Motor and sensory innervation of muscle Flashcards
What are the main functions of the brain in the motor system? (3)
- Initiation
- Integration
- Co-ordination
What are the main functions of the spinal cord in the motor system? (2)
- Simple reflexes
- Pattern generation
What is the function of the basal ganglia in the motor system?
Coordination of movement.
-proper initiation and dampens down movement
What is the function of the cerebellum in the motor system?
Sensory motor coordination and recognises error.
What are the main collections of UMNs in the brain involved in the motor system? (2)
- Motor cortex
- Brainstem centre
What is the role of the motor cortex?
Planning, initiating and directing voluntary movements.
What is the role of the brainstem centres?
Basic movements and postural control.
How is information from UMNs passed to LMNs?
Most goes via interneurons in the spinal cord.
-5% go directly to LMNs
Where is the 1* motor cortex located?
Precentral gyrus.
-frontal lobe
What is the role of the precentral gyrus?
Planning and initiating voluntary movement.
What are the main Brodmann’s area associated with the motor cortex?
Brodmann’s areas 4 and 6.
Where is Brodmann’s area 4 located?
1* motor cortex.
Where is Brodmann’s area 6 located?
Premotor cortex.
What are BETZ cells?
Pyramidal cells in 5th layer of the 1* motor cortex.
What is the main function of BETZ cells?
Provide indirect connection to spinal cord via corticospinal tract.
What is the main function of non-BETZ cells?
Provide direct connection from UMNs to LMNS.
What tracts to axons descend in from the motor cortex to the spinal cord?
- Corticobulbar tract
- to brainstem centres
- Corticospinal tract
- to spinal cord
How are the corticospinal and corticobulbar tracts arrange in the primary motor cortex?
Corticospinal tract = MEDIAL.
Corticobulbar tract = LATERAL.
What is another name for the corticospinal tract?
Pyramidal tract.
How do the axons of the corticospinal tract descend?
- 95% cross the midline in the MEDULLA»_space; descend contralaterally
- 5% descend ipsilaterally and cross in the SPINAL CORD
Where in the spinal cord are the contralateral and ipsilateral corticospinal tract axons located?
CONTRALATERAL - posterior / lateral cord
IPSILATERAL - ventral cord
Where do the axons of the corticospinal tract terminate?
Onto lower motor neurons in the ventral horn.
-directly / indirectly (through interneurons)
Where are lower motor neurons localised?
Lamina IX of ventral horn.
-dorsal horn = sensory
Where do efferent motor axons exit the spinal cord?
Via the ventral root and spinal nerve.
What are the 2 main types of LMNs?
Alpha
Gamma
Which LMNs are directly responsible for the generation of force by muscle?
Alpha motor neurons.
What are gamma motor neruons mainly responsible for?
Proprioception.
How many axons is each fibre within a muscle innervated by?
One.
Who introduced ‘the final common pathway concept’?
Charles Sherrington.
- defined motor neurons and units
- motor neuron = last ste
Why does the size of laminae IX vary down the cord?
Size reflects the demands of peripheral muscles and their innervation.
Where is lamina IX largest?
Cervical and lumbar regions.
-limb muscles
Approximately how many alpha motor neurons are there on each side of the spinal cord?
~150,000.
-many more muscles fibres than motor neurons»_space; need collateral nerves
What is a motor neuron pool?
Alpha motor neuron group that innervate a single muscle.
- form clusters in ventral horn
- more powerful muscle»_space; larger pool
How are LMNs to flexors and extensors organised within the spinal cord?
Motor neurons to flexors (e.g. biceps brachii) = DORSAL.
Motor neurons to extensors (e.g. triceps brachii) = VENTRAL.
How are LMNs to axial and distal muscles organised within the spinal cord?
Motor neurons to:
- axial muscles = MEDIAL.
- distal muscles = LATERAL.
What is a motor unit?
An alpha motor neuron and all the muscle fibres it innervates.
How many muscle fibres can each motor neuron innervate?
Each motor neuron can innervate multiple fibres.
-each muscle fibre is innervated by one axon
What is the smallest unit of force that can be activated to produce movement?
A motor unit.
Does damage to a single motor axon reduce the muscle’s ability to contract?
Not necessarily.
-muscles fibres innervated by a single nerve are evenly spread out
Why do motor unit vary in size?
Allows variability in force and level of control.
What do small motor units consist of and innervate?
- Consist of small motor neurons.
- Innervate small number of fibres
What do large motor units consist of and innervate?
- Consist of large motor neurons.
- Innervate large number of fibres
Give an example of when small motor units are used.
Extraocular muscles in eye.
- 3 muscle fibres per neuron
- precise, low strength movements
Give an example of when large motor units are used.
Gastrocnemius.
- ~1500 fibres per neuron
- generates large forces
What size motor units are recruited first?
Small motor units are recruited first, then large to produce increasing force in the muscle.
What are the types of motor unit based on the fibres they innervate?
- Slow motor units
- Fast fatigable motor units
- Fast fatigue resistant motor units
What are the main features of slow motor units? (4)
- Small motor units
- Small ‘red’ muscle fibres
- Low force
- Resistant to fatigue (^Hb)
What are the main features of fast fatigable motor units? (4)
- Large motor units
- Large paler muscle fibres
- Large forces
- Easily fatigued (less Hb)
What are the main features of fast fatigue-resistant motor units? (3)
- Intermediate type
- Moderate force
- Some fatigue
Which type of motor units always work at maximal force?
Slow motor units - resistant to fatigue.
fast fatigue start powerful but quickly fatigue
What sort of exercise uses fast fatigable and slow motor units?
Fast fatigable - weight lifting.
Slow - marathon.
What is fatigue mainly caused by?
Build up of lactic acid.
-anaerobic repiration
What is used to record motor unit activity?
Electromyograph (EMG).
How is an electromyograph performed?
Needle is inserted into selected muscle»_space; picks up electrical activity of motor unit.
-reflects synchronous recruitment
What does activation of a motor neuron at low frequency (
Muscle twitch.
-contraction followed by relaxation
What does increasing the frequency of motor neuron firing (>10Hz) lead to?
Summation of the twitch.
What does higher frequencies of motor neuron firing (>20Hz) lead to?
Smoothing of muscle contraction.
What do frequencies of motor neuron higher than 40Hz lead to?
Twitch summation»_space; tension generation.
-fused tetanus effect
What is the fused tetanus effect?
Muscle stimulated at such a high frequency»_space; constant tension.
-doesn’t happen under normal circumstances
Summarise what happens with increasing frequencies of action potentials.
10 Hz - summation
>20 Hz - smoothing of contraction
>40Hz - tension generation
What are the main sources of inputs to alpha motor neurons? (3)
- Sensory from peripheral proprioceptors
- Local from spinal interneurons (most)
- Descending from brain UMNs
What is the sensory apparatus of muscle?
The muscle spindle.
- parallel to fibres
- intrafusal fibres
What is the main function of the muscle spindle?
- Stretch reflex; detects changes in muscle length
- Contributes to proprioception
- Regulates contraction
What is muscle tone?
The continual partial contraction of muscles.
- always under some degree of stretch
- spindle / neuron association
What are the 2 main classes of muscle spindles?
- Chain
- Bag
What is the function and innervation of chain spindles?
- Encode static response of the fibres (fire as long as muscle is stretched)
- Innervated by Ia and II afferent fibres
What is the function and innervation of bag spindles?
- Encode dynamic response of fibres (change rate)
- Innervated by Ia afferent fibres
What are intrafusal fibres?
Spindles.
- have sensory functions
- do not generate tension
- receive motor innervation from gamma neurons
What are extrafusal fibres?
- Form bulk of muscle
- Generate tension
- Receive motor innervation from alpha neurons
What does activation of alpha motor neurons lead to?
Contraction / shortening of muscle fibres
» collapse of intrafusal spinal.
-Gamma MNs then match intrafusal and extrafusal lengths.
What is the golgi tendon?
A mechanoreceptor.
- detects changes in muscle tension (‘strain gauge’)
- contributes to proprioception
What is the golgi tendon innervated by?
Ib afferents.
What is the relative conduction velocities of afferents?
Ia > Ib > II.
-larger diameter»_space; faster conduction
What are the main physiological differences between muscle spindles and golgi tendons? (3)
- Spindles detect changes in muscle LENGTH, golgi tendons detect changes in muscle TENSION
- Isometric contraction; Ib (GT) afferent is active, Ia (spindle) is not
- Isotonic contraction; Ia in active, Ib is not
What does damage to descending motor pathways lead to?
Upper motor neuron syndrome.
-Babinski sign, spasticity, hyporeflexia
Why do babies have the Babinski sign for the first few days?
Corticosinal tract is immature.
-fanning of toes
