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Nervous System Key Points
I. Introduction:
A. Most highly organized system of the body
B. Fast, completx communication system that regulates thoughts, emoutions, movements, impressions,
reasonling, learning, memory, & choices.
C. basic Characteristics
1. Master control system
2. Master communications system
3. Regulates, maintains homeostasis
D. Functions
1. Monitors changes (stimuli) – sensory input
2. Integrates impules 0 integration
3. Effects responses – motor output
4. Mental activity
5. Homeostasis
II. Organization of the Nervous System
A. CNS (central nervous system)
1. Brain & spinal cord
2. Integrates incoming pieces of sensory information, evaluates the information & initiates the outgoing
responses.
3. NO potential for regeneration
B. PNS (peripherial nervous system) Made of 12 pairs of cranial nerves & 31 pairs of spinal nerves
1. Broken down into 2 parts:
2. Afferent (sensory) division
a Carries impulses from sensory receptors in the internal organs or skin
the CNS. Cell bodies
are always found in a ganglion outside the CNS.
3. Efferent (motor) division. Carries impulses from the CNS
to the viscera, muscles, or glands.
Is subdivided into 2 parts:
a. Somatic-voluntary control & reflex (skin, skeletal muscles, joints) carries information to skeletal
muscles.{involves one ganglia}{Responds to external stimuli}
b. Autonomic or Visceral –involuntary, regulates smooth muscles, cardiac muscles & glands (organs
within the ventral cavity). {involves two ganglia}[responds to internal stimuli]
Is subdivided into 2 parts.
i. Sympathetic: exit thoracic area of spinal cord & involved in preparing body for “fight or flight”
response. In this mode most of the time. “Fight-or-Flight” dilates pupils, reduces saliva, mucus,
perstalsis, intestinal motility, urine secretion. Increases heart rate, glycogen to glucose conversion.
ii. Parasympathetic: exit cervical & lumbar areas of spinal cord & coordinates the body’s normal
resting activities. Brings body’s physiological changes due to flight or flight back to normal. (“restingdigesting-repairing” – constricts pupils, reduces heart rate, increases mucus production, gastric juice
production, digestion, urine production, intestinal tract motility, & peristalsis.)
III. Histology of the Nervous Tissue
A. Basic Characteristics
1. Highly cellular
2. Two types of cells – neurons & supporting cells (neuroglia)
3. Neurons are nerve cells & the functional unit of the NS. NEURONS ARE NOT NERVES!!
a. Afferent neurons are sensory neurons where the impulse goes FROM a receptor in PNS - like in
skin, muscles, sense organs, joints, viscera - TO CNS (in other words, these conduct INCOMING information).
b. Somatic afferent neurons monitor the outside world and our position in it (many of these sensations
are at a conscious level).
c. Visceral afferent neurons monitor our internal environment and conditions that must be maintained
in order to be in homeostatic balance (many of these sensations are at an unconscious level). We will talk about
receptor types in Chapter 15, but they fall into three main types: Exteroceptors (info about the outside world
like touch, sight, hearing), proprioceptors (info about our body's position in space via monitors in skeletal
muscles and joints), and interoceptors (info about the internal environment like blood pressure or oxygen
concentration in blood).
d. Efferent neurons are motor neurons where the impulse goes FROM CNS TO effector in PNS - like to
muscle or gland cells.
e. Somatic efferent neurons innervate skeletal muscle (which is voluntary control, for the most part).
f. Visceral efferent neurons control all effectors other than skeletal muscle (cardiac muscle, glands,
smooth muscle). These neurons make up the autonomic nervous system as previously mentioned.
B. Structure of a neuron
1. Cell body: nucleus & cytoplasm containing neurofibrils that convey impulses.
2. Nissl Bodies: for protein synthesis; rough ER of neuron
3. NO Centrioles, therefore cannot divide by mitosis
4. Neuroglia: (nerve glue) support cells in CNS provide support, protection & access to nutrients, &
other valuable services for the NS. {nonexcitable}
a. Astrocytes: “nurse cells” made up of neuroglia cells that nurish & protect neurons. Star shaped
cells in CNS.
i. Most abundant neural cells. Form a barrier b/w capillaries & neurons to help protect the neurons
from harmful substances in the blood.
ii. make tight sheaths around the brain’s capillaries forming the blood-brain barrier that regulates
the chemical environment by picking up excess ions & recapturing released neurotransmitters.
5. Dendrites: receive incoming signals from sensory receptor & carry them towards cell body. Short,
tree-like fibers.
6. Axons: Long slender fibers that transmit signals away from the cell body. One per neuron.
a. Myelin Sheath: whitish, fatty lipoprotein material “insulating” or covering the axon
i. Myelinated fibers: speed up impulse conduction of nerve impulses.
ii. Unmyelinated fibers: conduct impulses slowly
iii. White matter: myelinated sheaths around axons of the PNS gives the tissue a white color &
forms myelinated nerves. (nerves=bundle of nerve fibers in the PNS) (nerve tract= bundle of nerve fibers in the
CNS)
iiii. Gray matter: concentration of cell bodies & unmyelinated fibers. (in PNS=ganglia; in
CNS=nuclei).
{Neurolemmacytes are most active during the first year of life, and spiral around an axon to leave a covering
called the neurolemma. This covering will also aid in repair. Oligodendrocytes myelinate in slightly different
ways in the CNS, they have fewer interrupting nodes, one cell may myelinate many axons, and there is little
neuron repair after injury. Myelination is still in progress in infants, so their responses aren't as quick or
coordinated. Some diseases (multiple sclerosis, Tay-Sachs) affect myelination, often causing "demyelination". }
b. Nodes of Ranvier: in the PNS, gaps along the myelin sheath of axons b/w ea. Schwann cell.
Also called neurofibral nodes.
c. Schwann Cells: neurolemmocytes (support cells) form myelin sheaths around axons of PNS.
i. As each Schwan cell wraps around the axon, its nucleus & cytoplasm are squeezed to the
perimeter to form the nuerilemma (sheath of Schwann) which is essential for nerve regeneration.
ii. Also acts as phagocytes (cell debris).
7. Microglia: (small ovoid thorny cells in CNS) glia cells that are spider like phagocytes that dispose of
debris, including dead brain cells & bacteria.
8. Ependymal Cells: squamous to columnar shaped, some ciliated in CNS that form a protective cushion
around the brain & spinal cord & the beating of the cilia helps to circulate the cerebrospinal fluid in the CNS.
9. Oligodendrocytes: form myelin sheaths around axons in CNS.
a. forms “white matter” of brain & spinal cord.
b. Multiple Sclerosis: disease of oligodendrocytes where hard lesions replace the myelin & affected
areas are invaded by inflammatory cells; nerve conduction is impaired; chronic deterioration of myelin of CNS
w/periods of remission & relapses; causes: autoimmunity or viral infection.
10. Satellite / Attendant Cells: (in PNS) surround & support neurons w/in the ganglia. Control chemical
environment.