Muscle TissueMuscle Tissue

Ch. 9Ch. 9

Three types of muscle tissueThree types of muscle tissue1. Skeletal: 1. Skeletal:

- Voluntary- Voluntary- Multi-nucleated cells with nuclei on the - Multi-nucleated cells with nuclei on the periphery periphery

- Found usually attached to long bones- Found usually attached to long bones- Short contractions- Short contractions- Quick twitch- Quick twitch- Many mitochondria. - Many mitochondria.

2. Smooth muscle: 2. Smooth muscle: - Involuntary- Involuntary- Spindle shaped cells- Spindle shaped cells- No striations- No striations- Single ovoid nucleus per cell- Single ovoid nucleus per cell- Found in blood vessels- Found in blood vessels- walls of hollow organs and GI tract and is - walls of hollow organs and GI tract and is associated associated with peristalsis; long contractions; with peristalsis; long contractions; slow twitchslow twitch

3. Cardiac muscle: 3. Cardiac muscle:

- Involuntary - Involuntary

- striated- striated

- Single nucleus per cell- Single nucleus per cell

- Intercalated discs - Intercalated discs

- Found only in the heart.- Found only in the heart.

Skeletal MuscleSkeletal Muscle

Smooth MuscleSmooth Muscle

Cardiac MuscleCardiac Muscle

Muscle tissue is derived from the Muscle tissue is derived from the mesoderm layer (middle germ layer)mesoderm layer (middle germ layer)

Characteristics of muscle tissue:Characteristics of muscle tissue:

1. Excitability (irritability): ability to respond 1. Excitability (irritability): ability to respond to to certain stimulicertain stimuli

2. Contractibility: ability to shorten and 2. Contractibility: ability to shorten and thickenthicken

3. Extensibility: able to stretch and extend 3. Extensibility: able to stretch and extend without without damagedamage

4. Elasticity: ability to return to original 4. Elasticity: ability to return to original shape after shape after being stretchedbeing stretched

Functions of muscle tissueFunctions of muscle tissue

1. Movement: muscle pushes against bone 1. Movement: muscle pushes against bone for for locomotion, the heart pumps blood locomotion, the heart pumps blood through through the blood vessels, urinary the blood vessels, urinary bladder emptiesbladder empties

2. Thermogenesis: 85% of body heat is 2. Thermogenesis: 85% of body heat is produced produced by skeletal muscle contractionby skeletal muscle contraction

3. Maintenance of posture: constant 3. Maintenance of posture: constant contractions contractions to keep us sitting or to keep us sitting or standing straight.standing straight.

4. Stabilize joints: as muscles pull on bone 4. Stabilize joints: as muscles pull on bone they they help to strengthen joints.help to strengthen joints.

Organization of muscle Organization of muscle tissuetissue

Muscles are covered in connective tissue Muscles are covered in connective tissue wrappings called epimysiumwrappings called epimysium

Bundle of muscles cells are termed a fascicle. Bundle of muscles cells are termed a fascicle. This fascicle is covered in perimysiumThis fascicle is covered in perimysium

Muscle (cell) fiber is covered in endomysium.Muscle (cell) fiber is covered in endomysium. Muscle fibers are composed of elongated Muscle fibers are composed of elongated

myofibrilsmyofibrils Basic contracting unit of a myofibril is Basic contracting unit of a myofibril is

sarcomeresarcomere

Microscopic anatomy of a skeletal muscle Microscopic anatomy of a skeletal muscle fiberfiber

Muscle cell is also called a muscle fiberMuscle cell is also called a muscle fiberMuscle fiber contains large number of rod Muscle fiber contains large number of rod

shaped myofibrils.shaped myofibrils.Myofibrils contain the contractile units called Myofibrils contain the contractile units called

sarcomeres.sarcomeres.Structures:Structures:a. Sarcolemma: plasma membranea. Sarcolemma: plasma membraneb. Sarcoplasm: cytoplasm containing stored b. Sarcoplasm: cytoplasm containing stored

sugars (glycogen), mitochondria and sugars (glycogen), mitochondria and myoglobin (Omyoglobin (O22 binding pigment in muscle). binding pigment in muscle).

c. Sarcoplasmic reticulumc. Sarcoplasmic reticulum - smooth ER - smooth ER

forming interconnecting tubules forming interconnecting tubules surrounding myofibrils.surrounding myofibrils.

d. Transverse (T) tubules - tubules d. Transverse (T) tubules - tubules running running between sarcoplasmic between sarcoplasmic reticulum and reticulum and penetrating deeply penetrating deeply into cell; conducts into cell; conducts "stimulus" into "stimulus" into cell.cell.

e. Terminal cisternae - terminal portions e. Terminal cisternae - terminal portions of of sarcoplasmic reticulum adjacent sarcoplasmic reticulum adjacent to to transverse tubules. transverse tubules.

f. Triads: one T-tubule and two terminal f. Triads: one T-tubule and two terminal cisternacisterna

Sarcomere anatomy:Sarcomere anatomy:

1.      A bands: area overlapping myosin 1.      A bands: area overlapping myosin and and actin filamentsactin filaments

2.      I bands: contains actin filaments 2.      I bands: contains actin filaments onlyonly

3.      Z discs (lines): separates 3.      Z discs (lines): separates sarcomeres sarcomeres and anchors the thin and anchors the thin filaments.filaments.

4.      H zone: part of the A band that 4.      H zone: part of the A band that contains only myosin fibers.contains only myosin fibers.

5.      M line: center of the H zone that 5.      M line: center of the H zone that holds holds the myosin fibers in place.the myosin fibers in place.

Myofibril filaments::Thick filaments: myosin containing a tail and Thick filaments: myosin containing a tail and

two (globular proteins) heads. Heads two (globular proteins) heads. Heads interact with thinner filaments called actin.interact with thinner filaments called actin.

Thin filaments: composed of actin. Thin filaments: composed of actin. - tropomyosin: two strands of protein - tropomyosin: two strands of protein that spiral around the actin filament. that spiral around the actin filament. - troponin: contains three subunits - troponin: contains three subunits

that that helps bind calcium. helps bind calcium.

Sliding filament theory of contractionSliding filament theory of contraction1954: Hugh Huxley1954: Hugh HuxleyThe sacromeres shorten and the distance The sacromeres shorten and the distance between Z lines is reduced between Z lines is reduced

Motor unit: motor neuron (somatic Motor unit: motor neuron (somatic nervous system) plus all of the nervous system) plus all of the muscle fibers it innervated.muscle fibers it innervated.

Point of innervation is termed Point of innervation is termed neuromuscular junction. neuromuscular junction.

When motor neuron fires, all muscle When motor neuron fires, all muscle fibers innervated by that motor fibers innervated by that motor neuron will contractneuron will contract

Anatomy of the neuromuscular junction:Anatomy of the neuromuscular junction:

Terminal axon: axonal ending of the Terminal axon: axonal ending of the motor neuronmotor neuron

Synaptic cleft: space between the Synaptic cleft: space between the terminal axon and the sarcolemma of terminal axon and the sarcolemma of the muscle fiberthe muscle fiber

Synaptic vesicle: vesicles located in the Synaptic vesicle: vesicles located in the terminal axonal bud contain the terminal axonal bud contain the neurotransmitter acetylcholine (ACh)neurotransmitter acetylcholine (ACh)

Motor end plate: section of sarcolemma Motor end plate: section of sarcolemma that is folded upon itself. Millions of that is folded upon itself. Millions of ACh receptors are found in the folds.ACh receptors are found in the folds.

Initiation of the muscle contractionI) Nerve impulse reaches the terminal axon I) Nerve impulse reaches the terminal axon

and Caand Ca++++ is allowed to enter via voltage is allowed to enter via voltage gated calcium channels. Cagated calcium channels. Ca++++ floods in floods in from the extracellular fluid.from the extracellular fluid.

II) Calcium triggers vesicles of ACh to fuse II) Calcium triggers vesicles of ACh to fuse with the axonal membrane and release with the axonal membrane and release (exocytosis) Ach into the synaptic cleft.(exocytosis) Ach into the synaptic cleft.

III) ACh binds to ACh receptors on the III) ACh binds to ACh receptors on the sarcolemma and creates an impulse that sarcolemma and creates an impulse that travels throughout the sarcolemma and travels throughout the sarcolemma and down T-tubulesdown T-tubules

IV) To prevent excessive contraction an IV) To prevent excessive contraction an enzyme, acetylcholinesterase, breaks enzyme, acetylcholinesterase, breaks down ACh into acetic acid and choline thus down ACh into acetic acid and choline thus stopping the flow of Ach into the binding stopping the flow of Ach into the binding sites located on sarcolemma.sites located on sarcolemma.

V) Resting sarcolemma is polarized V) Resting sarcolemma is polarized (voltage)(voltage)

(-) on the inside, (+) on the outside(-) on the inside, (+) on the outside

NaNa++ normally is not allowed to enter cell normally is not allowed to enter cell

VI) When ACh binds to receptors, gated VI) When ACh binds to receptors, gated ion channels allow Naion channels allow Na++ to flood into the to flood into the cell while Kcell while K++ diffuses out. More Na diffuses out. More Na++ is is pumped in than Kpumped in than K++ leaving. This creates leaving. This creates an electrical charge across the an electrical charge across the membrane:membrane:

depolarizationdepolarization

VII) Depolarization sets off an action VII) Depolarization sets off an action potential (propagation) down the potential (propagation) down the membrane surface.membrane surface.

VIII) Repolarization is resetting the VIII) Repolarization is resetting the membrane surface back to normal. membrane surface back to normal.

Na+ channels close while KNa+ channels close while K+ + channels channels remain open (Kremain open (K++ continues pump continues pump outside).outside).

The refractory period is when the The refractory period is when the muscle fiber is insensitive to further muscle fiber is insensitive to further stimulation until repolarization is stimulation until repolarization is complete.complete.

Action potentials are considered an Action potentials are considered an all or all or none responsenone response because once initiated, because once initiated, they are unstoppable. they are unstoppable.

Excitation-Contraction coupling: Electrical Excitation-Contraction coupling: Electrical impulse does not act directly on the impulse does not act directly on the myofibrils. They stimulate CAmyofibrils. They stimulate CA++ ++ to be to be released from within muscle cell.released from within muscle cell.

From motor neuron the action potential From motor neuron the action potential travels along to axon terminal; here ACh is travels along to axon terminal; here ACh is released causing depolarization of the released causing depolarization of the motor end plate; action potential motor end plate; action potential propagates along sarcolemma down T propagates along sarcolemma down T tubules. tubules.

Action potential triggers CaAction potential triggers Ca++++ release from release from terminal cisternae of sarcoplasmic terminal cisternae of sarcoplasmic reticulum (calcium ion gates open). reticulum (calcium ion gates open).

Calcium ions bind to troponin causing Calcium ions bind to troponin causing troponin to change shape and expose troponin to change shape and expose actin active sites. actin active sites.

Contraction occurs Contraction occurs Calcium levels decrease (due to change in Calcium levels decrease (due to change in

permeability) and tropomyosin blockage is permeability) and tropomyosin blockage is restored restored

Muscle twitch:Muscle twitch:

Myogram can record the phases of Myogram can record the phases of contraction of a muscle fiber.contraction of a muscle fiber.

Single action potential acting on a motor Single action potential acting on a motor unit is a muscle twitch.unit is a muscle twitch.

1. Latent period: muscle tension begins to 1. Latent period: muscle tension begins to increase, but contraction has not increase, but contraction has not occurred.occurred.

2. Contraction period: muscle shortens 2. Contraction period: muscle shortens demonstrating actin/mysoin activitydemonstrating actin/mysoin activity

3. Relaxation period: Muscle fiber returns to 3. Relaxation period: Muscle fiber returns to resting state.resting state.