Download Nervous System – Chapter 10

Nervous System – Chapter 10
I. Facts
A. Brain weights about 3 lbs
B. In the outer layer of the brain there are 14 billion cells
C. Receptors receive and effectors respond
II. Structure of the Nervous System
A. Three main divisions:
1. Central nervous system made of brain and spinal cord
2. Peripheral nervous system – branches
3. Autonomic nervous system – “cave man” response fight or flight
B. Basic unit of the nervous system is the neuron
1. Neuroglial cells – cells that surround nervous tissue
2. Parts of Neuron:
a. cell body – contains neuroplasm, a nucleus, and Nissl bodies
b. dendrite – a nerve fiber which is afferent sensory – carries impulse to nerve
cell body
c. axon – efferent motor or carries impulse away from nerve cell body
1. many axons are enclosed in sheaths formed by other cells
2. Schwann cells make up the sheath around the axon in peripheral nerves
C. Myelin sheath is made of many layers of a cell membrane
1. Facts:-Schwann cells form this insulating cover called a myelin sheath
a. composed of lipids and protein
b. conducts more rapidly than nerves without a sheath
c. believe to contain a source of energy for transmitting the impulse
d. only nerves with a myelin sheath are called white matter
2. Location of nerves with a myelin sheath
a. in the PNS
b. in neurons concerned with conduction (movement) in CNS (to and from the
brain)this myelin is made by oligodendrocytes
3. Functions of myelin sheath
a. controls rapid movements of body(increases speed of nerve impulse)
b. transmits many sensory signals of body to the brain
c. serves as an insulator
4. Neurolemma – a sheath which surrounds the myelin sheath – found only in
PNS
a. In order for a nerve to regenerate it must have a neurolemma.
D. Unmyelinated
1. Facts:
a. found in brain and spinal cord
b. cannot regenerate
c. made of gray matter
2. Functions (unmyelinated)
a. controls subconscious activities of the body (internal organs)
b. transmits sensory signals that do not require immediate action (delayed
reaction – soreness)
E. Neuroglial Cells
1. found in the tissue of the brain and spinal cord
2. there are more of them than are neurons
3. fill in spaces
4. capable of reproduction
a. astrocyte – star-shaped and found between nervous tissue and blood vessels
(repairman)
b. oligodendrocytes – resemble the astrocyte and functions in the formation
of myelin in the brain and spinal cord
c. microglia – support neurons and eat up bacteria
d. ependyma – covers spaces in brain – made of cells shape from squamous
to columnar cells .
III. Cell Membrane Potential
A. When nerve cells are at rest there is more sodium on the outside and potassium
on the inside
B. Potassium moves freely
C. Sodium has to be transported
IV. Nerve Impulse
A. Facts:
1. The speed of the impulse is slow compared to the electric current
2. Electrochemical change takes place down the membrane’s surface
3. Once the charge is started it is self-propogated
4. The neuron itself supplies energy for transmission
B. Action Potential – changing electrical voltage at the nerve cell membrane as the
impulse travels along it
C. How does a nerve charge?
1. Electrochemical conduction is due to the passage of ions
2. The ions are sodium (NA) and potassium (K)
3. The normal position is sodium on the outside and potassium on the inside
4. During transmission sodium moves in and potassium moves out—this is
called depolarization
5. The nerve goes back to the normal stage
D. Behavior of Nerve Impulse
1. Threshold – critical point when contact is made
2. Summation – adding a stimulus until the threshold is reached
3. The strength of the nerve impulse remains constant because the nerve
supplies the energy
4. All or none – all fibers respond in the nerve or none do
5. Impulse conduction
a. unmyelinated nerve fibers conduct an impulse over the entire nerve
surface
b. a myelinated fiber is different because myelin insulates
6. oxygen is necessary to maintain the concentrations of NA and K and for
cellular respiration
V. A synapse is where neurons connect.
A. Junction between the axon of one neuron and the dendrite of another
B. Facts about a synapse:
C. Synaptic transmission – process of crossing the gap
1. axons have synaptic knobs at their ends which contain synaptic vesicles
2. synaptic vesicles release a substance called neurotransmitter
3. acetycholine is the neurotransmitter
D. Excitatory and inhibitory actions
1. excitatory causes an increase in membrane permeability to sodium and this
triggers the nerve impulse
2. inhibitory will decrease sodium permeability and lower the charge
E. Factors affecting synaptic transmission:
1. epilepsy – “haywire” continuous impulses through the synapse (seizures)
2. drugs and chemicals
a. botulism – food poisoning – prevents acetylcholine from being released
b. caffeine – a stimulant
F. Neuropeptides – serve as neurotransmitters
1. enkepalins and endorphins relieve pain
2. substance P is pain producer
VI. Neurons and Nerves
A. Classification of Neurons
1. structural differences
a. multipolar neurons – have many processes so they have many dendrites –
found in brain and spinal cord
b. bipolar neurons have one axon and one dendrite (eyes and ears)
c. unipolar neurons – a single extension like in the ganglia (outside brain and
spinal cord)
2. functional differences
a. sensory neurons which are afferent and carry impulses to the CNS
b. motor neurons – efferent and carry impulses away from the CNS (effector –
another name)
c. interneurons – form links within the brain or spinal cord
d. receptor types – sensory:
1. exteroceptors – external on the body surface
2. proprioceptors – give muscle and joint sense
3. interoceptors – receive stimuli from internal organs
3. Types of nerves and nerve fibers
1. sensory nerves – conduct to CNS
2. motor nerves – conduct away from CNS
3. mixed nerves – have both
4. spinal nerves – connect spinal cord to body parts
5. cranial nerves – conduct from brain to body parts
6. somatic fibers – run to and from the skeletal muscles and skin
7. visceral fibers – run to and from internal organs
8. nerve fibers – made of axons
axons are extensions of neurons
nerve fibers make up a nerve
VII. Reflex Arcs and reflexes
A reflex leads directly from sensory to motor neurons through the spinal cord.
Example: direct movement; sticking pin to finger; reaction
A. reflexes are automatic, unconscious responses (you do not have to think to
protect yourself)
B. Examples: 1. withdrawal reflex
2. kneejerk reflex
Chapter 11
VIII. Coverings of CNS
A. Meninges – membrane around the brain and spinal cord made of three layers
1. dura mater – next to vertebra
2. arachnoid mater – middle layer
3. pia mater – next to spinal cord
4. subarachnoid space – contains cerebrospinal fluid
IX. Spinal Cord
A. Structure of spinal cord
1. gray matter
a. central or in the core of the spinal cord in the shape of an H or butterfly
b. neurons in this area act as an integrating center for incoming sensory and
outgoing motor responses
c. four arms of gray matter called horns and at intervals give rise to spinal
nerves
1. anterior horns are in the front and they give rise to anterior roots
which are efferent motor
2. posterior horns are in the back and give rise to posterior roots which
are afferent sensory
2. white matter – surrounds the gray matter and functions as a relay station to
and from the brain
a. white matter is divided in to 3-columned funiculi which are made of
longitudinal nerve fibers called tracts
b. Three columns:
1. anterior column – contains descending tracts from motor areas of the
brain which carry voluntary motor impulses to anterior horn cells
2. lateral columns – contain ascending tracts that carry sensations of
heat, cold, pain to the brain
3. posterior columns – contain ascending tracts that carry the sensations
of muscle and joint sense
X. Spinal Nerves – part of PNS
A. Facts:
1. 31 pairs spinal nerves
2. formed by combining fibers of posterior and anterior horns
B. There are 5 sets of spinal nerves
1. cervical – 8 pairs
2. thoracic – 12 pairs
3. lumbar – 5 pairs
4. sacral – 5 pairs
5. coccygeal – 1 pair
C. On the posterior root of each spinal nerve there is a swollen area called the
posterior (dorsal) root ganglia which supplies many sensory fibers to spinal
nerves.
D. Rami are branches of spinal nerves that run to the front and back
1. posterior primary rami goes to the back to the neck and front
2. anterior primary rami goes to the legs, arms, skin in front
E. Except in the thoracic region, the anterior branches of spinal nerves combine to
form complex networks called plexuses
1. cervical plexuses – deep in the neck from C1 to C4
2. brachial plexuses – C5 to T1
3. lumboscaral plexuses – T12 to S5