Summary

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Chapter: Anatomy and Physiology for Health Professionals: Control and Coordination: Neural Tissue

Nervous tissue includes neurons and neuroglial cells.


Summary

Nervous tissue includes neurons and neuroglial cells. The nervous system is divided into the CNS and PNS. Sensory functions receive stimulation from receptors concerning internal and external changes. Sensory information is used to carry out motor functions, which in turn stimulate effectors to respond. Neurog-lial cells include microglial cells, astrocytes, oligoden-drocytes, and ependymal cells. Astrocytes are the most abundant glial cells and are very important for main-taining the integrity of the CNS. Oligodendrocytes provide insulation around CNS axons. In the PNS, Schwann cells form myelin sheaths. A neuron con-sists of a cell body, dendrites, and an axon. Dendrites and cell bodies provide receptive surfaces. A single axon arises from the cell body and may be enclosed in a neurolemma and myelin sheath. Axons may have occasional branches known as axon collaterals, which usually extend in right angles. An axon with a myelin sheath is called myelinated.

In the CNS, dense groups of myelinated axons form the white matter. Those that are unmyelinated along with neuron cell bodies form the gray matter. Nerve fibers are classified by their diameter, degree of myelination, and speed of conduction and are divided into group A, group B, and group C fibers. Functional classifications of neurons include sensory (afferent) neurons, interneurons, and motor (effer-ent neurons). Structural classifications of neurons include multipolar, bipolar, and unipolar neurons. The surface of a cell membrane is usually electrically charged (polarized) compared with its inner contents. An action potential is a change in neuron membrane polarization and a return to its resting state. It is an all-or- none phenomenon, occurring completely or not at all. The absolute refractory period is the time between sodium channels opening until they begin to reset to their resting state.

Distribution of ions is determined in part by selective­ channels located in cell membranes. The difference in charges between the outside of the cell membrane and the inside of it in a resting cell is known as a resting potential. The cytosol of cells contains less sodium ions but more potassium ions compared with the extracellular fluid. Potassium ions diffuse out of cells very easily in comparison with how easily sodium ions can enter. Nerve cells respond with excit-ability to changes in surroundings. Depolarization opens sodium channels and then inactivates them. In repolarization, sodium channels are inactivating, with potassium channels open. In hyperpolarization, some potassium ion channels remain open, and sodium ion channels are reset.

In a nerve cell membrane, an action potential causes a local bioelectric current to reach other por-tions of the membrane. A wave of action potentials is a nerve impulse. Impulses are conducted over the entire surface of unmyelinated axons, but there is reduced impulse conduction in myelinated axons because of the insulation provided by the myelin. A synapse is a junction between two neurons. Presynaptic neurons carry impulses into synapses and postsynaptic neu-rons respond. Chemical synapses are more prevalent than electrical synapses, and are mostly comprised of axon terminals and neurotransmitter receptor regions. Less common electrical synapses consist of gap junctions. Synaptic fatigue occurs when demands for ACh cannot keep pace with the availability of this neurotransmitter.

Axons have synaptic knobs, which secrete ­neurotransmitters. Neurotransmitters reaching the postsynaptic neuron membrane are either ­excitatory or inhibitory. ACh is released at neuromuscular junctions­ and is eventually degraded to acetic acid and choline via the enzyme AchE. The way the nervous system processes and responds to nerve impulses is based on the organization of neurons in the brain and spinal cord. The aging process affects the entire nervous system in many different ways and usually reduces function as a result of slower impulse processing abilities.

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