Neuromuscular and Spinal Cord Flashcards
How can the postsynaptic neurone by altered?
- can be made less negative - EPSP (closer to firing threshold)
- can be made more negative - IPSP (further away from threshold for firing)
NMJ
A specialised synapse between the motor neuron and the motor end plate, the muscle fibre cell membrane
What allows saltatory conduction?
nodes of ranvier
Motor unit
- motor neurone and all the muscle fibres it innervates
- smallest functional unit in force production
- one motor neurone can innervate multiple muscle fibres, but a motor fibre cannot be innervated by multiple neurones?
How many muscle fibres does a motor neurone innervate?
On average about 600
What are the types of motor units?
- slow
- fast, fatigue resistant
- fast, fatiguable
Slow motor unit
- S, type I
- smallest diameter cell bodies
- small dendritic trees
- thinnest axons
- slowest conduction velocity
Fast fatigue resistant
- FR, type IIA
- larger diameter cell bodies
- larger dendritic trees
- thicker axons
- faster conduction velocity
Fast fatiguable
- FF, type IIB
- larger diameter cell bodies
larger dendritic trees - thicker axons
- faster conduction velocity
Which criteria can be used to classify the 3 motor unit types?
- amount of tension generated
- speed of contraction
- fatiguability
How can the brain regulate the muscle force production?
Using the 2 following mechanisms:
- recruitment
- rate coding
Recruitment
- recruitment of motor units has an order
- governed by the size principle -> smaller units are recruited first (generally the slow twitch units)
- as more force is required, more units are recruited
- allows for fine control (e.g. writing), under which low force levels are required.
Rate Coding
- A motor unit can fire at a range of frequencies.
- Slow units fire at a lower frequency.
- As firing rate increases, the force produced by the unit increases.
- Summation occurs when units fire at frequency too fast to allow the muscle to relax between arriving action potentials.
Neurotrophic factors
- Greek trophē, food
- Are a type of growth factor
- Prevent neuronal death
- Promote growth of neurons after injury
Effect of neurotrophic factors
- the activity of motor units is modifiable
- Motor unit and fibre characteristics are dependent on the nerve which innervates them.
- If a fast twitch muscle and a slow muscle are cross innervated, the soleus becomes fast and the FDL becomes slow.
- The motor neurone has some effect on the properties of the muscle fibres which it.
Plasticity
- Fibre types can change properties under many different conditions.
- Type IIB to IIA most common following training
- Type I to II possible in cases of severe deconditioning or spinal cord injury. Microgravity during spaceflight results in shift from slow to fast muscle fibre types
- Ageing associated with loss of type I and II fibres but also preferential loss of type II fibres. This results in a larger proportion of type I fibres in aged muscle (evidence from slower contraction times).
Name the main spinal cord motor tracts
- Pyramidal tracts: lateral (1a) and anterior (1b) corticospinal tracts
- Extrapyramidal tracts:
- rubrospinal tracts (2a)
- reticulospinal tract (2b)
- vestibulospinal tract (2c)
- olivospinal tract (2d)
Reflex
An automatic and often inborn response to a stimulus that involves a nerve impulse passing inward from a receptor to a nerve centre and then outward to an effector (as a muscle or gland) without reaching the level of consciousness.
- An involuntary coordinated pattern of muscle contraction and relaxation elicited by peripheral stimuli.
Tectum
Roof of midbrain
How do reflexes differ from voluntary movement?
- Reflexes differ from voluntary movements in that once they are released, they can’t be stopped.
- reaching level of conciousness
What are the components of the reflex arc?
- sensory receptor
- sensory neurone
- integrating center (one or more regions of the CNS that relay impulses from sensory to motor neurones)
- Motor neurone
- Effector (muscle/gland)
Monosynaptic (stretch) refelx
- e.g. patella reflex
1. stretching stimulates sensory receptor
2. sensory neurone is excited
3. within integrating center: sensory neurone…
a) activates motor neurone b) activates inhibitory interneurone that activates the motor neurone to antagonistic muscle and causes its relaxation
c) sends info to brain
4. motor neurone excited
5. effector (muscle) relieves the stretching
Hoffman (H-) reflex
- electric stimulation
- monosynaptic reflex
Polysynaptic reflexes
- multiple spinal cord segments can be activated
- e.g. flexion withdraws (e.g. stepping on nail)
Flexion withdrawl
- stepping on tack stimulates sensory receptor (dendrites of pain-sensory neurone)
- sensory neurone excited
- Within integrating center: sensory neurone activates interneurones in SEVERAL spinal cord segments
- motor neurones excited
- Effectors - flexor muscles contract and withdraw leg
Supraspinal control of reflexes
- Traditionally we think of reflexes as being automatic (knee jerk) and stereotyped behaviours (sneeze, cough) in response to stimulation of peripheral receptors.
- however, they can be influenced
- higher centres of brain exert inhibitory or excitatory regulation upon the stretch reflex
By what do higher centres influence reflexes?
- Activating alpha motor neurons (sitting in ventral horn)
- Activating inhibitory interneurons
- Activating propriospinal neurons (proprioceptive info coming back from the body about where we are in space)
- Activating gamma motor neurons (intrafusal, repsond to movement of muscles to tend or not, innate, control muscle fibre length based on afferent input)
- Activating terminals of afferent fibres (
What are the higher centres and pathways involved in supraspinal control of reflexes?
- Cortex – corticospinal (fine control of limb movements, body adjustments)
- Red nucleus – rubrospinal (automatic movements of arm in response to posture/balance changes)
- Vestibular nuclei – vestibulospinal (altering posture to maintain balance)
- Tectum – tectospinal (head movements in response to visual information).
Distribution of alpha motor neurones in the spinal cord
Ventral Horn:
- dorsal: flexors
- ventral: extensors
- medial: proximal muscles
- lateral: distal muscles
-> somatotropin mapping (functional) -> motor neurone pooling
Distribution of alpha motor neurones in the spinal cord
Ventral Horn:
- dorsal: flexors
- ventral: extensors
- medial: proximal muscles
- lateral: distal muscles
-> somatotropin mapping (functional) -> motor neurone pooling
In what order are motor units brought when provoking muscle activity?
S (I) -> FR (IIa) -> FF (IIb)
In what order are motor units brought when provoking muscle activity?
S (I) -> FR (IIa) -> FF (IIb)
What is the difference between II and IIb motor units?
- IIb are fatiguable
- IIb are activated after IIa when muscle activity is provoked
How can you visualise the different motor unit subtypes?
- using a histochemical stain (ATP-Myosin stain)
- different staining intensity in S, FR, FF
- allows also to count how many of each subtype are present
In what order are motor units brought when provoking muscle activity / order of recruitment?
S (I) -> FR (IIa) -> FF (IIb)
How can you visualise the different motor unit subtypes?
- using a histochemical stain (ATP-Myosin stain)
- different staining intensity in S, FR, FF
- allows also to count how many of each subtype are present
Rubrospinal tract (2a)
automatic movements of arm in response to posture/balance changes
Reticulospinal tract (2b)
coordinate automated movements of locomotion and posture (e.g. to painful stimuli)
Vestibulospinal tract (2c)
regulates posture to maintain balance, and facilitates mainly α motoneurones of the postural, anti-gravity (extensor) muscles
Pyramidal tracts function
control voluntary movements
lateral and anterior corticospinal tract
Flexion withdrawl & crossed extensor
- Inhibition across the midline
- Stimulation for other leg in terms of maintaining gate -> take weight on the other leg
- interneurones cross midline to allow for contraction of extensor muscle and extension of left leg (if you stepped on sth with your right leg)
Jendrassic manœuvre
- Reflexes can be influenced
- e.g. clench teeth or make a fist when having a patellar tendon tapped -> stronger reflex
Jendrassic manœuvre
- Reflexes can be influenced
- e.g. clench teeth or make a fist when having a patellar tendon tapped -> stronger reflex
When is supra spinal control of reflexes inhibitory/excitatory?
- Inhibitory control dominates in normal conditions (N).
- Decerebration reveals the excitatory control from supraspinal areas (D).
- Rigidity and spasticity can result from brain damage giving over-active or tonic stretch reflex.
When is supra spinal control of reflexes inhibitory/excitatory?
- Inhibitory control dominates in normal conditions (N).
- Decerebration reveals the excitatory control from supraspinal areas (D).
- Rigidity and spasticity can result from brain damage giving over-active or tonic stretch reflex.
Extrafusal muscle fibers
- under control of alpha motor neurones
- voluntary contraction
Intrafusal muscle fibers
- respond to changes in lengths of muscle spindles
- have sensory axons
- Reaction
Gamma motor neuornes
- change length based on input
- supply intrafusal fibers
- react to activity of the muscle itself
Gamma reflex loop
- What if the knee is extended and the muscle goes slack?
- The spindle is shortened to maintain its sensitivity
De-recruitment order
FF -> Fr -> S
reverse order to recruitment
Temporal vs spatial summation
Temporal: repeated inputs
Spatial: multiple simulations inputs (IPSPs, EPSPs)
