Gym Instructor Level 3 – The Nervous System

Although the following article about the nervous system is for those interested in the Gym Instructor level 3 course, it is still useful for anyone who exercises to know more about how the body works. For example strength training, plyometrics, stretching and PNF stretching all utilise the workings of the nervous system.

The following information could be included in the Exercise and Fitness Knowledge theory exam.

The Nervous System

The nervous system is essential to sensory perception, the perception of pain and pleasure, control of movements and regulation of body functions, such as breathing. At its centre are the brain and spinal cord which ultimately control all the nervous tissue in the body.

Basic Structure

Central Nervous System (CNS)

The brain and spinal cord. This is the control centre of the nervous system. It receives sensory input and formulates a response.

Peripheral Nervous System (PNS)

All the nerves that enter or exit the Central Nervous System.

The Central Nervous System (CNS) is comparable to the Central Processing Unit (CPU) of a computer. It acts on messages it receives from the nerves that go to it such as:

Sense Organs, which includes sight, hearing, touch, smell, taste and pain.
Receptors, which monitor changes in the body’s internal state.

The different types of receptor to be aware of for the level 3 gym instructor course are:

Thermoreceptors, which are present in all tissues and detect changes in temperature.

Baroreceptors, which monitor the pressure of blood flowing through blood vessels.

Proprioceptors, found in musculoskeletal tissue and detect changes in body position.

Chemoreceptors, track the concentrations of all nutrients, chemicals and waste products in the body. For example, detecting too much carbon dioxide in the blood.

The Peripheral Nervous System connects the Central Nervous System to the rest of the body. There are 43 pairs of major nerves:

12 pairs from the brain
31 pairs from the spinal cord

The Cranial Nerves, from the brain, mainly supply sense organs and muscles in the head, but also digestive organs, lungs and the heart. For example, by increasing / decreasing the heart rate.

The Spinal Nerves are:

Cervical, which include the head, face, vocal cords, neck, ears and eyes.
Thoracic, which include arms, hands, heart, lungs, gallbladder, liver, stomach, pancreas, spleen, kidneys, adrenal glands and small intestines.
Lumbar, which include large intestines, appendix, abdomen, bladder, reproductive organs, lower back, lower extremities and feet.

Sacral, which include hip, thigh, leg and foot.

Peripheral Nervous System

Nervous System ChartNerves that leave the spinal cord are mainly effectors (which I’ll explain in a moment) that cause an action in the target tissue, such as:

• Stimulating skeletal muscle to contract
• The contraction of smooth muscle (intestines) and cardiac muscle (heart)
• The release of hormones from endocrine glands (insulin from pancreas)

Some actions are under our conscious control, such as the contraction of skeletal muscle. Some actions are unconscious, such as hormone release.

The Nervous System

The Somatic Branch tends to involve consciously choosing to move the body, such as exercise. It is also called the Voluntary Branch.

The Autonomic Branch controls functions such as heart rate, blood pressure, hormone secretion and metabolic activity. It is automatic and unconscious. Also known as the Involuntary Branch.

Nerves going from receptors into the Central Nervous System are called Afferents.

Nerves leaving the Central Nervous System going to effectors (the part of the body that produces a response) are Efferents.

A good way to remember this is that Efferent nerves have an Effect upon the body.

In the Peripheral Nervous System there are two types of efferent nerves:

Sympathetic Nerves
Parasympathetic Nerves

Sympathetic Nerves

• Stimulate or increase activity and mobilise fuel reserves.
• Dominate during stressful times, such as intense exercise, and enable the fight-or-flight response.
• Cause a response by releasing a chemical from nerve endings called neurotransmitters. In the case of fight-or-flight this is adrenaline.
• Stimulates the breakdown of fat into fatty acids for energy and converts muscle glycogen into glucose.

Parasympathetic Nerves

• Calm things down and conserve fuel stores.
• Dominate when resting. Known as rest-and-digest.
• The neurotransmitter released is acetylcholine. With regular exercise, especially resistance training, receptors on muscle cells become more numerous and more sensitive to it. This makes it easier to stimulate muscle quickly, improving motor fitness (reaction times, agility, co-ordination).

Neuron – Nerve Cell

Nerve / Muscle Interactions

Nervous System NerveA nerve that goes to a muscle is called a Motor Neuron. They originate in the spinal cord. At the muscle it branches to every individual cell. Each motor nerve, plus all innervated muscles cells, is called a Motor Unit.

Innervate means to supply with nerves / stimulate to action.

Neuromuscular Connections

Muscle Structure - Nervous System

The All Or Nothing Response

When a motor neuron fires, every muscle cell in the motor unit contracts maximally. It is either on or off. Contraction strength is decided by the frequency of nerve impulses to muscle cells, which determines whether or not it fires, and the number of motor units recruited for the task. This is based on conscious estimation of the force required.

In other words, if you attempted to pick up an empty box fewer motor units would be recruited than if it was full and heavy. But if you didn’t know it was full your first attempt might fail because your conscious estimation would be wrong. The second time you tried you know the weight however, so you would be successful (assuming you were strong enough!).

 Neuromuscular Connections

For most daily movements, only a small amount of motor units in a muscle are needed for the job. The work is shared to avoid fatigue and the number of cells in a motor unit varies.

Large movements, such as the thigh, could have hundreds of cells in a motor unit. For example, when doing the squat exercise.

Small, delicate movements, such as fingers, could have just 2 or 3 cells in a motor unit. For example, using a mobile phone.

It is the scale of the movement that is important however, not the size of the muscle.


As mentioned earlier, proprioceptors are found in musculoskeletal tissue and detect changes in body position. This includes the amount of stretch or tension in a muscle. The spinal cord and brain receive this to provide information on the spatial arrangement of the body.

Signals are then sent back to muscles to relax or contract for posture and balance.

There are 2 types of proprioceptors: Muscle Spindle Cells and Golgi Tendon Organs.

Muscle Spindle Cells

These are deep inside muscles and monitor the degree and rate of stretch. If the stretch is too great the spinal cord and brain signal motor neurons in the muscle to contract, while motor neurons in the antagonist are inhibited to relax muscle. It is also known as the Stretch Reflex.

For example, if the stretch in the bicep was too great it would be told to contract, while the antagonist (opposite) muscle, the triceps, would relax.

The Knee Jerk Reflex and Plyometrics exploit this.

Knee Jerk Reflex

Knee Jerk Reflex - Nervous SystemTapping the muscles just below the patella (kneecap) stretches them slightly, which the muscle spindle cells detect, and the lower leg jerks up as the muscles are told to contract.


Woman Doing Plyometrics - Nervous SystemPlyometrics can be used to help improve strength and power. In the example below, the legs are bent and therefore the quad muscles in the thigh are stretched, which causes the stretch reflex to then contract those muscles maximally. The downside to plyometrics is an increased risk of injury, especially with beginners.

Golgi Tendon Organs

These are within collagen fibres of muscle tendon and convey messages to spinal cord about excessive stretching or contraction, especially rapid and forceful. The opposite of the stretch reflex – neurons inhibit muscle to relax while the antagonist contracts. PNF (Proprioceptive Neuromuscular Facilitation) stretching exploits this.

PNF Stretching

PNF Stretching - Nervous SystemThe example below is of PNF stretching the hamstring. The leg is raised to the limit of its range of movement and then made to contract isometrically (without getting longer or shorter) by pushing against the partner’s shoulder. This is held for several seconds and then the leg is relaxed by the golgi tendon organs which allows the partner to move the leg further, increasing the stretch. This can be repeated 2 or 3 times and then finally be held for 30 seconds to help achieve rapid improvements in flexibility.

So that’s the nervous system for the level 3 gym instructor course. To test your knowledge, click here for questions.

As always, any questions or feedback leave a comment below.

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