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Nervous System Ch 10
The Nervous System is composed of neural tissue, blood vessels and
connective tissue. This system allows us to experience the world, think
and to feel emotion.
Neural tissue : 2 types
a) neurons (nerve cells)- are specialized cells which
react to physical and chemical stimuli
b) neuroglia- which surround the neuron and help
nourish them and remove ions & neurotransmitters
that accumulate between neurons
The nervous system is divided into “2” groups
1) Central Nervous System (CNS) = brain and spinal cord
2) Peripheral Nervous System (PNS) = all the other nerves that connect
the CNS to other body parts.
These nerves include cranial & spinal nerves
There are 3 functions of the nervous system: sensory, integrative and
motor
Sensory : with the help of sensory receptors which detect changed inside
and outside the body, they gather info from the environment ex. from light,
sound, temperature, oxygen concentration.
Integrative : the sensory receptors change their info into nerve impulses
which by way of the PNS go to the CNS, where the signals are
“integrated” - meaning they are brought together to create sensations, add
to memory or help produce thoughts
Motor : once conscious or subconscious decisions are made they are
acted on by motor functions which use neurons to carry impulses from the
CNS to effectors. Effectors are muscles (respond) or glands (secrete) that
react when stimulated
The PNS motor portion is divided into:
1) Somatic nervous system : involves conscious activities employing
skeletal muscle
2) Autonomic nervous system : controls viscera, like heart and
glands under unconscious action
Neuron Anatomy
Each neuron is composed of many branched dendrites which receive
input from the body and a longer, single axon (nerve fiber) which carries
info away from the neuron in the form of electrical signals called a nerve
impulse ( See fig. 10.1 & 10.3 ) The cell body is also called the soma
which contains the nucleus
Between each neuron is a small space called a synapse. At the synapse,
biochemicals called neurotransmitters help send electrical messages from
neuron to neuron
Nerves are just bundles of axons
The larger axons of the PNS are enclosed in lipid-rich sheaths called
Schwann cells. Schwann cells are neuroglial cells that wrap around the
axon and protect it. They do not conduct impulses. The sheath is formed
by layers of cell membranes which are wound tightly like a bandage
wrapped around a finger. The layers are called myelin and the coating is
considered a “myelin sheath”.
“Nodes of Ranvier” are the narrow gaps in the myelin sheath between
Schwann cells (see fig. 10.3 / 10.4)
Axons with myelin sheaths are called myelinated and appear white, like
those seen in the “white matter” of the brain and spinal cord (multiple
sclerosis involves the loss of myelin)
Axons lacking myelin are called unmyelinated and appear gray, like those
seen in the “gray matter” of the brain and spinal cord
Neurons classified by their “function” depend on whether they carry info
into the CNS, carry info within the CNS or if they carry info out of the CNS
( see fig. 10.7 )
The 3 types of neurons based on function are: sensory, interneurons
and motor neurons.
1) Sensory neurons ( or afferent neurons ): carry nerve impulses from
the body to the brain or spinal cord
2) Interneurons: lie within the brain or spinal cord and transmit impulses
from one part of the brain or spinal cord to another
3) Motor neurons (or efferent neurons ): carry nerve impulses out of the
brain or spinal cord to effectors (muscles or glands) which either
contract or secrete substances
Cell Membrane Potential
A cell membrane is electrically charged or polarized so that the inside is
negatively charged with respect to the outside. This polarization is due to
an unequal distribution of positive and negative ions on either side of the
membrane.
The outer surface of the neuron membrane consists of many positive ions.
Sodium (Na+) and potassium ions (K+) are the major intracellular positive
ions. (see fig. 10.12). Remember that there is a small gap between
neurons called the synapse. Also a neuron that has positive ions on the
outside of the membrane is said to be a polarized nerve.
The difference in electrical charge between two points is measured in units
called volts. This represents a “potential difference” because it represents
stored electrical energy which can be used later to do work.
The potential difference across the cell membrane is called the membrane
potential, which is measured in millivolts.
A resting neuron is one that is not being stimulated to send a nerve
impulse and its membrane potential is called the resting potential. The
resting potential has a value of -- 70 millivolts. Itʼs negative to indicate the
excess negative charges on the inside of the cell membrane.
Figure 1 shows a simplified neuron and an impulse traveling through it.
Figure 2 shows a polarized neuron and the synapse.
Fig. 1: Simplified neuron
Fig. 2 Polarized neuron
When the membrane of the dendrite is activated by a stimulus, Na+ will
enter. These ions will diffuse along the membrane of the dendrite and
soma in the intracellular fluid. When the ions reach the junction of the
soma and the axon ( an area called the axon hillock), the membrane
becomes very permeable to Na+. There is a tremendous influx of Na+ in
the axon. The ions in the extracellular fluid of the axon will enter the axon
in a domino fashion. Therefore, when one positive ion enters (a process
called depolarization) an adjacent positive ion will then enter. This will
then cause the next positive ion to enter and so forth. As each ion enters
the neuron, a wave-like occurs. This wave-like activity is the impulse. This
traveling wave of activity is known as the action potential.
If a neuron is sufficiently depolarized the membrane reaches a level called
the “threshold potential” because events are set in motion to allow the
impulse to travel through the neuron. At threshold, an action potential is
produced.
Threshold can be defined as the amount of stimuli required to cause
depolarization. If a person has a high threshold, they will therefore,
require more stimuli in order to have depolarization and therefore in order
to respond to something. A neuron with a low threshold will have
depolarization with very little stimuli. Whenever a doctor gives a patient a
drug that numbs the feeling, the drug actually raises the threshold of the
neuron to the point where depolarization is not occurring. Without
depolarization, an impulse will not occur. Without an impulse, the brain
does not sense any pain.
Many times a single depolarizing stimuli is not sufficient for the membrane
to reach threshold. But, if another stimuli of the same type arrives before
the first subsides, the potential change is greater. This is called
summation. It allows several “subthreshold potentials” to combine and
reach threshold.
Refractory period: a period of time following the passage of a nerve
impulse where a threshold stimulus will not trigger another impulse. Takes
about 10 to 30 milliseconds to return to resting state. This time is required
for the membrane to change itʼs ion (Na+) permeability.
All-or-None Principle : This principle states that if a stimulus is strong
enough to cause depolarization, the entire neuron will depolarize in
succession. An impulse will travel the length of the neuron. If a stimulus is
not strong enough to cause depolarization, an impulse will not even
begin. An impulse will not travel part way through an axon and then quit.
It goes all the way or not at all.
Fig. 3: Depolarized neuron
To further understand how an impulse travels through a neuron, lets use
the domino analogy again. Think of the positive ions as dominoes. When
one positive ion enters the neuron, it causes the next ion to enter and then
the next, and so on. When you push one domino over, it causes the next
domino to fall and so on. As you watch each domino fall, you see a
wave-like activity. Figure 3 shows a depolarized neuron in sequence. To
help with your understanding, some of the dendrites have been left out.
Figure 3A shows a polarized neuron. The neuron is stimulated and
therefore, one positive ion begins to enter the dendrite. This is the
beginning of depolarization. This ion diffuses to the axon region. Figure
3B shows depolarization of the axon. The dotted arrow shows the
movement of the impulse down the axon. Figure 3C shows more ions
entering. The dotted arrow shows the impulse traveling a little farther
down the axon. This process continues until the impulse reaches the end
of the axon. At the end of the axon, there will be an influx of positive ions
but, in this case, it will be calcium ions. Calcium ions target the presynaptic
vesicle and cause the vesicle to release the neurotransmitter into the
synapse area. Figure 4 shows the release of the neurotransmitter. (see
fig. 10.15 & 10.18)
Fig. 4: The release of the neurotransmitter
As soon as the neurotransmitter ( acetylcholine in this example ) is
released from the presynaptic vesicle, it enters into the synaptic region. It
flows across the synapse and comes in contact with the membrane of the
next neuron in line. As soon as it stimulates the membrane of the next
neuron, it causes depolarization of the next neuron in sequence and the
impulse continues on to its destination.
In order to use the neuron a second time, all the positive ions must leave
the neuron and go back to the extracellular regions outside the neuron.
Also, the neurotransmitter must be decomposed so it too leaves the
synapse. Basically, everything needs to go back to its original position.
The process of returning the ions to the extracellular region is called
repolarization. This is an active process that requires ATP.
The dendrite end releases an enzyme called acetylcholinesterase.
This enzyme will decompose acetylcholine to form acetate and choline.
Acetate and choline can then be reabsorbed into the presynaptic vesicle
to recombine and make more acetylcholine. Figure 5 shows
repolarization and the breakdown of the neurotransmitter.
Fig. 5: Repolarization
After repolarization and after the breakdown of acetylcholine, everything
has returned to the original state. The positive ions are back to the outside
of the membrane to the polarized condition. The acetylcholine is
decomposed so the synapse region is clear once again. Now the same
set of neurons can be used again.
One Way Transmission
The nervous system is designed to ultimately protect the body from harm.
To ensure this, it is imperative that the impulse travel one direction through
a neuron to arrive at the correct destination. If impulses were allowed to
travel in any direction within a neuron, chaos would result. The impulse
must travel toward the axonʼs presynaptic region in order to be able to
successfully cross the synapse to continue its journey to its destination.
Neurotransmitters
The body produces about 30 different neurotransmitters. They are the
chemical messengers important to the transmission of a nerve impulse.
Most are made in the cytoplasm of the synaptic knobs and are stored in
the synaptic vesicles (see fig. 10.18)
Neurotransmitters are chemicals modified from amino acids or are
themselves short chains of amino acids. Acetylcholine is one of the most
common in skeletal muscle contractions. Other common ones include:
epinephrine, norepinephrine, dopamine and serotonin (see Table 10.4)
To Myelin or not to Myelin
( Myelinated vs Unmyelinated )
Unmyelinated axons conduct impulses over their entire surface.
Myelinated axons function differently. Because myelin serves to protect
and insulate the axon, it prevents all the flow of ions where it covers the
axon ( see fig. 10.16). However, at the nodes of Ranvier, between the
Schwann cells, the myelin is absent. At these nodes there are sodium and
potassium channels that allow for action potentials. The action potentials
appear to jump from node to node ( really the entire myelin covered
Schwann cell is stimulated ). This type of conduction is called saltatory
conduction.
Nerve impulses along myelinated axons happen many times faster than
conduction on unmyelinated axons.
Also the larger the diameter of the axon, the faster an impulse travels
through it.
Ex. a thick, myelinated motor neuron of a skeletal muscle travels ~ 120
meters per second but one which is thin and unmyelinated, like those of a
sensory neuron might only travel ~ 0.5 meters per second.