Human Physiology

Lesson 41 : Membrane Potentials

Classification of Nerve Fibers

Based on thickness of axons (nerve fibers) the nerve fibers are classified as:

Type	Diameter of axon Microns (µ)	Conduction Velocity meters/second	Example
IA	13	100	Sensory fibers from muscle spindle
IIA	9	50	Mechanoreceptors
IIIA	3	15	Deep pressure sensors of muscles
IVC	<1 (unmyelinated)	1	Pain fibers

Conduction Velocity: is velocity (distance tranvelled per second) of conduction of nerve impulse along a nerve fiber.

Velocity is proportional to the diameter of axon.

Velocity is greater in myelinated fiber than unmyelinated fiber of same diameter.

Sensory Processing of Nervous System

Sensory information from outside the body are gathered through special sensory organs like skin, eye, ear, nose, tongue etc. and passed on to central nervous system through various sensory nerves (Fig 41b). This kind of sensory experiences can either cause an immediate reaction from the brain or stored as memory for few minutes to few years for future use. Sensory nerves enter CNS through:

1. All level of spinal cord, through dossal root.
2. Reticular formation of pons, medulla and mid brain.
3. Cerebellum
4. Thalamus
5. Sensory cortex of cerebrum.

Motor processing of Nervous System

Most bodily activities of human are controlled through motor centers of brain and associated nerves. Various motor activities controlled by brain are:

1. Contraction of all skeletal muscles of body.
2. Contraction of all smooth muscles lf internal organs.
3. Secretion of exocrine and endocrine glands.

The organs (muscles and glands) affected by motor system are called effectors. Skeletal muscles are chiefly controlled by somatic nerves arising through spinal cord whereas smooth muscles and glands are controlled through autonomic nerves (Fig 41c). Motor centers are located in:

1. Spinal cord
2. Reticular formation in pons, medulla and mid brain.
3. Basal ganglion
4. Cerebellum
5. Motor cortex of cerebrum.