Muscle Lectures

Muscle (functions)

Movement, posture/joint stability, heat production, communication (facial muscles), control of body openings, support

Excitability

A muscle property; the ability to respond to an electrical stimulus

Conductivity

A muscle property; the ability to spread electrical excitation along the cell membrane

Contractility

A muscle property; the ability to shorten/generate force

Extensibility

A muscle property; the ability to be stretched beyond resting length

Elasticity

A muscle property; the ability to recoil to original length after being stretched

Skeletal Muscle

Striated, multinucleated, voluntary; cells called myofibers; attached to bones via tendons

Cardiac Muscle

Striated, typically uninucleated, involuntary, branched cells called cardiocytes; found only in heart wall; connected by intercalated discs

Smooth Muscle

Non-striated, uninucleated, involuntary, fusiform (spindle-shaped) cells; found in walls of hollow organs

Intercalated Discs

Junctions unique to cardiac muscle containing gap junctions and desmosomes; allow electrical signals to pass between cardiocytes

Myofibers

Individual skeletal muscle cells; elongated, cylindrical, multinucleated

Hypertrophy

Increase in size of existing muscle cells (NOT gaining new cells); how muscles grow with exercise

Myo- / Sarco-

Prefixes meaning “muscle” used in muscle anatomy terminology

Superficial Fascia

(Hypodermis/tela subcutanea) Separates muscles from overlying skin

Deep Fascia

Separates neighboring muscles from each other

Epimysium

Outermost connective tissue wrapping around an entire muscle; continuous with tendon; dense irregular CT

Perimysium

Connective tissue wrapping around a bundle of fibers (fascicle); dense irregular CT

Endomysium

Connective tissue wrapping around an individual muscle fiber (cell); areolar CT

Fascicle

A smaller bundle of muscle cells (fibers) within the whole muscle; wrapped by perimysium

Myofibril

Long, cylindrical contractile organelle within a muscle fiber; composed of myofilaments; exhibits striations

Myofilaments

Protein filaments that make up myofibrils; two types: thick (myosin) and thin (actin)

Sarcolemma

The plasma membrane of a muscle cell

Sarcoplasm

The cytoplasm of a muscle cell

Sarcoplasmic Reticulum (SR)

Specialized smooth ER in muscle cells; stores and releases Ca2+ to trigger contraction

T-Tubules (Transverse tubules)

Invaginations of the sarcolemma that carry action potentials deep into the muscle cell; link to SR

Terminal Cisternae

Enlarged ends of the SR that flank each T-tubule; release Ca2+ upon stimulation

Triad

Structure formed by one T-tubule flanked by two terminal cisternae; site of excitation-contraction coupling

Sarcomere

The functional contractile unit of a myofibril; extends from Z disc to Z disc; composed of overlapping thick and thin filaments

Z Disc (Z line)

Boundary of each sarcomere; thin (actin) filaments attach here

A Band

Dark band in sarcomere; spans the full length of the thick (myosin) filaments; contains overlap zone

I Band

Light band in sarcomere; contains only thin (actin) filaments; bisected by Z disc

H Zone

Central pale region within A band; contains only thick (myosin) filaments; no actin overlap

M Line

Center of the H zone; holds thick filaments in alignment

Thick Filament

Composed of myosin protein; has myosin heads with ATPase activity; binds actin during contraction

Thin Filament

Composed of actin (F-actin, G-actin), tropomyosin, and troponin complex; contains active sites for myosin binding

Myosin

Protein of thick filaments; heads bind actin and hydrolyze ATP to power the power stroke

Actin

Protein of thin filaments; contains binding sites for myosin heads

Troponin (Tn)

Regulatory protein on thin filament; binds Ca2+; causes conformational change that shifts tropomyosin

Tropomyosin (Tm)

Regulatory protein that covers actin binding sites at rest; shifts when troponin binds Ca2+, allowing myosin-actin binding

Titin

Elastic protein connecting thick filaments to Z discs; provides passive recoil and structural stability

Neuron

Nerve cell; conducts electrical signals (action potentials) to communicate with muscle cells

Dendrites

Branched extensions of a neuron that receive signals

Axon

Long process of a neuron that conducts action potentials away from the cell body

Axon Hillock

Junction of axon and cell body; site where action potentials are initiated

Myelin Sheath

Insulating layer around axon formed by Schwann cells; speeds up action potential conduction

Schwann Cells

Glial cells that form the myelin sheath around peripheral axons

Nodes of Ranvier

Gaps between Schwann cells on myelinated axons; site of saltatory conduction

Axon Terminals

Branched ends of the axon that form synapses with target cells

Resting Membrane Potential

The voltage across the cell membrane at rest; approximately -70 mV (inside negative)

Threshold Voltage

~-55 mV; the critical membrane voltage at which voltage-gated Na+ channels open and an action potential is triggered

Action Potential

An all-or-nothing electrical signal conducted along the axon; results from sequential Na+ influx (depolarization) and K+ efflux (repolarization)

Depolarization

Phase of action potential when Na+ rushes into the cell, making the inside positive

Repolarization

Phase of action potential when K+ rushes out of the cell, restoring negative membrane potential

Voltage-Gated Na+ Channels

Ion channels that open at threshold (-55 mV); allow Na+ to rush in, causing depolarization

Voltage-Gated K+ Channels

Ion channels that open slightly after Na+ channels; allow K+ to rush out, causing repolarization; slower to open

Synapse

Junction between two neurons, or between a neuron and a muscle cell; signal crosses via neurotransmitters

Synaptic Transmission

Process of sending a signal across a synapse using neurotransmitters

Neuromuscular Junction (NMJ)

The synapse between a somatic motor neuron’s axon terminal and a skeletal muscle fiber (myofiber)

Motor End Plate

The specialized region of the sarcolemma at the NMJ; contains ACh receptors

Synaptic End Bulb

The enlarged axon terminal at the NMJ; contains synaptic vesicles with ACh

Synaptic Cleft

The narrow space between the synaptic end bulb and the motor end plate across which ACh diffuses

Acetylcholine (ACh)

The neurotransmitter released at the NMJ; binds receptors on the motor end plate to trigger muscle action potential

Acetylcholinesterase (AChE)

Enzyme in the synaptic cleft that breaks down ACh, ending muscle stimulation and allowing relaxation

Motor Unit

A single somatic motor neuron and all the muscle fibers it innervates; basic unit of motor control

Recruitment

The process of activating more motor units to increase the force of muscle contraction

Somatic Motor Neuron

A neuron that innervates skeletal muscle; its axon terminals form NMJs

Excitation (muscle)

Step 1 of contraction: ACh released into synaptic cleft → binds receptors → action potential spreads along sarcolemma and down T-tubules

Excitation-Contraction Coupling

Step 2: Action potential travels down T-tubules → triggers SR to release Ca2+ into sarcoplasm → Ca2+ binds troponin → tropomyosin shifts → actin active sites exposed

Sliding Filament Theory

Mechanism of muscle contraction: myosin heads bind actin and pull thin filaments toward the M line (center of sarcomere), shortening the sarcomere

Cross-Bridge Cycle

The repeating cycle of myosin head binding actin, performing power stroke, detaching, and re-cocking; requires ATP and Ca2+

Power Stroke

Step in cross-bridge cycle when the myosin head pivots/rotates, pulling the actin filament toward the M line; ADP + Pi released

Cross-Bridge

The connection formed when a myosin head binds to an actin active site

Muscle Relaxation

Requires: cessation of nerve signal, breakdown of ACh by AChE, active transport of Ca2+ back into SR by Ca2+ pumps

Ca2+ Pump

Active transporter that returns Ca2+ into the SR after contraction ends; requires ATP (active process, unlike passive channels)

Origin

The attachment point of a muscle to the more stationary bone; typically proximal

Insertion

The attachment point of a muscle to the more movable bone; typically distal

Belly

The fleshy, contractile middle portion of a muscle between origin and insertion

Prime Mover (Agonist)

The muscle primarily responsible for producing a specific movement

Antagonist

A muscle that opposes or reverses the action of the prime mover

Synergist

A muscle that assists the prime mover; stabilizes joints or reduces unnecessary movements

Circular Fascicle Arrangement

Fibers arranged concentrically around an opening; functions as a sphincter (e.g., orbicularis oris)

Parallel Fascicle Arrangement

Fascicles parallel to long axis of muscle; high endurance, not as strong; strap-like (e.g., rectus abdominis)

Convergent Fascicle Arrangement

Fan-shaped; fibers converge at a common attachment site; direction of pull can be changed; e.g., pectoralis major

Pennate Fascicle Arrangement

Feather-shaped; fibers at oblique angle to tendon; stronger than parallel; types: unipennate, bipennate, multipennate

Unipennate

Pennate muscle with fibers on one side of the tendon (e.g., extensor digitorum)

Bipennate

Pennate muscle with fibers on both sides of the tendon (e.g., rectus femoris)

Multipennate

Pennate muscle with multiple tendon branches within the muscle (e.g., deltoid)

Slow Oxidative Fibers (Type I)

Slow-twitch; lots of mitochondria and myoglobin; aerobic respiration; fatigue-resistant; best for endurance activities

Fast Oxidative Fibers (Type IIa)

Fast-twitch oxidative-glycolytic; intermediate mitochondria and myoglobin; aerobic + anaerobic; used for walking/running

Fast Glycolytic Fibers (Type IIb)

Fast-twitch; anaerobic glycolysis; very little myoglobin; fatigue quickly; best for short bursts of strength/power

Myoglobin

Oxygen-binding protein in muscle cells; gives red muscle its color; stores O2 for aerobic respiration

Lever

A rigid bar (bone) that moves around a fixed point (fulcrum) when a force (muscle effort) is applied against a resistance (load)

Fulcrum

The fixed pivot point of a lever system; in the body, this is a joint

Effort

The force applied to move a lever; in the body, this is muscle contraction

Load/Resistance

The force that opposes movement in a lever system; in the body, this is the weight of a body part + any external load

First-Class Lever

Fulcrum is between effort and load (E-F-L); e.g., nodding the head (atlanto-occipital joint)

Second-Class Lever

Load is between fulcrum and effort (F-L-E); e.g., standing on tiptoe (ankle/calf raise)

Third-Class Lever

Effort is between fulcrum and load (F-E-L); most common in the body; e.g., biceps brachii flexing the forearm

Satellite Cells

Stem cells adjacent to skeletal muscle fibers; involved in limited muscle repair and regeneration

Tendon

Dense regular connective tissue that attaches muscle (epimysium) to bone