Transcript Neurons and Nerves

Neurons and Nerves
(Hand out)
prof.aza
prof. aza


The human nervous system has two main
divisions (Figure 01a): the central nervous
system (CNS), and the peripheral nervous
system (PNS), which includes the somatic motor
nervous system, and the sensory nervous
system.
The CNS consists of the brain and spinal cord. It
acts as the central control region of the human
nervous system, processing information and
issuing commands.
prof. aza


The autonomic nervous system (ANS) is the
command network the CNS uses to maintain the
body's homeostasis.
It automatically regulates heartbeat and controls
muscle contractions in the walls of blood
vessels, digestive, urinary, and reproductive
tracts. It also carries messages that help
stimulate glands to secrete tears, mucus, and
digestive enzymes.
prof. aza
Figure 01a
Nervous System
prof. aza


The nerves (Figure 01b) that are easily visible to
the unaided eye are not single cells. Rather,
they are bundles of nerve fibers (neurons) each
of which is itself a portion of a cell. The fibers are
all traveling in the same direction and are bound
together for the sake of convenience, though the
individual fibers of the bundle may have widely
differing functions.
There are no cell bodies in nerves; cell bodies
are found only in the CNS or in the ganglia.
Ganglia are collections of cell bodies within the
PNS.
prof. aza
Figure 01b
Neuron and
Nerve

prof. aza
The main
portion of
the neuron,
the cell
body, is not
too different
from other
cells. It
contains a
nucleus and
cytoplasm
The dendrites


Where it is most distinct from cells of other types
is that out of the cell body, long threadlike
projections emerge. Over most of the cell there
are numerous projections that branch out into
still finer extensions. These branching threads
are called dendrites ("tree" in Greek).
At one point of the cell, however, there is a
particularly long extension that usually does not
branch throughout most of its sometimes
enormous length. This is the axon (the axis).
prof. aza
Sensory neuron, motor neuron, and
interneuron


Figure 01b shows the three parts of the neurons:
dendrite(s), cell body, and axon. A dendrites conducts
nerve impulses toward the cell body, the part of a neuron
that contains the nucleus and other organelles. An axon
conducts nerve impulses away from the cell body.
There are three types of neurons: sensory neuron, motor
neuron, and interneuron. A sensory neuron takes a
message from the receptors in the sense organ to the
CNS. A motor neuron sends a message away from the
CNS to an effector, a muscle fiber or a gland. An
interneuron is always found completely within the CNS
and conveys messages between parts of the system
(Figure 3a).
prof. aza


In addition to neurons, nervous tissue contains glial cells
such as the Schwann cells covering the neurons with
sheath. These cells maintain the tissue by supporting
and protecing the neurons.
They also provide nutrients to neurons and help to keep
the tissue free of debris. The neurons require a great
deal of energy for the maintenance of the ionic
imbalance between themselves and their surrounding
fluids, which is constantly in flux as a result of the
opening and closing of channels through the neuronal
membranes.
prof. aza
Figure 01c Neurons
prof. aza
the energy consumption

Thus while the brain is only 2% of our
body weight, it consumes 20% of our
energy and moreover 80% of this energy
consumption is devoted to maintain the
imbalance.
prof. aza
Figure 01d Action
Potential
prof. aza
 Neurons
are dynamically
polarized, so that information
flows from the fine dendrites into
the main dendrites and then to
the cell body, where it is
converted into all-or-none signals,
the action potentials, which are
relayed to other neurons by the
axon, a long wire like structure.
prof. aza
 The
neuron is actually a very poor
conductor; the signal drops to 37% of
its original strength in only about 0.15
mm. Thus it needs amplification all
along its length in the form of sodiumpotassium pumps and gates (see
Figure 01d).
prof. aza
The amplification


The amplification is initiated by detection of
small changes in voltage across the membrane
with the opening of voltage-sensitive sodium
channels in the membrane of the neuron.
Sodium ions rush into the neurons from the extra
cellular fluid, resulting in a transient change in
the voltage difference between the neuron and
the surrounding environment. The action
potential travels like a wave from the cell body
down the neuron via the repeating
amplifications.
prof. aza
the action potential


Thus, the action potential enables the neuron to
communicate rapidly with other neurons over
sizable distances, sometime more than a meter
away with a speed from 20 -200 m/sec.
When the action potential reaches an axon
terminal (the synapse), it causes the terminals to
secrete a chemical messenger
(neurotransmitter), generally an amino acid or its
derivative, which binds to receptors in the postsynaptic neurons on the far side of the synaptic
cleft. When the postsynaptic potential has
reached a specific value an action potential is
triggered and the signal is passed to the next
neuron.
prof. aza