NERVOUS CONTROL OF MUSCULAR MOVEMENT
STRUCTURE AND FUNCTION OF THE NERVOUS SYSTEM
Divisions of The Nervous System
A. Central Nervous System
1. Brain - newer more sophisticated regions are piled on _____ of older, more
(i) ___Cerebrum____ constitutes about 80% of total brain weight
- cerebral cortex, basal nuclei
(ii) Diencephalon - thalamus, hypothalamus
c) __Brainstem____ - continuous with the spinal cord
- medulla, pons, midbrain
2. Spinal cord - long cylinder of nerve tissue which extends down from the
__brainstem__ to the second ___lumbar___ vertebrae. 45 cm long and 2 cm in
- Protected by the vertebral column and associated ligaments and muscles, the
spinal meninges and the cerebrospinal fluid.
B. Peripheral Nervous System
Consists of 12 pairs of __cranial__ nerves and 31 pairs of __spinal__ nerves
1. Afferent division - conveys information from the ___sensors__ in the
periphery to the central nervous system (CNS)
2. Efferent division
a) _Somatic__ nervous system - nerve fibers innervate _skeletal muscle__
b) _Autonomic__ nervous system - nerve fibers innervate _smooth___
and __cardiac__ muscle and glands
(i) sympathetic division [Stress, Fight/Flight – increase
(ii) parasympathetic division [Normal functions, opposite of
stress response –decrease activity]
Neuroglia – comprise about _90%__ of the cells within the CNS. They
occupy about __half__ of the volume of the brain. The four major types of
glial cells serve as the __connective____ tissue of the CNS and as such
help support the neurons both __physically____ and
It is estimated that there are approximately 100 billion neurons in the brain
and one trillion neuroglia. (More Neuroglia than neurons!)
Along with the endocrine system, the nervous system regulates and
coordinates the various functions of the body.
Basic Structure of a Nerve
Neuron – a nerve cell. Neurons are specialized to transmit __electrical___
signals. It consists of:
1. Cell body - soma - contains the _nucleus___
2. Axon - a long fiber that conducts impulses __away__ from the cell
body. The term "____nerve fiber____" is generally used in reference to
3. Dendrite - short projections from the cell body that transmit impulses
__toward____ the cell body
The main purpose of the neuron is to pass messages (impulses) from one
part of the body to another
Myelin sheath - a discontinuous sheath around the axon. It is composed
mainly of __lipid___ and _______protein__.
Nodes of Ranvier - spaces between the segments of myelin sheath
Myelinated nerve fibers have much __faster__ conduction velocities than
unmyelinated fibers 3
Synapse – the connection of an axon of one nerve to the cell body or
dendrites of another nerve.
Neurons can be divided into 3 functional classes:
1. Afferent neurons - carry impulses from the ___sensory___ receptors
into spinal cord or brain
2. Efferent neurons - transmit impulses from the CNS out to the
___effector______ organs such as muscles (motor neurons) and
3. Interneurons - lie entirely within the __CNS__. They account for
_99_% of all nerve cells.
Each spinal nerve is actually a nerve __trunk___ - it contains hundreds of
individual afferent and efferent nerve fibers that are bound together by
connective tissue sheaths.
The Nerve Impulse
Resting membrane potential - due to the __selective___ permeability
characteristics of the nerve cell membrane, a potential (voltage)
__difference__ exists between the inside and outside of the nerve fiber.
A high concentration of positive __sodium___ ions on the outside of the
nerve membrane causes it to be electrically ___positive___, while the
inside of the nerve is electrically __negative____.
Action potential - an appropriate stimulus suddenly causes _sodium__
ions to rush to the inside of the nerve reversal of polarity.
Once the action potential is _started__, it spreads along the entire length of
the nerve fiber
Nerve to Nerve Synapses
Nervous information is relayed across the ____synaptic cleft____ by
means of a chemical transmitter substance.
Transmitter substances can be either ___excitatory_____ or
___inhibitroy____ in their effects on the post synaptic membrane potential
[either creates an action potential or inhibits an action potential]
Spatial vs. temporal summation
Neuromuscular junction - nerve to __muscle__ synapse. The chemical
transmitter substance is ___acetylcholine____.
The spinal cord is enlarged in two regions for innervation of the limbs: 5
a) The __cervical___ enlargement which extends from the C4 through T1
segments of the spinal cord
b) The __lumbosacral____ enlargement which extends from the T11 through L1
segments of the spinal cord
Plexus – a network of converging and diverging nerve fibers, or blood vessels.
The brain and spinal cord are composed of gray matter and white matter. The
nerve cell bodies lie in and constitute the _gray__ matter while the
interconnecting tracts of nerve fibers (axons) form the __white___ matter.
Structure of spinal nerves – 31 pairs of spinal nerves are attached to the
spinal cord –
Each spinal nerve has a __dorsal___ root and a __ventral__ root connected to the
spinal cord. The dorsal roots contain __afferent___ (sensory) fibers that carry
information from the periphery to the spinal cord and brain. The ventral roots
contain ___efferent___ (motor) fibers to the skeletal muscle.
The cell bodies of the motor axons making up the __ventral___ roots are located
in the ventral gray horns of the spinal cord
The cell bodies of the sensory axons making up the __dorsal___ roots are outside
of the spinal cord in the spinal __ganglia__.
Ganglion – a collection of nerve cell _bodies__ located outside of the CNS.
Spinal cord injury – transection of the spinal cord results in loss of all sensatio