2. Muscular Contractions - NS Flashcards
Events during a muscle twitch after single nerve activation:
a) Latent period
b) Contraction
c) Relaxation
a) Latent period
Motor end-plate depolarisation
Depolarisation (AP) transmitted down T tubules
Ca2+ channels open in SR
[Ca2+] in the sarcoplasm
Ca2+ binds to troponin revealing myosin binding site on actin
b) Contraction
Myosin binds to actin, moves (powerstroke, ADP ejected), releases (new ATP binds) and reforms many times causing sarcomeres to shorten.
c) Relaxation
Ca2+ actively transported back into SR
Troponin-tropomyosin complex blocks myosin binding
Muscle fibre lengthens passively (relaxation)
SR
Sarcoplasmic reticulum
MF
Muscle fibre
Motor UNIT
Motor unit = 1 motor neuron & its muscle fibres
1 motor neuron branches and contracts ….
several muscle fibres
(Number of muscle fibres depends on the muscle)
Fine motor control requires a smaller ratio of…
Muscle fibres to nerve fibres
- extraocular muscles (the eye) 1:10
- the gastrocnemius (calf) 1:2000
Recruitment of force depends on the number of active muscle fibres
Muscles are made up of many motor units
One nerve impulse on one nerve =
activation of 1 motor unit = small contraction over whole muscle
Activation of more motor neurons = more motor units =
more muscle fibres = more contractile force
Gradation of force depends on the recruitment of motor units
3 Basic principles
1) The all or nothing principle
2) Threshold
3) Recruitment
1) The all or nothing principle
The skeletal muscle fibre/motor unit either operates or it does not
2) Threshold
If the threshold stimulus for a nerve is reached and the threshold for muscle contraction is reached, the muscle fibre will contract, otherwise it will not
3) Recruitment
The greater the force of contraction needed, the more motor units (one nerve and its associated innervated muscle fibres) are required. Each motor unit operates in an all or none fashion
Recruiting motor units by increasing stimulus intensity:
> Controls the force of contraction (in absence of internal changes, such as fatigue, fibres will contract ‘fully’ each time)
> The more motor units the bigger the twitch
Is there Maximum for tension at stimulus?
YES
Stimulation frequency and contractile force
Consider:
Lowest frequencies
Low frequencies (slightly higher than above described)
High frequencies
Lowest frequencies
Muscle fibres relax fully before next AP arrives
>
Twitches (tension returns to baseline – not shown)
Low frequencies (slightly higher than LOWER frequencies)
Next AP arrives before fibres are fully relaxed
AP
Action Potential
High frequencies (other graph)
No time for the muscle fibres to relax before the next AP arrives
Tetanus
Calciums role in high frequencies -
Ca2+ continually available, enabling MAXIMUM contraction
Skeletal Muscle Contraction Requires a Steady Supply of ATP; NEEDED FOR
Contraction (crossbridge forming and release),
Relaxation ( pump Ca2+)
Restore Na+ and K+ levels afer AP
Sources for skeletal muscle contraction
Phosphocreatine - A source of ATP
Carbohydrates
> Aerobic metabolism : producing about 30
ATP for each molecule of glucose
> Anaerobic glycolysis : glucose is
metabolized to lactate/lactic acid with a yield
of only 2 ATP per glucose
Lack of ATP not thought to contribute to muscle fatigue
= comes from other changes in the exercising muscle
Two types of skeletal muscle fibre
Speed & Fatigue Resistance
Control of Contraction force
All muscles in 1 motor unit are same type
Muscle as a whole made of multiple motor units of different types
Slow twitch fibres (minimal force)
As stimulus increases more neurones with higher thresholds begin to fire
> Fast twitch fibres
> Generate more force but fatigue more quickly
Muscle fibre types
1) Slow-twitch (SO or type I)
2) Fast-twitch (FG or type II)
1) Slow-twitch (SO or type I)
slow contraction
use aerobic metabolism
fatigue-resistant and well suited for prolonged aerobic exercise
2) Fast-twitch (FG or type II)
rapid contraction
use anaerobic metabolism
Activated in short-term sprint or any short-lived “burst” activity (stop-go activity).
Different types of exercise influence different types of muscles
e.g
- sprinting
-jogging
Mechanics of movement
a) Flexion moves bones closer together.
b) Extension moves bones away from each other.
Antagonistic muscle groups
> Move bones in opposite directions
> Contraction can pull on a bone
> Cannot push a bone away (Other groups exist)
How do we move?
Join movement
Steps of join movement:
1) Origin (bone): does not move
2) Insertion (bone): the point that moves
3) Bones & joints : levers and fulcrums on which muscles exert force to move or resist a load
4) Return to topic in somatosensory NS reflexes
Disorders
Myasthenia gravis endplate region of the postsynaptic membrane / weakness
Common Problems
(Disorders)
Muscle Cramp -
> hyperexcitability of somatic motor neurons -
motor unit go into a state of painful sustained
contraction
Overuse / fatigue
Disuse atrophy
Cardiac muscle
- Only found in the heart
- Striated
- Organized into sarcomere with same banding
organization - Muscle fibres are shorter usually contain only
one nucleus - Connected by intercalated discs
- Gap junctions and desmosomes
Gap Junctions:
channels between adjacent cardiac muscle fibres
Gap junctions job =
- allow depolarising current to flow from one
cardiac muscle cell to the next - quick transmission of action potentials and the
coordinated contraction of the entire heart - contract in a wave-like pattern so that the heart
can work as a pump.
Desmosome anchors
The ends of cardiac muscle fibres together
> cells do not pull apart during contraction
Heart: Pacemaker function
1) Contractions of the heart (heartbeats) are c
controlled by specialised cardiac muscle cells
called pacemaker (SAN)
2) Pacemaker cells respond to signals from the
autonomic nervous system (ANS) to speed up
or slow down the heart rate
3) Also responds to various hormones that
modulate heart rate to control blood pressure
4) Hear also has uniquely shaped action potential
(contractile cells)
Smooth Muscle
More variable than skeletal
Located:
> Blood vessel walls
> Walls of GI tract / associated organs
> Urinary system (walls of bladder and ureters)
> Respiratory system (airway passages)
> Reproductive system (both females and
males), and
> Ocular muscles (eye).
Smooth muscle contractions =
Some alternate between contraction and relaxation (phasic smooth muscle)
Some continuously contracted (tonically contracted)
Smooth muscle fibres
Lack striations – tissue appears uniform/bright
Small, spindle-shaped cells with a single nucleus
Smooth muscle - FILAMENTS:
Have actin and myosin contractile proteins, and generate force through thick and thin filaments
Thin filaments are anchored by dense bodies
Filaments occur in parallel with each other, but run obliquely : therefore get contraction in different directions
Smooth Muscle vs Skeletal : SIMILARITIES
Force - actin - myosin crossbridge / sliding filaments.
Contraction (cross bridge movements) initiated by an increase in free cytosolic Ca2+
Smooth Muscle vs Skeletal : DIFFERENCES
Layers of smooth muscle may run in several directions
Contract and relax much more slowly
Less energy to generate amount of force
Controlled by the autonomic nervous system
Most of calcium comes from outside cell
>No T-tubules
No troponin in actin filaments – use calmodulin
In skeletal muscle target for calcium is actin: in smooth muscle target for calcium is myosin
Smooth muscle contraction : Step by step
6 stages
- External Ca2+ ions enters cell
(opened calcium channels in the sarcolemma released from SR)
- Bind
to calmodulin
- Ca2+ / calmodulin complex then activates an enzyme called…
myosin (light chain) kinase (MLCK)
- MLCK in turn, activates the myosin heads by phosphorylating them
converting ATP to ADP and Pi, with the Pi attaching to the head
- The heads can then attach to actin-binding sites and pull on the thin filaments
Causes fibre to contract
Muscle contraction continues until ATP-dependent calcium pumps actively transport
Ca2+ out of the cell
> low concentration of calcium remains to maintain muscle tone.-Important around blood vessels
Smooth Muscle: Part of Autonomic System
Single-unit smooth muscle cells
Multi-unit smooth muscle cells
Single-unit smooth muscle cells
connected by gap junctions
cells contract as a single unit
Receptors are found all over the cell surface
Series of neurotransmitter-filled bulges called varicosities on axon
Forms loose motor units
Varicosity releases neurotransmitters into the synaptic cleft.
Bind to receptors on smooth muscle
Multi-unit smooth muscle cells
cell must be stimulated independently
Smooth muscle (hollow organs - except the heart) contains pacesetter cells -
spontaneously trigger action potentials
Triggers for smooth muscle contraction:
neural stimulation by the ANS,
hormones
local factors eg stretch receptors
Types of Muscle:
- Cardiac muscle
- Skeletal muscle
- Smooth muscle
