Skeletal Muscle Contraction and Function Overview

11 steps in skeletal muscle contraction

Nerve signal → ACh release → ACh binds to motor end plate → End-plate potential → Action potential → Ca²⁺ release from SR → Ca²⁺ binds troponin → Tropomyosin moves → Myosin binds actin → Power stroke → Myosin resets with ATP

Skeletal muscle relaxation

ACh is broken down, calcium returns to SR, tropomyosin re-covers actin, and muscle returns to resting length

5 main functions of skeletal muscle

Movement, posture, support, regulation of elimination, heat production

5 characteristics of skeletal muscle tissue

Excitability, conductivity, contractility, elasticity, extensibility

Connective tissue surrounding a whole muscle

Epimysium

Connective tissue surrounding a fascicle

Perimysium

Connective tissue surrounding a muscle fiber

Endomysium

Levels of muscle structure from largest to smallest

Muscle → Fascicle → Muscle fiber → Myofibril → Myofilament

Sarcolemma

The plasma membrane of a muscle fiber

T-tubules

Invaginations of the sarcolemma that carry action potentials deep into the muscle cell

Sarcoplasmic reticulum

Organelle that stores and releases calcium ions in muscle fibers

Sodium/potassium pump

Maintains resting membrane potential by pumping 3 Na⁺ out and 2 K⁺ in using ATP

Types of myofilaments

Thick (myosin) and thin (actin)

Arrangement of myosin in skeletal muscle

Myosin heads project outward; tails point toward the M line

Arrangement of actin in skeletal muscle

Twisted strands of actin with troponin and tropomyosin regulating binding sites

Parts of a sarcomere

Z disc, I band, A band, H zone, M line

What shortens during muscle contraction

I band and H zone

Motor unit

One motor neuron and all the muscle fibers it controls

Synaptic knob

The swollen end of a motor neuron that releases ACh

Synaptic cleft

The space between the motor neuron and the muscle fiber

Motor end plate

The region of the sarcolemma with ACh receptors

Muscle metabolism systems

Phosphagen system, anaerobic respiration, aerobic respiration

Sources of ATP in the phosphagen system

Stored ATP, creatine phosphate, and ADP + ADP via myokinase

Anaerobic respiration

ATP production without oxygen; fast, low yield (2 ATP)

Aerobic respiration

ATP production with oxygen; slower, high yield (30+ ATP)

Types of skeletal muscle fibers

Slow oxidative (Type I), fast oxidative (Type IIa), fast glycolytic (Type IIb)

Best muscle fiber type for endurance

Slow oxidative (Type I)

Muscle twitch

A single contraction in response to one stimulus

Threshold

The minimum stimulus required to cause a contraction

Latent period of a twitch

Delay between stimulus and start of contraction

Contraction period of a twitch

Time during which the muscle shortens

Relaxation period of a twitch

The time during which the muscle returns to its resting state

Time when muscle returns to resting length

The duration it takes for a muscle to return to its original length after contraction.

What is recruitment?

Activation of more motor units for stronger contraction.

What is an isometric contraction?

Muscle tension increases but length stays the same (e.g., holding a book still).

What is an isotonic contraction?

Muscle length changes while tension remains the same (e.g., lifting a weight).

List several causes of muscle fatigue.

Glycogen depletion, ion imbalance, lactic acid buildup, low oxygen, reduced calcium release.

Compare skeletal, cardiac, and smooth muscle.

Skeletal: striated, voluntary, multinucleate; Cardiac: striated, involuntary, 1-2 nuclei; Smooth: non-striated, involuntary, 1 nucleus.

What is hypertrophy?

Increase in muscle fiber size.

What is hyperplasia?

Increase in the number of muscle fibers.

Where is smooth muscle found?

In the walls of hollow organs like intestines, blood vessels, uterus, bladder.

How does smooth muscle structure differ from skeletal?

Smooth muscle has no striations, spindle-shaped cells, dense bodies instead of sarcomeres.

How is smooth muscle contraction different from skeletal?

Slower onset, longer duration, uses calcium from outside the cell, more energy efficient.

What does the frontalis muscle do?

Raises the eyebrows.

What does the orbicularis oculi do?

Closes the eyes.

What does the masseter do?

Closes the jaw.

What does the sternocleidomastoid do?

Rotates and flexes the head.

What does the buccinator do?

Compresses cheeks (blowing, sucking).

What does the platysma do?

Tenses the skin of the neck.

What does the orbicularis oris do?

Puckers lips (kissing muscle).

What do the zygomaticus major and minor do?

Elevate the corners of the mouth (smiling).

What does the pectoralis major do?

Flexes and adducts the arm.

What does the latissimus dorsi do?

Extends and adducts the arm.

What does the rectus abdominis do?

Flexes the vertebral column.

What does the erector spinae do?

Extends the vertebral column.

What does the trapezius do?

Moves the scapula and extends the neck.

What does the deltoid do?

Abducts the arm.

What does the levator scapulae do?

Elevates the scapula.

What do the rhomboids do?

Retract the scapula.

What does the gluteus maximus do?

Extends the thigh.

What does the iliopsoas do?

Flexes the thigh.

What do the quadriceps do?

Extend the knee.

What do the hamstrings do?

Flex the knee.

What does the tibialis anterior do?

Dorsiflexes the foot.

What do the gastrocnemius and soleus do?

Plantarflex the foot.

What does the biceps brachii do?

Flexes the elbow.

What does the triceps brachii do?

Extends the elbow.

What are the different types of joint movements?

Flexion, extension, abduction, adduction, rotation, supination, pronation, elevation, depression, inversion, eversion.

What does creatine phosphate do?

Quickly regenerates ATP from ADP during short bursts of activity.

What do calcium ions do in muscle contraction?

Bind to troponin, causing tropomyosin to uncover actin binding sites.

What do sodium ions do during contraction?

Enter the cell to depolarize the membrane and start the action potential.

What does acetylcholine do in muscle contraction?

Binds to receptors on the motor end plate to initiate the action potential.