Skeletal Muscle As A Target For Nervous System Control Flashcards
Muscle fibre: Sarcolemma
Plasma membrane
Receive electrical stimuli
Conducts an action potential to the internal structures via T tubules
Muscle fibre: transverse (T) tubules
Sarcolemma is invaginated to form membranous tunnels
Penetrate through the fibre
Conduct electrical stimuli from the Sarcolemma
Muscle fibre: sarcoplasmic reticulum
Special type of smooth endoplasmic reticulum
Contains large, concentrated stores of calcium
Muscle fibre: terminal cisternae
Sarcoplasmic reticulum becomes enlarged and forms large bands that wrap around the muscle fibres on either side of the T tubules
Action potential stimulated them to release calcium from the sarcoplasmic reticulum
Neuromuscular junction
- Action potential received
- Fusion of synaptic vesicle
- Sodium influx
Depolarisation of postsynaptic membrane
Neuromuscular junction: 1. Action potential received
Action potential at axon terminal causes VGCC to open
Ca2+ enters the axon terminal
Neuromuscular junction: 2. Fusion of synaptic vesicle
High Ca2+ causes vesicles containing acetylcholine to fuse with the membrane
Release acetylcholine into the synaptic cleft
Neuromuscular junction: 3. Sodium influx
Acetylcholine activates acetylcholine receptors
Conformational shape change
Opens sodium channels and Na+ enters the muscle cell
Neuromuscular junction: 4. Depolarisation of postsynaptic membrane
Action potential propagated along Sarcolemma and into the T tubules
Receptors on sarcoplasmic reticulum mediate the release of stored Ca2+ to begin muscle contraction
Excitation-concentration coupling
Action potential travels across Sarcolemma
Action potential down T tubules
Sarcoplasmic reticulum releases calcium via ryanodine sensitive channels
Calcium ions bind to troponin
Troponin change shape
Tropomyosin moves with troponin exposing the myosin binding site on actin
Cross-bridge cycle
- ATP hydrolysis
- Cross bridge formation
- Power stroke
- Detachment
Cross-bridge cycle: 1. ATP hydrolysis
Bonding of ATP causes a conformational change in the myosin head orientation
Brings it closer to the actin filament
A phosphate group is lost
ADP and phosphate groups remains
Cross-bridge cycle: 2. Cross bridge formation
Myosin head attaches to the actin
The remaining phosphate groups are released
Cross-bridge cycle: 3. Power stroke
Myosin head pivots and rotates
Releasing the ADP
Generates force and pulls the actin filament to the centre of the sarcomere
Prepares myosin head to receive another ATP
Cross-bridge cycle: 4. Detachment
New ATP binds to the myosin head
Causing it to detach from the actin
Ready for ATP hydrolysis
Sarcomere
Functional unit of a myofibril
Myofilament - consisting of a complex arrangement of contractile proteins
I-band
Z-line
M-line
H-zone
A-band
Sarcomere: contractile proteins
Myosin
Actin
Contractile proteins: myosin
Thick filament
In A-band and H-zone
Interact with actin to create movement
Head, tail and neck
Acts with actin to shorten the cell
Contractile proteins: actin
Thin filament
Stays anchored
Bound to by the myosin molecule
Act with myosin to shorten the cell
Sarcomere: regulatory proteins
Tropomyosin
Troponin
Regulatory proteins: Tropomyosin
Long molecule
Twist around each filament of actin
Involved in uncovering of myosin head binding sites on the actin filament
Regulatory proteins: troponin
Involved in moving Tropomyosin away from the myosin binding sites on actin
Binding of Ca2+ causes a conformational shape change that moves Tropomyosin away from the myosin binding sites
Sarcomere: structure protein
Titin
Structural protein: Titin
Large, singular protein coiled at one end
Sits between the M-line and Z-line
Acts as a spring for actin
Attaching them to the Z-line
Sliding filament theory
Muscle contraction - myosin binds onto actin forming chemical bonds (cross bridges)
Myosin ‘walk’ along the actin and pull them towards the centre of the sarcomere
Causes sarcomere shortening
H-zone - smaller
I-band - smaller
A-band - same
Motor units
Consists of motor neurone and all the muscle fibres it innervates
Overall muscle movement - recruitment of different size units, different strength of units and quantity of the units
Frequency of stimulation: summation
Wave summation - an increase in the number of stimuli (in quick succession) to a muscle fibres
Causes an increase in the force of contraction
Frequency of stimulation: incomplete tetanus
Muscle is stimulated after the absolute refractory period but before the muscle can relax
Tension of the muscle increases to its maximum
How to improve the smoothness of motor actions
Frequency coding
Recruit motor units with larger and larger cells
Recruit more and more motor units
Rigorous mortis
ATP synthesis stops
Sarcoplasmic reticulum pump stops
So actin/troponin complex activated
No ATP available for cross bridge detachment
Muscle stiffness - 3-12 hours to develop
Enzymatic breakdown of proteins can reverse this
Muscle ageing
Sarcopenia - muscle loss
Myosin production
Mitochondrial malfunction
Motor axon atrophy - nerve cell die back
Atrophy of muscle fibres
Spinal reflex
Sensory receptor
Sensory neurone
Synapsis and interneurones
Motor neurone
Effector
Ipsilateral reflex
Motor impulse leave spinal cord via the motor neurone on the same side as the sensory impulses that enter via the sensory neurone
Contralateral reflex
Motor impulse leaves spinal cord via the motor neurone on the opposite side as the sensory impulses that enters via the sensory neurone
Stretch/ myotactic reflex
- Muscle spindles
- Sensory neurone
- Excitatory synapse
- Motor neurone
- Skeletal muscle
Stretch/ myotactic reflex: 1. Muscle spindles
Sensory receptor (muscle spindle) is activated
Slightly stretching of the muscle
Stretch/ myotactic reflex: 2. Sensory neurone
Fires action potential along neurone into spinal cord
Stretch/ myotactic reflex: 3. Excitatory synapse
Sensory neurone synapses ipsilaterally with motor neurone in anterior grey horn of spinal cord
Stretch/ myotactic reflex: 4. Motor neurone
Action potential along neurone to neuromuscular junction in the stretched skeletal muscle fibre
Stretch/ myotactic reflex: 5. skeletal muscle
Muscular contraction
Relieving the stretching of the muscle
Tendon reflex
- Golgi tendon organ
- Sensory neurone
- Inhibitory interneurone
- Motor neurone
- Skeletal muscle
Tendon reflex: 1. Golgi tendon organ
Sensory receptors (Golgi tendon organ) activated
Located within a tendon at its junction within a muscle
Activated in response to increased tension
Tendon reflex: 2. Sensory neurone
Action potential along neurone into spinal cord
Tendon reflex: 3. Inhibitory interneurone
Sensory neurone synapses with inhibitory interneurone which synapses ipsilaterally with a motor neurone in anterior grey horn of spinal cord
Tendon reflex: 4. Motor neurones
Interneurones release inhibitory neurotransmitters that inhibit the motor neurone
Reducing chance of action potential
Tendon reflex: 5. Skeletal muscle
Relaxation of the muscle attached to the stretched muscle tendon
Reducing tension in tendon and protecting it from damage
Flexor reflex
- Pain receptor
- Sensory neurone
- Interneurone
- Motor neurone
- Flexor muscles
Flexor reflex: 1. Pain receptor
Free nerve endings of pain-sensitive neurones activated
Response to painful stimulus
Flexor reflex: 2. Sensory neurone
Action potential along neurone to spinal cord
Flexor reflex: 3. Interneurones
Sensory neurone synapses with ascending and descending interneurones
Flexor reflex: 4. Motor neurones
Interneurones synapses ipsilaterally with motor neurone in anterior grey horn of spinal cord
Triggering action potential to neuromuscular junction in flexor muscle fibres
Flexor reflex: 5. Flexor muscles
Contraction
Triggers withdrawal of the limb away from the painful stimuli
Extensor reflex
- Pain receptor
- Sensory neurone
- Interneurone
- Motor neurone
- Extensor muscles
Extensor reflex: 1. Pain receptor
Free nerve endings of a pen-sensitive neurone activated
Response to painful stimulus
Extensor reflex: 2. Sensory neurone
Action potential along neurone into spinal cord
Extensor reflex: 3. Interneurone
Sensory neurone synapses with interneurones which synapses with motor neurones contralaterally in anterior grey horn of spinal cord
Extensor reflex: 4. Motor neurones
Action potential along neurone to the neuromuscular junction in extensor muscle fibres of the unharmed limb
Extensor reflex: 5. Extensor muscles
Contraction
Limb to stabilise the body during the withdrawal action
