17,18-skeletal muscle Flashcards
What are the 4 tissue types ?
- Nervous
- Muscle
- Connective
- Epithelial
What are the functions of skeletal muscle ?
- Produce heat:help genererate heat during contraction->helps maintain normal body temp
- Movement of body : responsible for all locomotion and manipulation
- Stabilise joints:help strenghen & stabilise joints of skelaton
- Maintain posture & bidy position:working continuosly
What are general functions of musclesc ?
- Excitability =reieve and respond to stimulas by changing memebrane potential
- Contractility =ability to shorten forcibly when stimulated
- Extensibility :ability to stretch/extend muscle fibres shorten when contracting ;conversly can be stretched when relaxed even beyond resting length
- Elasticity: ability of muscle cell to spring back-recoil
Organisational levels of skeletal muscles ?
Muscle fascile
muscle fibre
myofibril
arrangements of myofilaments
actin & myosin
Organisation of sarcomeres ?
A band =dark regions
I band =light regions
middle of I band=Z band
Actin ?
Made of 3 proteins: globular (G) actin, tropomyosin & troponin.
o Chain of 200 G actin subunits form a strand: fibrous (F) actin.
o Each G actin subunit has a myosin myofilament active site
→ myosin head can bind at this site (as during contraction).
o When a muscle is relaxed, tropomyosin (long, fibrous protein)
covers/blocks the active sites on G actin subunits:
o For a muscle to contract, tropomyosin must move.
o Troponin consists of 3 subunits:
(1) a subunit anchors troponin to
actin;
(2) a subunit prevents tropomyosin from uncovering the
active sites in relaxed muscle;
(3) a subunit that binds Ca2+ ions
Myosin ?
Thick filament
Composed of many myosin molecules (shaped like golf clubs).
o Each molecule is made of 2 myosin heavy chains, wound together to form a rod portion; note: 2 myosin heads.
o 4 light myosin chains are attached to the heads of each
myosin molecule.
What is dystrophin ?
links the thin actin myofilaments to
integral proteins of the sarcolemma (i.e. plasma membrane of the muscle fibre):
o Loss of dystrophin → muscle fibres damage
more easily; compromises the sarcolemma.
In DMD its mutated -x linked
Symptoms fo DMD ?
muslce weakness
muscle atrophy
contractures
What is the sliding filament theory ?
changes in banding pattern -myofilaments slide past each other
Cross bridges –>each actin molecule chain is able to bind to 1 myosin head region
Sliding filament model :myofilaments ?
Thick & thin filaments slide past each other without shortening
Ca2+ions are key for muscle contraction
Action potentials cause release of Ca2+ from SR –>in at the sarcoplasm ,Ca2+ binds troponin –>causes tropomyosin to move –>exposes the active binding on actin myofilaments —> myosin heads can now bind to these sites to form cross-bridges
Muscles contract when cross-bridges move (→ mechanical component of muscle contraction: rapid sequence of events, cause sarcomeres to shorten)
What is step 1 cross bridge cycle ?
Before each cycle, myosin head is in its resting position:
o Myosin stores chemical energy from ATP breakdown that occurred during
the previous cycle; appropriate stimulus causes release of Ca2+ from SR…
Cross bridge cycle step 2 ?
Myosin head remains in its resting position until muscle fibre is stimulated by incoming action potentials (originating from a motor neuron):
o Excitation-contraction is initiated: Ca2+ binds → active sites exposed →
myosin heads bind → cross-bridges are formed.
Cross bridge cycle step 3 ?
Cross-bridge formation triggers rapid movement of myosin heads at their
hinged position:
o Power stroke: movement of the myosin heads → this causes the actin
myofilament to be ‘pulled’ past the myosin myofilament (towards H zone).
Cross bridge cycel step 4 ?
Binding of ATP to the myosin head causes it to detach from actin myofilament
Cross bridge cycle :step 5 and 6 ?
Breakdown of (bound) ATP by the myosin head supplies the energy needed for the recovery stroke: myosin heads return to their resting position.
Sliding filament model & the length-tension relationship of skeletal muscle ?
- Latter leads to a decline in ability of
skeletal muscle to generate tension.
o When muscle is at its natural resting length, actin and myosin
myofilaments overlap optimally and form the max number of cross-bridges
o When muscle is stretched, degree of overlap between myofilaments is decreased and so now the number of
cross-bridges falls.
Types of muscle contraction ?
isotonic =Muscles can shorten
Isometric = muscle can not shorten
Isotonic contraction :eccentric & concentric ?
eccentric contraction :skeletal muscle generates force as it lengthens
occurs in lower limbs when running downhill
Concentric contractions :skeletal muscle shortens
smooth muscles ?
Also contain actin and myosin but not arranged in regular sarcomeres
each has loose matrix of contractile proteins attached to plasma membrane at dense patches
consists of sheets of small spindle-shaped cells,
linked by mechanical and electrical (gap) junctions
Smoothe muscle contraction ?
2 types of smooth muscle:
(1) single unit, and (2) multi-unit:
o (1) shows myogenic, i.e. regular spontaneous contractions → pacemaker activity originates from interstitial cells of Cajal (ICC).
o (2) little spontaneous activity (activated by motor nerves).
* Still, as for cardiac and skeletal muscle, relative movement / sliding
of actin and myosin myofilaments → smooth muscle contraction.
* However, contractile response of smooth muscle is slower, and is
regulated by myosin light chain kinase (MLCK) enzyme activity:
o Involves Ca2+ but binds calmodulin (not troponin-tropomyosin).