Neuromuscular and spinal cord Flashcards
Give another name for a presynaptic terminal.
Bouton
How long is a synaptic cleft?
10-50nm
What is the role of a synapse?
Allow for communication/contact between neurones or neurone –> muscle
How does the contact ratio of synapses vary?
1:1 muscle
103:1 in the CNS
What is summation?
The graded effects of combined EPSPs and IPSPs on the neurone membrane potential.
The degree of summation determines whether an AP will be generated.
What is the difference between an IPSP and an EPSP.
An EPSP makes the neurones potential less negative, vice versa.
What is a neuromuscular junction?
Specialised synapse between a motor neurone, a motor end plate and the muscle fibre cell membrane.
Diagram of NMJ.
How is the NMJ activated?
AP arrives at terminal bouton.
Ca2+ influx.
ACh vesicles fuse with pre-synaptic membrane + diffuses across cleft.
ACh binds to post-synaptic receptors.
Na+ influx stimulated.
AP in muscle fibre.
what are miniature end-plate potentials?
At rest individual vesicles release ACh at a very low rate causing minimal stimulation.
Where are alpha motor neurons found?
Brainstem and spinal cord.
What do alpha motor neurons innervate?
The muscle fibres of skeletal muscle - activation causes contraction.
What is a motor neuron pool?
all the alpha motor neurons innervating a single muscle.
Cross section of vertebrae showing where in grey matter alpha motor neurons are located.
What is a motor unit?
A motor neuron and all the muscle fibres innervated by that particular motor neuron.
Stimulation of one motor unit causes contraction of all muscle fibres in that unit.
How many motor neurons and skeletal muscle fibres does a human have?
420,000 motor neurons
250 million skeletal muscle fibres.
How many muscles fibres are supplies by each motor neuron?
Approx. 600
Table comparing types of motor unit:
Slow (type 1)
Fast, fatigue resistant (type IIA)
Fast, fatiguable (type IIB).
Compare cell body diameter, no. dendritic trees, axon thickness, velocity of conduction,
In what 3 ways can different motor units be classified?
Amount of tension generated.
Velocity of conduction.
Fatiguability of motor unit.
Compare the tension, contraction speed, and fatiguability of Type I, Type IIA and Type IIB motor units.
Type IIB - fast twitch, high tension, high fatigue.
Type IIA - less fast twitch, moderate tension, somewhat fatigue resistant.
Type I - slow twitch, low tension, fatigue resistant.
What two mechanisms does the brain use to regulate the force that a single muscle produces?
Recruitment and rate coding.
Explain recruitment.
Recruit different motor units to build the force of contraction.
Smaller units recruited first - slow twitch.
Slow –> FR –> FF.
Explain rate coding.
Different motor units fire at different frequencies.
Slow units fire at a lower frequency.
As firing rate increases, force produced by the unit increases.
Summation occurs when units fire too fast to allow muscle to relax between arriving APs.
What are the roles of the motor cortex, brainstem, and cerebellum/basal ganglia in the motor system?
Motor cortex - receive info from other cortical areas and relays them to brainstem and thalamus.
Brainstem - passes command to spinal cord for body muscles and directly to muscles of face, head and neck.
Cerebellum/basal ganglia - adjust commands from other parts of motor system before outputting them to the thalamus and brainstem.
What are neurotrophic factors?
growth factors that prevent neuronal death and promote growth after injury
Explain normal relative to crossed innervation.
normal innervation - slow units have slow nerve fibres innervating them, while fast units have rapid ones.
Crossed innervation - fast and flow innervation is swapped so slow units become faster and fasr units become slower (e.g. Soleus faster, FDL slower).
Define plasticity and give some examples of it
The ability of fibre type to change.
IIB –> IIA after training
I –> II after severe deconditioning or spinal cord injury (or space microgravity)
Ageing - preferential loss of type II.
What is the role of pyramidal corticospinal tracts?
Control voluntary movements
What is the role of reticulospinal tracts?
coordinate automated movements of locomotion and posture e.g. To painful stimuli
What is the role of the vestibulospinal tract?
regulate posture to maintain balance and uses anti-gravity muscles.
What is the role of the rubrospinal tract?
automatic movements of arm in response to posture/balance changes.
Which tags correspond to the:
lateral and antrerior corticospinal tracts (pyramidal)
rubrospinal, reticulospinal, vestibulospinal, olivospinal tracts (extrapyramidal)?
- a,b
- a,b,c,d
What is a reflex?
involuntary coordinated pattern of muscle contraction and relaxation elicited by peripheral stimuli.
Diagram showing components of a reflex arc.
Compare monosynaptic stretch reflexes with polysynaptic flexion withdrawal reflexes.
Polysynaptic involves activated interneurons at several spinal cord levels
Diagram showing monosynaptic reflex.
Give the steps of a polysynaptic flexion withdrawal reflex.
- step on painful stimulus –> sensory receptor stimulated –> sensory neurone stimlated.
- sensory neurone activates interneurons in several spinal cord segments within integrating centre
- interneurons as/descend several levels and stimulate motor neurones from several levels to contract flexor muscles to move leg.
Explain crossed extension.
interneurons cross the cord and stimulate effectors on opposite limb to contract and extens other limb.
Define hyporeflexia.
below normal/absent reflexes. mostly associated with LMN disease.
Define hyperflexia.
supra-normal reflexes.
Explain supraspinal control of reflexes.
Higher CNS centres regulate stretch reflex. Under normal conditions, inhibitory control dominates.
Decerebration: reveals excitatory control from supraspinal areas - leading to rigidity and spasticity (overactive and tonic stretch reflex)
Via which neurons do higher centres of the CNS regulate reflexes?
- Activating alpha motor neurons
- Activating inhibitory interneurons
- Activating propriospinal neurons
- Activating gamma motor neurons
- Activating terminals of afferent fibres
What spinal pathways are involved in reflex regulation?
Rubrospinal: red nucleus for arms in balance
Vestibulospinal: vestibular nuclei for balance
Tectospinal: head movements due to visual information
