LO8 of Unit 1 – The nervous system and it’s relationship to exercise (2021)
The nervous system and its relation to exercise
Learning outcomes
By the end of this section, you will be able to:
8.1 Describe the role and functions of the nervous system
8.2 Describe the principles of muscle contraction
8.3 Explain the ‘all or none law’/motor unit recruitment
8.4 Identify how exercise can enhance neuromuscular connections and improve motor fitness
8.1
The role and functions of the nervous system
The nervous system includes all the neural tissue in the body; it controls all bodily functions. It is the source of human behaviour, is responsible for your emotions, what you think and feel and how you learn and remember. It controls an unbelievable myriad of complex functions, often without you knowing about it. It is continually receiving a stream of internal (from your muscles and organs) and external (visual, sound and tactile) sensory messages which all need processing. The nervous system then allows you to act on this information by coordinating and controlling your muscles’ motor output.
The nervous system is made of two primary divisions:
- The central nervous system (CNS)
- The peripheral nervous system (PNS)
The central nervous system (CNS)
Structure: Brain and spinal cord
Function:
- To receive sensory information from the body and the external environment.
- To process information to see what is immediately necessary and relevant, ready to send to the appropriate brain areas.
- To coordinate the appropriate response by controlling your thoughts, behaviours or actions.
[IMAGE 1]
chart of the organisation of the nervous system
The peripheral nervous system (PNS)
Structure: Cranial and spinal nerves
Function: Communication lines between the CNS and the rest of the body.
Sensory division
Structure: Somatic (to the muscles) and visceral (organs) nerve fibres
Function: Sends sensory signals from receptors to the CNS
Motor division
Structure: Somatic (to the muscles) and visceral (organs) nerve fibres
Function: Sends signals from the CNS to muscles
The autonomic nervous system (ANS)
Structure: Visceral (to the organs) motor (involuntary)
Function:
- Conducts impulses from the CNS to cardiac and smooth muscle plus glands
- Effect heart and respiration rate, digestion and sexual arousal etc.
The somatic nervous system (SNS)
Structure: Somatic (to the muscles) motor (voluntary)
Function:
- Sends impulses from the CNS to skeletal muscles
Parasympathetic division
Conserves energy
Sympathetic division
Mobilises body systems during activity (‘flight or fight’)
8.2
Motor unit recruitment and ‘all-or-none’ law
For a muscle (or group of muscles) to contract, it must first be innervated (supplied) by a functioning nerve and receive a motor impulse. No nerve or motor impulse means no muscle contraction. A single nerve can attach to as few as 5-10 muscle fibres, or as many as a few thousand depending on the muscle’s role or location in the body.
A single motor neuron and all the skeletal muscle fibres it innervates is known as a motor unit (MU). This unit forms the basic functional unit of skeletal muscle. When stimulated by nerve impulses, muscle contraction is under varying force levels.
There are many MUs in a single muscle, and their total number depends on the muscle’s role and overall function. For example, the innervation estimation for the eye’s extraocular muscles is vastly different from the number required by the lower limb’s medial gastrocnemius muscle. They range from less than a dozen fibres for the eye, compared to over a thousand muscle fibres for the gastrocnemius. The eye muscles must produce fine levels of control with precise movements but little strength, whereas gastrocnemius is involved in locomotion such as walking and running.
Within a muscle, axons from countless individual nerve fibres branch out and connect with many different individual muscle fibres. The axon attachment to fibres is distributed over a relatively wide area within the muscle to ensure that the motor unit’s contractile force is spread evenly.
[IMAGE 2]
Motor units are classified as being slow, intermediate or fast-twitch. They are named after their contraction properties’ speed. The rate in which motor units’ contracts and relaxes is dependent on the size and the features of the nerve, which determines whether the muscle fibres act in a slow or fast-twitch manner. Muscles can graduate, control or increase force through the following mechanisms:
- Recruiting more motor units
- Increasing synchronisation (together at the same time)
- Increasing firing frequency (number of nerve impulses) of already-recruited motor units
All mechanisms are used during voluntary movements. What determines the number of MUs recruited is dependent on the activity or task to be performed. For example, hand muscles need fine motor control, producing enough dexterity force (pianist).
[IMAGE 3]
Image of the three types of motor units
In contrast, large leg and trunk muscles recruit MUs at very high forces. These distinctions among different types of MUs indicate how the nervous system produces movements appropriate for different circumstances.
[IMAGE 4]
Photo of a piano player
8.3
The ‘all-or-none’ law and the Henneman size principle
The ‘all-or-none’ law
The ‘all-or-none’ law states that a single nerve impulse will cause all of the muscle fibres innervated by that nerve (i.e. within a motor unit) to either contract or not contract at all.
Motor nerve fibres respond to stimuli. If the stimulus’s strength is above the nerve fibre’s firing threshold (slow, intermediate and fast-twitch), it will trigger a nerve impulse.
Once the nerve impulse is generated, it travels at speed towards the MUs muscle fibres (all myofibrils and myofilaments) causing them to contract.
Henneman size principle
The Henneman size principle states that motor units are recruited in an orderly manner from smallest (slow twitch) to largest (fast-twitch), depending on the activity’s effort.
Small or slow-twitch MUs don’t produce much force, and they are slow to act, however, they are resistant to fatigue. In contrast, fast-twitch units produce a lot of force, quickly, but they are easily fatigued.
[IMAGE 5]
Henneman graph
Key point!
Slow motor units have lower activation thresholds than fast motor units, which have a much higher activation threshold. Slow-twitch MUs are involved in activities that require sustained efforts such as standing and postural control. Large, fast-twitch MUs, on the other hand, are
only recruited when rapid movements requiring great force are made, such as jumping and sprinting
This sequence allows for smoother, more controlled movements and ensures that the body acts as efficiently as possible from an energy standpoint.
1.4
The effects of exercise on the neuromuscular system
Regular exercise profoundly affects the nervous system by enhancing neuromuscular connections, thereby improving motor fitness.
Effect of regular exercise on the nervous system:
- Develop efficient motor programmes
- Improve neuromuscular efficiency (the ability of the nervous system to communicate effectively with the muscular system to produce, reduce and stabilise against force resulting in optimal movement)
- Improve the ability to recruit motor units simultaneously
- Improve the ability to recruit additional motor units
- Increase rate coding (the capacity to increase firing rate (motor unit discharge rate) to express more strength)
- Increase the ability to inhibit antagonists
- Decrease autogenic inhibition from Golgi tendon organs
- Improve proprioception
- Improvements in motor fitness include enhanced agility, balance, coordination, power, reaction time and speed.
[IMAGE 6]