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Overview and Organization of the Nervous System
- The divisions of the nervous system have been categorized as either structural (central nervous system [CNS] and peripheral nervous system [PNS]) or functional (somatic nervous system and autonomic nervous system [ANS]).
- The CNS is contained within the brain and spinal cord.
- The PNS is composed of cranial and spinal nerves, which carry impulses toward the CNS (afferent) and away from the CNS (efferent) to target organs or skeletal muscle.
Cells of the Nervous System
- The neuron and neuroglial cells constitute nervous tissue. The neuron is specialized to transmit and receive electrical and chemical impulses, whereas the neuroglial cell provides supportive functions. The neuron is further divided into unipolar, pseudounipolar, bipolar, and multipolar categories according to its structure and particular mechanics of impulse transmission.
- The neuron is composed of a cell body, dendrite(s), and an axon. A myelin sheath around selected axons forms an insulation that allows quicker nerve impulse conduction.
The Nerve Impulse
- The region between the neurons is the synapse, and the region between the neuron and muscle is the myoneural junction.
- Neurotransmitters are responsible for chemical conduction across the synapse, and myoneural junction nerve impulse is regulated predominantly by a balance of inhibitory postsynaptic potentials (IPSPs) and excitatory postsynaptic potentials (EPSPs), temporal and spatial summation, and convergence and divergence.
The Central Nervous System
- The brain is contained within the cranial vault and is divided into three distinct regions: (1) forebrain, (2) hindbrain, and (3) midbrain.
- The forebrain comprises the two cerebral hemispheres and allows conscious perception of internal and external stimuli, thought and memory processes, and voluntary control of skeletal muscles. The deep portion of the forebrain is termed the diencephalon and processes incoming sensory data. The center for voluntary control of skeletal muscle movements is located along the precentral gyrus in the frontal lobe, whereas the center for perception is along the postcentral gyrus in the parietal lobe. The Broca area (inferior frontal gyrus) and the Wernicke area (superior temporal gyrus) are major speech centers.
- The hindbrain allows sampling and comparison of sensory data, which are received from the periphery and motor impulses of the cerebral hemispheres, for the purpose of coordination and refinement of skeletal muscle movement.
- The midbrain is primarily a relay center for motor and sensory tracts, as well as a center for auditory and visual reflexes.
- The spinal cord contains most of the nerve fibers that connect the brain with the periphery. Reflex arcs are completed in the spinal cord and influenced by the higher centers in the brain.
- The CNS is protected by the scalp, bony cranium, meninges, vertebral column, and cerebrospinal fluid (CSF). CSF is formed from blood components in the choroid plexuses of the ventricles and is reabsorbed in the arachnoid villi (located in the dural venous sinuses) after circulating through the brain and subarachnoid space.
- The paired carotid and vertebral arteries supply blood to the brain and connect to form the circle of Willis. The major branches projecting from the circle of Willis are the anterior, middle, and posterior cerebral arteries. Drainage of blood from the brain is accomplished through the venous sinuses and jugular veins.
- The blood-brain barrier is provided by tight junctions between the cells of brain capillaries and surrounding supporting cells.
- Blood supply to the spinal cord originates from the vertebral arteries and branches arising from the aorta.
The Peripheral Nervous System
The PNS relays information from the CNS to muscle and effector organs through cranial and spinal nerve tracts arranged in fascicles (multiple fascicles bound together form the peripheral nerve).
The Autonomic Nervous System
The ANS is responsible for maintaining a steady state in the internal environment. Two opposing systems make up the ANS: (1) the sympathetic nervous system responds to stress by mobilizing energy stores and prepares the body to defend itself, and (2) the parasympathetic nervous system conserves energy and the body’s resources. Both systems function, more or less, at the same time.
Aging & the Nervous System
- Major structural changes with aging include a decrease in number of neurons and a decrease in brain weight and size.
- Deposition of lipofuscin and the presence of multiple
- neurofibrillary tangles are common cellular changes with aging.
- A progressive slowing of neurologic function occurs with advancing age.