Muscle Exam Concepts

Skeletal muscle

Organs attached to bones and skin

Elongated cells called muscle fibers

Striated

Voluntary

Cardiac muscle

Only in heart; bulk of heart walls

Striated

Can contract without nervous system stimulation

Involuntary

Smooth muscle

In walls of hollow organs

Not striated

Can contract without nervous system stimulation

Involuntary

Epimysium

Dense irregular connective tissue wrapping whole muscle, outermost

Perimysium

Dense irregular connective tissue wrapping fascicle

Houses many blood vessels and nerves, middle

Endomysium

Areolar connective tissue wrapping individual fiber

Delicate layer for electrical insulation, capillary support, binding of neighboring cells, innermost

Tendon

Cordlike structure of dense regular connective tissue, muscle to bone

Aponeurosis

Thin, flattened sheet of dense irregular tissue, muscle to muscle

Parts of a muscle cell (fiber)

Sarcoplasm (cytoplasm)

Multiple nuclei (individual cells are multinucleated)

Sarcolemma (plasma membrane)

T-tubules (transverse tubules) that extend deep into the cell

Myofibrils

Sarcoplasmic reticulum

Terminal cisternae: blind sacs of sarcoplasmic reticulum that serve as reservoirs for calcium ions

Two cisternae with T-tubule in between = triad

Thick filaments

Consist of bundles of many myosin protein molecules

Thin filaments

Consist of bundles of actin regulated by troponin and tropomyosin

Sacromeres

Repeated units on myofilaments

I bands

Light-appearing regions that contain only thin filaments

H zone

Only thick filaments present; no thin filament overlap

A band

Dark-appearing region that contains thick filaments and overlapping thin filaments

M line

Attachment site for thick filaments

Z disc

Attachment site for thin filaments

Steps in skeletal muscle contraction

ACh from the synapse causes chemically gated sodium channels to open
Sodium ions flow into the cell, changing the potential from -90 mV to -65 mV

Voltage gated sodium channels open and potential drops to 30mV
Voltage gated sodium channels close and potassium channels open, potassium ions leaves, potential returns to -90mV

Action potential opens voltage gated calcium channels in T-tubules, releasing calcium from terminal cisternae
Calcium ions bind to troponin and tropomyosin, myosin and actin power row

Isometric contraction

Although tension is increased, it is insufficient to overcome resistance

Muscle length stays the same

Isotonic contraction

Muscle tension overcomes resistance resulting in movement

Tone stays constant, but length changes

Smooth muscle fibers

Caveolae store calcium
Sparse sarcoplasmic reticulum, outside is source of calcium ions
Intermediate filaments with no sarcomeres
MLCK

Steps of smooth muscle contraction

Stimulus leads to opening of voltage-gated Ca2+ channels

Ca2+ enters sarcoplasm and binds to calmodulin

Calcium-calmodulin complex binds to myosin light-chain kinase (MLCK)

MLCK phosphorylates myosin head

Crossbridges form, pull on actin, similar to skeletal muscle but more slowly

First class lever

Fulcrum is between effort and resistance

Second class lever

Resistance is between fulcrum and effort

Third class lever

Effort is between fulcrum and resistance

Brachial plexus

Anterior rami of C5-T1