Neuromuscular functionMuscle contraction

How does the muscular system

contribute to human movement?

Topic 4: Movement Analysis

Nervous system connecting with muscular system

• The central nervous system (CNS) consists of brain and spinal cord and is

where most sensing and control takes place.

• Sensory neurons carry signals to the CNS from receptors that sense various

factors such as body temperature, blood pressure, blood oxygen and

carbon dioxide levels (and more). =>Afferent neurons

• Motorneurons are nerves which carry information from the CNS to the

muscles and which tell muscles to contract or relax. => Efferent

neurons

Skeletal muscle has important functions

• Movement: skeletal muscle attach to bones, against which they pull to

enable movement

• Support and Posture: the muscles are seldom fully relaxed and are often

in a constant state of slight contraction. This is known as muscle tone

• Heat production: the contraction of skeletal muscle involves the

production of energy. In breaking down glycogen to provide this energy

heat is released. This is why the body gets hot while exercising

(Taken from: Sport & PE, Students book: A complete guide to Advanced Level Study)

The structure of Skeletal MuscleThe muscle belly is surrounded by a layer of epimysium (1.15a) a thick connective tissue surrounding the entire surface of the muscle. This is continuous and eventually forms tendons which join the muscle onto bones.The muscle belly is composed of many bundles of fibers know as fasciculi, these are covered by the perimysium. Each fiber within a single fascicle contains many smaller fibers called myofibrils (1.15b) which provide the contractile unit. Myofibrils have characteristic dark and light bands (striations) which represent a sarcomere (1.15c). This pattern is repeated along the length of the myofibril

Prefixes sarco and myo both refer to muscle. Therefore if you see a word with either of these prefixes you should immediately think MUSCLE.

So, does muscle contract by itself?

One motoneuron (nerve)

cannot stimulate the whole

muscle, but is only capable of

stimulating a numbers of

fibers within it. The

motoneuron and the muscle

fibers it stimulates, is called a

motor unit.

No, for skeletal muscle to contract it needs an interaction with the nervous system (neuromuscular interaction). When a muscle is required to contract, an electrical impulse is emitted from the central nervous system. The electrical impulse, or action potential, begins at the brain and is transmitted via the spinal cord and by nerve cells called motor neurons.

Motor units• A motor unit is a motor neuron

and all of the muscle cells

(muscle fibers) it stimulates.

The strength of a muscle

contraction is determined by

the size and number of motor

units being stimulated.

• The number of muscle fibers

innervated by a single

motorneuron may be high as

2000 or as small as 10.

• Why the difference in the

innervation ratio?

Diagram of motor unit

www.tokresource.org

Selective contraction

• This is why the muscle fibers are

surrounded by a membrane called the

endomysium, This is very important in

the physiology of muscle contraction

because it electrically insulates the

individual muscle cells from each other.

• At the ends of the muscle all of the

connective tissue sheaths (epimysium,

perimysium, and endomysium) converge

to form a tendon which will connect the

muscle to its attachment site.

Motor units allow for selective contraction of muscle fibers so that we may control the strength and extent of muscle contraction. Without motor units a nerve impulse to the muscle would result in the entire muscle contracting to its full extent. That would make every motion that we make an “all or none” motion. This type of movement would make life nearly impossible.

Role of neurotransmitters in human movement

• Neurotransmitter: A chemical that is released from a nerve cell

which thereby transmits an impulse from a nerve cell to another nerve,

muscle, organ, or other tissue. A neurotransmitter is a messenger of

neurologic information from one cell to another. (http://www.medterms.com)

• Neurotransmitters like Acetylcholine are the chemicals which allow

the transmission of signals from one neuron to the next across

synapses. They are also found at the axon endings of motor neurons at

the motor end plate, where they stimulate the muscle fibers.

• Acetylcholine/cholinestrase in the neuromuscular junction

• Neuromuscular junction

• Muscle contraction

The structure of Actin and Myosin

Sarcomeres have a highly organized structure, and at the most fundamental level the sarcomere is composed of two protein-based myofilaments:• A thick myosin filament• A thinner actin filament

The interaction and overlapping of these two myofilaments enables muscles to contract through the sliding filament theory

http://www.youtube.com/watch?v=Ct8AbZn_A8A

Skeletal muscle (Sarcomere, myosin and Actin)

Summary• Electrical signal from CNS triggers a release of acetylcholine in the synapse -> Influx

of Sodium ions and efflux of potassium ions reach a threshold for an action

potential -> This action potential travels along the myofibril over the sarcolemma

(through the T-tubules) and causes the sarcoplasmic reticulum underneath to

release calcium ions (Ca2+) into the sarcomeres -> Sarcomeres are composed of two

protein-based myofilaments, actin (thin) and myosin (thick) The Calcium ions opens

binding sites on tropomyosin (string of protein) connected to troponin coiled around

actin filaments. The myosin head connects to the actin and using the energy from

a chemical called ATP (adenosine triphosphate) that is on the myosin heads it bends

myosin head “sliding” actin along (sliding filament theory) -> When the energy is

released to cock the head of myosin ATP is broken into ADP (adenosine

diphosphate) and phosphate. Nerve no longer stimulated, an enzyme called

acetylcholinesterase breaks down the acetycholine and it returns to the synaptic

vesicles.

Control of muscle force

Done by recruiting motor units in two ways:

• Size principle – smaller motor units are recruited first, larger

motor units later when larger forces are required.

• Frequency (rate coding) of motor unit recruitment – higher

rate of activation of motor units causing higher force in muscle

Measuring muscle activation

• To measure the electrical

activation of a muscle in an

athlete, detremining the timing

of contraction, force of

contraction and fatigue, is

useful.

• This is done with

Electromyography (EMG), using

electrodes in or on the surface

of the muscle.

• Is this invasive/accurate?

Raw EMG signal (action potential in muscle) and rectified signal.

Fast twitch & slow twitch muscles

• Type 1 –

• Type 2a

–

• Type 2b -

Slow twitch, slow transmission speeds, small muscle forces, fatigue resistant (red fibers)

Fast twitch, fast contractions, stronger contractions, fatigue resistant (pink, Fast.Oxidative.Glycolitic. fibers)

Fastest contractions, largest forcesFatigue quick (white, F.T.G. fibers)

Taken from Sport and PE, A complete guide to advanced level study.

Major differences in structure and function between slow- and fast twitch muscle fibers

• Speed of contraction – slow-twitch muscle fibers contract at a rate of about

20% when compared with fast-twitch muscle fibers

• Muscle fiber force – fast-twitch fibers are bigger in size than slow-twitch

fibers, have larger motor neurones and therefore can generate high force

rapidly

• Muscle endurance – slow-twitch fibers are capable of resisting fatigue

whereas fast-twitch are easily fatigued

• Mitochondrial density – Slow-twitch (and type 2a) muscle fibers have higher

mitochondrial density compared to fast-twitch muscle fibers coloring the

muscle red. Fast-twitch fibers are white. (type 2a is pink)

• Glycogen stores – Fast-twitch muscle fibers have far bigger glycogen stores

than slow-twitch

Glycogen

General characteristics

• Polysaccharide (C6,H10,O5)

• Stored primarily in the liver and

muscle tissue

• Readily converted to glucose needed

by the body to satisfy energy needs

• Supplies energy during heavy work

• Stored with water (1gr Carb with 3gr

Water)

• CNS is dependent on hepatic (from

liver) glycogen

Effects on performance

• Increased storage can double

duration of exhaustive work

• Low or depleted glycogen

stores- limits exercise intensity

- decreases time to exhaustion

- increases rating of perceived exhaustion during

physical activity

http://www.exrx.net/Nutrition/Glycogen.html http://student.biology.arizona.edu/honors99/group7/glycogen.jpg

Which muscle fibers have a higher glycogen content?

Fast twitch and slow twitch muscle fibers

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