Muscle Tissue Ch. 9

Three types of muscle tissue 1. Skeletal: - Voluntary - Multi-nucleated cells with nuclei on the periphery - Found usually attached to long bones - Short contractions - Quick twitch - Many mitochondria. 2. Smooth muscle: - Involuntary - Spindle shaped cells - No striations - Single ovoid nucleus per cell - Found in blood vessels - walls of hollow organs and GI tract and is associated with peristalsis; long contractions; slow twitch

3. Cardiac muscle: - Involuntary - striated - Single nucleus per cell - Intercalated discs - Found only in the heart.

Skeletal Muscle

Smooth Muscle

Cardiac Muscle

Muscle tissue is derived from the mesoderm layer (middle germ layer) Characteristics of muscle tissue: 1. Excitability (irritability): ability to respond to certain stimuli 2. Contractibility: ability to shorten and thicken 3. Extensibility: able to stretch and extend without damage 4. Elasticity: ability to return to original shape after being stretched

Functions of muscle tissue 1. Movement: muscle pushes against bone for locomotion, the heart pumps blood through the blood vessels, urinary bladder empties 2. Thermogenesis: 85% of body heat is produced by skeletal muscle contraction 3. Maintenance of posture: constant contractions to keep us sitting or standing straight. 4. Stabilize joints: as muscles pull on bone they help to strengthen joints.

Organization of muscle tissue Muscles are covered in connective tissue wrappings called epimysium Bundle of muscles cells are termed a fascicle. This fascicle is covered in perimysium Muscle (cell) fiber is covered in endomysium. Muscle fibers are composed of elongated myofibrils Basic contracting unit of a myofibril is sarcomere

Microscopic anatomy of a skeletal muscle fiber Muscle cell is also called a muscle fiber Muscle fiber contains large number of rod shaped myofibrils. Myofibrils contain the contractile units called sarcomeres. Structures: a. Sarcolemma: plasma membrane b. Sarcoplasm: cytoplasm containing stored sugars (glycogen), mitochondria and myoglobin (O2 binding pigment in muscle).

c. Sarcoplasmic reticulum - smooth ER forming interconnecting tubules surrounding myofibrils. d. Transverse (T) tubules - tubules running between sarcoplasmic reticulum and penetrating deeply into cell; conducts "stimulus" into cell. e. Terminal cisternae - terminal portions of sarcoplasmic reticulum adjacent to transverse tubules. f. Triads: one T-tubule and two terminal cisterna

Sarcomere anatomy: 1.      A bands: area overlapping myosin and actin filaments 2.      I bands: contains actin filaments only 3.      Z discs (lines): separates sarcomeres and anchors the thin filaments. 4.      H zone: part of the A band that contains only myosin fibers. 5.      M line: center of the H zone that holds the myosin fibers in place.

Myofibril filaments: Thick filaments: myosin containing a tail and two (globular proteins) heads. Heads interact with thinner filaments called actin. Thin filaments: composed of actin. - tropomyosin: two strands of protein that spiral around the actin filament. - troponin: contains three subunits that helps bind calcium. Sliding filament theory of contraction 1954: Hugh Huxley The sacromeres shorten and the distance between Z lines is reduced

Motor unit: motor neuron (somatic nervous system) plus all of the muscle fibers it innervated. Point of innervation is termed neuromuscular junction. When motor neuron fires, all muscle fibers innervated by that motor neuron will contract

Anatomy of the neuromuscular junction: Terminal axon: axonal ending of the motor neuron Synaptic cleft: space between the terminal axon and the sarcolemma of the muscle fiber Synaptic vesicle: vesicles located in the terminal axonal bud contain the neurotransmitter acetylcholine (ACh) Motor end plate: section of sarcolemma that is folded upon itself. Millions of ACh receptors are found in the folds.

Initiation of the muscle contraction I) Nerve impulse reaches the terminal axon and Ca++ is allowed to enter via voltage gated calcium channels. Ca++ floods in from the extracellular fluid. II) Calcium triggers vesicles of ACh to fuse with the axonal membrane and release (exocytosis) Ach into the synaptic cleft. III) ACh binds to ACh receptors on the sarcolemma and creates an impulse that travels throughout the sarcolemma and down T-tubules IV) To prevent excessive contraction an enzyme, acetylcholinesterase, breaks down ACh into acetic acid and choline thus stopping the flow of Ach into the binding sites located on sarcolemma.

V) Resting sarcolemma is polarized (voltage) (-) on the inside, (+) on the outside Na+ normally is not allowed to enter cell VI) When ACh binds to receptors, gated ion channels allow Na+ to flood into the cell while K+ diffuses out. More Na+ is pumped in than K+ leaving. This creates an electrical charge across the membrane: depolarization VII) Depolarization sets off an action potential (propagation) down the membrane surface.

VIII) Repolarization is resetting the membrane surface back to normal. Na+ channels close while K+ channels remain open (K+ continues pump outside). The refractory period is when the muscle fiber is insensitive to further stimulation until repolarization is complete. Action potentials are considered an all or none response because once initiated, they are unstoppable.

Excitation-Contraction coupling: Electrical impulse does not act directly on the myofibrils. They stimulate CA++ to be released from within muscle cell. From motor neuron the action potential travels along to axon terminal; here ACh is released causing depolarization of the motor end plate; action potential propagates along sarcolemma down T tubules. Action potential triggers Ca++ release from terminal cisternae of sarcoplasmic reticulum (calcium ion gates open). Calcium ions bind to troponin causing troponin to change shape and expose actin active sites. Contraction occurs Calcium levels decrease (due to change in permeability) and tropomyosin blockage is restored

Muscle twitch: Myogram can record the phases of contraction of a muscle fiber. Single action potential acting on a motor unit is a muscle twitch. 1. Latent period: muscle tension begins to increase, but contraction has not occurred. 2. Contraction period: muscle shortens demonstrating actin/mysoin activity 3. Relaxation period: Muscle fiber returns to resting state.