MOVEMENT Flashcards
What is the NMJ?
The neuromuscular junction - synapse between nerve and muscle fibre (acetlycholine released)
Where do some acetlycholine receptors bind?
To nicotinic receptors (proteins embedded in muscle fibre)
What happens when Ach binds?
It binds and simultaneously lets Na+ in
How many subunits does a nicotinic receptor contain?
5 subunits
What needs to occur for the channel to open?
One molecule of Ach binding to each alpha subunit
What is another term for postsynaptic membrane?
Motor end plate
Where are lower motor neurons found ?
Ventral horn of spinal cord
What do lower motor neurons do?
Directly command muscle contraction
Are lower M.Ns distributed evenly throughout the body?
NO! Innervation of more than 50 muscles of arm originates from C3-T1 (ventral horn is ‘swollen’ )
Where are the motor neurons that innervate distal and proximal musculature found?
C and L-S segments of spinal cord
What is the first way the CNS controls muscle contraction?
By varying the firing rate of motor neurons
How does the alpha neuron communicate with the muscle fibre?
- By releasing neurotransmitter Ach at NMJ
- Ach released from presynaptic terminal causes response (EPSP-but not the same as in normal neuron-neuron synapses) in muscle fibre- end plate potential
What does one poostsynaptic AP cause?
A twitch (rapid contraction and relaxation of muscle fibre)
What does a sustained contraction require?
Continual barrage of APs
What does high frequency presynaptic activity cause?
Temporal summation of postsynaptic responses
What is the second way the CNS grades muscle contraction?
-Recruitment (recruiting additional synergistic motor units)
What can occur in a muscle with a large number of SMALL motor units (small alpha motor neurons) ?
The muscles can be more finely controlled by the CNS
What are small neurons more exited by?
Signals descending from brain
What is orderly recruitment of motor neurons due to?
The variation in motor neuron size
What are the three main sources of input to alpha motor neurons?
- Dorsal root ganglion
- Upper motor neurons in motor cortex and brain stem
- Interneurons in spinal cord (largest input and could be excitatroy or inhibitory, for motor programs)
What do the dorsal root ganglion cells do?
Innervate the muscle spindle- sensory and muscle length feedback
What are the upper motor neurons in the motor cortex and brain stem important for?
- Initiation and control of voluntary movement
What are slow twitch muscle fibres?
- Dark red
- Large number of motochondira
- Fatigue resistant
- Slow to contract
Where are slow twitch muscles found?
Antigravity muscles (gastrocnemius soleus) of legs, the torso, and wings of birds
What are fast twitch fibres?
- Contract rapidly
- Fatigue faster
- Fewer mitochondria
- Anaerobic metabolism e.g. arm muscles
What are the two types of fast twitch muscles?
- fatigue resistant (FR) and fast fatigable (FF) q
What are FR (fatigue resistant) fibres classified by?
Moderately strong and fast contractions and relatively resistant to fatigue
- Intermediate size
What are FF (fast fatiguable)fibres classified by?
- They generate the stronges, fastest contractions but are quickly exhausted when stimulated at high frequency for long periods
- Largest diameter so fastest conducting
Do different types of muscle fibres coexist in muscles?
YES!
Do different types of motor neurons coexist in muscles?
NO!! Each motor unit contains muscle fibres of only a single type
e.g. one type of slow motor unit that contains only slowly fatiguing red fibres
- also two types of motor units containing EITHER FR FF
What can too muhc Ach lead to?
Desenistisation of receptors and block of nueromuscular transmission
What are myofibrils surrounded by?
Sarcoplasmic reticulum (sac that stores Ca2+)
How do APs gain access to the SR?
T tubules (network of tunnels)
What are T tubules?
- Like inside out axons
- membrane contains voltage sensitive clusters of 4 Ca2+ channels called tetrads
- T tubule membrane linked to calcium release channel in SR membrane
What is the molecular basis of muscle contraciton?
Myofibril divided into disks (z lines)
What is a sarcomere?
Segment composed of two z lines with myofibril between
What is anchored to Z lines?
- thin filaments (actin)
- the actin proteins face one another and never come into contact
What is between the thin filaments?
THICK filaments (myosin protein)
When does muscle contraction occur?
When thin filaments slide along thick filaments and bring Z lines toward one another
In contraction the sarcomere…
Becomes shorter in length
What do the myosin heads do?
- ‘walk along’ the actin filament
- binds actin molecules and conformational change occurs that causes them to pivot (causes thick filament to move with respect to the thin filament)
How does the myosin head ‘unlock’?
ATP binds to the myosin heads and heads disengage and unlock so process can repeat
What happens when muscle is at rest?
Myosin can’t interact with actin
How does Ca2+ initiate contraction with respect to the complex?
- Binds to TROPONIN which then shifts tropomyosin position and then exposes sites where myosin can bind to actin
How long does contraction continue?
Based on amount of Ca2+ and ATP being available
Why do muscles become stiff after death?
- Because muscle cells starved of ATP
- No ATP prevents detachment of myosin heads and leaves myosin attachment sites on actin filaments exposed for binding (permanent attachments for thick and thin filaments form)
What are the critical steps in contraction?
- AP
- Ach release
- Na+ entry triggers Navs
What happens when AP propagates in muscle fibre?
- The T tubules allow AP to reach all parts of the muscle fibre simultaneously
What is a DHP receptor?
- Voltage gated Ca2+ channel
What happens when the AP reaches the DHP receptor?
- There is physical coupling between DHP receptor and ryanodine receptor
- When DHP opens in response to voltage gradient, also causes ryanodine receptor to open (ca2+ channel on the lateral sac of the SR)
Why do your muscles get ‘warm’ when you exercise?
Energy demands associated with ATP pumping Ca2+ back in
What occurs in crossbridge cycling?
- 4 states
- The RATE at which this occurs depends on the ATP (e.g. muscle cells with lots of mitochondria will go through it quicker)
What is Ca2+ critical for?
Cross bridge cycling
What is the latent period known as?
Gap between when the AP has arrived and when you start seeing force
What does the latent period depend on ?
- AP conduction time
- Time for Ca2+ to be released
Why does force last for so long?
Takes some time for troponin to move off tropomyosin (1st step)
What limits the maximum force generation?
- takes time for cross bridging to occur
- Passive muscle tension
(fused and unfused tetanus)
What effect does a nicotinic Ach receptor antagonist have?
- nicotinic receptors on skeletal muscle
- weakness and inability to generate contractions
- competitive antagonist prevents Ach binding
- So flaccid paralysis
What effect does massive release of acetylcholine, norepinephrine and GABA have?
- Cramps, muscle contractions. Rarely death, only death in small animals, also spasms
What happens if you prevent Ach release?
Flaccid paralysis with cramps (e.g in Botox)
What does the titin filament account for?
PASSIVE TENSION
- has spring like properties
What is active tension?
The amount of tension generated on the sarcomere
- Depends on the length
What is the optimal length dependent on?
- The overlap between actin and myosin
What is the max length dependent on?
No overlap of actin/myosin
What is the min length limited by?
- Thin filament overlap
- Overlap of thick filaments and Z disks
What is isotonic contraction?
Muscle changes length whilst maintaining constant tension
What is isometric contraction?
- Muscles develops tension without changing length