muscle physiology

General Features

multinucleated cells (syncytium: from fusion), very long compared to other cells, not wide

sarcolemma

special name for plasma membrane

sarcoplasm

rich in glycogen and myoglobin

myoglobin

stores oxygen; similar to hemoglobin

special structures

myofibrils and sarcoplasmic reticulum

muscle cell contains many

parallel myofibrils

myofibrils have

DARK bands (A bands) and LIGHT bands (I bands) that cause

"striated" appearance of muscle

A band and I band result from the arrangement of overlapping and non-

overlapping regions of two types of

myofilaments

myosin

thick filaments

actin

thin filaments

sarcomere

smallest contractile unit of muscle cell

Z-line -

connection of actin filaments; dividing line between two adjacent sarcomeres

M-line -

connection of myosin filaments

H-zone -

non-overlapping region of the myosin filaments around the M-line

A-band -

length of myosin filaments

I band -

length of non-overlapping actin filaments

Each muscle cell (fiber) is composed of many

myofibrils

Each myofibril contains

hundreds of accordion-like sarcomeres laid end-to-end.

Muscle contraction occurs when the

sarcomeres contract by the sliding motion of actin and myosin filaments

sacroplasmic reticulum

smooth ER that houses Ca 2+ , surrounds each myofibril, fused together at H zone and A/I Bands

Transverse (T) Tubules

passegeways from extracellular space to the terminal cisternae of SR, passage of nerve message directly to SR, and passage of glucose, oxygen, and salt to fibers

Terminal cisternae (of sacroplasmic reticulum)

specialized sacs of sacroplasmic reticulum that border the T Tubules in muscle cells

Molecular Structure of Actin & Myosin Filament

thick filaments (myosin filaments) 12-16 nm

a. composed of about 200 myosin proteins

i. myosin has a golf club like shape

ii. 2 heads (cross bridges) - can bind to the actin filaments and use

ATP

iii. tail - shaft of the thick filament

What triggers muscle contraction in the sliding filament model?

Ca++ released from sarcoplasmic reticulum

What does Ca²⁺ bind to?

Troponin (specifically the TnC subunit).

What happens when Ca²⁺ binds to troponin?

Troponin changes shape and moves tropomyosin, exposing actin binding sites.

What is the attachment step in muscle contraction?

Myosin head (with ADP + Pi) binds to actin.

What is the power stroke?

Myosin head pivots, pulling actin filament; ADP + Pi are released.

What causes detachment of myosin from actin?

ATP binds to myosin head, causing it to detach.

What is re-cocking of the myosin head?

ATP is broken down into ADP + Pi, releasing energy to reset the myosin head.

Why does contraction continue in a “walking” motion?

Some myosin heads are always attached while others cycle, creating continuous movement.

What stops muscle contraction?

Lack of ATP or re-uptake of Ca²⁺ into the sarcoplasmic reticulum.

What is rigor mortis?

Muscles harden because myosin remains attached to actin without ATP to detach.

How much does one contraction cycle contribute?

One cycle produces about 1% muscle shortening. motion continues until no more ATP is present or Ca 2+ levels frop by re uptake into SR

neuromuscular junction

nerve/muscle intersection, synapse (actual point of contact) that includes motor neuron and muscle cell

synaptic vesicles

sacs that contain acetylcholine (ACh- neurotransmitters). this process is exocytosis (releasing ACh)

synaptic cleft

space between the axon terminal and the sarcolemma of the

muscle cell

motor end plate

highly folded part of sarcolemma beneath the synaptic cleft; rich in ACh receptors

whats the presynaptic and postsynaptic nerve in the neuromuscular junction

presynaptic; motor neuron. postsynaptic: muscle cell

threshold (in neuromuscular junction)

voltage needed to open Na+ channels , most common is Na+ . THIS IS NEEDED TO TRIGGER AN ACTION POTENTIAL!

what happens before action potential?

threshold caused by depolarization

What happens during action poteintial?

the voltage gated sodium channels open to allow Na+ in the cell becoming positive making inside positive furthering depolarization AS SOON AS IT HITS 3 MILLISECONDS IT CLOSES.

What happens when the voltage gated sodium channels closes

potassium voltage channels open and postassium can rapidly leave. NOT THE SAME AS LEAKAGE CHANNELS

Whats the point of potassium channel opening?

Positive charged ions can leave and repolarization can occur ( goes toward resting membrane potential)

pathway of action potential

the voltage gated sodium channels open to allow Na+ in the cell becoming positive making inside positive AS SOON AS IT HITS 3 MILLISECONDS IT CLOSES.potassium voltage channels open and potassium can rapidly leave for repolarization

refractory period

when doing an action potential and can’t trigger another at a certain time, causing the action potential to be unidirectional

absolute refractory period

no matter what cannot have another action potential

relative refractory period

can have another action potential but needs a stronger stimulus.

what happens when the action potential reaches the end of axon terminals

the effect of axon terminals becoming depolarized causing the next action potential to repolarize then depolarize

The depolarization from the axon terminals causes what

• The depolarization opens voltage-gated calcium channels

• These are located on the presynaptic axon terminal membrane

After the voltage -gated calcium channels open what happens?

• Calcium flows into the axon terminal

• Calcium causes synaptic vesicles to fuse with the membrane

Why is Calcium so important

Ca²⁺ diffuses into the axon terminal (from outside the neuron) Ca²⁺ triggers exocytosis of ACh vesicles

What happens after ACh vesicles is triggered

ACh is released into synaptic cleft

What happens ACh is released into synaptic cleft?

it binds to sodium receptor chemically gated channel on motor end plate

What happens when ACh binds to sodium receptor chemically gated channel on motor end plate?

Sodium ions flow into the skeletal muscle cell, causing depolarization called an end-plate potential

after depolarazation what happens?

muscle action potential is fired going to sacrolemma

After sacrolemma ?

Action potential goes into T-tubules

• The electrical signal dives deep into the muscle fiber

• This ensures the whole muscle cell gets the message

The action potential triggers what from where

Voltage change triggers calcium release from calcium channel in sacroplasmic reticulum and SR releases Ca²⁺ into the muscle cell

what happens when calcium is released into the muscle cell

Calcium starts contraction, Ca²⁺ binds TNC site on troponin, Tropomyosin moves, Actin binding sites exposed, and Myosin binds → contraction starts

after ACh gets released what controls it

. after binding to ACh Receptors on sarcolemma, ACh is quickly broken down by

an enzyme known as Acetylcholinesterase (AChE

repolarization takes how long and contraction?

3 milliseconds, contraction can last up to 100 milliseconds

myasthenia gravis

autoimmune disease where immune system attacks ACh

Receptor

ACh Antagonists

chemicals that block an ACh receptor

snake venoms

curare and other venoms

latent period -

time between excitation & contraction

Calcium Sequesters

bind Ca++ in the cell so it will not form Calcium Phosphate

crystals

A Skeletal Muscle CELL (Fiber) will contract in an

All-or-None fashion when

ITS motor neuron stimulates it to fire by releasing ACh!

. Motor Unit

a single motor neuron and all of the muscle cells stimulated by it muscle cells per motor neuron = 4 - 400

large motor unit

muscles of posture and gross movement (gluteus maximus): more muscle cells

per neuron

smaller motor unit

muscles of fine control (fingers, eyes and face): fewer muscle cells per neuron

Muscle Twitch

the response of a muscle to a single short electrical stimulus

strong twitch -

many motor units activated; weak twitch - few motor units are

activated

latent period (3 ms) of muscle twitch

time after stimulation for coupling to occur and contraction

to start

contraction period (10 - 100 ms)

from beginning of contraction to maximum force (tension)

relaxation period (10 - 100 ms)

time from maximum force to original relaxed state

Frequency of Stimulation (Wave Summation) -

- a motor unit may be stimulated over and over again so no relaxation period is possible and frequency of stimulation cannot be greater than 1 every 3 ms (REFRACTORY PERIOD)

tetanus

smooth muscle contraction that occurs when summation is so great that the relaxation period disappears

Summation of Multiple Motor Units -

as strength of stimulus is increased, more and

more motor units are activated in the muscle itself

threshold stimulus -

level of stimulus at which first motor units are activated

. maximal stimulus

level of stimulus at which all motor units of a muscle are

activated

recruitment

more activation of additional motor units ot increase the overall force produced by muscle

Asynchronous Motor Unit Summation

motor units activated in different cycles

"average out to produce a smooth muscle contraction