192 The Nervous System
STUDY OUTLINE
Functions of the Nervous System
- Detect changes and feel sensations.
- Initiate responses to changes.
- Organize and store information.
Nervous System Divisions
- Central nervous system (CNS)—brain and spinal
cord. - Peripheral nervous system (PNS)—cranial nerves
and spinal nerves.
Nerve Tissue—neurons (nerve fibers) and
specialized cells (Schwann, neuroglia)
- Neuron cell body contains the nucleus; cell bodies
are in the CNS or in the trunk and are protected by
bone. - Axon carries impulses away from the cell body;
dendrites carry impulses toward the cell body. - Schwann cells in PNS: Layers of cell membrane
form the myelin sheath to electrically insulate neu-
rons; nodes of Ranvier are spaces between adjacent
Schwann cells. Nuclei and cytoplasm of Schwann
cells form the neurolemma, which is essential for
regeneration of damaged axons or dendrites. - Oligodendrocytes in CNS form the myelin
sheaths; microglia phagocytize pathogens and
damaged cells; astrocytes contribute to the
blood–brain barrier (see Table 8–1). - Synapse—the space between the axon of one neu-
ron and the dendrites or cell body of the next neu-
ron. A neurotransmitter carries the impulse across
a synapse and is then destroyed by a chemical inac-
tivator. Synapses make impulse transmission one
way in the living person.
Types of Neurons—nerve fibers
- Sensory—carry impulses from receptors to the
CNS; may be somatic (from skin, skeletal muscles,
joints) or visceral (from internal organs). - Motor—carry impulses from the CNS to effectors;
may be somatic (to skeletal muscle) or visceral (to
smooth muscle, cardiac muscle, or glands). Visceral
motor neurons make up the autonomic nervous
system. - Interneurons—entirely within the CNS.
Nerves and Nerve Tracts
- Sensory nerve—made only of sensory neurons.
- Motor nerve—made only of motor neurons.
3. Mixed nerve—made of both sensory and motor
neurons.
4. Nerve tract—a nerve within the CNS; also called
white matter.
The Nerve Impulse—see Table 8–2- Polarization—neuron membrane has a () charge
outside and a () charge inside. - Depolarization—entry of Naions and reversal of
charges on either side of the membrane. - Impulse transmission is rapid, often several meters
per second.
- Saltatory conduction—in a myelinated neuron
only the nodes of Ranvier depolarize; increases
speed of impulses.
The Spinal Cord- Functions: transmits impulses to and from the
brain, and integrates the spinal cord reflexes. - Location: within the vertebral canal; extends from
the foramen magnum to the disc between the 1st
and 2nd lumbar vertebrae. - Cross-section: internal H-shaped gray matter con-
tains cell bodies of motor neurons and interneu-
rons; external white matter is the myelinated axons
and dendrites of interneurons. - Ascending tracts carry sensory impulses to the
brain; descending tracts carry motor impulses away
from the brain. - Central canal contains cerebrospinal fluid and is
continuous with the ventricles of the brain.
Spinal Nerves—see Table 8–3 for major
peripheral nerves- Eight cervical pairs to head, neck, shoulder, arm,
and diaphragm; 12 thoracic pairs to trunk; 5 lum-
bar pairs and 5 sacral pairs to hip, pelvic cavity, and
leg; 1 very small coccygeal pair. - Cauda equina—the lumbar and sacral nerves that
extend below the end of the spinal cord. - Each spinal nerve has two roots: dorsal or sensory
root; dorsal root ganglion contains cell bodies of
sensory neurons; ventral or motor root; the two
roots unite to form a mixed spinal nerve.
Spinal Cord Reflexes—do not depend directly
on the brain- A reflex is an involuntary response to a stimulus.
- Reflex arc—the pathway of nerve impulses during a