Nervous tissue is one of the four basic tissues of the body, and it specializes in receiving information from the external and internal milieu, integrating

it, analyzing it, and comparing it with stored experiences and/or predetermined (reflex) responses, to select and effect an appropriate reaction.

•The reception of information is the function of the sensory component of the peripheral nervous system

(PNS).

•The processes of integration, analysis, and response are performed by the brain and spinal cord comprising the central nervous system (CNS) with its gray matter and white matter.

•The transmission of the response to the effector organ is relegated to the motor component of the PNS.

Therefore, it should be appreciated that the PNS is merely a physical extension of the CNS, and the separation of the two should not imply a strict dichotomy.

The nervous system may also be divided functionally into somatic and autonomic nervous systems. The somatic nervous system exercises conscious control over voluntary functions, whereas the autonomic nervous system controls involuntary functions. The autonomic nervous system is a motor system, acting on smooth muscle, cardiac muscle, and some glands. Its three components, sympathetic, parasympathetic, and enteric nervous systems, usually act in concert to maintain homeostasis.

•The sympathetic nervous system prepares the body for action as in a “fight or flight” mode,

•the parasympathetic system functions to calm the body and provides secretomotor innervation to most exocrine glands;

•the enteric nervous system is more or less a standalone system that is responsible for the process of digestion.

It is interesting to note that the enteric nervous system is very large, it has about the same number of neurons as those located in the spinal cord.

The actions of the enteric nervous system are modulated by the sympathetic and parasympathetic components of the autonomic nervous system.

The CNS is protected by a bony housing, consisting of the skull and vertebral column, and the meninges, a tri- ple-layered connective tissue sheath.

•The outermost meninx is the thick fibrous dura mater.

•Deep to the dura mater is the arachnoid, a nonvascular connective tissue membrane.

•The innermost, vascular pia mater is the most intimate investment of the CNS.

•Located between the arachnoid and the pia mater is the cerebrospinal fluid (CSF).

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BLOOD-BRAIN BARRIER

The selective barrier that exists between the neural tissues of the CNS and many blood-borne substances is termed the blood-brain barrier. This barrier is formed by the fasciae occludentes of contiguous endothelial cells lining the continuous capillaries that course through the neural tissues.

•Certain substances (i.e., O2, H2O, CO2, and selected small lipid-soluble substances and some drugs) can penetrate the barrier.

•Other substances, including glucose, certain vitamins, amino acids, and drugs, among others, access passage only by receptor-mediated transport and/or facilitated diffusion.

•Certain ions are also transported via active transport. It is also believed that some of the perivascular neuroglia may play a minor role in the maintenance of the blood-brain barrier.

NEURONS

The structural and functional unit of the nervous system is the neuron, a cell that is highly specialized to perform its two major functions of irritability and conductivity. Each neuron is composed of a cell body (soma, perikaryon) and processes of varied lengths, known as axons and dendrites, usually located on opposite sides of the cell body (see Graphic 7-2). A neuron possesses only a single axon. However, depending on the number of dendrites a neuron possesses, it may be

•unipolar (a single process but no dendrites—rare in vertebrates, but see below),

•bipolar (an axon and one dendrite), or

•the more common multipolar (an axon and several dendrites).

•An additional category exists where the single dendrite and the axon fuse during embryonic development, giving the false appearance of a unipolar neuron; therefore, it is known as a pseudounipolar neuron, although recently neuroanatomists began to refer to this neuron type as a unipolar neuron.

Neurons also may be classified according to their function. Sensory neurons receive stimuli from either the internal or external environment then transmit these impulses toward the CNS for processing. Interneurons act as connectors between neurons in a chain or typically between sensory and motor neurons within the CNS. Motor neurons conduct impulses from the CNS to the targets cells (muscles, glands, and other neurons).

152 N E R V O U S T I S S U E

Information is transferred from one neuron to another across an intercellular space or gap, the synapse. Depending on the regions of the neurons participating in the formation of the synapse, it could be axodendritic, axosomatic, axoaxonic, or dendrodendritic.

•Most synapses are axodendritic and involve one of many neurotransmitter substances (such as acetylcholine) that is released by the axon of the first neuron into the synaptic cleft.

•The chemical momentarily destabilizes the plasma membrane of the dendrite, and a wave of depolarization passes along the second neuron, which will cause the release of a neurotransmitter substance at the terminus of its axon.

This type of a chemical synapse is an excitatory synapse, which results in the transmission of an impulse.

Another type of synapse may stop the transmission of an impulse by stabilizing the plasma membrane of the second neuron; it is called an inhibitory synapse.

into the cell, and at that point, the resting potential is reversed, so that the inside becomes positive with respect to the outside.

• In response to this reversal of the resting potential, the Na+ channel closes and for the next 1 to 2 ms cannot be opened (the refractory period).

•Depolarization also causes the opening of voltagegated K+ channels (note that these are different from the potassium leak channels) through which potassium ions exit the cell, thus repolarizing the membrane and ending not only the refractory period of the Na+ channel but also the closure of the voltage-gated potassium channel.

The movement of Na+ ions that enter the cell causes depolarization of the cell membrane toward the axon terminal (orthodromic spread). Although sodium ions also move away from the axon terminal (antidromic spread), they are unable to affect sodium channels in the antidromic direction, since those channels are in their refractory period.

Membrane Resting Potential

The normal concentration of K+ is about 20 times greater inside the cell than outside, whereas the concentration of Na+ is 10 times greater outside the cell than inside. The resting potential across the neuron cell membrane is maintained by the presence of potassium leak channels in the plasmalemma.

•These potassium leak channels are always open, and it is through these channels that K+ ions diffuse from inside the cell to the outside, thus establishing a positive charge on the outer aspect and a negative (less positive) charge on the internal aspect of the cell mem-

brane, with a total differential of about 40 to 100 mV.

•Na+ ions can also traverse this channel, but at a 100-fold slower rate than potassium ions.

•Although the majority of the establishment of the membrane potential is due to the potassium leak channel, the action of the Na+-K+ pump does contribute to it to a certain extent.

Action Potential

The action potential is an electrical activity where charges move along the membrane surface. It is an all- or-none response whose duration and amplitude are constant. Some axons are capable of sustaining up to 1,000 impulses/second.

•Generation of an action potential begins when a region of the plasma membrane is depolarized.

•As the resting potential diminishes, a threshold level is reached, voltage-gated Na+ channels open, Na+ rushes

Myoneural Junctions

Neurons also communicate with other effector cells at synapses. A special type of synapse, between skeletal muscle cells and neurons is known as a myoneural junction. The axon forms a terminal swelling, known as the axon terminal (end-foot), that comes close to but does not contact the muscle cell’s sarcolemma.

•Mitochondria, synaptic vesicles, and elements of smooth endoplasmic reticulum are present in the axon terminal.

•The axolemma involved in the formation of the synapse is known as the presynaptic membrane, whereas

•the sarcolemmal counterpart is known as the postsynaptic membrane.

The presynaptic membrane has sodium channels, voltage-gated calcium channels, and carrier proteins for the cotransport of Na+ and choline.

The postsynaptic membrane has acetylcholine receptors as well as slight invaginations known as junctional folds.

A basal lamina containing the enzyme acetylcholinesterase is also associated with the postsynaptic membrane.

•As the impulse reaches the end-foot, sodium channels open, and the presynaptic membrane becomes depolarized, resulting in the opening of the voltagegated calcium channels and the influx of Ca+ into the end-foot.

• The high intracellular calcium concentration causes the synaptic vesicles, containing acetylcholine,