Overview of the Nervous System Among the 11 body systems, the nervous system and the endocrine system play the most important roles in maintaining homeostasis

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Overview of the Nervous System

Among the 11 body systems, the nervous system and the endocrine system play the most important roles in maintaining homeostasis.

Neurology is the branch of medical science that deals with the normal functioning and disorders of the nervous system.

The central nervous system (CNS) consists of the brain and spinal cord. The peripheral nervous system (PNS) consists of all nervous tissue outside the CNS. Components of the PNS include nerves, ganglia, enteric plexuses, and sensory receptors. The PNS is divided into a somatic nervous system (SNS), autonomic nervous system (ANS), and enteric

nervous system (ENS). The SNS consists of sensory neurons that conduct impulses from somatic and special sense receptors to the

CNS and motor neurons from the CNS to skeletal muscles. The ANS contains sensory neurons from visceral organs and motor neurons that convey impulses from the CNS

to smooth muscle tissue, cardiac muscle tissue, and glands. The motor part of the ANS consists of two branches, the sympathetic division and the parasympathetic division. In general, the sympathetic division helps support exercise and emergency actions, or “fight-or- flight”

responses, and the parasympathetic division takes care of “rest-and-digest” activities. The ENS consists of neurons in enteric plexuses in the gastrointestinal (GI) tract that function somewhat

independently of the ANS and CNS. The ENS monitors sensory changes in and controls operation of the GI tract.

3 basic functions of the nervous system are detecting stimuli (sensory function); analyzing, integrating, and storing sensory information (integrative function); and responding to integrative decisions (motor function).


Histology of Nervous Tissue

Nervous tissue consists of two types of cells: neurons and neuroglia. Neurons are cells

specialized for nerve impulse conduction and provide most of the unique functions of the

nervous system, such as sensing, thinking, remembering, controlling muscle activity.

Neuroglia support, nourish, and protect the neurons and maintain homeostasis in the

interstitial fluid that bathes neurons. Most neurons have three parts. The dendrites are the main receiving or input region.

Integration occurs in the cell body. The output part typically is a single axon, which

conducts nerve impulses toward another neuron, a muscle fiber, or a gland cell. Structurally, neurons are classified according to the number of processed that

extend from the cell body. There are 3 classifications: multipolar, bipolar, or unipolar.

Functionally, neurons are classified as sensory (afferent) neurons, motor (efferent) neurons, and interneurons. Sensory neurons carry sensory information into the CNS. Motor neurons carry information out of the CNS to effectors (muscles and glands). Interneurons are located within the CNS between sensory and motor neurons.


Histology of Nervous Tissue

Neuroglia or glia (glue) make up about half the volume of the CNS. Neuroglia support, nurture, and protect neurons and maintain the interstitial fluid that bathes them. Neuroglia in

the CNS include astrocytes, oligodendrocytes, microglia, and ependymal cells - Know function Neuroglia in the PNS include Schwann cells and satellite cells. Two types of neuroglia produce myelin sheaths: Oligodendrocytes myelinate axons in the CNS, and Schwann

cells myelinate axons in the PNS. White matter is composed primarily of myelinated axons. Gray matter of the nervous system contains neuronal cell bodies, dendrites, and axon terminals of neurons,

unmyelinated axons, and neuroglia. In the spinal cord, gray matter forms an H-shaped inner core that is surrounded by white matter. In the brain, a

thin, superficial shell of gray matter covers the cerebrum and cerebellum.

Clusters of Neuronal Cell Bodies A ganglion (plural is ganglia) refers to a cluster of neuronal cell bodies located in the PNS. As mentioned

earlier, ganglia are closely associated with cranial and spinal nerves and are located in the dorsal root ganglia.

Bundles of Axons A nerve is a bundle of axons that is located in the PNS. Cranial nerves connect the brain to the periphery;

spinal nerves connect the spinal cord to the periphery.


Action Potentials

Neurons communicate with one another by means of nerve action potentials, also called nerve impulses. Generation of action potentials depends on the existence of a membrane potential and the presence of voltage-

gated channels for Na+ and K+. The resting membrane potential (the electrical charge across the plasma membrane) is –70 mV. A cell that

exhibits a membrane potential is polarized. The resting membrane potential arises due to an unequal distribution of ions on either side of the plasma

membrane and a higher membrane permeability to K+ than to Na+. The level of K+ is higher inside and the level

of Na+ is higher outside, a situation that is maintained by sodium–potassium pumps. During an action potential, voltage-gated Na+ and K+ channels open in sequence. Opening of voltage-gated Na+

channels results in depolarization, the loss and then reversal of membrane polarization (from –70 mV to +30

mV). Then, opening of voltage-gated K+ channels allows repolarization, recovery of the membrane potential to

the resting level. According to the all-or-none principle, if a stimulus is strong enough to generate an action potential, the impulse

generated is of a constant size. During the refractory period, another action potential cannot be generated. Nerve impulse conduction that occurs as a step-by-step process along an unmyelinated axon is called

continuous conduction. In saltatory conduction, a nerve impulse “leaps” from one node of Ranvier to the next along a myelinated axon.

Axons with larger diameters conduct impulses faster than those with smaller diameters; myelinated axons conduct impulses faster than unmyelinated axons.


Action Potentials

The propagation speed of a nerve impulse is not related to stimulus strength. larger, myelinated fibers conduct impulses faster due to size & saltatory conduction

Fiber types A fibers largest (5-20 microns & 130 m/sec)

myelinated somatic sensory & motor to skeletal muscle B fibers medium (2-3 microns & 15 m/sec)

myelinated visceral sensory & autonomic preganglionic C fibers smallest (.5-1.5 microns & 2 m/sec)

unmyelinated sensory & autonomic motor

Neurons communicate with other neurons and with effectors at synapses in a series of events known as

synaptic transmission. At a synapse, a neurotransmitter is released from a presynaptic neuron into the synaptic cleft and then binds to

receptors on the plasma membrane of the postsynaptic neuron. An excitatory neurotransmitter depolarizes the postsynaptic neuron’s membrane, brings the membrane potential

closer to threshold, and increases the chance that one or more action potentials will arise. An inhibitory neurotransmitter hyperpolarizes the membrane of the postsynaptic neuron, thereby inhibiting action

potential generation. Neurotransmitter is removed in three ways: diffusion, enzymatic destruction, and reuptake by neurons or

neuroglia. Important neurotransmitters include: acetylcholine, glutamate, aspartate, gamma aminobutyric acid (GABA),

glycine, norepinephrine, dopamine, serotonin, neuropeptides, and nitric oxide.

Excitatory & Inhibitory Potentials

The effect of a neurotransmitter can be either excitatory or inhibitory a depolarizing postsynaptic potential is called an EPSP

it results from the opening of ligand-gated Na+ channels the postsynaptic cell is more likely to reach threshold

an inhibitory postsynaptic potential is called an IPSP it results from the opening of ligand-gated Cl- or K+ channels it causes the postsynaptic cell to become more negative or hyperpolarized the postsynaptic cell is less likely to reach threshold

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Overview of the Nervous System Among the 11 body systems, the nervous system and the endocrine system play the most important roles in maintaining homeostasis