Muscle Physiology 1 Functional Features of Muscles Contractility

Muscle Physiology 1

Functional Features of Muscles – Contractility • Long cells shorten and generate pulling force – Excitability • Electrical nerve impulse stimulates the muscle cell to contract – Extensibility • Can be stretched back to its original length by contraction of an opposing muscle – Elasticity • Can recoil after being stretched

Types of Muscle Tissue • Skeletal – Attached to bones – Makes up 40% of body weight – Responsible for locomotion, facial expressions, posture, respiratory movements, other types of body movement – Voluntary in action; controlled by somatic motor neurons • Smooth – In the walls of hollow organs, blood vessels, eye, glands, uterus, skin – Some functions: propel urine, mix food in digestive tract, dilating/constricting pupils, regulating blood flow, – In some locations, autorhythmic – Controlled involuntarily by endocrine and autonomic nervous systems • Cardiac – Heart: major source of movement of blood – Autorhythmic – Controlled involuntarily by endocrine and autonomic nervous systems

Function of Muscle 1. Produce Movements- Muscle pulls tendons to move the skeleton 2. Maintenance of posture – enables the body to remain sitting or standing 3. Joint stabilization 4. Guard entrances and exits: Encircle openings to digestive and urinary tracts. Control swallowing and urination 5. Heat generation Muscle contractions produce heat Helps maintain normal body temperature

Basic Features of a Skeletal Muscle • Connective tissue and fascicles – Connective tissue sheaths bind a skeletal muscle and its fibers together • Epimysium – dense regular connective tissue surrounding entire muscle • Perimysium – surrounds each fascicle (group of muscle fibers) • Endomysium – a fine sheath of connective tissue wrapping each muscle cell Connective tissue sheaths are continuous with tendons

Basic Features of a Skeletal Muscle • Nerves and blood vessels – Each skeletal muscle supplied by branches of • One nerve • One artery • One or more veins – Nerves and vessels branch repeatedly – Smallest nerve branches serve • Individual muscle fibers

Motor Units • Composed of one motor neuron and all the muscle fibers that it innervates • There are many motor units in a muscle • The number of fibers innervated by a single motor neuron varies (from a few to thousand) • The fewer the number of fibers per neuron the finer the movement (more brain power)

Proprioceptors Muscle spindles are sensory receptors in muscle that control muscle tone and prevent injury from overstretching of the muscle. They are found in all muscles and are tonically active, firing increases as the muscle stretches. Figure 13 -3 a–b

Proprioceptors Golgi tendon organs are sensory receptors in muscle that respond to tension changes in the muscle and attempt to prevent injury from excessively strong contractions. When Golgi sensory neuron fibers the efferent signal in inhibitory and thus there is a loss in contraction strength Figure 13 -3 a, c

Skeletal Muscle

Microscopic Anatomy of Skeletal Muscle • Myofibrils -cylindrical structures within muscle fiber – Are bundles of protein filaments (=myofilaments) • Two types of myofilaments 1. Actin filaments (thin filaments) 2. Myosin filaments (thick filaments) – At each end of the fiber, myofibrils are anchored to the inner surface of the sarcolemma – When myofibril shortens, muscle shortens (contracts)

Sarcomeres • Basic unit of contraction of skeletal muscle – Z disc (Z line) – boundaries of each sarcomere – Thin (actin) filaments – extend from Z disc toward the center of the sarcomere – Thick (myosin) filaments – located in the center of the sarcomere • Overlap inner ends of the thin filaments • Contain ATPase enzymes

Sarcomere Structure • A bands – full length of the thick filament – Includes inner end of thin filaments • H zone – center part of A band where no thin filaments occur • M line – in center of H zone – Contains tiny rods that hold thick filaments together • I band – region with only thin filaments – Lies within two adjacent sarcomeres

The Proteins of Muscle • Myofibrils are built of 3 kinds of protein – contractile proteins • myosin and actin – regulatory proteins which turn contraction on & off • troponin and tropomyosin – structural proteins which provide proper alignment, elasticity and extensibility • titin, myomesin, nebulin and dystrophin

Structure of Actin and Myosin 10 -15

Neuromuscular junction • Motor neurons innervate skeletal muscle tissue – Neuromuscular junction is the point where nerve ending and muscle fiber meet

Transmission of Nerve Impulse to Muscle · Neurotransmitter – chemical released by nerve upon arrival of nerve impulse · The neurotransmitter for skeletal muscle is acetylcholine · Neurotransmitter attaches to receptors on the sarcolemma · Sarcolemma becomes permeable to sodium (Na+) · Sodium rushing into the cell generates an action potential · Once started, muscle contraction cannot be stopped

Transmission of Nerve Impulse to Muscle • When an action potential reaches the presynaptic terminal of the motor neuron acetylcholine is released in synaptic cleft

Mechanism of Contraction

Muscle relaxation • Ach is removed from the receptors by acetylcholinesterase • Ligand-gated Na+ channels close • Na/K pumps reestablish the RMP • Ca++ ions leave troponin and are brought back into the cisternae (this process needs energy) • Tropomyosin moves back over the actin active site • The myosin heads release their binding to actin • The filaments passively move back into resting position

SLIDING FILAMENT THEORY It has the following steps: 1. Before contraction begins, An ATP molecule binds to the myosin head of the cross-bridges. 2. The ATPase activity of the myosin head immediately cleaves the ATP molecule but the products (ADP+P) remains bound to the head. Now the myosin head is in a high energy state and ready to bind to the actin molecule. 3. When the troponin-tropomyosin complex binds with calcium ions that come from the sarcoplasmic reticulum, it pulls the tropomyosin so that the active sites on the actin filaments for the attachment of the myosin molecule are uncovered. 4. Myosin head binds to the active site on the actin molecule.

SLIDING FILAMENT THEORY 5. The bond between the head of the cross bridges(myosin) & the actin filaments causes a the bridge to change shape bending 45° inwards as if it was on a hinge, stroking towards the centre of the sarcomere, like the stroking of a boat oar. This is called a POWER STROKE. 6. This power stroke pulls the thin filament inward only a small distance. 7. Once the head tilts, this allows release of ADP & phosphate ions. 8. At the site of release of ADP, a new ATP binds. This binding causes the detachment of the myosin head from the actin. 9. A new cycle of attachment-detachment-attachment begins. 10. Repeated cycles of cross-bridge binding, bending and detachment complete the shortening and contraction of the muscle.

Types of Skeletal Muscle Fibers • Skeletal muscle fibers are categorized according to – How they manufacture energy (ATP) – How quickly they contract • Skeletal muscle fibers – Are divided into 3 classes • Slow oxidative fibers (Type I) – Red Slow twitch • Fast glycolytic fibers (Type IIx) – White fast-twitch • Fast oxidative fibers (Type IIa) – Intermediate fibers

Types of Muscle Contractions · Isotonic contractions · Myofilaments are able to slide past each other during contractions · The muscle shortens · Isometric contractions · Tension (forces) in the muscles increases · The muscle is unable to shorten (not appear to be moving)

Rigor Mortis • Rigor mortis is a state of muscular stiffness that begins 3 -4 hours after death and lasts about 24 hours • After death there is Excessive release of calcium out of the sarcoplasmic reticulum in muscle that allow myosin heads to bind to actin • Since ATP synthesis has stopped, crossbridges cannot detach from actin until proteolytic enzymes begin to digest the decomposing cells.