1-The nervous system:
The nervous system can be divided into two parts:
1- Central nervous system (CNS)
That consists of:
a- Brain b- Spinal cord
c- Peripheral nervous or (nerve fiber)
There are two types of nerves:
1. Afferent nerves are transmitted information to brain or spinal cord (Sensory information).
2. Efferent nerves are transmit information from brain or spinal cord to muscles and glands (Control information).
2- Autonomic nervous system
Control various internal organs such as the heart.
2-The Neuron:
1. It’s the basic structural unit of nervous system.
2. Its specialized cell for reception, interpretation, and transmission of
electrical messages.
The neuron structure is shown in the fig.1
Fig.1: Schematic of a motor neuron.
3- Electrical potentials of nerve:
1. Resting potential:
A cross the surface of membrane of neuron is a potential difference (?V). This potential is due to the movement and unequal distribution of ions across the cell membrane of the neuron. (The negative ions on the inside of the membrane than outside). This neuron is said polarized. Inside the membrane is (-60 to -90) mV more negative than outside.
The resting potential = (-60 to -90) mV + + + + + + + + + + + + +
_ _ _ _ _ _ _ _ _ _ _ _ _ _
Polarized or resting potential
_ _ _ _ _ _ _ _ _ _ _ _ _ _
+ + + + + + + + + + + + +
2. Action potential:
When the neuron is stimulated by; (heat, cold, light, sound ……..) a large momentary change in the resting potential occurs at the point of stimulation. This potential is called Action potential.
The action potential for most neurons and muscles cells lasts about a few msec. The action potential for cardiac muscle may lasts from 150 to 300 msec.
4- Direction of action potential
An axon can transmit in either direction. But the synapse that connects it to another neuron only permits the action potential to move along the axon way from its own cell body.
Type of nerve fibers:
a- Myelinated nerves: The axons are covered with fatty insulated layer called myeline. It has small un insulated gaps called Nodes of Ranvier.
1. It’s the most common type.
2. Conduct action potential much faster than unmyelinated nerves.
3. Mylin sleeve (Fatty layer) is a very good insulater.
4. The myelinated segment of an axon has very low capacitance (charge storage).
b- Un myelinated nerves: The axon have no myeline sleeve.
5- Stimulation and action potential:
If the axon is slightly stimulated at some point a small action potential is take place at some point. A small action is sent along the axon in both directions but it is attenuated rapidly.
If the stimulation is great enough, the action potential is transmitted with out attenuation, as shown in the following:
1. The action potential decrease in amplitude as it travels through mylinated segment.
2. The reduced signal then acts like stimulus at the next of Ranvier (gap) to restore the action potential to its original size and shape.
3. This process repeats along the axon (saltatory conduction). The action potential seems to jump from one node to the next.
6- Speed of action potentials
The factors affect the speed of the action potentials are;
1. The resistance (R) within the core of the membrane.
R ? , i.e, R ?
Area= ?r2
r= radius
So, R ?
That’s mean: Resistance of axon decreases as diameter increases and that cause increased the velocity of the action potential.
Axon with large diameter has higher velocity of propagation than an axon with small diameter.
2. The capacitance (C) across the membrane.
Principle: The greater the stored charge on the membrane (high capacitance), the longer it takes to depolarized it; and thus the slower the propagation speed.
But; the charge stored in a myelinated nerve fiber is very small compared with an un myelinated fiber. Thus myelinated fiber has a low capacitance (C).
The conduction speed and action potential in the myelinated fiber is many time faster.
The dvantages of myelinated nerves
1. They produce high propagation velocities in axons of small diameter.
2. A large number of nerve fibers can thus be packed into a small bundle to provide for many signal channels.
The electrical signals from muscles - The Electromyogram (EMG):
EMG: Is an electrical device which is used to record the action potentials from muscles during movement.
- A muscle is made up of many motor units.
- Motor unit: Consist of a single branching neuron from the brain stem or spinal cord + (25 - 2000) muscle fibers it connects to via motor end plates.
- The resting potential across the membrane of a single muscle fiber = the resting potential across the nerve fiber.
- An action potential that travels along an axon and is transmitted across the motor end plates into the muscle fiber causing them to contract.
- Single muscle cells are usually not monitored in an EMG, because it is difficult to isolate a single fiber.
EMG electrodes:
A. Surface electrode: Attached to the skin, measures the electrical signals from many motor units.
B. Needle electrode: This is inserted under the skin, measure single motor unit activity.
Method for obtaining an EMG:
Instrument arrangement can be shown below:
EMG can be obtained from muscles or motor units that are transmitted by:
i. Electrical stimulation
ii. Voluntary contraction
Q/ Compare between voluntary contraction and unvoluntary
(Electrical stimulation)?
Sol/
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Voluntary contraction
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Unvoluntary contraction (electrical stim.)
|
|
1.The contraction speed last over 100
msec.
2. All motor units don’t fire at the same
time.
3. Each motor unit may produce several
action potential.
4. The stimulation time is undefined.
|
1.Stimulation time may last for only 0.1
to 0.5 msec.
2. Motor unit fire at nearly the same time.
3. The stimulation is directly given to
muscle cells then they produce same
action potential.
4. Stimulation time is well defined.
|
For the above reasons an external electrical stimulation is preferred then this can be used to measure:
1. Latency period: The time between stimulation and the beginning of the
response.
2. Symmetrical muscles of the body are compared to each other, or to
those of individuals to determine whether the action potentials and
latency periods are similar.
3. It is possible to excite the sensory nerves that carry information to the
CNS. The reflex system can be studies by observing the reflex
response at the muscle
4. The velocity of the action potential in motor nerves can be determined.
For this two stimuli are applied at two locations, and the latency period for each response is measured (Show the figure below). The difference between the two latency periods (is the time required for an action potential to travel the distance between them) the velocity of the action potential is this distance divided by this time. The normal velocity is between (40 and 60) m/sec; a velocity below 10 m/sec would indicate a problem.