Electricity within the Body pdf - D. Hadi

College of Medicine

Dept. of Physio &Medical Physics

Electricity within the body

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Physical phenomena involving electricity and magnetism have been observed since

ancient times.

There are two aspects of electricity and magnetism in medicine: -

1. Electrical and magnetic effects generated inside the body.

2. Applications of electricity and magnetism to the surface of the body.

The electricity generated inside the body serves for the control and operation of

nerves, muscles, and organs.

The nervous system plays a fundamental role in nearly every body function.

Basically, a central computer (the brain) receives internal and external signals and

(usually) makes the proper response.

The nervous system and the neuron

The nervous system can be divided structurally into two parts: -

1. The central nervous system.

2. The peripheral nervous system.

On the other hand, the nervous system can be divided functionally into two parts: -

1. The somatic nervous system (SNS) [Voluntary].

2. The autonomic nervous system (ANS) [Involuntary]

The autonomic nervous system itself reasonably can be classified to:

1. Sympathetic nervous system

2. Parasympathetic nervous system

Time: 2 Hours

May 4, 2020

Course: MPH2

Dr Hadi Al-Sagu

The autonomic nervous system controls various internal organs such as the heart,

intestines, and glands. The control of the autonomic nervous system is essentially

involuntary.

The Peripheral Nervous System (PNS) consist mainly of nerves that connect the CNS

to every other part of the body.

Therefore nerves can be divided based on the directionality of the carried signal:

Afferent neurons are sensory neurons that carry nerve impulses from sensory stimuli

(organs) towards the brain or spinal cord (CNS), while Efferent neurons are motor

neurons that carry neural impulses away from the brain or spinal cord (CNS) towards

muscles (organs) to cause movement.

Neuron

The dendrites are the parts of the neuron specialized for receiving information from

stimuli or from other cells. If the stimulus is strong enough, the neuron transmits an

electrical signal outward along a fiber called an axon. The axon, or nerve fiber, which

may be as long as 1m, carries the electrical signal to muscles, glands, or other

neurons.

The basic structural unit of the nervous system is

the neuron; a nerve cell specialized for reception,

interpretation, and transmission of electrical

messages. There are many types of neurons.

Basically, a neuron consists of a cell body that

receives electrical messages from other neurons

through contacts called synapses located on the

dendrites or on the cell body.

The diagram is the neuron

Examination of the axons of various neurons with an electron microscope indicates

that there are two different types of nerve fibers: -

1. The membranes of some axons are covered with a fatty insulating layer

called myelin that has small uninsulated gaps called nodes of Ranvier

every few millimeters; these nerves are, referred to as myelinated

nerves. Myelin helps to prevent action potentials, which are the

electrical signals that travel along axons, from decaying due to the

electrical current leaking out through the axonal membrane. Myelinated

axons thus conduct action potentials more quickly and efficiently than

unmyelinated axons.

2. The axons of other nerves have no myelin sleeve (sheath), and these

nerves are called unmyelinated nerves. Like these sorts of nerves can be

marked at babies due to the lack of mature myelin sheaths. As a result,

babies’ movements are jerky or shaky and uncoordinated. Their

movements become smoother and coordinated once the myelin sheaths

develop.

Electrical potentials of nerves

Across the surface or membrane of every neuron is an electrical potential

(voltage) difference due to the presence of more negative ions on the inside of the

membrane than on the outside. The neuron is said to be polarized. The inside of the

cell is typically -60 to -90 mV more negative than the outside. This potential difference

is called the resting potential of the neuron. When the neuron is stimulated, a large

The diagram is the neuron

momentary change in the resting potential occurs at the point of stimulation. This

potential change, called the action potential, propagates along the axon.

Electrical potentials of nerves

Why this potential difference exists?
The two basic reasons are:

1. The chemical compositions of the fluids inside and outside the cell are different.
2. The membrane allows some ions to entre and leave the cell more easily than others.

The action potential sequence is essential for neural communication and the process

involves several steps:

1. A stimulus is received by the dendrites of a nerve cell. This causes the

channels to open. If the opening is sufficient to drive the interior potential

from -70 mV up to -55 mV, the process continues.

2. Having reached the action threshold, more Na

channels (sometimes called

voltage-gated channels) open. The Na

influx drives the interior of the cell

membrane up to about +30 mV. The process to this point is called

depolarization.

The figure explain the potential difference (-70) across the cell membrane

3. The Na+ channels close and the K+ channels open. Since the K+ channels are

much slower to open, the depolarization has time to be completed. Having both

Na+ and K+ channels open at the same time would drive the system toward

neutrality and prevent the creation of the action potential.

4. With the K+ channels open, the membrane begins to repolarize back toward its

rest potential.

5. The repolarization typically overshoots the rest potential to about -90 mV. This

is called hyperpolarization and would seem to be counterproductive, but it is

actually important in the transmission of information. Hyperpolarization

prevents the neuron from receiving another stimulus during this time, or at least

raises the threshold for any new stimulus. Part of the importance of

hyperpolarization is in preventing any stimulus already sent up an axon from

triggering another action potential in the opposite direction. In other words,

hyperpolarization assures that the signal is proceeding in one direction.

6. After hyperpolarization, the Na

pumps eventually bring the membrane back

to its resting state of -70 mV.

Two primary factors affect the speed of propagation of the action potential are:

1. The resistance within the core of the membrane.

2. The capacitance (or the charge stored) across the membrane.

The diagram shows the six sequenced stages of the action potential