What is the connection between adjacent neurons

The nervous system is one of two key control systems of the body that sends and receives messages, enabling us to function properly. 

It is able to do this because of the nerves that connect the brain and spinal cord with our peripheral regions (legs and arms) and viscera (organs).  This connection allows the action potentials (impulses) to travel throughout the body, much like the wiring in your house.

What are nerves?

Nerves are made up of cable-like bundles of nerve cells (neurons) and each neuron has three main parts, these are:

1.      dendrites

2.      cell body

3.      axon

The dendrites receive impulses from sensory receptors or other neurons and send them towards the cell body, which contains the nucleus.

Impulses are then conducted along the axons full length away from the cell body to connect with the dendrites of another neuron, muscle, organ or gland of some kind.

Some axons have a fatty covering called a myelin sheath as seen in the adjacent image.  A myelin sheath is formed by Schwann cells that wrap around the axons.  Schwann cells are spaced evenly apart, the gaps in-between them are called the ‘nodes of ranvier’.  Impulses can travel faster along neurons that have a myelin sheath as an impulse can jump from node to node.

What do nerves do?

All nerves have two main functions, these are:

1. Receive signals:  Nerves receive signals because they are irritable, meaning that they can respond to a stimulus.  Once a stimulus is received by a nerve cell it converts it to a neural impulse which passes along the nerve.  For example, if I touch something hot the nerve endings in my finger get ‘irritated’ and send a message (impulse) back to my brain to say ‘Ouch!’
2. Transmit nerve impulses:  Nerves are able to transmit impulses along the length of the nerve cell in the form of an electrical signal (like power lines).  These electrical signals move from one end of the nerve cell to another and can pass from nerve to nerve (power pole to power pole).

What are the properties of neurons?

Neurons have two special properties that enable nerves to carryout their two main functions, these are:

 1.      Excitability / irritability:  This means they can be stimulated to produce an impulse (action potential) – a tiny electrical current.

 2.      Conductivity:  This means neurons are also able to transfer an impulse along the full length of their axons and then on to other neurons, muscles or glands.  Like the electrical wiring in your house.

Neurons are always excited by a stimulus first, before that stimulus is conducted to the next nerve, muscle or gland.  A stimulus comes about by energy being delivered to the membrane of a neuron.  There are several ways in which a stimulus can be received in order to excite a neuron.

How can a neuron be stimulated?

1.      A sound wave exciting neurons in the inner ear (E.g. A personal trainer providing instruction to a client on how to perform an exercise).

2.      Heat exciting neurons in the skin (E.g. Information sent to the brain telling the sympathetic nervous system to increase sweating to cool the body).

3.      Light exciting neurons in the eye (E.g. A personal trainer providing a visual demonstration of an exercise for the client to repeat).

4.      Pressure or length change exciting the neurons in the muscles (E.g. Lifting a barbell to do a bench press and feeling the load as increased pressure).

What types of nerves are there?

There are three different types of nerves, these are explained below and can be seen in the following image:

1.      Sensory neurons send impulses from sensory receptors to inform the CNS of a stimulus.  A sensory receptor is a structure that recognises a change in its environment.  Sensory neurons have long dendrites and short axons.  The dendrites of a sensory neuron are found outside the spinal cord in the skin, muscle or gland of their particular sensory receptor.  Their axons end in the spinal cord where they connect with the dendrites of other neurons.

2.      Motor neurons send impulses from the CNS to the muscle, organ or gland they command to take action.  Motor neurons have short dendrites and long axons.  Their dendrites and cell body are in the spinal cord and their axons are found in the muscle, organ or gland that they innervate.  They can be over one metre long.

3.      Inter neurons receive information from other neurons and pass this information on to other neurons.  Their dendrites and axons are of similar length and are both found completely within the spinal cord or brain (CNS).

In summary neurons are either:

1.                  Sensory – receive information from the PNS

2.                  Inter – pass information on in the CNS

3.                  Motor – send information from the CNS

How are nerves organised and ‘joined’?

Neurons meet with other neurons, sensory receptors and tissues at synapses. 

Synapses are junctions that allow neurons to communicate with each other, sense receptors, muscles and glands. 

It is these synapses that make it possible for the CNS to form inter connected circuits. 

Communication across a synapse occurs in one direction only.  Chemicals called neurotransmitters are released from the axon terminals of the pre synaptic neuron into the synapse (also called a synaptic cleft) and are received by the dendrites of the post synaptic neuron. 

The synapse is the control switch of the nervous system.  It is here where information or commands sent as impulses can be passed on, changed or stopped.

A synapse is created in several different ways depending on where in the body it’s located.  It consists of the axon terminals of the pre synaptic neuron on one side, a small gap, called the synaptic cleft and the dendrites of the post synaptic neuron on the other side.  Or it consists of a sensory receptor on one side, the synaptic cleft and the dendrites of a sensory neuron on the other side.  Or it consists of the axon terminals of a motor neuron on one side, the synaptic cleft and the muscle, gland or organ on the other side.

How do nerves conduct messages?

Impulses that are sent along neurons are called action potentials.  An action potential (also known as an impulse) is the electrical current that travels the length of an axon when a neuron has been stimulated.

At rest the membrane inside of a neuron has a negative charge.  This is called the resting membrane potential.  The outside of the neurons membrane is positively charged at rest, as can be seen in the below diagram.

To send an action potential across the length of a neuron the charge across the membrane is reversed briefly (this is also known as depolarisation) and the charge within the membrane briefly becomes positive.  As the charge passes, the membrane then re-polarises and returns to its resting (negatively charged) potential. 

In the axon of an unmyelinated neuron an action potential travels along as a wave, as illustrated below.

A brief reversal of charge travelling the length of an unmyelinated axon

In a myelinated neuron the potential jumps around the Schwann cell from node to node, as can be seen in the below diagram.

The nodes of ranvier in myelinated neurons allow the action potential to jump along the axon at a higher speed than action potentials in unmyelinated neurons. 

How can nerves produce a ‘graded’ response?

We don’t always want nerves to fire the next nerve because sometimes we want a graded response that primes the system for action.  For example if we were doing some warm up sets of squats prior to some very heavy sets, the warm up sets would prime most of the nerves in the relevant muscles, but the load (weight) wouldn’t be significant enough to require them all to fire.

By not having all the nerves’ firing during the warm up precious energy is conserved for the heavy sets.  By having the relevant nerves ‘primed’ for the heavy sets the messages will be ready to go directly to the target muscles ‘on mass’ when the load gets heavy, making for a more effective and efficient lift.

A graded potential is local depolarisation (i.e. at the dendrites involved) that may not meet the threshold required to depolarise the membrane of the neuron.  Each neuron has a threshold that must be met before an action potential travels the length of the neurons axon.  All graded potentials travel from the dendrites toward the cell body and on to the axon hillock.  

The axon hillock is at the end of the cell body and it controls the firing of the neuron.  If the graded potential is enough to meet the threshold, it will depolarise the membrane and an action potential will travel the length of the axon.  If it is not enough there will be no depolarisation across the membrane and an action potential will not occur. 

Action potentials are self-propagated and once started they progress along the axon membrane. It is all-or-none; there are not different degrees of action potentials. You either have one or you don't. 

How can nerves and their arrangement reduce or increase a response to a stimulus?

Once an action potential has fired it will synapse with another neuron, muscle or gland.  It will not always depolarise the neuron membrane, instead it could hyperpolarise it.  When a neuron is hyperpolarised it is inhibited.

Hyperpolarising makes the neuron membrane more negative.  This means the threshold to depolarise the neuron is harder to reach making it less likely for an action potential to fire.  

An action potential travels the length of the axon reaching the axon terminals.  At this point the axon meets with another neuron, muscle or organ at a synapse. The action potential is transmitted through the synapse by the neurotransmitters.  The neurotransmitters are received by receptors on the other side of the synapse.  This transmission across the synapse can be inhibitory or excitory depending on the type of synapse. 

The combination of the neurotransmitter and the receptor of the post-synaptic neuron determine the mode of transmission of the action potential.

If the transmission is inhibitory, the message that is received on the post-synaptic neuron is an Inhibitory post synaptic potential (IPSP), which results in the neuron membrane becoming hyperpolarised and therefore less likely to fire.

If the transmission is excitory, the message that is received on the post-synaptic neuron is an Excitory post synaptic potential (EPSP).   This will lead toward a depolarisation of the neuron membrane and an increase in the likely hood of firing.  The adjacent image shows an inhibitory neuron (C), two excitory neurons (A and B) and the resultant depolarisation of neuron D.

The post-synaptic neuron can synapse with more than one pre-synaptic neuron, as seen in the above image.  The arrangement of neurons synapsing together is set out according to the functions of the structures (muscles, glands, organs etc) the neurons service. 

An action potential will also only be generated in the post-synaptic neuron if the threshold is met.  This is achieved if the sum of all potentials, excitory and inhibitory, is received at the axon hillock and is above the threshold, as shown in the below image.

If excitory and inhibitory potentials are received at the same time or in very quick succession then the inputs are added together (summated).  If there are enough excitory potentials to meet the threshold an action potential will be generated, as seen in the below image.  Conversely if there are more inhibitory potentials then the threshold will not be met and the post-synaptic neuron will not propagate an action potential and the message will be blocked.

The summation of potentials can actually occur is two ways depending on how many pre-synaptic neurons are sending action potentials to the post-synaptic neuron.

The two ways in which potentials can be summated are through temporal and spatial summation, as described and shown in the following table.

Temporal summation refers to two or more action potentials arriving frequently from one single pre-synaptic neuron, as seen in the below image.

Spatial summation refers to two or more action potentials arriving at the same time from different pre-synaptic neurons, as seen in the below image.

Why are IPSPs important?

The location of inhibitory inter neurons in the spinal cord make it possible for action potentials to be inhibited. 

Because we don’t want to be continuously acting on the multitude of messages from sensory neurons, inhibitory inter neurons are strategically placed within the network of connections between neurons.

In this way the CNS can create an IPSP (inhibitory post-synaptic potential) which increases the amount of stimulus required for the action potential to continue through the post-synaptic neuron.  An example of this is our ability to stop from pulling our arm away when given an injection.

Inhibitory post synaptic potentials are also important for allowing the body to perform movement.  When one muscle group contracts it is often necessary for its antagonist muscle group to relax. 

The antagonists are inhibited from contracting in what is called reciprocal inhibition.  An example of this is your triceps relaxing when you do a bicep curl, if they didn’t you would not be able to bend your elbow.

Can the brain ‘prime’ or ‘tune in’ to a stimulus to help reflexes and movements?

The brain can cause EPSP’s (excitory post-synaptic potentials) to make reflexes or movements more likely to occur.  When we visualise or anticipate movements we can ‘prime’ the reflex or movement.

An example is when you are going to catch a ball, you are usually prompted to have your hands up and ready, with fingers spread.  Often children will be told to ‘get ready, watch the ball’.  In this way coaches are constantly asking kids to ‘prime’ or ‘tune in’ to stimuli that are important for performance of a task.

The same principles apply to training clients.  For example if you are teaching a client to do back squats you may say things such as, “remember to squeeze your glutes and keep your chest up”.  These are essentially cues that will get your client thinking about those technique points and will result in EPSP’s being sent to ‘prime’ the movement.

Learned reflexes

Many reflexes can be learnt by the CNS.  The brain facilitates reflex pathways when we learn and practice new skills.  It is important to learn and practice correctly as once a technique becomes learned it is facilitated as an established reflex.  To unlearn something is very difficult.

This is a particularly important point when you think about all the new exercises that we will pattern in to our clients nervous systems in the first few sessions we have with them.  Perfect technique early will pay massive dividends long term and it’s just as easy to create as poor technique.

Posture is also an example of a learned reflex.  If a certain posture is assumed often it is difficult to change, we can shift to a better posture but slip back into a hunched position when not consciously thinking of it.  Examples of different postures can be seen in the following image.

Learning to ride a bike or throw a ball are also learned reflex pathways.  They are learned by repeating them over and over when you are young.

Before an action is facilitated as a learned reflex pathway it is clumsy or uncoordinated.  When the action is performed repetitively we become more efficient at it as our CNS sends out the correct EPSPs and IPSPs automatically.  Eventually having a complete map of the movement recorded which we can execute quickly and efficiently whenever required.

What is the connection between adjacent neurons called?

What is the connection between two neurons?

What is adjacent neurons?

Which part transmits signals to adjacent neurons?