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This text describes the functions and structure of the nervous system, including sensory input, integration of information, motor output, and the classification of the central and peripheral nervous systems. It also provides information on neurons and neuroglia, as well as common disorders of the nervous system.
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NERVOUS SYSTEM • Functions A. Sensory Input – monitors all information about change occurring inside and outside the body B. Integration – processes and interprets the information received and integrates it in order to make decisions C. Motor Output – commands responses by activating muscles, glands, and other parts of the nervous system
Structural Classification A. Central Nervous System (CNS) 1. brain 2. spinal cord B. Peripheral Nervous System (PNS) 1. spinal nerves 2. cranial nerves
Nervous System Cells A. Neurons 1. Structure: a. dendrites – receive impulses and conduct them to cell body. b. cell body- contains the nucleus and organelles; regulates cell. c. axon – conducts impulses away from cell body by means of axonal terminals. i. Axon terminals - distal terminations of the branches of an axon ii. Synaptic end bulbs - terminal enlargements of nerve fibers that are in contact with the dendrites or cell bodies of other nerve cells, muscles, or glands.
iii. myelin – lipid-protein covering that protects, insulates, and speeds transmissions iv. Schwann cells (outside CNS) – segments along the axon separated from one another by Nodes of Ranvier (produce myelin sheaths) v. Nodes of Ranvier - are the gaps (approximately 1 micrometer in length) formed between the Schwann cells. vi. neurolemma – outermost layer (sheath) around Schwann cell in PNS
2. Types of Neurons a. sensory (afferent) – conduct impulses to the brain and spinal cord b. motor (efferent) – conduct impulses away from brain and spinal cord to muscles and glands c. interneurons (association) – conduct impulses from sensory neurons to motor neurons
Neuroglia (glial) – function in support, insulation, and protection 1. Types in the CNS a. Astrocytes – star-shaped cells that anchor small blood vessels to neurons b. Microglia – small cells that move in inflamed brain tissue and carry on phagocytosis c. Oligodendroglia – form myelin sheaths around axons in the CNS d. Ependymal cells – line the cavities of the brain & spinal cord and produce cerebrospinal fluid.
2. Types in the PNSa. Schwann Cells- produce myelin sheaths for the PNS b. Satellite Cells – support neurons in ganglia of PNS C. Axon bundles 1. In the PNS the bundles form White Matter. They are white because of the myelin, the white matter propagates the never signals. 2. In the CNS the cell bodies and unmyelinated axons are called Gray Matter. This tissue performs integrative functions.
Disorders of Nervous Tissue 1. Multiple Sclerosis – characterized by myelin loss in central nerve fibers resulting in conduction impairment a. loss of muscle control/coordination, visual impairment, and speech problems b. autoimmune disease triggered by a viral infection c. chronic, prolonged, can be relapsing/remitting d. most common in women age 20-40 e. no known cure 2. Tumors – general name for nervous system tumor is neuroma a. most neuromas are gliomas (neuroglial tumor) b. multiple neurofibromatosis – characterized by numerous benign tumors that can progress to disfiguring, crippling soft tissue tumors 1. inherited (a.k.a. Elephant Man’s disease)
Nerve – bundle of peripheral nerve fibers (axons) bundled together like strands of a cable (in CNS referred to as tracts) A. White matter – tissue composed mainly of myelinated axons (nerves/tracts) B. Gray matter – tissue composed of cell bodies and unmyelinated fibers C. Coverings – fibrous connective tissue 1. Endoneurium – surrounds individual fibers within a nerve 2. Perineurium – surrounds a group (fascicle) of nerve fibers 3. Epineurium – surrounds the entire nerve
Reflex Arcs – conduction of nerve impulse results in a reflex A. responses can be contraction by a muscle or secretion by a gland B. simplest reflex arcs are two-neuron: sensory neurons synapsing in the spinal cord with motor neurons. Ex: “knee-jerk” response C. three-neuron arcs consist of sensory neurons synapsing with interneurons that synapse with motor neurons. Ex: withdrawal response
Nerve Impulse – a self-propagating wave of electrical disturbance that travels along the surface of a neuron membrane A.Physiology – neurons have two major functional properties: 1.Irritability – the ability to respond to a stimulus and convert it into a nerve impulse (electrochemical event)
a. stimuli causes change in the neuron plasma membrane permeability allowing sodium ions (Na+) to enter the cell – depolarization b. action potential/nerve impulse (all-or-none response) continues over the entire length of the cell c. electrical conditions of the resting stage (polarized) are restored by the diffusion of potassium ions (K+) out of the cell – repolarization d. ion concentration is restored by the sodium-potassium pump
2. Conductivity – the ability to transmit the impulse to other neurons, muscles, or glands. a. neurotransmitters released by axonal terminals of a presynaptic neuron into synaptic cleft. b. neurotransmitter binds to specific molecules in the membrane of a postsynaptic neuron. This stimulates impulse conduction by the membrane. c. names of neurotransmitters – acetylcholine, catecholamines (norepinephrine, dopamine, serotonin)
d. Parkinson’s disease – chronic disorder brought on by a deficiency of the neurotransmitter dopamine 1. signs include: - rigidity and trembling of the head and extremities - forward tilt of the trunk - shuffling manner of walking 2. treatments include: - L-dopa - surgical grafting of normal dopamine-secreting neurons - surgery to cauterize affected areas
