Muscular System : Unit 3.1

Muscular System: Microanatomy and Physiology

EHS Unit 3 Day 1

Goals

• Identify key structures in the microscopic anatomy of skeletal muscles.

• Discuss the process of muscle contraction.

Muscular System Overview

• This is the term for the collective skeletal (voluntary) muscles. There have three functions.

– Allow movement (works with???)

– Give shape to the body, posture and compresses the vital organs.

– Generates heat to regular temperature (what other system regulates temp??)

Characteristics of Muscle Tissue

• Excitability– Able to respond to impulse from nerve cell

• Contractility– Able to shorten in length to produce movement.

• Extensibility– Ability to stretch beyond resting length.

• Elasticity– Ability to return to resting length after contracting

or stretching.

Structure of a Skeletal Muscle

• Made of of Myocytes

– Myocytes are long, cylindrical cells with multiple nuclei.

– Sarcoplasm is the special cytoplasm of muscle cells that contains myoglobin (carries oxygen) and glycosomes (stores energy).

– These cells are also filled with fibers called myofibers.

Myofibrils

• These contain the contracting units of the muscle. – They are banded.

– The space from one Z band to the next is a sarcomere.

– Bands are created by the pattern of Myosin (thick) and Actin (filaments)

– During muscle contraction, the sarcomere shortens from both ends:

The Process of Muscle Contraction

• Occurs in Stages:

1. Nerve Stimulus and Release of ACh at Neuromuscular Junction

2. Generation of Action Potential Across Sarcolemma (muscle cell membrane)

3. Muscle Fiber Contraction

4. Relaxation

Nerve Stimulus

• An impulse originating in the central nervous system travels through the peripheral nervous pathway to the neuromuscular junction.

– At the synapse, the nerve releases ACh (acetylcholine), a neurotransmitter.

– ACh receptors in the muscle bind with the ACh molecules.

– ACh cases Ion channels to open, allowing for generation of action potential.

Action Potential

• This is the term for the electrical impulse that travels along the muscle unit. – It is generated by the movement of ions across the

sarcolemma.

– The movement of K+ and Na+ ions create this. (A wave of depolarization followed by repolarization)• When stimulated, ACh opens ion gates which allow Na+

to move in rapidly. (Depolarization)

• Once ACh is broken down, Na+ gates close and K+ gates open, allowing potassium to move out of the cell.

Fiber Contraction

• The action potential stimulates the release of calcium ions in the cell.

– Calcium allows the myosin heads to bind to the actin strands.

– ATP and phosphate are then used to produce the contractions (the myosin pulls in a rowing motion towards the middle of the sarcomere)

Relaxation

• Following contraction, the cell must return to it’s original shape.

– ATP is used, broken into ADP and phosphate.

– Ca+ ions are returns to their location.

– Actin and Myosin return to original location.

– Relaxation is a necessary break in the contraction cycle, because ions need to return to original location, ATP and Oxygen need to be replenished.

Aerobic Vs. Anaerobic Contraction

Aerobic

• Requires oxygen

• Uses stored ATP

• Then, used direct phosphorylation of ADP when exertion increases.

Anaerobic

• When oxygen is depleted and ATP or ADP production cannot keep up with demands (prolonged or extremely intense exercise) .

• Generates energy via glycolysis.

• Produces lactic and pyruvate acids as waste products.

Muscle Fatigue

• This occurs when the muscle can no longer respond to nerve stimulus.

– With prolonged exercise, acids (waste products) build up in the cell and prevent the contraction mechanism from working.

– This can also be caused by severe electrolyte imbalance that prevents generation of action potential. (What do we say we need if we have cramps?)