Nervous System

Nervous System Divisions of the Nervous System Is divided anatomically into the central nervous system (CNS), consisting of the brain and spinal cord, and the peripheral nervous system (PNS), consisting of 12 pairs of cranial nerves and 31 pairs of spinal nerves, and their associated ganglia. Is divided functionally into the somatic nervous system, which controls primarily voluntary activities, and the visceral (autonomic) nervous system, which controls primarily involuntary activities. Is composed of neurons and neuroglia , which are nonneuronal cells such as astrocytes, oligodendrocytes, and microglia. Controls and integrates the activity of various parts of the body. Neurons Neurons: Are the structural and functional units of the nervous system (neuron doctrine). Are specialised for the reception, integration, transformation, and transmission of information. Components of neurons Consist of cell bodies (perikaryon or soma) and their processes, dendrites and axons. Cell bodies are located in the gray matter of the CNS, and their collections are called ganglia in the PNS and nuclei in the CNS. Dendrites are usually short and highly branched and carry impulses toward the cell body. Axons are usually single and long, have fewer branches (collaterals), and carry impulses away from the cell body. Classification of neurons 1. Unipolar (pseudounipolar) neurons Have one process , which divides into a central branch that functions as an axon and a peripheral branch that serves as a dendrite. Are called pseudounipolar because they were originally bipolar. The two processes fuse during development to form a single process that bifurcates at a distance from the cell body. Are sensory neurons of the PNS and found in spinal and cranial nerve ganglia. 2. Bipolar neurons Have two processes (one dendrite and one axon); are sensory; and are found in the olfactory epithelium, the retina, and the inner ear. 3. Multipolar neurons Have several dendrites and one axon and are most common in the CNS (e.g.,motor cells in anterior and lateral horns of the spinal cord, autonomic ganglion cells). Ganglion Is a collection of neuron cell bodies outside the CNS , and a nucleus is a collection of neuron cell bodies within the CNS. Other components of the nervous system Cells that support neurons Include Schwann cells and satellite cells in the PNS. Are called neuroglia in the CNS and are composed mainly of three types: astrocytes; oligodendrocytes , which play a role in myelin formation and transport of material to neurons; and microglia , which phagocytose waste products of nerve tissue. Myelin Is the fat-like substance forming a sheath around certain nerve fibers. Is formed by Schwann cells in the PNS and oligodendrocytes in the CNS. Synapses Are the sites of functional contact of a neuron with another neuron, an effector (muscle, gland) cell, or a sensory receptor cell. Are classified by the site of contact as axodendritic, axoaxonic, or axosomatic (between axon and cell body). Subserve the transmission of nerve impulses, commonly from the axon terminals (presynaptic elements) to the plasma membranes (postsynaptic elements) of the receiving cell. Central Nervous System (CNS) Brain Is enclosed within the cranium, or brain case. Has a cortex , which is the outer part of the cerebral hemispheres, and is composed of gray matter. This matter consists largely of the nerve cell bodies , dendrites, and neuroglia. Has an interior part composed of white matter , which consists largely of axons forming tracts or pathways, and ventricles, which are filled with cerebrospinal fluid (CSF). Spinal cord Is cylindrical , occupies approximately the upper two thirds of the vertebral canal, and is enveloped by the meninges. Has cervical and lumbar enlargements for the nerve supply of the upper and lower limbs, respectively. Has centrally located gray matter , in contrast to the cerebral hemispheres, and peripherally located white matter. Grows more slowly than the vertebral column during fetal development, and hence, its terminal end gradually shifts to a higher level. Has a conical end known as the conus medullaris and ends at the level of L2 (or between L1 and L2) in the adult and at the level of L3 in the newborn. Meninges Consist of three layers of connective tissue membranes (pia, arachnoid , and dura mater) that surround and protect the brain and spinal cord. Contain the subarachnoid space , which is the interval between the arachnoid and pia mater, filled with CSF. Peripheral Nervous System (PNS) Cranial nerves Consist of 12 pairs and are connected to the brain rather than to the spinal cord. Have motor fibers with cell bodies located within the CNS and sensory fibers with cell bodies that form sensory ganglia located outside the CNS. Emerge from the ventral aspect of the brain (except for the trochlear nerve, cranial nerve IV). Contain all four functional components of the spinal nerves and three additional components Spinal nerves Consist of 31 pairs : 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. Are formed from dorsal and ventral roots; each dorsal root has a ganglion that is within the intervertebral foramen. Are connected with the sympathetic chain ganglia by rami communicantes. Contain sensory fibers with cell bodies in the dorsal root ganglion (general somaticafferent [GSA] and general visceral afferent [GVA] fibers), motor fibers with cell bodies in the anterior horn of the spinal cord (general somatic efferent [GSE]fibers), and motor fibers with cell bodies in the lateral horn of the spinal cord (general visceral efferent [GVE] fibers) between T1 and L2. Are divided into the ventral and dorsal primary rami. The ventral primary rami enter into the formation of plexuses (i.e., cervical, brachial, and lumbosacral); the dorsal primary rami innervate the skin and deep muscles of the back. Functional components in peripheral nerves General somatic afferent (GSA) fibers Transmit pain, temperature, touch, and proprioception from the body to the CNS. General somatic efferent (GSE) fibers Carry motor impulses to skeletal muscles of the body. General visceral afferent (GVA) fibers Convey sensory impulses from visceral organs to the CNS. General visceral efferent (GVE) fibers (autonomic nerves) Transmit motor impulses to smooth muscle, cardiac muscle, and glandular tissues. Special somatic afferent (SSA) fibers Convey special sensory impulses of vision, hearing, and equilibration to the CNS. Special visceral afferent (SVA) fibers Transmit smell and taste sensations to the CNS. Special visceral efferent (SVE) fibers Conduct motor impulses to the muscles of the head and neck. Arise from branchiomeric structures such as muscles for mastication, muscles for facial expression, and muscles for elevation of the pharynx and movement of the larynx. Autonomic Nervous System The autonomic nervous system is Divided into the sympathetic (thoracolumbar outflow), parasympathetic (craniosacral outflow), and enteric divisions. Composed of two neurons, preganglionic and postganglionic, which are GVE neurons. Sympathetic nerve fibers Have preganglionic nerve cell bodies that are located in the lateral horn of the thoracic and upper lumbar levels (L2 or L1-L3) of the spinal cord. Have preganglionic fibers that pass through ventral roots, spinal nerves, and white rami communicantes. These fibers enter adjacent sympathetic chain ganglia, where they synapse or travel up or down the chain to synapse in remote ganglia or run further through the splanchnic nerves to synapse in collateral ganglia, located along the major abdominal blood vessels. Have postganglionic fibers from the chain ganglia that return to spinal nerves by way of gray rami communicantes and supply the skin with secretory fibers to sweat glands, motor fibers to smooth muscles of the hair follicles (arrectores pilorum), and vasomotor fibers to the blood vessels. Function primarily in emergencies , preparing individuals for fight or flight , and thus increase heart rate, inhibit GI motility and secretion, and dilate pupils and bronchial lumen. Parasympathetic nerve fibers Comprise the preganglionic fibers that arise from the brainstem (cranial nerves III,VII, IX, and X) and sacral part of the spinal cord (second, third, and fourth sacralsegments). Are, with few exceptions, characterized by long preganglionic fibers and short postganglionic fibers. Are distributed to the walls of the visceral organs and glands of the digestive system but not to the skin or to the periphery. Decrease heart rate, increase GI peristalsis, and stimulate secretory activity. Function primarily in homeostasis , tending to promote quiet and orderly processes of the body. Enteric division Consists of enteric ganglia and plexus of the GI tract, including the myenteric(Auerbach's) and submucosal (Meissner's) plexuses. Plays an important role in the control of GI motility and secretion. ◄.....Go back to the Gross Anatomy homepage