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Nervous System

Organization of the Nervous System

1. Structural: All nervous system structures are classified as part of the CNS (brain and spinal cord) or PNS (nerves and ganglia).

2. Functional: Motor nerves of the PNS are classified on the basis of whether they stimulate skeletal muscle (somatic division) or smooth/cardiac muscle and glands (autonomic division) (Figure 7.1).

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Nervous Tissue: Structure and Function

1. Supportive connective tissue cells

   1. Neuroglia support and protect neurons in the CNS. Specific glial cells are phagocytes; others myelinate neuron processes in the CNS or line cavities.

   2. Schwann cells myelinate neuron processes in the PNS (Figure 7.2).

2. Neurons

   1. Anatomy: All neurons have a cell body containing the nucleus and processes (fibers) of two types; (1) axons (one per cell) typically generate and conduct impulses away from the cell body and release a neurotransmilter, and (2) dendrites (one to many per cell) typically carry electrical currents toward the cell body. Most large fibers are myelinated; myelin increases the rate of nerve impulse transmission.

   2. Classification

      1. On the basis of function (direction of impulse transmission) there are sensory (afferent) and motor (efferent) neurons and association neurons (interneurons). Dendritic endings of sensory neurons are bare (pain receptors), or are associated with sensory receptors (Figure 7.3).

      2. On the basis of structure, there are unipolar, bipolar, and multipolar neurons; the terminology reveals the number of processes extending from the cell body. Motor and association neurons are multipolar; most sensory neurons are unipolar. The exceptions are sensory neurons in certain special sense organs (ear, eye), which are bipolar.

   3. Physiology

      1. A nerve impulse is an electrochemical event (initiated by various stimuli) that causes a change in neuron plasma membrane permeability, allowing sodium ions (Na+) to enter the cell (depolarization). Once begun, the action potential, or nerve impulse, continues over the entire surface of the cell. Electrical conditions of the resting state are restored by the diffusion of potassium ions (K+) out of the cell (repolarization). Ion concentrations of the resting state are restored by the sodium-potassium pump (Figure 7.4).

      2. A neuron influences other neurons or effector cells by releasing neurotransmitters, chemicals that diffuse across the synaptic cleft and attach to membrane receptors on the postsynaptic cell. The result is opening of specific ion channels and activation or inhibition, depending on the neurotransmitter released and the target cell (Figure 7.5).

      3. A reflex is a rapid, predictable response to a stimulus. There are two types: autonomic and somatic. The minimum number of components of a reflex arc is four: receptor, effector, and sensory and motor neurons (most, however, have one or more interneurons) (Figure 7.6). Normal reflexes indicate normal nervous system function.

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Central Nervous System

1. The brain is located within the cranial cavity of the skull and consists of the cerebral hemispheres (forebrain), diencephalon, brain stem structures, and cerebellum (Figure 7.7).

   1. The two cerebral hemispheres form the largest part of the brain. Their surface, or cortex, is gray matter and their interior is white matter. The cortex is convoluted and has gyri, suici, and fissures. The cerebral hemispheres are involved in logical reasoning, moral conduct, emotional responses, sensory interpretation, and the initiation of voluntary muscle activity. Several functional areas of the cerebral lobes have been identified. The basal nuclei, regions of gray matter deep within the white matter of the cerebral hemispheres, modify voluntary motor activity. Parkinson's disease and Huntington's chorea are disorders of the basal nuclei.

   2. The diencephalon is superior to the brain stem and is enclosed by the cerebral hemispheres. The major structures include the following:

      1. The thalamus encloses the third ventricle and is the relay station for sensory impulses passing to the sensory cortex for interpretation.

      2. The hypothalamus makes up the "floor" of the third ventricle and is the most important regulatory center of the autonomic nervous system (regulates water balance, metabolism, thirst, temperature, and the like).

      3. The epithalamus includes the pineal body (an endocrine gland) and the choroid plexus of the third ventricle.

   3. The brain stem is the short region inferior to the hypothalamus that merges with the spinal cord.

      1. The midbrain is most superior and is primarily fiber tracts.

      2. The pons is inferior to the midbrain and has fiber tracts and nuclei involved in respiration.

      3. The medulla oblongata is the most inferior part of the brain stem. In addition to fiber tracts, it contains autonomic nuclei involved in the regulation of vital life activities (breathing, heart rate, blood pressure, etc.).

      4. The cerebellum is a large, cauliflower-like part of the brain posterior to the fourth ventricle. It coordinates muscle activity and body balance.

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2. Protection of the CNS

   1. Bones of the skull and vertebral column are the most external protective structures.

   2. Meninges are three connective tissue membranes: dura mater (tough outermost), arachnoid mater (middle weblike), and pia mater (innermost delicate) (Figure 7.8). The meninges extend beyond the end of the spinal cord.

   3. Cerebrospinal fluid (CSF) provides a watery cushion around the brain and cord. CSF is formed by the choroid plexuses of the brain. It is found in the subarachnoid space, ventricles, and central canal (Figure 7.9). CSF is continually formed and drained.

   4. The blood-brain barrier is composed of relatively impermeable capillaries.

3. Brain dysfunctions

   1. Head trauma may cause concussions (reversible damage) or contusions (nonreversible damage). When the brain stem is affected, unconsciousness (temporary or permanent) occurs. Trauma-induced brain injuries may be aggravated by intracranial hemorrhage or cerebral edema, both of which compress brain tissue-

   2. Cerebrovascular accidents (CVAs, or strokes) result when blood circulation to brain neurons is blocked and brain tissue dies. The result may be visual impairment, paralysis, and aphasias.

   3. Alzheimer's disease is a degenerative brain disease in which abnormal protein deposits and other structural changes appear. It results in slow, progressive loss of memory and motor control plus increasing dementia.

   4. Techniques used to diagnose brain dysfunctions include the EEG, simple reflex tests, pneumo-encephalography, angiography, and CT, PET, and MRI scans.

4. The spinal cord is a reflex center and conduction pathway. Found within the vertebral canal, the cord extends from the foramen magnum to L1 or L2. The cord has a central bat-shaped area of gray matter surrounded by columns of white matter, which carry motor and sensory tracts from and to the brain.

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Peripheral Nervous System

1. A nerve is a bundle of neuron processes wrapped in connective tissue coverings (endoneurium, perineurium, epineurium) (Figure 7.10).

2. Cranial nerves: Twelve pairs of nerves that extend from the brain to serve the head and neck region. The exception is the vagus nerves, which extend into the thorax and abdomen.

3. Spinal nerves: Thirty-one pairs of nerves are formed by the union of the dorsal and ventral roots of the spinal cord on each side. The spinal nerve proper is very short and splits into dorsal and ventral rami. Dorsal rami serve the posterior body trunk; ventral rami (except T1 through T12) form plexuses (cervical, brachial, lumbar, sacral) that serve the limbs.

4. Autonomic nervous system: Part of the PNS, composed of neurons that regulate the activity of smooth and cardiac muscle and glands. This system differs from the somatic nervous system in that there is a chain of two motor neurons from the CNS to the effector. Two subdivisions serve the same organs with different effects (Figure 7.11).

   1. The sympathetic division is the "fight-or-flight" subdivision, which prepares the body to cope with some threat. Its activation results in increased heart rate and blood pressure. The pre-ganglionic neurons are in the gray matter of the cord. The postganglionic neurons are in sympathetic chains or in collateral ganglia. Postganglionic axons secrete norepinephrine.

   2. The parasympathetic division is the "housekeeping" system and is in control most of the time. This division maintains homeostasis by seeing that normal digestion and elimination occur, and that energy is conserved. The first motor neurons are in the brain or the sacral region of the cord. The second motor neurons are in the terminal ganglia close to the organ served. Postganglionic axons secrete acetylcholine.

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Developmental Aspects of the Nervous System

1. Maternal and environmental factors may impair embryonic brain development. Oxygen deprivation destroys brain cells. Severe congenital brain diseases include cerebral palsy, anencephaly, hydrocephalus, and spina bifida.

2. Premature babies have trouble regulating body temperature because the hypothalamus is one of the last brain areas to mature prenatally.

3. Development of motor control indicates the progressive myelination and maturation of a child's nervous system. Brain growth ends in young adulthood. Neurons die throughout life and are not replaced; thus, brain mass declines with age.

4. Healthy aged people maintain nearly optimal intellectual function. Diseaseóparticularly cardiovascular diseases the major cause of declining mental function with age.