Download VII. The Nervous System

VII. The Nervous System
A. Cells of the Nervous System
1. Neurons- cells specialized for transmitting
chemical and electrical signals from one location in
the body to another
a) Cell body- contains most of the
cytoplasm, the nucleus and other
organelles
b) Dendrites- convey signals to the
cell body, Usually short,
numerous and extensively
branched
c) Axon- conducts impulses away
from the cell body. They are
tipped with synaptic terminals
which release neurotransmitters
1)Neurotransmitters- chemicals that cross the
synapse to relay a message to a new neuron
2) Synapsegap
between a
synaptic
terminal
and the
dendrites
of another
neuron
2. Types of Neurons:
a) Sensory neuronsconvey information
about the external or
internal environment
to the central
nervous system
b) Motor Neuronsconvey impulses
from the CNS to the
effector cells
c) Interneuronsintegrate sensory
input and motor
output (located in the
CNS)
3. Supporting Cells- structurally support, protect
insulate and assist neurons. They do not conduct
impulses. They out number neurons 10-50X
a) Glial Cells- supporting cells of the central nervous
system
1) Astrocytes- encircle
capillaries in the brain to
control the ionic
environment around
neurons
2) Oligodendrocytes- form
myelin sheaths that
insulate nerve processes
b) Schwann cells- form myelin sheath in the
peripheral nervous system to provide electrical
insulation and speed the rate of nerve impulse
transmission.
B. Transmission of Electrical
Signals along a Neuron
1. The Origin of Electrical Membrane Potential
a) Membrane potential- the difference in charge
between the cytoplasm and extracellular fluid
due to a differential distribution of ions
b) Resting Potential- about -70 mV in a nontransmitting neuron
c) Ion Distribution
1) Inside Cell: High K+, A-; Low Na+, Cl- = Negative Charge
2) Outside Cell: Low K+; High Na+, Cl- = Positive Charge
d)Sodium-Potassium Pumps actively move ions
against their concentration gradient to maintain
proper resting potential and counteract diffusion
2. Action Potential- rapid change in membrane
potential caused by selective opening and closing of
ion gates
a)Gated ion Channels allow neurons to change
their membrane potential in response to stimuli
b) Different ion channel affect the neuron:
1) Hyperpolarization- gates
allow K+ to leave the cell
causing the inside to
become more negative
2) Depolarization- gates
allow Na+ to enter the
cell making the inside
less negative
c) An action potential has four phases
1)Resting State:
no channels are
open
2) Large depolarizing phaseNa+ gates are
opened. K+
gates are
closed. The
influx of Na+
causes the
interior of the
cell to become
positively
charged
3) Steep repolarizing phase-
Na gates close.
K+ gates open.
Loss of K+
causes cell
interior to return
to a negative
state
4) Undershoot phase- is a time when membrane
potential is temporarily more
negative than resting state
due to the lag in closing K+
gates
There is a Refractory Period- during undershoot phase.
During this period the neuron is insensitive to stimulus
and will not fire. This limits the maximum rate at which a
neuron can fire
3. Propagation of Action Potential
a)Strong depolarization in
one area results in the
depolarization of the
neighboring area
b)Action potential does not
travel down the axon but
is regenerated at each
position
c) The impulse travels in
one direction due to the
refractory period at the
previous position
4. Speed of Propagation of Action Potential
a) The larger the diameter of a neuron, the faster the
impulse
b) Saltatory Conduction- the impulse jumps from one
node of Ranvier to the next, skipping myelinated regions
c) Nodes of Ranvier- gaps in the myelinated sheath
where ion gates are concentrated
C. The Synapse: Transmission
between Cells
1. Synapse- tiny space between neurons that control
communication between those neurons.
a) Presynaptic Cell- is the transmitting cell
b) Postsynaptic cell- is the receiving cell
2. Electrical Synapse- action potential spreads directly from
presynaptic to postsynaptic cell via gap junction. Very
uncommon
Chemical Synapse Animation
3. Chemical Synapse- a chemical called a
neurotransmitter is released from the presynaptic
cell and binds to receptors on a postsynaptic cells
causing it to fire.
a) An action potential arriving at the synaptic terminal at the end of an
axon causes Ca+2 to rush through voltage sensitive channels
b) The sudden in rush of Ca+2 causes synaptic vesicles which contain
neurotransmitters to fuse with the presynaptic membrane releasing
neurotransmitters into the synaptic cleft (the space between neurons)
c) Neurotransmitters diffuse to the postsynaptic membrane where they
bind to specific receptors and trigger the opening of ion gates
d) This may cause hyperpolarization or depolarization depending on
which ion gates are opened
e) The neurotransmitter is quickly degraded by enzymes and recycled to
the presynaptic cells
4. Nervous Integration
a) One neuron receives signals from numerous
adjacent neurons
b) Excitatory postsynaptic Potential (EPSP)
are caused by neurotransmitters that open Na+
gates triggering depolarization
c) Inhibitory postsynaptic Potential (IPSP) are
caused by neurotransmitters which open K+ or
Cl- gates causing hyperpolarization
d) A single EPSP is rarely strong enough to
trigger an action potential, although and
additive effect, summation, from several
terminals can trigger a neuron to fire
Summation
1) Temporal summation is when chemical transmission from
one or more synaptic terminals occur so close in time that
the additive effect causes the neuron to fire
2) Spacial Summation occurs when several neurons stimulate
the postsynaptic cell at the same time and cause it to fire
3) EPSPs and IPSP can summate also, but each counters the
other’s affect preventing the neuron from firing
D. The Vertebrate Nervous System
1. Peripheral Nervous system- consists of
sensory and motor neurons
a) Somatic Nervous System (voluntary)
carries messages from the central nervous
system to skeletal muscles
b) Autonomic Nervous System controls
involuntary functions and is divided into:
1) Sympathetic Nervous System increases
function
2) Parasympathetic Nervous System decreases
function
2. Central Nervous
a) Cerebellum - the part of the brain below the back of the
cerebrum. It regulates balance, posture, movement, and
muscle coordination.
b) Corpus Callosum - a large bundle of nerve fibers that
connect the left and right cerebral hemispheres. In the
lateral section, it looks a bit like a "C" on its side.
c) Frontal Lobe of the Cerebrum - the top, front regions of
each of the cerebral hemispheres. They are used for
reasoning, emotions, judgment, and voluntary movement.
d) Medulla Oblongata - the lowest section of the brainstem
(at the top end of the spinal cord); it controls automatic
functions including heartbeat, breathing, etc.
e) Occipital Lobe of the Cerebrum - the region at the back
of each cerebral hemisphere that contains the centers of
vision and reading ability (located at the back of the head).
f) Parietal Lobe of the Cerebrum - the middle lobe of each
cerebral hemisphere between the frontal and occipital
lobes; it contains important sensory centers (located at the
upper rear of the head).
g) Pituitary Gland - a gland attached to the base of the brain
(located between the Pons and the Corpus Callosum) that
secretes hormones.
h) Pons - the part of the brainstem that joins the hemispheres
of the cerebellum and connects the cerebrum with the
cerebellum. It is located just above the Medulla Oblongata.
i) Spinal Cord - a thick bundle of nerve fibers that runs from
the base of the brain to the hip area, running through the
spine (vertebrae).
j) Temporal Lobe of the Cerebrum - the region at the lower
side of each cerebral hemisphere; contains centers of
hearing and memory (located at the sides of the head).