Section 2: The Neuromuscular System

All of the internal and external muscles in the body and the nerves serving them make up the neuromuscular system. Every movement your body makes requires communication between the brain and muscles, some of which you don’t even have to think about, such as your digestive muscles breaking down ingested food.

The muscular system

There over 600 muscles in the body, making up around 40% of a person’s total weight.

The bones and joints create the framework of levers (bones) and pivots (joints) which give the body the potential to move, but this framework cannot move on its own. It is the contraction and relaxation of muscles that bring about movement.

The muscular system produces a continuous and wide-ranging number of actions, such as bodily movements (e.g. walking and jumping) and the powering of internal processes (e.g. contraction of the heart muscle and focusing of the eye).

Types of muscle tissue

There are three types of muscle tissue and each one has a different function. The three types are:

Cardiac muscle (myocardium), e.g. the heart.
Smooth muscle, e.g. the walls of the small intestine.
Skeletal muscle (striated), e.g. the hamstrings or triceps.

The table below describes the key characteristics of the different types of muscle tissue:

	Cardiac	Smooth	Skeletal
Control	Involuntary, not under conscious control (autonomic nervous system).	Involuntary, not under conscious control (autonomic nervous system).	Voluntary, under conscious control (somatic nervous system).
Appearance	Striated (striped or streaked).	Smooth, spindle-shaped.	Striated (striped or streaked).
Location examples and function	The heart, to ensure continuous rhythmic beating in order to push oxygen around the body.	The digestive system, to break down ingested food and drink. The walls of blood vessels, to control the volume of blood flow.	Biceps, triceps, quadriceps, etc. to create bodily movement. Some muscles contract to stabilize the body and prevent unwanted movement.

Characteristics of muscle tissue

Muscle tissue has four key characteristics:

The heart contracts to pump blood and relaxes to fill with blood. The skeletal muscles work in pairs and contract and relax to create movement of the skeleton.

Muscle contraction occurs in response to different stimuli, such as neurotransmitters and hormones. Skeletal muscle is controlled by the somatic nervous system. Smooth muscle is controlled by the automatic nervous system. Contraction of the heart is controlled by the SAN.

Muscle is elastic; it can stretch and then recoil to its original shape. Skeletal muscle is like an elastic band; if the muscle is pulled too far it can tear.

Skeletal muscle

Skeletal muscles make the human body move. They sit just underneath the skin, shortening and lengthening by pulling on bones; this is normally achieved using a tendon to pull them in different directions.

Structure of a Skeletal Muscle

Each bundle of individual muscle fibres (fasciculi) is wrapped in connective tissue called perimysium, and each single fibre within the bundle is wrapped in connective tissue called endomysium.

Inside the individual fibres, there are smaller myofibrils and within each myofibril are strands of myofilaments (actin and myosin). It is the action of myosin and actin working together that brings about movement.

The numerous fibres and connective tissues continue throughout the length of the muscle. Layers of connective tissue converge to form tendons, which are strong inelastic and strap-like. The tendon attaches to the periosteum, which is the sheath that surrounds the bone.

To summarise:

Whole muscle is wrapped in epimysium.
Bundles of fibres, or fasciculi, are wrapped in perimysium.
Single muscle fibres are wrapped in endomysium.
Myofibrils are located inside single fibres.
Myofilaments – myosin and actin – are located inside sarcomeres.

Sliding Filament Theory

The sliding filament theory was proposed by Huxley in 1954 to explain the contraction of skeletal muscle. The theory states that the myofilaments, actin (a thin protein strand) and myosin (a thick protein strand) slide over each other, creating a shortening of the sarcomere (the contractile units in the muscle where myosin and actin are found), which causes the shortening or lengthening of entire muscle. The myofilaments do not decrease in length themselves.

This proposed action is accomplished by the unique structure of the protein, myosin. The myosin filaments are shaped like gold clubs and form cross bridges with the actin filaments. Each myosin molecule (there are many) has two projecting heads. These heads attach to the actin filaments and pull them in closer.

Stimulus from the nervous system and the release of adenosine triphosphate (ATP) – the high-energy molecule stored on the myosin head – provide the impetus for the head to ‘nod’ in what is termed the ‘power stroke’. It is nodding action which ‘slides’ the thin actin filaments over the thick myosin filaments. The myosin head then binds with another ATP molecule, causing it to detach from the actin-binding site, which is known as the ‘recovery stroke’. It is then able to attach to the next binding site and perform the same routine.

Skeletal muscle fibres

Skeletal muscle fibres vary in terms of structure and function. Two distinct fibre types have been identified and classified by their contractile and metabolic characteristics:

The table below highlights the key structural and functional features of both muscles fibres, as well as examples of activities through which they are more effectively utilized:

Fibre type	Structural features	Functional features	Activities
Slow twitch or type I.	Smaller diameter. Large myoglobin content. Many mitochondria (cells where aerobic energy produced). Many capillaries to deliver blood and oxygen. Red in colour.	Increased oxygen delivery. Produce less force. Long-term contractions. Resistant to fatigue. Aerobic.	Maintaining posture, i.e. stabilization. Endurance-based activities. Lower-intensity, aerobic activities.
Fast twitch or type II.	Larger diameter. Smaller myoglobin content. Fewer mitochondria. Fewer capillaries. White (pale) in colour.	Decreased oxygen delivery. Generate more force. Short-term contractions. Less resistant to fatigue Anaerobic.	Rapid, intense movements. Strength training. Sprinting. Anaerobic training.

Muscle fibre considerations

Most people have a mixture of different fibres in their skeletal muscles. The actual percentage of fibre types for each individual is determined by genetics and heredity. This means that different people may have relatively more of one variety in a specific area. These genetic differences significantly contribute to athletic abilities.

For example, the large leg muscles of marathon runners have a higher percentage of slow twitch fibres (about 80%), while those of sprinters contain a higher percentage of fast twitch fibres (about 60%).

The proportion of slow and fast twitch fibres is also determined by the role of the muscle. The muscles of the neck and back, for example, have a key role to play in the maintenance of correct posture and so they have a high proportion of slow twitch fibres. By contrast, the muscles of the shoulders and arms are often called upon to generate considerable force and are not continually active in posture; consequently, these muscles have a higher proportion of fast twitch fibres. Leg muscles often have large numbers of both fast and slow twitch muscles, since they need to continually support the body and play a role in movement.

Effects of exercise on muscle fibres

Exercise and the type of training undertaken also affect muscle fibre proportions. Specific types of training will increase the size and capacity of certain types of muscle fibres:

Intense exercise that brings about anaerobic metabolism will boost muscular strength and mass, and increase the size of fast twitch fibres.
Moderate-intensity, aerobic endurance exercise that increases the volume of blood and oxygen to the muscle will develop the aerobic capacity of slow twitch fibres.

Intermediate fibres

Fast and slow twitch muscle fibres cannot be converted into each other. However, there is one type of fast twitch fibre (intermediate fibres) that can adapt in different ways depending on the type of training performed. In response to cardiovascular training, intermediate fibres will adapt and respond like slow twitch fibres. In response to resistance training, they will adapt and respond like other fast twitch fibres.

Location of Skeletal Muscles

The main anterior and posterior skeletal muscles are shown below:

Muscle actions and movement

When standing upright and erect, a range of skeletal muscles are in a state of continuous tone or tension. Muscles are contracting, resisting the force of gravity and preventing the body from falling to the floor. For example, the muscles in the neck keep your head upright to prevent it from dropping to the front, back or side. Skeletal muscles with a postural role typically have a higher proportion of slow twitch fibres.

How skeletal muscles create movement

To create specific movement of the joints, the following has to happen:

Muscles receive a message from the brain to shorten.
Muscles exert a force and pull on the bones.
As one muscle contracts and shortens, it works in pairs with an opposing muscle, which relaxes and lengthens.

Origins and insertions

Each muscle has a start and end point known as the origin and insertion.

Origin

The muscle attachment site on a bone(s) that serves as a relatively fixed, motionless anchor point. This end is called the origin of a muscle and is described as the proximal attachment, i.e. the one nearest to the centre midline of the body. Muscles may have more than one origin, e.g. quadriceps (four) and triceps (three).

Insertion

The end of the muscle attached to the bone that usually moves during contraction is called the muscle insertion. The insertion is described as the distal attachment, i.e. the one furthest away from the centre midline. Muscles usually have a single insertion.

Types of muscle contractions

Muscles work and contract in different ways. They can contract and shorten, contract and lengthen or contract and stay the same length, with no movement occurring. A number of terms are used to help distinguish between these different types of muscular activity:

Isotonic

Muscles move under tension by either shortening and lengthening. These terms are known as:

Concentric contraction: the muscle shortens under tension, i.e. the insertion moves towards the origin, for example the curling/upward phase of the bicep curl.
Eccentric contraction: the muscle lengthens under tension, i.e. the insertion moves away from the origin, for example the straightening/downward phase of the bicep curl.

Isometric

The muscle remains the same length under tension, for example, holding a squat at the bottom of the movement.

Roles of different muscles during movement

Efficient human movement is dependent on the coordinated activity of whole groups of muscles and will involve varying combinations of different muscle actions happening simultaneously. During any movement, different muscles can be working in the following ways:

Agonist of prime mover: This is the muscle(s) that contracts and causes a desired action, e.g. the biceps brachii contracts during a bicep curl or quadriceps contract during a leg extension.
Antagonist: This is the opposing muscle(s) to the agonist that is relatively relaxed, e.g. the triceps brachii during a bicep curl or the hamstrings during a leg extension.
Synergist: This is the muscle(s) that contracts to assist or modify the movement of the prime mover, e.g. during hip extension the hamstrings act as synergists for the gluteus maximus.
Fixators: These are the muscles that contract to stabilize the part of the body that remains fixed, e.g. shoulder girdle muscles stabilize the scapula to allow for efficient movement at the shoulder joint when the arm moves.

Joint Movements caused by Concentric Contractions

When specific muscles contract and shorten (concentric muscle work) they pull on bones to create an action or movement at the joints they cross. The table below identifies the different joint actions and movements that are brought about when specific muscles contract and shorten (concentric muscle work) while working in the role of a prime mover.

Muscle	Location	Origin (start point)	Insertion (end point)	Primary concentric actions
Deltoids.	Shoulder.	Clavicle and upper scapula.	Upper humerus.	Abduction, flexion and extension, horizontal flexion and extension and internal and external rotation of the shoulder joint.
Biceps brachii.	Front of the upper arm.	Another surface of the scapula.	Upper radius.	Flexion of the elbow and supination of the forearm.
Triceps brachii.	Back of the upper arm.	Posterior upper humerus and the scapula.	Upper ulna.	Extension of the elbow.
Latissimus dorsi.	Sides of the back.	Lower seven thoracic vertebrae, inferior angle of the scapula, thoracolumbar fascia and the iliac crest.	Anterior upper humerus.	Adduction, extension and internal rotation of the shoulder joint.
Trapezius.	Upper back.	Base of skull and spinous processes of C7-T12.	Lateral clavicle and upper surface of the scapula.	Elevation, retraction and depression of the shoulder girdle; extension, lateral flexion and rotation of the neck.
Rhomboids.	Mid-back.	Spinous processes of C7-T5.	Medial border of the scapula.	Retraction and elevation of the scapula.
Pectoralis major.	Chest.	Medial clavicle and sternum.	Upper humerus.	Flexion, horizontal flexion, adduction and internal rotation of the shoulder joint.
Erector spinae.	Either side of spine.	Sacrum, ilium, ribs and vertebrae.	Ribs, vertebrae and base of the skull.	Extension and lateral flexion of the spine.
Rectus abdominis.	Along the centre of the abdomen.	Pubis.	Cartilage of 5^th–7^th ribs and base of the sternum.	Flexion and lateral flexion of the spine and tilting the pelvis posteriorly.
Internal obliques.	Sides of the abdomen, deeper to external obliques.	Iliac crest and thoracolumbar fascia.	Lower three ribs, pubic crest and the fascial connection to the linea alba.	Rotation and lateral flexion of the spine.
External obliques.	Sides of the abdomen, closer to the surface – superficial.	Outer surface of the 5^th–12^th ribs.	Iliac crest, the pubis and the fascial connection to the linea alba.	Rotation and lateral flexion of the spine.
Transversus abdominis.	Abdomen.	Iliac crest, thoracolumbar fascia and lower six ribs.	Pubis and fascial connection to the linea alba.	Compressing and supporting the abdominal contents. Deep stabilizer of the spine.
Diaphragm.	Beneath the ribcage.	Base of the sternum, inner surface of the lower six ribs and the upper three lumbar vertebrae.	Central tendon of the diaphragm.	Drawing the central diaphragmatic tendon downwards and increasing volume of the thorax.
Intercostals.	Between ribs.	Inferior border of the ribs and costal cartilages.	Superior border of the rib below.	Elevate the ribs to aid inspiration and draw the ribs down to aid expiration.
Hip flexors.	Through the pelvis and onto the femur.	Iliac fossa and all lumbar vertebrae.	Lesser trochanter of the femur.	Flexion and external rotation of the hip.
Gluteus maximus.	Bottom – buttocks.	Coccyx, sacrum and iliac crest.	Upper femur and iliotibial band (ITB).	Extension, external rotation and abduction of the hip.
Abductor group. Gluteus medius and minimus.	Outside of the upper thigh/hip.	Outer surface of the ilium.	Upper femur and upper tibia (via the ITB).	Abduction of the hip.
Adductor group.	Inner thigh.	The pubis and ischium.	Upper, mid and lower femur.	Adduction and internal rotation of the hip.
Quadriceps group.	Front of the thigh.	Anterior inferior iliac spine (AIIS) and the femur.	Anterior, upper tibia via the patella.	Flexion of the hip and extension of the knee.
Hamstrings group.	Back of thigh.	Ischium and posterior surface of the femur.	Head of the fibula and upper, medial surface of the tibia.	Extension of the hip, flexion of the knee and tilting the pelvis posteriorly.
Gastrocnemius.	Calf.	Posterior, lower femur.	Calcaneus.	Plantarflexion of the ankle and flexion of the knee.
Soleus.	Calf.	Upper, posterior and tibia.	Calcaneus.	Plantarflexion of the ankle.
Tibialis anterior.	Front of the lower leg.	Lateral, upper tibia.	1^st metatarsal and medial tarsal.	Dorsiflexion and inversion of the ankle.

The Nervous System

All the internal communication and coordination is the responsibility of the nervous system. Its primary role is to maintain a constant balance of the internal environment, known as homeostasis. It achieves this with the help of the brain and a huge, complex network of electrical nerves and chemical messages that run throughout the body.

How the nervous system functions

To sum up the role of the nervous system, it is quite simply to:

Gather information (sensation).
Analyse the gathered information (integration).
Respond appropriately to the information (response).

Sensation

The nervous system gathers information about the internal and external environment. A vast array of sensors throughout the body (including the eyes, ears and internal proprioceptors) gather information about the internal environment (e.g. carbon dioxide levels in the blood) and the external environment (e.g. air temperature and space available).

Integration (interpretation and analysis)

The nervous system interprets and analyses the information gathered from the sensors and decides on the most appropriate action. Many of these ‘decisions’ are automatic (involuntary) without conscious control, e.g. digestion. Others are consciously controlled, e.g. voluntary muscle action.

Response

The nervous system responds to the information analysed by initiating an appropriate reaction. Responses may include muscle contraction to perform a movement or lift a weight, or glandular secretion. The nervous system works closely with the endocrine system, which is responsible for releasing hormones (chemicals) to maintain homeostasis.

Structure of the nervous system

The nervous system consists of two primary divisions:

Central Nervous System (CNS)

The CNS is the control base for the whole nervous system. All nerve impulses that stimulate muscles to contract and create movement of the body originate from the CNS.

The CNS is comprised of the brain and the spinal cord.

The Brain

The spinal cord consists of cervical, thoracic, lumbar and sacral segments, named according to the portions of the vertebral column through which they pass.

The spinal cord is the communication link between the brain and the peripheral nervous system (PNS). It integrates incoming information and produces responses via reflex mechanisms (reflex arc).

Peripheral Nervous System (PNS)

The PNS consists of all the branches of nerves that lie outside the spinal cord. Its role is to transport messages through its network of nerve cells, to and from CNS.

The peripheral nervous system subdivides into the:

Somatic System which controls voluntary (conscious) movement of the skeletal muscles, e.g. standing, walking and lifting a weight.
Autonomic system which controls involuntary functions, e.g. digestion and heart rate.

Neurons

Neurons (also called nerve cells) are responsible for transmitting electrical messages.

Spinal nerves are divided into motor and sensory neurons.

Structure of a neuron

An individual neuron consists of:

The cell body which directs the activities of the neuron.
The nucleus, which stores the cell’s genetic information, i.e. DNA. In simple terms, it tells the cell ‘what to do’.
Dendrites, which pick up impulses and transmit these to the cell body.
The axon, which transmits messages away from the cell body.
The myelin sheath, which insulates the axon to speed up the transport of messages.

The combined work of the muscular and nervous systems

Without the nervous system, bodily movement would not occur as muscles quite simply would not know what to do.

To lift a weight, for example:

The eyes gather information (estimate how heavy the weight is and where and how it is positioned). This information is sent to the CNS to be processed.
The brain sends information on how to position the body, which muscles to contract and the number of motor units to recruit to perform the lift.
Once they are stimulated to contract, the muscles pull on the bones and create appropriate movement of the joints, through the sliding action of the myofilaments (myosin and actin).

This, in effect, is the neuromuscular system at work.

Motor unit recruitment and the ‘all or none’ law

A motor unit consists of a single motor neuron and all the muscle fibre it innervates (activates). A single motor neuron may be responsible for innervating thousands of muscle fibres, depending on its location and function. This concept is known as the innervation ratio.

When an impulse is sent down a neuron, all the muscle fibres within that motor unit are innervated. The motor unit activates all of its fibres or none at all. This is known as the ‘all or none’ law.

Number and size of motor units

The number and size of motor units in specific areas of the body depend on their role and function.

Muscles responsible for strength and large force generation, such as the quadriceps and gluteals, tend to have motor units with a larger innervation ratio e.g. 1:2000. Muscles involved in finer, intricate movement, such as the fingers, tend to have a much lower innervation ratio e.g. 1:50.

The hands, for example, have a lot of small motor units and these supply fewer fibres to enable finer, more intricate movement, e.g. playing a musical instrument or using a computer.

The legs and muscles involved in maintaining posture have fewer motor neurons, but these are larger and supply more muscle fibres. Maintaining posture and movement of the legs to walk, kick, run and jump is less intricate than the movement of the hands.

A motor unit is typically made up of one type of muscle fibre (slow or fast twitch) spread throughout the muscle:

For tasks requiring less effort, smaller neurons controlling slow twitch fibres are recruited.
For tasks requiring more effort, larger neurons controlling fast twitch fibres are recruited.

Neuromuscular sensory organs

There are different muscle sense organs that form part of the autonomic system:

Joint receptors are found in the ligaments and joint capsule. They inform the brain about the position of the joint.
Muscle spindles are found in the muscle belly and inform the brain about the length of a surrounding muscle fibre – this helps to prevent overstretching and resultant damage.
Golgi tendon organs (GTOs) are found in the tendons and tell the brain how much tension a muscle is under. In extreme cases, where the muscle cannot withstand the amount of tension, the GTOs will cause the muscle to relax to avoid injury.

The lifecycle of the Neuromuscular System

Early years

This is the period of most significant growth for all of the body’s systems, including the neuromuscular system.

Over this period, neural pathways (motor connections) increase rapidly in number to develop specific movement patterns and motor skills, such as coordination and balance. Postural and stabilizing muscles also grow very quickly to progress a newborn baby from not having the ability to hold any body part upright, to having head control and being able to sit up on their own, and eventually walking within the first 12-18 months of life.

The two main factors that influence the rate of neuromuscular development in early years are:

Genetics: Each individual has a genetic potential for maximum growth, which is shaped by the genes passed on by our parents and grandparents.
Environment: Opportunities to support neuromuscular development, or restrictions that could hinder it strongly affect the potential for neuromuscular development in early years. For example, a child that spends more time taking part in physical activity, especially if it has a particular focus on developing motor skills will be much more likely to fulfill their genetic potential for growth than a child that is restricted to a more sedentary lifestyle.

Muscle, as a percentage of body mass, increases from about 42% to 54% in boys between the ages of 5 and 11; in girls, it increases from about 40% to 45% between the ages of 5 and 13 and thereafter declines (Malina et al., 2004).

Pubescent Period

Up until puberty, neuromuscular development is fairly similar between girls and boys – this changes dramatically during adolescence. The growth of new neural pathways slows down significantly in both sexes, however the growth of muscle tissue (hypertrophy) increases at a much higher rate in boys and girls. This is due to a surge in sex hormones (testosterone in boys and estrogen in girls); testosterone primarily stimulates muscle and bone growth in males, whereas in females, estrogen stimulates increases in bone, muscle and female specific fat tissue in preparation for bearing children.

Adulthood and later years

It is commonly accepted that we continue to grow in different ways until our mid-20s, with many individuals finishing their development even earlier. The neuromuscular system is the same; neural pathways and muscular growth that aren’t related to exercise come to a halt around the age of 25. If we continue to challenge the neuromuscular system, there is potential for further growth beyond our mid-20s.

One of the effects of ageing on the nervous system is the loss of neurons. By the age of 30, the brain begins to lose thousands of neurons each day. The cerebral cortex can lose as much as 45% of its cells and the brain can weigh 7% less than in the prime of our lives. Because of this, the processing of information slows down. Additionally, the voluntary motor movements slow down.

Loss of cells from the motor system occurs during the normal ageing process, leading to a reduction in the complement of motor neurons and muscle fibres. The latter age-related decrease in muscle mass has been termed ‘sarcopenia’ and is often combined with the detrimental effects of a sedentary lifestyle in older adults.

Clear evidence of this ageing effect is seen when voluntary or stimulated muscle strength is compared across the adult lifespan, with a steady decline of approximately 1-2% per year occurring after the sixth decade (Vandervoort, 2002).