Download Nervous System

The Nervous System – Structure and Function
The nervous system is built from a huge number of
neurons. While details differ, they all have the same
basic architecture:
There is an input end (frequently highly branched)
called a dendrite.
There is a cell body, where the nucleus and much of
the metabolic machinery is located.
There is an output end, called an axon. It is frequently
branched at its tip. Each tip branch ends in a terminal
(synaptic) knob.
In this example, the
dendrites are short and the
axon is long. These are the
relative lengths seen in a
typical motor neuron (one
that innervates voluntary
muscle, like the fingers I’m
using to type). There is one
‘extra’ structure: the myelin
sheath, each ‘sausage’ is a
separate Schwann cell that
insulates the axon and
speeds conduction.
Other types of neurons have different
lengths of axons and dendrites, but
the same set of structures. A sensory
neuron has a very long dendrite,
reaching from the tip of your toe to
the lower part of your spinal cord,
but a short axon, extending only into
the spinal cord.
The lengths of dendrites and axons
are approximately equal in your
brain, and both are branched.
The nervous system is divided into two major
divisions:
The Central Nervous System (or CNS) made
up of:
Cerebrum
Cerebellum
Medulla Oblongata
Spinal Cord
and the Peripheral Nervous System (or PNS)
made up of:
Somatic (Voluntary) and the
Autonomic (Involuntary)
Sympathetic and Parasympathetic
How do the PNS and CNS function together? The
knee-jerk reflex…
In words first:
1.Your doctor hits your knee just below the kneecap.
That stretches a stretch receptor (a sensory neuron)
in the patellar tendon.
2. The axon of that neuron carries the signal into the
spinal cord through its dorsal root.
3. There is a synapse from the stretch receptor onto a
spinal interneuron. Its axon has at least two
synapses: one excites a motorneuron connecting to
an extensor muscle and one inhibits the motorneuron
for the opponent flexor muscle.
4. The signal is also transmitted through the spinal
cord up to your brain. You feel that your knee has
been struck, but you’ve already responded by…
5. Contraction of the extensor muscle and relaxation
of the flexor muscle.
Diagrammatically:
This diagram has one error.
It shows the motor neuron
innervating the same muscle
as the stretch receptor.
Really, the stretch receptor
is in a tendon, at one end of
the muscle, and the reflex
involves a completely
different muscle contracting.
A little more now about Schwann cells and the myelin
sheath…
Schwann cells are wrapped around the axon so that
there are many layers of cell membrane insulating it.
There are small gaps, called nodes of Ranvier,
between individual Schwann cells. Conduction
velocity goes from about 5m/s without the myelin
sheath to ~150m/s with it.
The signal carried along the length of the dendrite and
axon is a nerve impulse. It is a short (~10ms)
electrical wave that passes down the dendrite and
axon.
To understand the impulse, you first need to learn how
neurons maintain a resting potential. The cell
membrane of the neuron has proteins in it that act as
ion-specific channels that are
described as “gated” or voltagedependent (K and Na), as well as
a voltage independent K channel.
When the cell is at rest, the sodium channel is closed.
The voltage-independent potassium channel permits
ion movement, and the ATP-powered sodiumpotassium pump pumps sodium out and potassium in,
but it’s a coupled pump and moves more sodium out
than potassium in.
Net result: organic ions (- charged) and more Na+
outside than K+ inside leaves the interior of the cell at
a relative
voltage of
~-70mv.
Now something happens to reduce the potential on the
membrane to a threshold.
The stimulus could be any of a number of things: the
stretching of the stretch receptor membrane, a stimulus
coming from another nerve cell by way of the synapse,
…
The stimulus opens some of the Na channels. If enough
sodium moves in to reduce the membrane potential to
about -50mv, then the gated Na channels open and
much more Na moves into the nerve cell. So much
moves in that the interior becomes momentarily
positive.
At peak the Na channels are closed. Now the voltagedependent K channels open, and potassium rushes out
of the cell.
The membrane potential once more becomes
negative, even more negative (-75 to -80mv) than at
rest. The K channels close.
However, now the K+
and Na+ ions are on the
‘wrong’ sides of the cell
membrane. The coupled
pump exchanges them,
and restores the resting
condition.
Propagation of the nerve impulse:
The nerve impulse is a local event, occurring at one
point along an axon. To communicate, it must move
down the axon to the point of communication with
another cell.
To understand how, all you need to recognize is that
when the sodium rushes in, it spreads (in both
directions) along the axon.
The sodium depolarizes a region further along the axon
at least to the threshold level. Sodium then rushes in
there, and all the rest of the stages of an impulse.
But that sodium also spreads out ….
Now we come to the point where information must be
communicated from one neuron to another. This
happens at synapses. In us virtually all synapses are
chemical.
Inside the synaptic knob are large numbers of synaptic
vesicles. Inside them are chemical transmitters. When
an impulse arrives at the synaptic knob, it causes a
number of these vesicles to fuse with the membrane,
releasing their chemical contents into the synaptic
cleft between the neurons.
The chemical diffuses to the post-synaptic membrane
(that of the receiving cell), and opens ion channels,
depolarizing or hyperpolarizing the cell.
The chemical is called a neurotransmitter. It is
rapidly broken down on the post-synaptic membrane
to limit how long it affects the receiving cell.
If the neurotransmitter depolarizes the cell, it is an
excitatory transmitter; if it opens K channels, it
causes a hyperpolarization and makes an impulse less
likely – it is acting as an inhibitory transmitter.
Many different chemicals act as neurotransmitters.
Among the most important are acetylcholine (ACH),
serotonin, dopamine, and gammaaminobutyric
acid (GABA). Most are exciteatory in some places
and inhibitory in others.
This diagram (opening sodium channels) represents the
way that ACH acts on a muscle cell membrane,
exciting it.
Most of the drugs that police are interested in
(morphine, heroin, cocaine, amphetamines, LSD,
mescaline, …) have their effects by functioning as,
blocking, or altering chemical synaptic activity.
Similarly, medicine uses drugs with known effects to
treat various types of mental difficulty. Prozac
increases serotonin presence at synapses. Valium and
its relatives activate synaptic receptors for GABA.
Long-term use changes the synaptic chemistry. Both
psychological and physical addiction can occur.
In the CNS (and many other places) there are many
synapses connecting to a receiving cell. How it
responds depends on how the potentials caused by
synapses add together. The process is called
summation.
Summation is clearly critical to CNS and brain
function. We put together many sources of information
to determine appropriate responses. The “putting
together” is summation.
Some summation occurs in the spinal cord and
associated dorsal root ganglia. Much more occurs in
the cerebrum and cerebellum.
The brain and spinal cord function together.
However, the tracts in the spinal cord and regions of
the brain involved in controlling the right side of your
body travel up the left half of the spinal cord and are
represented in the left half of the cerebrum.
The cerebrum has the cell bodies of its neurons in the
‘surface’ layer, called the cerebral cortex. It’s
sometimes also called the gray matter. The axons and
dendrites of these cells are myelinated, and are
principally in a thick, deeper layer called the white
matter (due to the whiteness of the myelin).
Extending outward from the brain itself are a set of
12 cranial nerves. There are many mnemonics to
remember them (you don’t have to). The one I
remember is:
On old Olympus’ towering tops a Finn and German
viewed some hops.
The first letter of each of the 12 words is the first
letter of the corresponding cranial nerve. For
example, the first 3 Os stand for:
I. olfactory
II. optic
III.oculomotor
There are also spinal nerves coming to and leaving
from each segment of the spinal cord.
Your sensory input and motor control are fully
represented on the cortex of your cerebrum:
Touch sensation is arrayed on the somatosensory
cortex.
Control of voluntary muscle is similarly, but not
identically arrayed on the motor cortex.
The details of how each is arrayed have been learned
by stimulating the cortex of people undergoing brain
surgery. The resulting ‘body images’ are referred to as
a sensory and a motor homunculus:
motor
control
sensory
input
Other parts of the brain are vital to how we function.
The reticular formation, an extended area in the
brainstem, filters information and, as a result, is critical
to wakefulness (alertness) and sleep.
The brain doesn’t stop when you’re asleep. Rather, it is
partially cut off from sensory input. How alert you are
to external information is quite evident in an EEG or
electroencephalogram. External electrodes record a
kind of summation of overall brain activity.
β waves
 waves
δ waves
Emotions are associated with the amygdala and the
hypothalamus.
An important aspect of laying down and bringing up
memories seems associated with the hippocampus.
However, tests on conscious surgery patients have
found stimulating areas in the temporal lobe also bring
up memories.