Muscle physiology

Three muscle types

skeletal, cardiac, smooth

structure of cells

body location

function

how activated to contract

How do muscles differ

elongated muscle cells (muscle fibers)

contraction dependent on microfilaments (Actin and myosin)

sarcolemma

sarcoplasma

how are muscles similar

sarcolemma

plasma membrane of a muscle fiber

sarcoplasm

cytoplasm of a muscle fiber

Skeletal muscle

A muscle that is attached to the bones of the skeleton and provides the force that moves the bones.

1. attach/cover bones

2. longest fibers

3. multinucleated

4. striations

5. voluntary

6. contract rapidly

7. tires easily

8. adaptable

9. wind-up toy- runs down

Skeletal muscle overview

sarcomere

Contractile unit of a muscle fiber

true or false: there are multiple sarcomeres per muscle fiber

true

where does a sarcomere run from

Z disc to Z disc

what filaments are in a sarcomere

thin (actin) and thick (myosin)

titin filament

connect myosin to z disc

why does skeletal muscle tire easily

it needs rest after a short period of activity it quickly uses ATP resources and has a slow vs fast twitch

Why can't mature skeletal muscle cells divide?

They are specialized cells that have lost the ability to go through cell division

How do skeletal muscles grow if they can't divide?

Muscle growth happens through hypertrophy, which is stimulated by hormones and exercise

What are satellite cells in skeletal muscle

stem cells located around muscle fibers that can regenerate or repair damaged muscle tissue by forming new fibers

1. only in heart walls

2. striations- thick + think filaments

3. involuntary- ANS + pacemaker cells

4. Steady rate

5. coordinated rhythm

cardiac muscle overview

Automatic Nervous System cells are found in

smooth and cardiac muscle (involuntary)

Pacemaker cells are

found in cardiac muscle

coordinated rhythm of cardiac muscle

contracts smooth steady pattern to keep blood flow properly

1. walls of hollow organs

2. no striations (still have thick + thin filaments)

3. involuntary

4. slow contractions

5. last long time

6. steady engine going along

smooth muscle overview

unique properties of smooth muscle

excitability, contractility, extensibility, elasticity, conductivity

excitability

ability to receive and respond to stimuli (can be chemical)

response of smooth muscle

make and transmit electrical currents (Action potentials) along the sarcolemma, causing the muscle to contract and produce movement

neuromuscular junction

The connection between motor neuron and muscle fiber where the nerve impulse signal is transmitted to muscle to trigger contraction

action potential

An electrical signal that travels along the sarcolemma, initiating the process that leads to muscle contraction

contract

shorten when stimulated

extensibility

the ability to be stretched or extended

elastic

resume to resting length

conductivity

The ability of a muscle to carry an electrical signal so the whole muscle can contract together

wave of excitation

levels of organization from largest to biggest

epimysium

bicep brachii

perimysium

fascicle

endomysium

sarcolemma

microfibril (organelle)

sarcomere

myofilaments

actin and myosin

Muscle fibers

long cells with multiple flattened nuclei inside the cell membrane

what is the sarcolemma

muscle cell membrane with tunnel-like infolding called tubules that carry signals into the cell

what is found inside the sarcoplasm

myofibrils, glycogen for energy storage, and myoglobin for binding oxygen

what does the sarcoplasmic reticulum do

(smooth ER) around myofibrils that store and release calcium to trigger muscle contraction

what makes up a triad in muscle cells

T-tubule and two terminal cisternae

What happens when calcium floods out of the sarcoplasmic reticulum

it triggers muscle contraction

what are thick filaments made of

contractible protein myosin

what do thick filaments do

Bind to actin and cause contraction (uses lots of ATP)

what are thin filaments made of

globular (G) actin with active sites and tropomyosin and troponin proteins

tropomyosin

blocks actin's active sites when the muscle is relaxed

troponin

regulatory protein that binds to actin, and moves tropomyosin off actin's active sites for contraction

what are elastic filaments made of and function

springy proteins called titin, they anchor thick filaments to the Z disc to prevent overstretch of sarcomere

main contractile proteins

myosin and actin

what proteins regulate contraction

troponin and tropomyosin

how does calcium trigger contraction

calcium binds to tropoinin which moves tropomyosin which then exposes actin active sites

what causes muscle striations

alternating dark A and light I bands in the sarcomere

what is a sarcomere

the segment from one Z disc to the next- functional unit of muscle

what happens when a muscle contracts

sarcomeres shorten as Z discs are pulled closer together; filaments slide

how is skeletal muscle activated

by a nerve signal

what are somatic motor neurons

nerve cells whose axons stimulate skeletal muscle fibers

motor unit

A motor neuron and all of the muscle fibers it innervates

small motor units

fine degree of control

large motor units

for strength

What is a neuromuscular junction

point of contact between a motor neuron and a skeletal muscle cell

what neurotransmitter is released at the NMJ

Acetylcholine (ACh)

Main components of the NMJ

Synaptic knob, synaptic cleft, basal lamina

synaptic knob

swollen end of nerve fiber that releases ACh

synaptic cleft

tiny gap between nerve and muscle cells

basal lamina

thin layer of collagen and glycoprotein over all of muscle fiber

what do cholinesterase inhibitors do

prevent breakdown of Ach; cause spastic paralysis

what is tetanus

blocks glycine release, which causes overstimulation of muscles; spastic paralysis

flaccid paralysis

a state in which the muscles are limp and cannot contract

what happens when there is a polarized muscle cell

a difference in charge- more Na+ outside and more K+/anions in the cell

what is the resting membrane potential

Difference in electrical charge across the membrane at rest; The muscle cell’s charged-up resting state before any signal arrives.

what happens when the muscle cell is stimulated

Ion gates open, Na+ diffuses in, K+ rushes out which causes quick voltage change called an action potential

What happens when a nerve signal reaches the synaptic knob

Voltage-gated calcium channels open, calcium enters, and triggers exocytosis of ACh into the synaptic cleft

What happens when Ach binds to receptors on the muscle fiber

Ligand- gated sodium and potassium channels pop open and sodium rushes in, which activates the action potential

How is action potential created from the end plate potential

voltage-gated channels open and sodium rushes in first and an action potential spreads across muscle fiber

What happens when the action potential spreads across the sarcolemma

It travels down the T-tubules

How is calcium released inside the muscle fiber

Action potential triggers the sarcoplasmic reticulum to release Ca+

What does calcium bind to during contraction

troponin, which shifts tropomyosin revealing actin active sites

What activates the myosin heads?

Myosin ATPase hydrolyzes ATP "cocking" the head into position

Why is ATP important for cross bridge formation

Myosin must bind ATP to release its old position, then bind actin and form a new cross bridge

whats the power stroke

myosin releases ADP and Phosphate and pulls actin toward the center, shortening the sarcomere

how does the myosin head reset

A new ATP molecule binds, detaches, and prepares for the next stroke

how does stimulation stop

nerve signals stop and acetylcholinesterase removes Ach from receptors

What happens to calcium during relaxation

it is pumped back into the SR using ATP and bound to calsequestrin

calsequestrin

molecule that binds calcium within the sarcoplasmic reticulum

how do actin sites get re-covered

loss of Ca+ allows troponin/tropomyosin to block active sites again

what helps the muscle return to resting length

elastic recoil and antagonistic muscle action

why does rigor mortis occur

no ATP is available to release attached actin and myosin molecules

isometric contraction

no shortening; muscle tension increases but does not exceed load

isotonic contraction

muscle shortens because muscle tension exceeds load

concentric

tension while shortening

eccentric

tension while lengthening