Neurology 9 - Neuromuscular and Spinal Cord Flashcards
Describe the transmission across synapses
- Can make membrane potential less negatve (excitatory post synaptic potential)
- Can make membrane potential more negative (inhibitory post synaptic potential)
- Added together this is summation
Where are the alpha motor neurons found?
- Brainstem and ventral horn of the spinal cord
- Innervate the extrafusal muscle fibres of skeletal muscles
- Activation causes voluntary muscle contraction
- Motor neuron pool contains all alpha motor neurons innervating a single muscle
What is a motor unit?
- Smallest functional unit with which to produce force
- The name given to a single motor neuron together with all the muscle fibres that it innervates.
- Stimulation of one motor unit causes contraction of all of the muscle fibres innervated
List the types of motor unit
- Slow (S type 1, smallest cell bodies, small dendritic trees, thin axons)
- Fast, fatigue resistant (FR type IIA, large diameter and dendritic trees, thicker axons)
- Fast, fatiguable (FF type IIB, large diameter and dendritic trees and thicker axons).
How are the motor unit types classified?
- Amount of tension generated
- Speed of contraction
- Fatiguability
What are the two mechanisms by which the brain regulates the force produced by a muscle?
Recruitment and rate coding
What is recruitment?
- Motor units are not randomly recruited.
- Governed by the “Size Principle”. Smaller units are recruited first (these are generally the slow twitch units).
- As more force is required, more units are recruited.
- This allows fine control (e.g. when writing), under which low force levels are required.
What is rate coding?
- A motor unit can fire at a range of frequencies. Slow units fire at a lower frequency.
- As the 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.
What are neurotrophic factors?
- A type of growth factor
- Prevent neuronal death and promote growth of neurons after injury
- Activity of the motor unit can be modified by these factors, to increase speed of contraction of a muscle usually innervated by slow neurones for example.
Which fibre types change?
- Type IIB to type IIA is the most common
- Type I to II is possible in severe spinal cord injury (and during spaceflight)
- Ageing is associated with loss of type II preferentially as well as some type I, resulting in slower contraction times
- Governed by neurotrophic factors
List the extrapyramidal tracts
- Extrapyramidal tracts are outside of the pyramids
- Rubrospinal causes automatic movements of the arm in response to posture change
- Reticulospinal tract coordinates automated movements of locomotion and posture
- Vestibulospinal tract maintains balance by regulating posute, and facilitates alpha motorneurones of the postural anti–gravity muscles
What is a 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.
What are the components of a reflex arc?
- Sensory receptor
- Sensory neuron
- Integrating center
- Motor neuron
- Effector
Describe the pathway of the monosynaptic (stretch) reflex
- Sensory receptor stimulated by stretch which triggers contraction of the same muscle via a single synapse
- At the same time, via an interneuron, antagonistic muscles are inhibited so they relax. This takes longer.
What is the Hoffman reflex?
- Electrical stimulation causing a monosynaptic reflex
- Eg. A precise way of measuring the patellar reflex
- M wave is triggered at higher intensity stimulus, causing a faster response than hitting the patellar
- Used to determine conduction times
Describe the pathway of flexion withdrawal (polysnaptic reflex).
- Sensory receptor to neuron following stepping on a pin
- To integrating centre which activates interneurons in other segments and motor neurones- therefore, polysynaptic
- Results in flexor muscle contraction to withdraw leg
- Inhibition across the midline and stimulation of the other leg in order to maintain the balance
What is the Jendrassik manoevre?
- Higher centres of the CNS exert inhibitory and excitatory regulation upon the stretch reflex.
- Inhibitory control dominates in normal conditions (N).
- Decerebration reveals the excitatory control from supraspinal areas (D).
- Therefore, you can increase reflexes. (The patient clenches the teeth and interlocks the fingers, and the patellar reflex will increase)
List the ways higher centres influence reflexes.
- Activating alpha motor neurons
- Activating inhibitory interneurons
- Activating propriospinal neurons
- Activating gamma motor neurons
- Activating terminals of afferent fibres
List 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).
What is the gamma reflex loop?
- Extrafusal voluntary muscle outside (alpha motor neurones)
- Intrafusal supplied by the gamma motor neurones, reacting to contraction of the extrafusal muscle. They are like springs.
What is the babinski sign?
- Looks for upper motor neuron dysfunction
- Stimulus to the lateral aspect of the foot and balls of the toes
- Great toe will extend and the small toes will fan out when there is nerve damage, while normally the toes will flex
What is a neuromuscular junction?
A specialised synapse between the motor neuron and the motor neuron end plate (muscle fibre cell membrane)
How are neuromusclar junctions activated?
- Action potential reaches terminal bouton increasing calcium influx
- ACh vesicles fuse with membrane
- ACh binds to receptors on postsynaptic membrane, resulting in an action potential due to sodium influx
- Acetylcholine can be released randomly to release miniature end-plate potentials
What can cause hyperreflexia?
- Stroke affecting the descending control of movement
- Upper motor neuron issues
What can cause hyporeflexia?
- Caused by damage to the lower motor neurones
Describe the arrangement of alpha motor neurones in the spinal cord
- In the ventral root there is functional mapping
- Extensor muscles more anterior
- Flexor muscles more posterior
- Proximal neurones are medial
- Distal musculature motor neuron pool more lateral
Describe the functional differences between the 3 types of motor unit
- Type IIb motor units are fast twitch, high tension and high fatigue
- Type IIa motor units are fast twitch, moderate tension and fatigue resistant
- Type 1 motor units are slow twitch, low tension and fatigue resistant
