Skeletal MuscleContraction

If the body is to maintain homeostasis and function effectively, its trillions of cells must work together in a coordinated fashion. If each cell behaved without regard to what others were doing, the result would be physiological chaos and death.

This is prevented by two communication systems the nervous system which is specialized for the rapid transmission of signals from cell to cell, and the endocrine system, which is specialized for sending chemical messengers, the hormones, through the blood.

The most important aspect of both systems is that they detect changes in an organ, modify its physiology, and modify that of other organs. Thus, these systems functionally coordinate the organs of the body and play a central role in maintaining homeostasis.

The fundamental purpose of the nervous system is

(1) to receive information from receptors—cells and organs specialized to detect changes in the body and its external environment;

(2) to process this information and determine the appropriate response, if any—a step called neural integration; and

(3) to issue commands to effectors, cells and organs (mainly muscle and gland cells) that carry out the body’s responses.

The nervous system has two major anatomical subdivisions

The central nervous system (CNS) consists of the brain and spinal cord, which are enclosed and protected by the cranium and vertebral column.

The peripheral nervous system (PNS) consists of all the nervous system except the brain and spinal cord. It is composed of nerves and ganglia. A nerve is a bundle of nerve fibres wrapped in fibrous connective tissue. Nerves emerge from the CNS through foraminaof the skull and vertebral column and carry signals to and from other organs of the body.

The peripheral nervous system is functionally divided into sensory and motor divisions, and each of these is further divided into somatic and visceral subdivisions.

• The sensory division carries sensory signals by way of afferent nerve fibres from sensory receptors (cells and organs that detect stimuli) to the CNS.

• The visceral sensory division carries signals mainly from the viscera of the thoracic and abdominal cavities, such as the heart, lungs, stomach, andurinary bladder.

• The somatic sensory division carries signals from receptors in the skin, muscles, bones, and joints.

Skeletal Muscle

• Human body contains over 400 skeletal muscles– 40-50% of total body weight

• Functions of skeletal muscle– Force production for locomotion and breathing– Force production for postural support– Heat production during cold stress

There are three general types of muscle tissues:

Skeletal muscle responsible for movement

Cardiac muscle responsible for pumping blood

Smooth muscle responsible for sustained contractions in the blood vessels, gastrointestinal tract and other areas in the body

Structure of Skeletal Muscle:Connective Tissue Covering

• Epimysium (upon, outside)– Surrounds entire muscle

• Perimysium (around)– Surrounds bundles of muscle fibres

• Fascicles (bundle)

• Endomysium (within)– Surrounds individual muscle fibres

A muscles contraction (also known as a muscle twitch or simply twitch) occurs when a muscle fibre generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten or remain the same.

Though the term 'contraction' implies a shortening or reduction, when used as a scientific term referring to the muscular system contraction refers to the generation of tension by muscle fibres with the help of motor neurons.

Locomotion in most higher animals is possible only through the repeated contraction of many muscles at the correct times.

Contraction is controlled by the central nervous system (CNS), which comprises the brain and spinal cord.

Voluntary muscle contractions are initiated in the brain, while the spinal cord initiates involuntary reflexes.

For voluntary muscles, contraction occurs as a result of conscious effort

originating in the brain. The brain sends signals, in the form of action

potentials, through the nervous system to the motor neuron that innervates the

muscle fibre The best-understood example of an action potential is that

which is generated on the membrane of the axon of a neuron,.

Action Potential is a self-regenerating wave of electrochemical activity that allows nerve cells to carry a signal over a distance.

Action potentials (also known as nerve impulses or spikes) are pulse-like waves of voltage that travel along several types of cell membranes.

The nervous system is a network of specialized cells that communicate information about an animal's surroundings and itself. It processes this information and causes reactions in other parts of the body.

Motor Neuron

The myelin sheath is an insulating layer around a nerve fiber, somewhat like the rubber insulation on a wire. It is formed by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system.

1. Presynaptic terminal2. Sarcolemma3. Synaptic vesicle4. Nicotinic acetylcholine receptor5. Mitochondrion

Structure of Skeletal Muscle:Microstructure

• Sarcolemma (flesh – husk)– Muscle cell membrane

• Myofibrils (muscle – fibres)– Threadlike strands within muscle fibres– Actin (thin filament)

• Troponin• Tropomyosin

– Myosin (thick filament)

• In the case of some reflexes, the signal to contract can originate in the spinal cord through a feedback loop with the grey matter.

• Involuntary muscles such as the heart or smooth muscles in the gut and vascular system contract as a result of non-conscious brain activity or stimuli endogenous to the muscle itself.

• Other actions such as locomotion, breathing and chewing have a reflex aspect to them; the contractions can be initiated consciously or unconsciously, but are continued through unconscious reflex.

A sarcomere (Greek sárx = "flesh", méros = "part") is the basic unit of a muscle's cross-striated myofibril. Sarcomeres are multi-protein complexes composed of three different filament systems.

The thick filament system is composed of myosin protein which is connected from the M-line to the Z-disc by Titin It also contains myosin-binding protein C which binds at one end to the thick filament and the other to Actin.

The thin filaments are assembled by actin monomers bound to nebulin. Which also involves tropomyosin; a dimer which coils itself around the F-actin core of the thin filament.

Nebulin and Titin gives stability and structure to the sarcomere.

A muscle cell from a bicep may contain 100,000 sarcomeres. The myofibrils of smooth muscle cells are not arranged into sarcomeres.

Structure of Skeletal Muscle:The Sarcomere

• Further divisions of myofibrils– Z-line– A-bandA– I-band

• Within the sarcoplasm– Sarcoplasmic reticulum

• Storage sites for calcium– Transverse tubules

Skeletal and cardiac muscles are called striated muscle because of their striped appearance under a microscope which is due to the highly organized alternating pattern of A band and I band.

While nerve impulse profiles are, for the most part, always the same, skeletal muscles are able to produce varying levels of contractile force.

This phenomenon can be best explained by Force Summation. Force Summation describes the addition of individual twitch contractions to increase the intensity of overall muscle contraction.

Force SummationThis can be achieved in two ways:

(1) by increasing the number and size of contractile units simultaneously, called multiple fibre summation, and

(2) by increasing the frequency at which action potentials are sent to muscle fibres, called frequency summation

Multiple fibre summation – When a weak signal is sent by the CNS to contract a muscle, the smaller motor units, being more excitable than the larger ones, are stimulated first.

As the strength of the signal increases, more motor units are excited in addition to larger ones, with the largest motor units having as much as 50 times the contractile strength as the smaller ones. As more and larger motor units are activated, the force of muscle contraction becomes progressively stronger.

A concept known as the size principle allows for a gradation of muscle force during weak contraction to occur in small steps, which then become progressively larger when greater amounts of force are required.

Frequency summation - For skeletal muscles, the force exerted by the muscle is controlled by varying the frequency at which action potentials are sent to muscle fibres. Action potentials do not arrive at muscles synchronously, and during a contraction some fraction of the fibres in the muscle will be firing at any given time.

Typically when a human is exerting a muscle as hard as they are consciously able, roughly one-third of the fibres in that muscle will be firing at once, but various physiological and psychological factors can affect that. This 'low' level of contraction is a protective mechanism to prevent avulsion of the tendon - the force generated by a 95% contraction of all fibres is sufficient to damage the body.

As a muscle contracts, the Z disks come closer together; the width of the I bands decreases; the width of the H zones decreases, but there is no change in the width of the A band.

Conversely, as a muscle is stretched, the width of the I bands and H zones increases, but there is still no change in the width of the A band.

Muscular Contraction

• The sliding filament model– Muscle shortening occurs due to the movement of

the actin filament over the myosin filament– Formation of cross-bridges between actin and

myosin filaments– Reduction in the distance between Z-lines of the

sarcomere

ACTIN AND MYOSIN

Myosin is a molecular motor that acts like an active ratchet.

Chains of actin proteins form high tensile passive 'thin' filaments that transmit the force generated by myosin to the ends of the muscle.

Myosin also forms 'thick' filaments. Each myosin 'paddles' along an actin filament repeatedly binding, ratcheting and letting go, sliding the thick filament over thin filament.

Cross-Bridge Formation in Muscle Contraction

ExcitationExcitation is the process in which action potentials in the nerve fiber lead to action potentials in the muscle fibre.

1. A nerve signal arrives at the synaptic knob and stimulates voltage-gated calcium channels to open. Calcium ions enter the synaptic knob.

2. Calcium ions stimulate exocytosis of the synaptic vesicles, which release acetylcholine (ACh) into the synaptic cleft. One action potential causes exocytosis of about 60 synaptic vesicles, and each vesicle releases about 10,000 molecules of ACh.

3. ACh diffuses across the synaptic cleft and binds to receptor proteins on the sarcolemma.

4. These receptors are ligand-gated ion channels. When ACh (the ligand) binds to them, they change shape and open an ion channel through the middle of the receptor protein. Each channel allows Na todiffuse quickly into the cell and K to diffuse outward. As a result of these ion movements, the sarcolemma reverses polarity—its voltage quickly jumps from the RMP of 90 mV to a peak of 75 mV as Na enters, and then falls back to a level close to the RMP as K diffuses out. This rapid fluctuation in membrane voltage at the motor end plate is called the end-plate potential (EPP).

5. Areas of sarcolemma next to the end plate have voltage-gated ion channels that open in response to the EPP. Some of the voltage-gated channels are specific for Na and admit it to the cell, while others are specific for K and allow it to leave. These ion movements create an action potential. The muscle fiber is now excited.

6. A wave of action potentials spreads from the end plate in all directions, like ripples on a pond. When this wave of excitation reaches the T tubules, it continues down them into the sarcoplasm.

7. Action potentials open voltage-regulated ion gates in the T tubules. These are physically linked to calcium channels in the terminal cisternae of the sarcoplasmic reticulum (SR), so gates in the SR open as well and calcium ions diffuse out of the SR, down their concentration gradient and into the cytosol.

8. The calcium ions bind to the troponin of the thin filaments.

9. The troponin-tropomyosin complex changes shape and shifts to a new position. This exposes the active sites on the actin filaments and makes them available for binding to myosin heads.

Types of Muscle Contraction

• Isometric– Muscle exerts force without changing length– Pulling against immovable object– Postural muscles

• Isotonic (dynamic)– Concentric

• Muscle shortens during force production– Eccentric

• Muscle produces force but length increases

• Classification of voluntary muscular contractions

• In the case of eccentric contraction, the force generated is insufficient to overcome the resistance placed on the muscle and the muscle fibres lengthen as they contract or an eccentric contraction is used as a means of decelerating a body part or an object.

• In the case of concentric contraction, the force generated is sufficient to overcome the resistance, and the muscle shortens as it contracts.

Types of Muscle Contraction

Alternatively, muscle contractions can be categorized as isometric or isotonic.

An isometric contraction occurs when the muscle remains the same length despite building tension; an example of this is muscle contraction in the presence of an afterload.

Isotonic contractions occur when tension in the muscle remains constant despite a change in muscle length. This can occur only when a muscle's maximal force of contraction exceeds the total load on the muscle.

Types of Muscle Contraction

FIBRE TYPES

Skeletal muscle is divided into several subtypes

Type I, slow oxidative, slow twitch, or "red" muscle is dense with capillaries and is rich in mitochondria and myoglobin, giving the muscle tissue its characteristic red colour. It can carry more oxygen and sustain aerobic activity.

Capillaries are the smallest of a body's blood vessels, measuring 5-10 μm in diameter, which connect arterioles and venules, and enable the interchange of water, oxygen, carbon dioxide, and many other nutrient and waste chemical substances between blood and surrounding tissues

Mitochondria are sometimes described as "cellular power plants" because they generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of chemical energy.

Type II, fast twitch muscle, has three major kinds that are, in order of increasing contractile speed

• Type II, fast twitch muscle, has three major kinds that are, in order of increasing contractile speed:

• Type IIa, which, like slow muscle, is aerobic, rich in mitochondria and capillaries and appears red.

• Type IIx (also known as type IId), which is less dense in mitochondria and myoglobin. This is the fastest muscle type in humans. It can contract more quickly and with a greater amount of force than oxidative muscle, but can sustain only short, anaerobic bursts of activity before muscle contraction becomes painful (often incorrectly attributed to a build-up of lactic acid). N.B. in some books and articles this muscle in humans was, confusingly, called type IIB.

Fibre Types and Performance• Power athletes

– Sprinters– Possess high percentage of fast fibres

• Endurance athletes – Distance runners– Have high percentage of slow fibers

• Others– Weight lifters and non-athletes– Have about 50% slow and 50% fast fibers

Receptors in Muscle• Muscle spindle

Muscle spindles are sensory receptors within the belly of a muscle, which primarily detect changes in the length of this muscle. They convey length information to the central nervous system via sensory neurons. This information can be processed by the brain to determine the position of body parts. Changes in length detected by muscle spindles also plays an important role in regulating the contraction of muscles, by preventing unwanted stretching. This property is demonstrated by the stretch reflex.

– Detect dynamic and static changes in muscle length– Stretch reflex

• Stretch on muscle causes reflex contraction

Muscle Spindle

Golgi tendon organ (GTO)

The Golgi organ (also called Golgi tendon organ, neurotendinous organ or neurotendinous spindle), is a proprioceptive sensory receptor organ that is located at the insertion of skeletal muscle fibres into the tendons of skeletal muscle.

Monitor tension developed in muscle

Prevents damage during excessive force generation

Stimulation results in reflex relaxation of muscle

Golgi Tendon Organ