Body functions are controlled by a rapid-response nervous system
and also by hormones that act more slowly.
Nervous tissues are made of nerve cells (neurons) and other
cells (neuroglia).
A nerve impulse begins as a depolarization
of the normally polarized cell membrane, and travels as a wave of
depolarization.
Neuroglia cells aid in the structural support and nourishment of the neurons.
Structure of nerve cells (neurons):
- Cell body: includes both nucleus (normal in
structure, normally round) and cytoplasmic portion (perikaryon)
surrounding nucleus.
Most of the perikaryon contains many
grain-like vesicles of rough endoplasmic reticulum (Nissl
granules) rich in RNA.
Exception: axon hillock,
at the base of the axon, is largely Nissl-free.
- Dendrites (often numerous): branching processes whose membranes
conduct nerve impulses toward the cell body. Interior cytoplasm resembles
cytoplasm of cell body, but with fewer Nissl granules.
- Axon: long, cylindrical process, essentially unbranching over
most of its length, usually conducting impulses away from the cell body
and usually more rapidly than in dendrites. Many axons are surrounded
by a myelin sheath of fatty Schwann cells, providing
electrical insulation by wrapping plasma membrane around the axon in
the manner of a jelly roll. Impulses in myelinated axons travel much
faster by saltatory conduction (jumping) from one constriction
(node of Ranvier) to the next.
Nature of the nerve impulse:
- In the "resting" (non-conducting) neuron, a sodium pump
actively transports Na+ ions across the cell membrane to
the outside. Potassium (K+) ions enter the cell to neutralize
the electric charge, but they do not quite compensate, so the membrane
is polarized with a net negative charge of about 60 millivolts (-60 mV)
inside the cell.
- Changes that hyperpolarize the membrane, or that depolarize it
only slightly, travel a short distance along the membrane before they decay.
They may locally alter the probability of an action potential.
- Neurotransmitters released by neighboring neurons turn off the
sodum pump, creating a depolarization of the membrane. This is more
than enough to interrupt the sodium pump in the next section of membrane,
which then depolarizes, and the process repeats itself rapidly along the
membrane, resulting in a nerve impulse, which is essentially a
wave of depolarization.
- Electrical recordings show a reverse polarization of characteristic shape
called a spike or action potential.
- After the nerve impulse has passed, the sodium pump resumes its action
and reestablishes normal polarity after a short delay called the
refractory period.
Neuroglia: Cells of the nervous system, other than neurons. These cells
provide structural support and nutrition to the neurons.
Most nervous tissue is avascular (no blood vessels inside), so nutrients must
diffuse in and wastes must diffuse out. The neuroglia aid in this process,
often by sending out a perineural foot to surround the neuron and a
perivascular foot to surround a nearby blood vessel.
Examples: Astrocytes, oligodendrocytes (oligodendroglia), microglia,
Schwann cells.
Organization of the nervous system:
- Central nervous system (CNS): the brain and spinal cord
- Peripheral nervous system (PNS): peripheral nerves, including
both cranial nerves (originating in the brain) and spinal
nerves (originating in the spinal cord)
- Special sense organs: eye, ear, taste buds, nasal epithelium
- Nerves are bundles of axons outside the CNS; tracts
are similar bundles within the brain.
- Ganglia are clumps of nerve cell bodies outside the CNS;
nuclei are similar clumps in the brain.
|