BIOL 207 Exam 3 Muscular System

Muscle Types

Skeletal Muscles, Smooth Muscles, Cardiac Muscles

General function of the muscular system

generate movement through the interaction of contractile muscle fibers (myofilaments)

Two main protein filaments

Actin & Myosin

Motor unit

motor neuron & muscle fibers (cells) connected to it

Somatic Motor Neurons

nerve cell that transmits signals from the central nervous system to skeletal muscles, controlling voluntary movements

Neuromuscular Junction

functional synapse between a motor neuron & the plasma membrane of a muscle fiber

Sarcolemma

plasma membrane surrounding a myocyte

T-tubule

inward fold of the sacrolemma

Sacroplasmic reticulum

Ca2+ storage

Protein Channels of the Neuromuscular Junction

Voltage gated Na+ & Ca2+ channels

Explain Neuromuscular Junction

1. Action potential travels to the synaptic bulb 2. Depolarization opens up V-gated Ca2+ channels 3. Ca2+ influx triggers exocytosis of Ach from the synaptic terminals 4. Ach binds to cholinergic receptors & opens up V-gated Na+ channels @ the synaptic cleft 5. Depolarization of motor endplate begins as Na+ enters the cell

What is the purpose of the Neuromuscular Junction?

To depolarize the muscle

Muscle excitation

x3 Na+ enters the cell & x2 K+ leaves the cell

Muscle fatigue

K+ depletion in muscle cells

What is the benefit of muscle fatigue?

Triggers the rest response

What must happen after muscle excitation?

excitation-contraction coupling

Role of K+ in muscle excitation

Maintains RMP (-90mV), muscle fatigue, & diffuses through leakage channels & Na+/K+ pumps on the sarcolemma (restoring MP after fatigue)

Explain Calcium Release

1. All-or-none MP propagates through the sacrolemma. 2. Depolarization @ T-tubules activates V-gated Ca2+ channels (DHP receptors). 3. These bind to Ca2+ release channels (RyR) on SR. 4. Ca2+ diffuses into sacroplasm

Ca2+-Induced Ca2+ Release Mechanism

High concentration activates more Ca2+ channels, increasing intracellular Ca2+ further (a positive feedback mechanism)

What is the purpose of Ca2+ Release?

to trigger muscle contraction

Calcium Reuptake

Ca2+ release is passive; reuptake requires more ATP. SERCA pumps actively transport Ca2+ back into SR once stimulation stops

Sarcomere

functional unit of muscle; made of actin (thin) & myosin (thick). (Supporting proteins; M-line & Titin)

Role of Calcium in Contraction

Ca2+ binds troponin → tropomyosin shifts → actin binding sites exposed → myosin binds actin

Power Stroke

1. Myosin binds actin (cross-bridge formation). 2. Phosphate dissociates → power stroke occurs (myosin bends). 3. ADP release → ATP binds, causing detachment. 4. ATP hydrolysis resets myosin head

What is the purpose of the Power Stroke?

to shorten/lengthen muscle fibers

Sliding Filament Theory

Muscle fibers shorten as sarcomeres shorten; filaments slide past each other during contraction. (A-band stays same length; I-band & H-zone shorten)

Hypocalcemia

Ca2+ deficiency → muscle cramps, confusion, memory problems, muscle aches (Caused by parathyroid hormone/vitamin D deficiency)

Hypercalcemia

Ca2+ excess → usually caused by excess PTH or cancer; decreases neuronal excitability.

Graded Contractions

strength of contraction depends on number of motor units activated

Recruitment

Increasing # of active motor units to increase contraction strength

Twitch

single contraction-relaxation cycle of a muscle fiber

Summation

repeated stimulation before complete relaxation; increases contraction strength

Effect of Summation on Ca2+

Low Ca2+ during rest; High Ca2+ during summation

Tetanus

sustained muscle contraction

Incomplete (Unfused) Tetanus

relaxation time decreases between twitches; force increases

Complete (Fused) Tetanus

No relaxation; maximum force achieved

Asynchronous Activation

different motor units fire @ different times - allows sustained contractions & prevents fatigue

Fatigue

reduction in ability to generate force due to buildup of extracellular K+, depletion of glygogen/ATP, & reduced Ca2+ release

Skeletal Muscle Stimulation

Each motor unit = 1 motor neuron + muscle fibers. Each muscle fiber is stimulated @ its own neuromuscular junction (cannot stimulate adjacent fibers)

Cardiac Muscle Structure

fibers are branched & interconnected via intercalated discs. Discs contain desmosomes (structural support) & gap junction (electrical synapse)

Two Myocardia

Atrial myocardium & ventricular myocardium

Significance of Interconnectivity between myocardial cells

allows uniform, synchronous contractions of cardiac cells

Myocardial Stimulation

depolarization controlled by Autonomic neurons (regulate rate of depolarization) & Pacemaker cells (specialized myocardial cells that depolarize rhythmically)

How is myocardium stimulation different from skeletal muscle at the Neuromuscular Junction?

Uniform, synchronous contractions, pacemaker cells (heart is self stimulating), & Ca2+ used for depolarization

Cardiac Conduction Pathway

1. SA nodes depolarize & activate pacemaker cells. 2. AV node depolarizes atrial myocardium → atria contract. 3. Bundle of His carries signal between ventricles. 4. Purkinje fibers depolarize ventricles → ventricular myocardium contracts

How is myocardium stimulation different from skeletal muscle at the Ca2+ Release?

The channel doesn’t touch the SR. Skeletal has extra (3) channels while myocardium only has 2 because they have graded potentials

Calcium role in Cardiac Muscle

V-gated Ca2+ channels open; Ca2+ influx triggers additional Ca2+ release from SR into sarcoplasm

How is myocardium stimulation different from skeletal muscle at the Power Stroke?

There is no difference