NEURON AND MUSCLE POTENTIALS

BY

UMAR SHU’AIBU

12/15/2014

INTRODUCTION

Muscle is a soft tissue found in most animals. they , produced a contraction that changes

both the length and the shape of the cell. Muscles function to produce force

and motion. They are primarily responsible for maintaining and

changing posture, locomotion as well as movement of internal organs, such as the

contraction of the heart and the movement of food through the digestive system. The

Movement of the muscle generate potential known as muscle potential.

The neuron also known as a nerve cell is an electrically excitable cell that processes and

transmits information through electrical and chemical signals. neurons are the core

components of the nervous system. neuron respond to touch, sound, light and all other

stimuli affecting the cells of the sensory organs that then send signals to the spinal cord

and brain.

NEURON POTENTIAL

The neuron potential result from the electrochemical activity of excitable cells

also known as neuron. These cells are surrounded by body fluids having a high

Cl concentration. Neuron acts as a constant current source when stimulated,

and creates an ionic current within the body fluid. This current induces

electrical potentials within the human body. This potential decrease in

amplitude with increasing distance from the excitable cell.

Resting neurons maintain a difference in charge across their cell membrane with

negative charges inside, and a positive charges outside. When neuron is

stimulated this polarity is reversed. The neuron potential is divided into

resting potential and action potential which can be explain as follow

NEURON RESTING POTENTIALResting membrane potential is the difference in voltage of the fluids inside a cell

and outside a cell, which is usually between -70 to -80 millivolts (mV). All

cells have this difference, but it is particularly important in relation to nerve and

muscle cells, since any stimulus that changes the voltage and makes it different

from the resting membrane potential is what allows the cells to transmit

electrical signals. If resting potential rises above threshold, an action potential

starts to travel from cell body down the axon

5

OUTSIDE

INSIDE

K+ = Potassium; Na+ = Sodium; Cl- = Chloride;

Na+

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RESTING POTENTIAL

INITIALIZATION OF ACTION POTENTIAL

Stimulation of a neuron opens some of the membrane proteins (a.k.a. Na+gates)

allows to pass freely into the cells free flow of Na+ into the cell causes a

reversal of membrane polarity polarity reversal is called the action

potential Stimulation of neuron due to environmental changes opens some of

the membrane protein there by allowing Na+ to pass feely into the cell. The

free flow of into the cell causes a reversal of membrane polarity. This

polarity reversal is called action potential.

This result to the changes in potential from -70mV to about 40mV. spike in voltage

causes the K+ gate open and Na+ gate to close . This make K+ ions rush out of

the neuron. The inside becomes negative again. This is repolarization.

So many K+ ions get out that the charge goes below the resting potential. While the

neuron is in this state it cannot react to additional stimuli. This is termed as

refractory period which last for about 0.5.

The sequence of depolarization and repolarization generates a small electrical current

in this localized area. The current affects the nearby protein channels for Na+ and

causes them to open. When the adjacent channels open, Na+ions flood into that area

of the neuron and an action potential occurs. This in turn will affect the areas next

to it and the impulse passes along the entire neuron. The electric current pas.ses

outward over the membrane in all directions.

MUSCLE POTENTIAL

The action potential will continue to flow along the axon until it reaches

neuromuscular junction as can be shown below

The neuromuscular junction consist of two part. Ie presynaptic terminal on the side of

the axon, and the postsynaptic terminal on the side of the muscle, between them

existed a gap called synaptic cleft. To this end, a chemical known as acetylcholine

will be released( also known as neurotransmitter) this will allow the action potential

to cross the gap and move to the target muscle fibre.

MUSCLE FIBRE EXCITATION

Each muscle fibre has one neuromuscular junction, receiving input from just

one efferent neuron . The neuro transmitter allows Na+ ions to enter the

cell, causing a depolarizing excitatory postsynaptic potential (EPSP) that is

above the threshold potential, this triggered an action potential in the muscle

fibre. the EPSP is always well above threshold. This means that under normal

circumstances, an action potential in a somatic efferent neuron always elicits an

action potential in the muscle fibre. Consider the diagram below.

The above figure shows a muscle cell EPSP in response to a single action

potential in a somatic efferent neuron . The amount of Ach(neuro transmitter)

released with one neuronal action potential is enough to depolarize the muscle

fibre well above the threshold for eliciting an action potential. The degree that

the EPSP exceeds threshold is known as the safety factor.

The term 'safety factor' refers to the ability of neuromuscular transmission to

remain effective under various physiological conditions and stresses.

This is a result of the amount of transmitter released per nerve impulse being

greater than that required to trigger an action potential in the muscle fibre.

The safety factor is a measure of this excess of released transmitter. This

means that any action potential in the neuron will be enough to triggered

action potential in the muscle fibre(also known as sarcolemma).

a single action potential in a motor neuron can activate hundreds of muscle

fibers in synchrony, the resulting currents sum to generate a potential known

as the compound action potential, which is the summed action potentials

of all the muscle fibers in the motor unit. This is also termed as the muscle

potential as can be shown in the figure below.

The action potential in the muscle will cause the muscle contraction( also known as

twitch). The twitch is divided into three period.

1. Latent period

brief delay between the stimulus and the muscle contraction The latent period is

less than 2milliseconds in humans

2. Period of contraction

3. Period of relaxation

This can be as shown in the figure below

If the muscle is allowed to relax completely before each stimulus than the muscle

will contract with the same force.

If the muscle is stimulated again before it has completely relaxed, then the force

of the next contraction increases.

i.e. stimulating the muscle at a rapid frequency increases the force of contraction.

This is called summation