Skeletal and Smooth Muscle Flashcards
Skeletal Muscle
Striated/striped appearance
Actin/myosin
Ca++ binds to troponin
Stimulated by somatic motor neurons
Ca++ comes from SR
Smooth Muscle
Smooth appearance
Actin/myosin
Ca++ binds to calmodulin
Stimulated by autonomic neurons, hormones, or other chemicals.
Ca++ comes from extracellular fluid
Acetylcholine (Ach)
Bind to nicotinic receptors which allow Na+ to enter muscle cell.
describe the processes that take place at the NMJ and
the generation of an end -
plate potential
when an action potential travels down the axon , the depolarization
triggers the opening of Ca Channels on the axon .Ca++ flows
in and stimulates the exocytosis of Ach. Ach travels to cleft
and binds to nicotinic receptors , which open and allow Na+ to
enter muscle cell . This is a motor end plate potential .
explain excitation -
contraction coupling in skeletal muscles
when enough EPSPs fire to depolarize the motor neuron to threshold,
an AP is fired depolarizing the axon ,spreading to the terminal .
This voltage changes triggers the opening of voltage-gated bait
channels , causing Ca++ to flow into the axon terminal . Ca++ triggers
exocytosis of synaptic vesicles containing Ach . Ach flows across the synaptic cleft onto the motor end plate of the muscle ,binding to nicotinic Ach receptors which are Na+ channels . These open in response to Ach binding , allowing Na+ to flow into the cell , causing a motor end plate potential .When the potential difference reaches threshold , an AP will fire causing VG Na+ channels to open and depolarize the cell . The depolarization spreads down the muscle into the T tubules and detected by the DHP receptors , which are physically linked to RYR receptors on the SR .
In response, the RYR opens allowing Ca++ to flow out of SR and into sarcoplasm. Ca++ then binds to troponin , which is connected to tropomyosin covering the active site of actin.
Troponin and Ca pull back tropomyosin , exposing active site , allowing myosin to bind using ATP. Forming a crossbridge and pulling actin ,
a power stroke helps to contract the muscle.
Skeletal Muscle Relaxation
Ca++ is constantly being pumped back into SR by an ATPase
Repolarization phase of AP allows DHP to reset, which closes RYR, preventing more Ca++ from leaving SR
Decreased Ca++ levels in the cytosol mean less Ca++ bound to troponin
Troponin without Ca++ allows tropomyosin to cover actin binding sites
Myosin cannot bind/form cross-bridges
explain the length -
tension relationship in skeletal muscle
*
explain force generation in skeletal muscle
as the sarcomere shortens , more tension is developed until no more crossbridges can form , then
tension begins to decrease. (tension =
force development)
describe what a motor unit is and how skeletal muscles can
create graded contractions of varying force
a motor unit is a somatic motor neuron and all the muscle cells it innervates.
Muscles can generate graded contractions by recruiting certain groups / amounts of muscle fibers depending on the need .
Single Unit Smooth Muscle
Cells are connected by gap junctions
Signal down neuron affects all cells and leads to all of them contracting as a single unit
Multi unit Smooth Muscle
Cells are not connected by gap junctions.
Each cell independently stimulated by a neuron.
Actin
binds to myosin to form crossbridges to cause tension to develop
ATP
binds to myosin causing it to change position and attach to actin and pull, causing muscles to contract.
Ca++
binds to troponin which is connected to tropomyosin which uncovers the actin sites so it can bind to myosin.
Ca++ ATPases
transport calcium ions from the cytosol back to the sarcoplasmic reticulum (SR) following muscle contraction
DHP receptors
voltage gated calcium channels that detect action potentials and are linked to RYR receptors
Myosin
binds to actin allowing for a crossbridge to form
Nicotinic receptors
open and allow Na to enter muscle
RYR receptors
open allowing calcium to flow our of SR into sarcoplasm
Synaptic vesicles
release Ach
Tropomyosin
uncovers the actin sites so it can bind to myosin.
Troponin
pulls back tropomyosin along with Ca+ to expose actin binding site
Voltage-gated Ca++ channels
cause calcium to flow into axon terminal.
Voltage-gated Na+ channels
Open and depolarize cell when threshold is reached.
Length-Tension in Skeletal Muscle
Tension (or force) development depends on cross-bridge formation.
Motor Unit:
A somatic motor neuron and all the muscle cells it innervates.
Smooth Muscle contraction
smooth skeletal
triggered by hormone , voltage , mechanical strecth
calcium induced calcium
release .
calmodulin activated by Ca+
activates MLCK → phosphorylates myosin
* contracts in a spiral , 1b *
RELAX : sodium calcium exchanger
skeletal muscle contraction
triggered by AP
RYR releases Ca++
troponin reveals active site
release .
contracts toward middle * →←
sodium calcium exchanger
