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Nervous System: Anatomy and Function

Read this page and discover how the nervous system controls everything we do - with significant implications for exercise instruction.

Nervous system terms

Firstly, with the nervous system there are many new terms you will likely come across, whether its in this section, in other anatomy text books and in the fitness industry.  To help you understand some of these new terms we have defined them in the following table  (bear in mind that you may need to read on for some of these definitions to make sense!):




CNS stands for central nervous system and includes the brain and spinal cord.


PNS stands for peripheral nervous system and includes all nerves outside of the CNS


Neurons (nerve cells) are the building blocks of all nerves.  Individual nerves are made up of thousands of neurons.

Action potential

Action potentials are the electrical impulses (messages) that travel throughout the nervous system


EPSP stands for excitory post-synaptic potential.  It is a positive change that makes a nerve more likely to fire an action potential


IPSP stands for inhibitory post-synaptic potential.  It is a negative change that makes a nerve less likely to fire an action potential


This refers to the charge within a neuron becoming positive and reaching threshold.  Resulting in an action potential being fired.


This refers to the charge within a neuron becoming more negative and an action potential being blocked from firing.


This refers to the neuron returning to its resting/relaxed state after it has depolarised at sent an action potential.

Temporal summation

This refers to two or more action potentials arriving in quick succession from one neuron.

Spatial summation

This refers to two or more action potentials arriving at the same time from two or more different neurons.

personal training nervous system diagram

What is the nervous system?

The nervous system is a control system of the body and is a bit like a computer.  The brain is similar to the software and is responsible for making decisions and the nerves are like the hardware or wiring that communicates those decisions with the rest of the body.


What does the nervous system do?

The nervous system along with the endocrine (hormonal) system works to control all activities within the human body.  It does this by communicating messages between the brain and the body very quickly using nerve impulses (action potentials). 

The four main functions of the nervous system are:

1.      Control of body’s internal environment to maintain ‘homeostasis’

An example of this is the regulation of body temperature.  As we exercise we create heat, in order to maintain a relatively constant core temperature the nervous system sends messages to the blood vessels to dilate (expand), increasing blood flow to the skin, and increasing sweating to help disperse the accumulating heat.

2.      Programming of spinal cord reflexes

An example of this is the stretch reflex.  This reflex functions to protect us from injury.  If we were out jogging and accidentally ran into a pot-hole and rolled our ankle, the stretch reflex would instantly sense the stretch in the muscles around the ankle and send messages to those muscles telling them to contract and resist the stretch.  This reflex serves to protect the ankle from breaking and results in a minor sprain rather than a severe break.

3.      Memory and learning

You didn’t learn to read or write overnight did you?  A certain amount of repetition was required to learn and memorise these key functions.  The same applies with exercise.  New movements, especially complex ones, take time for the nervous system to learn.  Remember this when teaching new exercises to people – a certain amount of repetition will need to occur before their nervous system gets it right!

4.      Voluntary control of movement

Every voluntary movement that a person performs is under the direct control of the nervous system as the nervous system sends the messages to the particular body parts to move.  If the movement has been repeated numerous times (walking for most of us…) the movement will be very efficient.  If however the movement is new and still requires some repetition then we would expect the movement to be less efficient and in some cases look awkward and ungainly (such as a person learning the squat for the first time).


Why is the nervous system important?

personal training nervous system functionThe nervous system is integral to our ability to function in everyway.  As we know muscle creates movement by contracting and pulling on our bones.  However it is the nervous system that is responsible for stimulating the muscles and causing them to contract.  Without the neural impulses of the nervous system, muscle would simply not work.

When someone experiences a severe trauma to their spinal cord, it will often result in paralysis of their body below the point of trauma.  For example if the spinal cord is damaged above the nerves that stimulate their lower body (legs etc), then they will not be able to walk again.  This is because the messages which are intended for the legs can no longer reach them.  In essence it is like the power cable to your house being cut and the lights going out.

The nervous system is not just responsible for stimulating muscle; it stimulates every tissue and organ within the body.  It is therefore important that you understand the nervous system so that you can train clients safely and effectively.

The nervous system and fitness

The nervous system and fitness go hand in hand.  Completing an activity that you have done thousands of times like running, learning a new skill such as squatting or simply thinking about the activity you are about to do all utilise the nervous system.personal training dumbbell bench press

For example when a client learns a new exercise, such as the dumbbell bench press, you may find that the movement is quite awkward and difficult for them.  This is because their nervous system is trying to learn something new.  However the more they repeat and refine the same movement the more efficient and smooth it becomes, until it is second nature.

When working with clients, a lot of the initial gains come from improvements in the nervous system as it learns new movement patterns and becomes more efficient at doing its job.  It is also worth noting however, that if you give a client an exercise that is too advanced for them they may be deterred from exercising.  This is because the experience of not being able to do the exercise, feeling vulnerable while lifting weights or being excessively sore the next day may put them off.

Ensuring your client is challenged sufficiently and appropriately to achieve their goals is fundamental when working in fitness.  To help you do this understanding the nervous system and your clients ability is important.  This is because it will help you pick the correct types of exercise and intensities for clients, as well as know when to progress or regress an exercise.


How is the nervous system organised?

The nervous system has many divisions, each division has their own distinct purpose.  The diagram that follows represents the nervous system and its various divisions, followed by a explanation of each division.

personal training nervous system flow chart

personal training spinal cord

Central Nervous System (CNS)

The structures of the CNS are the brain and spinal cord.  Their job is to integrate information coming back from the peripheral nervous system and to respond automatically or make decisions on actions that should be taken.  You can think of the CNS as the ‘head office’ of the body, it works consciously and subconsciously to control all activities within the body.

personal training PNS System


Peripheral Nervous System (PNS)

The structures of the PNS include the cranial nerves (nerves of the head) and spinal nerves. 

Their job is to communicate information between the CNS and the rest of the body. 


Sensory (afferent) divisionpersonal training sensory

The sensory (also known as afferent) division of the nervous system contains nerves that come from the viscera (internal organs) and the somatic areas (muscles, tendons, ligaments, ears, eyes and skin). 

These nerves conduct impulses to the PNS/CNS providing information on what is happening within and outside the body.  The senses include; hearing, sight, touch, and proprioception (the awareness of where you are in space and what position you’re in).


Motor (efferent) division

The motor (also known as efferent) division of the nervous system contains motor nerves. 

These nerves conduct impulses from the CNS and PNS to the muscles, organs and glands’ effecting what happens in those tissues.

personal training somatic system


Somatic nervous system

The somatic division of the nervous system contains nerves which end in the skeletal muscles. 

These nerves conduct impulses which control the skeletal muscles in response to a directive that comes from the brain.  This conscious control means we call the activity of this division ‘voluntary’.


personal training autonomic system

Autonomic nervous system

The autonomic division of the nervous system contains nerves which end in the viscera (internal organs).  They are therefore called visceral motor nerves.

These nerves conduct impulses which control the heart, lungs, smooth muscle in blood vessels, digestive tract and glands.  These nerves are active without conscious input from the brain so are said to be ‘involuntary’. 


Sympathetic division

The sympathetic division of the nervous system is part of the autonomic nervous system.  It works ‘automatically’ to mobilise the body’s systems during activity (for example the fight or flight response). 

If you have ever had a fright and afterward realise your heart is still beating rapidly, you’re tense and your palms are sweating, then you have experienced the activity of the sympathetic portion of the autonomic nervous system.

Parasympathetic division

The parasympathetic division of the nervous system is part of the autonomic nervous system as well.  It works ‘automatically’ to inhibit or relax the body’s systems.  It promotes digestion and other ‘housekeeping’ functions when the body is at rest.  The following diagram highlights how the sympathetic and parasympathetic divisions have different effects on various organs.

personal training parasympathetic diagram


Central Nervous System – what does the brain and spinal cord do?

The brain

The brain is organised into areas responsible for processing information, making decisions and then carrying out the appropriate task.  As you know from the previous section it can do this consciously and subconsciously.  Examples of some of these tasks are:

1.      maintaining homeostasis

2.      interpreting sensory information

3.      creating motor responses (movement)

4.      learning

5.      thinking

6.      talking

By looking at the following diagram you can see that the brain has distinguishable anatomical divisions that operate simultaneously.  Essentially the brain is modular by design, with each module responsible for a particular function, but the brain also has the ability to integrate information in a split second between modules. 

personal training the brain

The spinal cord

The spinal cord provides an ASCENDING pathway for sensory information travelling from sensory receptors in the body up to the brain and a DESCENDING pathway for motor information travelling down from the brain to the motor units. 

The spinal cord also acts like a switchboard for reflexes or movements requiring speed.  With reflexes, motor responses (movements) are initiated at the spinal cord rather than the brain.  This is because with reflexes speed is the absolute key – precious time would be lost if the message had to travel to and from the brain.  Withdrawal and jerk are the most common types of reflexes.  

An example of a withdrawal reflex is putting your hand on a hot element and moving it away before the sensation of pain is registered.  In this reflex sensory information is relayed to the spinal cord through a sensory neuron. The spinal cord then sends a command via a motor neuron back to the motor unit telling it to contract the muscle and move the hand off the element. personal training reflexes

An example of the jerk reflex is the kick of the leg resulting from a tap on the patella tendon.  This usually happens at the doctors when they are testing function.

As shown on the adjacent diagram a stretch receptor in the muscle senses the tap on the patella and sends this information to the spinal cord via a sensory nerve.  The spinal cord sends a command via a motor nerve to the motor unit in the muscle (in this case the quadriceps) to contract, causing a rapid contraction or ‘jerk’.

How do the different nervous system divisions connect in order to work together?

Understanding what each of the nervous system divisions are and their individual roles is important, but it’s also vital to know how they are wired in to work together in order to function as a whole.   This is because it is the nervous system divisions working together that allow it to function and effectively carryout it’s various tasks.

The Central Nervous System (CNS) is connected to the rest of the body by the sensory and motor nerves of the Peripheral Nervous System (PNS).   Sensory nerves relay information to the CNS; motor nerves execute motor commands from the CNS.  The nerves of the PNS are split into the Cranial and Spinal nerves.

Cranial nerves branch out of the brain spersonal training detailed spinal cord ending information and commands directly between the brain and structures in the head, neck, thoracic and abdominal cavities including eyes, ears, nose, mouth throat, heart, lungs, liver and all the other abdominal and thoracic organs, as well as some muscles of the shoulder and neck. 

Spinal nerves branch out from the spinal cord then branch off to make all the nerves of the trunk, arms and legs. 

The spinal nerves are classified according to the region in which they branch off the spinal cord; the cervical spinal nerves branch off from the neck region, the thoracic nerves branch off from the mid torso region and the lumbar and sacral spinal nerves branch off from the lower back and hip regions.

Spinal nerves carry information and commands to and from the spinal cord, trunk, arms and legs.  

The motor and sensory tracts of the spinal cord carry information and commands between the spinal nerves and the brain.


How does voluntary movement happen?

To create voluntary movement the CNS processes sensory information supplied by the eyes, ears and other sense organs and receptors of the body.  It then selects the appropriate response, plans, and then carries out the movement by sending nerve impulses through the motor branch of the peripheral nervous system.

The sympathetic branch of the autonomic nervous system is simultaneously increasing the activity of the heart and lungs to supply extra oxygen and nutrients to the skeletal muscles and at the same time, is decreasing the activity of the digestive system to allow increased blood flow to skeletal muscles.

Worked example; going for a jog. 

As soon as I start thinking about jogging and what’s involved such as putting on my shoes, what route I’ll run and so on, my sympathetic nervous system is getting me ready internally by increasing my heart rate, changing my blood pressure and exciting nerves and muscles that will soon be involved.  Remember, this is before I’ve even left my house!

personal training runnerOnce I’m on the road I start to jog nice and easy.  To jog my brain has recalled a motor pattern (a known set of muscle instructions that I learnt and refined between the ages of one and five) and sent the signals out through the CNS, PNS, motor and somatic system to the muscles.  I start to jog without any problem at all.

As I jog along my sensory division is sending signals via my PNS back to my brain at light speed.  It’s telling me my calves are a bit tight, watch that pothole, get ready to turn; you’re getting warmer and so on.

By the time I’ve reached the end of my street my sympathetic nervous system has me breathing at a good rate, my heart rate is keeping up nicely and pumping blood to those working muscles, my sweat glands are starting to open up to cool me off and so on.

The sensory division within the working muscles is sensing an increase in temperature, tension and cell by-products and the sympathetic nervous system responds by making the blood vessels in that area dilate (expand) so more blood can flow in and flow out.  The tension in the tendons and muscles is constantly monitored and if pain increases the brain gets the message and I may choose to stop and stretch.

Worked example; learning to squat.personal training squat technique

What about a movement for which the brain doesn’t have an established motor pattern to recall, such as learning to do a squat holding dumbbells for the first time.  As the instructor demonstrates and tells me what to do, the sensory receptors in my eyes and ears send the instructors information (“keep your knees shoulder width apart, weight on heals, look straight ahead, and keep your back straight” etc) via my PNS to my CNS for processing.

The CNS sends information to the motor division of my nervous system, specifically to the sympathetic division of the autonomic (involuntary) system, which starts to prepare me for my turn by accelerating my heart, secreting a little adrenaline to help me and reducing the function of my internal organs so more blood is available for my working muscles.

Then the instructor encourages me to have a go…

My autonomic system keeps functioning but now my somatic (voluntary) system starts to kick in as well.  The brain tries to recall those important instructions and remember what the ‘picture’ of good technique shown by the instructor was.  It sends messages to the muscles to bend knees, hips, and distribute weight over the heels.  But as this is a new pattern for me sometimes those messages don’t quite go to the target area and the movement isn’t quite correct. 

The instructor prompts and reminds me, which combined with sensory information from my muscles that tell me I just about fell over, all feeds back to my CNS and modified messages are sent back to the somatic system that improves my technique.

Sounds complicated doesn’t it?  Well learning new movements are complicated so try to remember this when instructing people.  Try to keep your instruction clear and simple so you don’t overload people’s nervous system with too many commands.  Integrate patterns they may have used already and can recall (e.g. ‘pretend you’re sitting down into a chair’ for squat instruction) and provide clear feedback that encourages correct technique and helps to remedy poor technique and most of all…be patient!

This is a very brief overview of just a fraction of what the nervous system is doing whenever we move.  You’ll get more of an understanding as you read through the pages here at

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