Summation and Synaptic Potentials (An Overview)

Did you know whenever we move any muscle of our body, thousands of electric impulses reach specific parts of our brain? These impulses are passed from one neuron to the next by a delicate sodium/potassium balance and synapses between neurons. This complex procedure of information exchange and their corresponding orders are carried out within seconds.

Summation and synaptic potentials

Our
central nervous system (CNS) consists of the brain and the spinal cord.

The
other organs of the nervous system falls under the peripheral nervous system
(PNS). The nervous system is made up of many small cells called neurons.  The information exchange in the body, in the
form of electric signals, through neurons make movements of body possible. The
information sending neuron is called Pre-Synpatic Cell and the information
recieving information in the form of electric signals is called Post synpatic
or recieving cell.

Action Potential

To
know better, how information is passed through neurons we need to understand
action potential.

What
exactly is action potential?

Your
body has nerves that connect to your brain just like wires of your room
connected to switchboard. When you want to move your hand or any parts of the
body, it is just like putting the switches of the switchboard on. Your brain is
switched on to send information to that part of the body which needs movement, through
neurons ( example: switching on the fan means to send electricity from the
switchboard to the fan through wires.)

Here
the switchboard or wire doesn’t say to the fan “ it is your time to move”. It
simply sends electricity which moves the fan. Akin to this the neurons do not
say to the hands “ Now you should move”. It simply send electrical information
to the hand for movement.

The
nerve cells or neurons contain charged particles. They move through action
potential. To understand what is action potential , we need to understand what
is electric potential.

Electric
potential is the amount of work required to move a single positive charge from
one point to other within the electric field.

Similarly
action potential is the amount of work required to move a single charged
particle from one neuron to other neuron.

Source: https://www.khanacademy.org/science/biology/human-biology/neuron-nervous-system/a/the-synapse

These
charged particles (ions) move from one cell to other cell through the outer
covering (cell membrane) of the neurons.

There are small gates through which the charge particles travel.

The
action potential definition according to physiology goes as: A part of the
process of firing of the neuron where the neutral membrane opens to allow
positively charged ions inside the cell and negatively charged ions flow
outside.

Definition of synapse (psychology)

It can be defined as the flow of information from
one neuron to another through a small gap between two axons or dendrites.
The neurons communicate
with one another via the synaptic junctures.

Synapses are pathways of neural information exchange, where an action triggers the pre-synaptic neuron to initiate the release of neurotransmitters. Thousands of synapses innervate the central nervous system and PSPs produced by each synapse is summed together to determine the behaviour of the post-synaptic neuron.

Process Explainer

  1. Mosquito
    tickles your hand. ( Stimuli)
  2. Brain
    sends signals in the form of electrical impulses passing through neurons to
    raise your hand and scratch the skin. ( Response)
  3. Stimuli
    — Signal sent to brain about the tickling of mosquito—-Signal sent by brain
    to take action or scratch your skin—- Response ( scratching)

What
is Summation (Temporal Vs Spatial Summation)

One of the fundamental aspects of human physiology is the summation. The
interaction betweentemporal vs
spatial summation plays a significant role in determining our body
functionality and the process in which it triggers the neurons to respond.

Let us look at some of the key differences between temporal and spatial summation:

Table 1: Temporal vs Spatial
summation

If we have
to define summation in
physiological terms, it will go as:

Summation is a part of physiology that deals with signals and inputs that help the skeletal system and the muscles to interact. It is the cumulative effect of electrical impulses on a neuromuscular junction (junction of a nerve cell and muscle cell).

Or

Summation
is the added effect of electrical impulses or signals on a neuro-muscular
junction( the point where neurons or nerve cells meet with muscle cells of hand
, feet etc.), causing that mascular part of the body to take action.

Temporal Summation

A temporal summation occurs when one presynaptic
neuron releases neurotransmitters several times over a specific period. In this
case, a single presynaptic neuron is involved in the production of multiple sub
thresholds until it becomes sufficient to generate the firing of an action
potential on the postsynaptic neuron. The action potential is generated when
the sum of the sub thresholds becomes equal to the suprathreshold.

Spatial Summation

Spatial summation takes place when multiple presynaptic neurons combine to release neurotransmitters (e.g. acetylcholine) that exceed the threshold of the postsynaptic neuron. For example, neuron A and neuron B individually release insufficient neurotransmitters. However, when the quantities are combined, the result might exceed the threshold and generate an action potential. As the distance between the dendritic input and axon hillock decreases, the chances of firing of an action potential increase in the postsynaptic cell.

Primary Functions of A Wave Summation

A wave summation is a combination of responses from
a motor unit where two or more stimuli are applied in quick succession. A
single stimulus makes a motor unit of a muscle generate a twitch response.
If a second stimulus is applied to the muscle before its response to the
first one is lost, it generates greater tension in the muscles.

This does not give the muscle enough time to relax and the contractions build on one another producing a wave pattern. Thus, whenever a muscle receives the next stimulus without recovering from the former, the contraction will be the summation of the two. This response is called wave summation.

Bodily functions possible due to Wave
Summation.

  •  increase muscle tension
  •  prevent muscle relaxation
  • produce smooth, continuous muscle contraction

The term threshold

The cumulative effect of multiple stimuli on a single cell generates an action potential. This action potential determines how the stimuli will react with neurons. Are you looking for an answer to the query “what is an action potential and how does it function?” We have just the answer for you.


Source:https://www.researchgate.net/figure/Typical-Nerve-Action-Potential_fig2_303389501

The action potential definition according to physiology goes as: A part of the process of firing of the neuron where the neutral membrane opens to allow positively charged ions inside the cell and negatively charged ions flow outside.

Have you ever wondered about the process in which
neurons transmit signals throughout the body? It involves a complicated
transmission process of firing of a neuron. In other words, it is the reverse
polarisation of a muscle cell.

An excitation-contraction coupling is maintained by
cells that use ion movement among the neural membranes. Movement of differently
charged particles across the cell membranes leads to a spike potential when
they are charged by a stimulus.

It is this spike potential that sets the action potential running. Action Potentials only happen when all the factors combine to reach an energy threshold. If it doesn’t, the action potential doesn’t occur at all and the body fails to conduct successful movement of muscles.

Source https://eprojects.isucomm.iastate.edu

It can be defined as the critical level at which
the membrane potentials depolarise to initiate an action potential.

Take a look at the picture to find the part marked
as resting potential. This resting potential gives us the necessary information
about a neuron when it is at rest. The neuron gains an action potential when
another nerve cell sends information through an axon out of the cell body.

It is the sum of the
graded potentials that determine whether the action potential will be generated
or not. If the sum of the graded potential is of sufficient voltage and reaches
the threshold value, only then an action potential will be generated at the
axon hillock.

If a neuron does not reach this critical threshold level, then no action
potential will be fired. If the neuron reaches the threshold, a full action
potential is fired. This is also known as the all or none law.

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