Porth's Essentials of Pathophysiology, 4e

826

Nervous System

U N I T 1 0

the resting ionic concentrations on each side of the mem- brane. Membranes of excitable cells must be sufficiently repolarized before they can be re-excited. During repo- larization, the membrane remains refractory (i.e., does not fire) until repolarization is approximately one-third complete. This period, which lasts approximately one half of a millisecond, is called the absolute refractory period . During one portion of the recovery period, the membrane can be excited, although only by a stronger- than-normal stimulus. This period is called the relative refractory period . The excitability of neurons can be affected by condi- tions that alter the resting membrane potential, moving it either closer to or farther from the threshold potential. Hypopolarization increases the excitability of the post- synaptic neuron by bringing the membrane potential closer to the threshold potential so that a smaller sub- sequent stimulus is needed to cause the neuron to fire. Hyperpolarization brings the membrane potential fur- ther from the threshold and has the opposite, inhibitory effect, decreasing the likelihood that an action potential will be generated. SynapticTransmission Neurons communicate with each other through struc- tures known as synapses . There are two types of synapses in the nervous system: electrical and chemical. Electrical synapses permit the passage of current-carrying ions through small openings called gap junctions that pen- etrate the cell junction of adjoining cells. Although not important in synaptic transmission in the nervous sys- tem, gap junctions are important in cell-to-cell commu- nication in smooth and cardiac muscle. Chemical synapses, which are the more common type of synapse, involve special presynaptic and postsynap- tic membrane structures, separated by a synaptic cleft. The presynaptic terminal secretes one and often several chemical transmitter molecules (e.g., neurotransmitters, neuromodulators). The secreted neurotransmitters dif- fuse into the synaptic cleft and bind to receptors on the postsynaptic membrane. In contrast to an electrical syn- apse, a chemical synapse permits only one-way commu- nication. Chemical synapses are divided into two types: excitatory and inhibitory. In excitatory synapses, bind- ing of the neurotransmitter to the receptor produces depolarization of the postsynaptic membrane; and in inhibitory synapses it reduces the postsynaptic neuron’s ability to generate an action potential. The process of neurotransmission involves the syn- thesis, storage, and release of a neurotransmitter; the reaction of the neurotransmitter with a receptor; and ter- mination of the receptor action. Neurotransmitters are synthesized in the cytoplasm of the axon terminal. The synthesis of transmitters may require one or more enzyme-catalyzed steps (e.g., one for acetylcholine and three for norepinephrine). Each neuron generally produces only one type of neurotransmitter. After synthesis, the neu- rotransmitter molecules are stored in the axon terminal in tiny, membrane-bound sacs called synaptic vesicles .

+20

Overshoot

0

Threshold potential Depolarization Repolarization

– 20

– 40

Resting potential

– 60

Membrane potential (mv)

– 80

Time (msec)

A

C

B

(approximately −60 mV in large nerve fibers) represents the membrane potential at which neurons or other excit- able tissues are stimulated to fire. When the threshold potential is reached, the gatelike structures in the ion channels open. Below the threshold potential, these gates remain tightly closed. The gates function on an all-or- none basis, meaning they are either fully open or fully closed. Under ordinary circumstances, the threshold stimulus is sufficient to open many ion channels, trigger- ing massive depolarization of the membrane (the action potential). Depolarization is characterized by a rapid change in polarity of the resting membrane potential, which was negative on the inside and positive on the outside, to one that is positive on the inside and negative on the outside. During the depolarization phase, the membrane suddenly becomes permeable to sodium ions. The rapid inflow of sodium ions produces local electric currents that travel through the adjacent cell membrane, caus- ing the sodium channels in this part of the membrane to open. In neurons, sodium ion gates remain open for approximately a quarter of a millisecond. During this phase of the action potential, the inner side of the mem- brane becomes positive (approximately +30 to +45 mV). Repolarization is the phase during which the polar- ity of the resting membrane potential is reestablished. This is accomplished with closure of the sodium chan- nels and opening of the potassium channels. The out- flow of positively charged potassium ions across the cell membrane returns the resting membrane potential to negativity. The sodium/potassium–adenosine triphos- phatase (Na + /K + -ATPase) pump gradually reestablishes FIGURE 34-4. Time course of the action potential recorded at one point of an axon with one electrode inside and one outside the plasma membrane.The rising part of the action potential is called the spike. (A) The rising phase plus approximately the first half of the repolarization phase is the absolute refractory period. (B) The portion of the repolarization phase that extends from the threshold to the resting membrane potential represents the relative refractory period. (C) The remaining portion of the repolarization phase to the resting membrane potential is the negative afterpotential.