Muscle Physiology

A&P Chapter 10

Describe the general functions of muscles

Movement, heat production, posture

Explain the characteristics of skeletal muscle

Voluntary, striated, long cells with multiple nuclei

Excitable, contractable, extendable

Sarcolemma

Muscle cells’ plasma membrane

Sarcoplasm

Muscle cells’ cytoplasm

Sarcoplasmic reticulum

Muscle cells’ calcium storage unit

T Tubules

Inward extensions of the sarcolemma that carry nerve impulse into the center of the musxle cell

Discuss the myofilaments of skeletal muscle

thick = myosin only

thin = actin, tropomyosin, troponin

sarcomere

unit of muscle fibers, composed of alternating thick and thin between z lines

Discuss the first step in the Sliding Filament Theory: Excitation

• Nerve impulse travels along the plasma membrane of the neuron, and when it gets to the end, neurotransmitters are released and cross the synapse

• Neurotransmitters bind to receptors in the sarcolemma, and NI begins traveling the sarcolemma

• NI travels inward at T Tubules to stimulate the sarcoplasmic reticulum

Discuss the second step in the Sliding Filament Theory: Contraction

• NI stimulates calcium release

• Ca++ binds to troponin, so tropomyosin can move, exposing actin

• Myosin can bind to actin

• ADP+P release makes myosin bend forcefully, causing sarcomere to contract

• ATP binds to myosin, so it releases, re-cocks, and re-binds to myosin, continuing the contraction

  • It keeps going as long as there’s calcium and ATP

Discuss the third step in the Sliding Filament Theory: Relaxation

• When the nerve impulse is done, the SR begins to pump calcium back into its sacs. The Ca++ is stripped from the troponin

• Troponin returns to its original site, blocking actins active site

• Myosin cannot bind to actin so the contraction can’t continue

• The muscle fiber stops contracting and returns to its long, relaxed position

Describe the All-or-None Principle

There must be stimulated with enough energy to completely contract, otherwise it won’t contract at all

Where energy comes from for a muscle contraction?

ATP is stored in muscle cells, but only enough for 2-4 seconds of contraction. So the cells have to be able to synthesize ATP really fast, so there’s tons and tons of mitochondria

Where is oxygen stored?

In the blood it circulates as hemoglobin, but is stored in muscle as myoglobin

Describe a motor unit.

A motor neuron and all the muscle fibers it attaches to. The more fibers, the less precise the movement.

tonic contraction

Continued, partial contraction. Results in firmness/tone instead of recognizable movement. Tonic contractions maintain posture.

Isotonic contraction

Contraction where the tone (tension) of a muscle stays the same, but the length changes. This happens when little to no load is placed on a muscle.

Isometric contraction

Muscle tenses without changing the length of the muscle. Too much load is placed on the muscle

Twitch contraction

Quick jerk of muscle when all motor units of a muscle are contracted simultaneously

Treppe

Gradual, stair like increase in the strength of a muscle contraction. This happens when the muscle is stimulated at brief, constant intervals. Muscles get stronger after contracting a few times, which is why athletes warm up.

Tetanus contraction

Prolonged contraction of a muscle after rapidly repeated motor impulses, so muscle can’t relax between contractions.

Graded strength principal

Skeletal muscles contract w/ varying degrees of force at varying times, so you can use the appropriate amount of force

Describe the length-tension relationship

Muscle fibers need to be at their optimal length before a contraction in order to produce the strongest contraction. If it’s too short, the sarcomeres are already too compressed to produce a strong full contraction. If it’s too long, the sarcomeres can’t reach each other to contract enough.

Oxidative fibers

Aerobic respiration, lots of myoglobin, lots of mitochondria, can contract for a long time, fatigue resistant

RED

Glycolytic fibers

Anaerobic respiration, few mitochondria, little myoglobin, large glycogen reserves, low contractibility duration, fatigue easily

WHITE

Cardiac muscle

Striated, involuntary, one nucleus, intercalated disks

Contractions are not stimulated by nerve impulses, they come from within the muscle itself. The muscles don’t fatigue. Self-stimulating = myogenic

Smooth muscle tissue

Not striated, shorter, located in the viscera, help with peristalsis