential (see Figure 1-10C). Depolarizations from many quanta summate to produce a full end-plate potential in the muscle membrane, which is an example of an excitatory postsynaptic potential. A single motor nerve impulse normally produces an end-plate potential that exceeds the threshold for action potential generation in the muscle cell membrane.

Acetylcholine within the synaptic cleft is rapidly broken down to choline and acetic acid by the enzyme acetylcholinesterase, which is anchored to the muscle cell basement membrane. Choline is taken up by presynaptic nerve terminals and is reused for acetylcholine synthesis. Table 1-5 summarizes the effects of several toxins and drugs that interfere with the neuromuscular junction.

Myasthenia gravis is an autoimmune disease in which antibodies are directed against nicotinic acetylcholine receptors, reducing the number at the end plate. Acetylcholine release is normal, but the postsynaptic membrane is less responsive and results in muscle weakness. Treatment of myasthenia gravis is with agents such as neostigmine, which inhibit acetylcholinesterase in the cleft, thereby prolonging the action of acetylcholine at the neuromuscular junction.

Skeletal Muscle Function

The generation of action potentials in the skeletal muscle cell membrane (sarcolemma) triggers a sequence of events that result in force development by the muscle. The unique ability of muscle to generate force when stimulated results from the presence of motor proteins inside muscle cells.

Sarcomeres

Skeletal muscles consist of muscle columns, each of which consists of a bundle of muscle cells (also called fibers or myocytes) (Figure 1-11A). Muscle cells are multinucleate and are bounded by the sarcolemma. Each myocyte contains several cylindrical myofibrils, which display a distinctive pattern of light and dark bands under the light microscope. This striated appearance arises from the orderly arrangement of structural and contractile proteins. Each repeating motif in the striated pattern is called a sarcomere, which is the fundamental contractile unit of skeletal muscle. Each sarcomere has the following elements (see Figure 1-11B):

• A Z disk bounds the sarcomere at each end.

• Thin filaments, composed of actin, tropomyosin, and troponins, project from each Z disk toward the center of the sarcomere.

• Thick filaments, composed of myosin, are present in the center of the sarcomere and are overlapped by thin filaments.

• Sarcomeres line up end-to-end within a single myofibril. The darker areas that can be seen microscopically are denoted as A bands and correspond to the location of thick filaments. Lighter areas at the ends of sarcomeres are denoted as I bands and correspond to thin filaments where no overlap with thick filaments occurs.

Molecular Components of Sarcomeres

The orderly array of thin and thick myofilaments produces the characteristic striations of skeletal muscle. Thin filaments have three major components (see Figure 1-11C):

1. The backbone of a thin filament is a double-stranded helix of actin.

2. The helical groove on the actin filament is occupied by tropomyosin. Skeletal muscle contraction is regulated via a protein complex that consists of tropomyosin plus attached troponin subunits.

3. Troponin is a heterotrimer consisting of troponins T, C, and I; troponin T anchors the trimer to tropomyosin; troponin C binds Ca²⁺, which allows muscle contraction to occur; and troponin I inhibits interaction between actin and myosin if the intracellular Ca²⁺ concentration is low.

Thick filaments are composed of myosin molecules, which are the molecular motors responsible for the generation of force. Myosin molecules are composed of the following major parts:

• The myosin head contains the actin-binding site plus elements necessary for ATP binding and hydrolysis. The heads are cross-bridges that bind to actin during muscle contraction.

• Myosin heads are connected to the tail of the molecule via a hinge. The hinge allows the movement of cross-bridges, which is the basis of force generation.

The giant protein titin is important for maintaining sarcomere structure and runs from the Z disk to the M line at the center of the sarcomere. In the region of the I band, titin is extensible and is largely responsible for the passive tension that is measured when a relaxed muscle is stretched (see Muscle Mechanics).

Sliding Filament Theory

The mechanism of active force generation in all muscle types is based on thin filaments being pulled over thick filaments (Figure 1-12). In a relaxed skeletal muscle, contraction is inhibited by the tropomyosin-troponin complexes, which obscure the active site on actin and prevent cross-bridge binding (Figure 1-12, panel 2). When the muscle is stimulated, the intracellular Ca²⁺ concentration increases and Ca²⁺ binds to troponin C. The resulting conformational change exposes active sites on actin (Figure 1-12, panel 3). A cycle of events now occurs in which myosin cross-bridges bind to actin, perform a powerstroke, detach, become cocked again, and then reattach (Figure 1-12, panel 4). The cycle repeats in the continued presence of sufficient Ca²⁺ and ATP. As a result, thin filaments from each end of the sarcomere move toward the center by sliding over thick filaments, causing neighboring Z disks to approach each other. In the example shown in Figure 1-12, muscle shortening has occurred and the sarcomere length is reduced (compare panels 1 and 5).

When death occurs, ATP production by mitochondria stops and rigor mortis (stiffening of the muscles) sets in. ATP is necessary for the myosin heads to detach from the actin filaments after a powerstroke occurs. Once ATP production ceases, the cross-bridges are locked in place, which results in stiff muscles. Rigor mortis is a sign of death.

Excitation-Contraction Coupling

In all types of muscle, the key event causing contraction is increased intracellular Ca²⁺ concentration. In skeletal muscle, the source of Ca²⁺ for contraction is the sarcoplasmic reticulum (SR). Muscle contraction only occurs after an increase in cytosolic Ca²⁺, which follows the generation of a muscle action potential (Figure 1-13). The only physiologic stimulus for skeletal muscle action potentials is the end-plate potential, developed in response to motor nerve impulses.

The contractile apparatus of each sarcomere is surrounded by the SR, providing a short diffusion distance for Ca²⁺ to its site of action. The membrane system of the SR is completely enclosed within the cell and is comprised of longitudinal tubules, which surround the contractile apparatus and terminate in lateral sacs (terminal cisternae). Lateral sacs are closely associated with invaginations of the muscle cell plasma membrane called T tubules. The T tubules extend deep into the cell to form blind-ended tubes containing ECF. The lateral sacs from two neighboring sarcomeres converge on a T tubule to form a structure known as a triad (Figure 1-14A). Triads are located at the junction of the A band and the I band of every sarcomere.

Action potentials in T tubules induce Ca²⁺ release from the lateral sacs. The Ca²⁺ release mechanism involves voltage sensors in the T-tubule membrane (L-type Ca²⁺ channels), which are linked to Ca²⁺ release channels (ryanodine receptors) in the SR (see Figure 1-14B). The voltage sensors respond to depolarization in the T tubule with conformational changes that result in the opening of the Ca²⁺ release channels of the SR. The intracellular [Ca²⁺] increases from about 10⁻⁸ M at rest to about 10⁻⁵ M during a muscle contraction. To return a muscle to the relaxed state, Ca²⁺ uptake occurs in the longitudinal tubules via Ca²⁺-ATPases of the SR. Following Ca²⁺ reuptake, most stored Ca²⁺ is found in the lateral sacs, where it is weakly bound to the protein calsequestrin.

Force of Contraction

The force of skeletal muscle contraction is controlled by altering the firing pattern of motor nerves to the muscle. The effect of increasing action potential frequency to a muscle is known as temporal summation (Figure 1-15). At low stimulation frequency, the muscle briefly generates force and then relaxes. At high stimulation frequency, the muscle does not have time to relax between stimuli. Contractions fuse into a plateau of active force called a tetanic contraction; the steady high force level is called the tetanic force. Force does not increase if the stimulation frequency is further increased because cross-bridge attachment is already maximal; at this point, the muscle is said to be tetanized.

If a greater force of muscle contraction is needed, the number of active motor neurons increases. Recruitment of motor units is called spatial summation and is organized according to the “size principle.” Small motor neurons, which reach only a few muscle fibers, are more excitable than large motor neurons and are recruited first. A weak contraction is produced initially because only a few muscle fibers comprise the motor unit of small motor neurons. Large motor neurons are less excitable and require a stronger stimulus from the central nervous system. When large motor neurons are recruited, a large number of muscle fibers are stimulated to produce a strong contraction.

Tetanus toxin, which is produced by the bacterium Clostridium tetani, can result in tetany throughout all the skeletal muscles of the body. The bacterium lives in the soil, and once it contaminates a dirty wound, the tetanus toxin is released. The toxin travels to the spinal cord where it blocks inhibitory nerves, allowing the excitatory motor neurons to fire rapidly. Rapidly firing motor neurons summate to produce tetany, a potentially fatal condition!

Skeletal Muscle Diversity

There are two types of muscle fibers: slow twitch (type I) and fast twitch (type II) (Table 1-6). Expression of different isoforms of myosin and other proteins accounts for different shortening speeds of fast and slow twitch fibers. Muscles have different proportions of slow and fast twitch fibers, which are classified according to their function. For example, postural muscles contain a higher proportion of slow twitch fibers because they must maintain tone and resist fatigue. Extraocular muscles are required to make fast, brief movements of the eye and therefore contain a high proportion of fast twitch fibers. There are genetic differences in the general proportions of muscle fiber types that are expressed among individuals, which accounts in part for the tendency for a person to be either a better sprinter (more fast twitch fibers) or have higher endurance (more slow twitch fibers).

Muscle Mechanics

The sliding filament mechanism provides the potential for a muscle to develop force; the characteristics of the resulting contraction depend on a combination of intrinsic muscle properties and the external constraints placed on the muscle. There are two general modes of muscle contraction:

1. An isometric, or fixed-length contraction, occurs if both ends of a muscle are fixed. In this case, the muscle is able to develop tension but it cannot shorten.

2. An isotonic, or fixed-load contraction, occurs if a muscle is able to shorten to carry a given load.

Length-Tension Relationship

Muscle length is an important determinant of the force of contraction. The length-tension (length-force) relation is studied under isometric conditions. A resting muscle behaves like a rubber band, requiring force to stretch it to different lengths. The preload is the amount of force applied to a resting muscle before stimulation, creating passive tension (Figure 1-16A, blue curve). When the muscle is stimulated, active tension is added to the passive tension. The amount of active tension produced is the difference between passive tension and total tension (Figure 1-16A, red curve).

There is an optimal range of resting muscle length (preload) that produces maximal contraction. At very short or very long muscle lengths, active force development declines, producing the characteristic length-tension relationship of striated muscle. The plateau of the length-tension relation (Figure 1-16B, point y) reflects optimal overlap of thin and thick filaments within sarcomeres, providing maximum myosin cross-bridge binding. On the ascending limb (Figure 1-16B, between points x and y), the muscle is too short, resulting in double overlap of thin filaments and attachment of fewer cross-bridges. On the descending limb of the curve (Figure 1-16B, points y and z), the muscle is too long, resulting in inadequate overlap of thin and thick filaments. An additional mechanism contributing to the length-tension relationship in cardiac muscle is the increased Ca²⁺-sensitivity of troponin C as muscle length increases.

Skeletal muscles operate at the plateau of their length-tension relation because preload is set at the optimal level by bony attachments at each end of the muscle. By contrast, preload is not fixed in cardiac muscle but varies according to the amount of venous return. Preload is a key determinant of the force of cardiac muscle contraction (see Chapter 4).

In a healthy heart, an increased preload will stretch the cardiac myocytes, resulting in a more forceful and speedy contraction. However, in a diseased heart (e.g., left ventricular hypertrophy), cardiac muscle can become stiff, impairing its ability to stretch. Increasing preload in this setting will mainly increase the pressures in the heart without the benefit of improving contractile force or speed.

Force-Velocity Relationship

Muscle performance can be studied under isotonic conditions to produce a force-velocity relation. The resting muscle length is set by adding a given preload, and the initial velocity of shortening is then measured as the muscle is stimulated to lift a range of additional weights. The weight a muscle must lift upon stimulation after resting length has been established is called afterload. The speed of shortening decreases as the total load increases (Figure 1-17A). Extrapolating this curve to a theoretical zero load indicates the maximum possible speed of shortening (V_max),_.which reflects the rate of myosin ATPase activity. If preload is increased, there is an increased speed of shortening for any given total load, but without any change in V_max (see Figure 1-17B).

In cardiac muscle, increased force and speed of contraction can be produced without changing preload. This is called increased contractility and can be identified by an increase in V_max (see Chapter 4).

Smooth muscle lines the walls of most hollow organs, including organs of the vascular, gastrointestinal, respiratory, urinary, and reproductive systems. Smooth muscle is an important therapeutic target because it regulates variables such as blood flow, ventilation of the lungs, and gastrointestinal motility.

Homeostasis can be disrupted by too much or too little smooth muscle tone, resulting in a pathologic condition. For example, excessive smooth muscle contraction in the respiratory tract can cause difficulty breathing, such as occurs during an asthma attack. An example of inadequate tone occurs in vascular smooth muscle during septic shock; an overwhelming infection releases inflammatory mediators that cause dilation of systemic blood vessels, resulting in severe hypotension.

Ultrastructural Features of Smooth Muscle

Smooth muscle cells are not striated in appearance (as are skeletal and cardiac muscle) because thin and thick filaments are not organized as sarcomeres. A network of dense bodies in the cytoplasm of smooth muscle cells serves as attachment points for actin filaments; thick filaments overlap thin filaments in an irregular array (Figure 1-18A). The irregular arrangement of actin and myosin allows smooth muscle cells to generate force over a larger range of preloads than is possible in striated muscle. There is no T tubule or muscle triad structure in smooth muscle; SR is present but it has an irregular arrangement.

Histologically, there are two general types of smooth muscle:

1. Visceral smooth muscle is the most common type, in which cells are arranged in large bundles. Cells within a bundle behave as a functional syncytium due to the presence of gap junctions between cells.

2. Multiunit smooth muscle cells function individually because the cells are not connected by gap junctions. There is a dense nerve supply from autonomic neurons (see Chapter 2). A characteristic of multiunit smooth muscle is fine control over force development in a manner similar to spatial summation in skeletal muscle. For example, multiunit smooth muscle is found in the iris, where it provides fine control over the diameter of the pupil.

Excitation-Contraction Coupling

The contraction of smooth muscle begins when the intracellular Ca²⁺ concentration increases (see Figure 1-18B). The source of Ca²⁺ is usually from the ECF. Ca²⁺ enters the cell through voltage-gated Ca²⁺ channels when the cell membrane becomes depolarized. Depolarization of the membrane potential to threshold for Ca²⁺ channel opening may result from excitatory neurotransmitters or hormones or from spontaneous oscillations in membrane voltage. Contraction can also result from second messenger-stimulated Ca²⁺ release from the SR.

The regulation of smooth muscle contraction involves myosin phosphorylation because smooth muscle myofilaments do not contain troponins. The relative activity of the antagonistic enzymes myosin light chain kinase and myosin light chain phosphatase determines whether myosin is activated. When smooth muscle is stimulated and the intracellular [Ca²⁺] increases, the result is the formation of Ca²⁺-calmodulin complexes. Activated calmodulin stimulates myosin light chain kinase to phosphorylate and thereby activate myosin. When Ca²⁺ levels decrease, the muscle relaxes because the activity of myosin light chain phosphatase predominates, causing myosin to become dephosphorylated.

An important feature of some smooth muscles (e.g., sphincters) is the ability to maintain force over long periods. The maintenance of muscle tone without high rates of ATP consumption is possible because cross-bridges can remain attached to actin for extended periods. This is called the latch bridge state, and results from slow rates of cross-bridge detachment in smooth muscle.