Nerves are the white cable-like structures (located outside the central nervous system) over which the nerve impulses travel in roughly the same way the electrical waves travel over electrical wires. A nerve consists of nerve fibers (grouped in bundles, or fasciculi), together with their blood vessels and lymphatic vessels and fiber coverings. The porineurium is the thin covering of afsciculus, with thin strands (the endoneurium) extending among the nerve fibers; the epineurium, comprising the membrane that covers the nerve, contains blood vessels and adipose tissue, encloses the fasciculi, and contains the nervi, nerverum, or sensory nerve fibers.
Bundles of nerve fibers maybe connected within a single nerve, or the fibers may divide into branches connecting with other nerves to form a nerve network, or plexus. Inside the spinal cord and brain, however, when the nerve fibers connect to form bundles, these are called tracts instead of nerves. In the cord, ascending tracts carry afferent impulses to the brain, while descending tracts carry efferent impulses from the brain. In the brain, projection fibers connect the cerebral cortex and brain stem or the spinal cord, commissural fibers connect both sides of the brain and association fibers connect brain parts on the same side. Injury to a group of projection tracts (the internal capsule) of the brain may cause paralysis from hemorrhage (apoplexy or stroke).
A characteristic of nerve tissue is reflected in the popular reference to gray matter as a sign of intelligence or “using one’s head”. The brain and parts of the spinal cord contain masses of gray nerve fibers and cell bodies which act as integrating centers for organizing nerve impulses and thus make possible the higher mental processes of reasoning, or judging and understanding. The brain and the cord also contain white matter, the group of nerve fibers (with a fatty, myelin sheath) which serves mainly as of nerve fibers (with a fatty, myelin sheath) which serve mainly as conductors or nerve impulse.
The nerve tissue possesses the properties of irritability (the ability to respond to a stimulus), conductivity (the ability to conduct nerve impulses), and integration (the ability to organize and regulates nerve impulses).
Types of Stimulus
- Other force, like radiation
Characteristics of an effective stimulus:
- Strong enough to cause a response (minimal strength)
- Must occur suddenly
- Must continue its action for a minimal period of time at a minimal note of occurrence
Similarities between a nerve tissue and muscle tissue:
- Both follow the all-or-none response
- A more powerful stimulus affects more fibers (nerve or muscle) producing a greater flow of (nerve or muscle) impulses
- Both are unable to react unless rested briefly between stimulus
- Toxic substances or lack of 02 may impede their activity
Differences: Nerva fibers are not subject to fatigue, unlike muscle tissue.
Methods of blocking a nerve impulse:
- Application of extreme cold
- Pressure on the nerve. Ex.: crushing the phrenic nerve to stop movements of the diaphragm is part of one treatment for intractable TB.
- Electrical or chemical application.
- Anesthesia (cause loss of consciousness by interfering with nerve function).
All nerve impulses are fundamentally alike and can be regarded in the same way as electrical or other types of waves. The nerve impulses differ from one another primarily as to the areas of the body which they activate. Examples:
- Light rays affect the optic nerve which transmits the nerve impulses to the cortex of the brain producing the visual sensation.
- Sound waves affect the acoustic or auditory nerve which transmits nerve impulses to the tempral lobe of the brain, producing the cessation of hearing.
Reflex Action – the nerve mechanism responsible to most activities of the organism.
Types of Reflex Action:
- Automatic, specific and designed to perform a definite, useful function
- Conditioned reflex
- Inborn reflexes (present at birth)
- Defecation of, etc.
Important reflexes checked by physicians for diagnosis:
- Knee jerk or patellar reflex – the leg extends in response to tapping the patellar tendon.
- Achilles reflex – the foot extends in response tapping the tendon of Achilles.
- Corneal reflex – the eye closes in response to touching the corneal.
- Pupiflary reflex – the pupil of the eye constricts in response to the bright light.
- Babiuska reflex – the toes extend in response to stroking the sole of the foot.