7.1 Organisation of the Motor System and Spinal Reflexes Flashcards
What components of NS control:
a) reflexes
b) posture
c) voluntary movement
a) LMN and spinal cord circuits
b) Brain stem and spinal cord
c) Cortex, brain stem & spinal cord
Compare ‘open’ and closed’ loops that control voluntary and involuntary muscle control
1) Closed loops – reflex control
* Axial muscles (muscles of trunk and head) balance, posture, locomotion
2) Open loops – Sensory cue or desire to move
* Distal muscles, fine motor skills
Where is the brain centre for movement control located?
Movement control sits just anterior to the central sulcus located in the pre-central gyrus of the frontal lobe
State the function of each:
- premotor cortex
- primary motor cortex
- primary somatosensory cortex
premotor cortex: motor planning and sequencing for contralateral body side
primary motor cortex: controls contralateral body motor functions
primary somatosensory cortex: recieves contralateral sensory input from the body (incl taste)
State the function of each:
- posterior parietal cortex
- primary visual cortex
- primary auditory cortex
posterior parietal cortex: integration of sensory input (sterogenosis)
primary visual cortex: recieves contralateral visual field info from both eyes
primary auditory cortex: recieves bilateral auditory sensory input
Label the following image
Which regions of the spinal cord are important in control of movement?
Anterior and lateral funiculus
Label the image of an axon below
Which tracts are involved in conscious vs unconscious elements of the NS?
Conscious: Pyramidal/ Corticospinal tracts
Unconscious: Extrapyramidal tracts (4)
What are the 4 Extrapyramidal tracts?
Label these on the image below
1) Rubrospinal
2) Tectospinal
3) Vestibulospinal
4) Reticulospinal
What are the 2 components of the motor cortex?
1) Cerebellum
2) Basal ganglia
Brifely explain how signals decend via the Pyramidal tracts
CC ➞ basal ganglion ➞ cortiocospinal tracts (pyramidal) ➞ synapses in spinal cord with peripheral motor neuron ➞ connects to muscle via NMJ
Compare Pyramidal to Extrapyramidal tracts
Pyramidal tracts: originate in cerebral cortex, carrying motor fibres to the spinal cord and brain stem. They are responsible for the voluntary control of the muscles of the body and face.
Extrapyramidal tracts: originate in the brain stem, carrying motor fibres to the spinal cord. They are responsible for the involuntary and automatic control of all muscles, such as muscle tone, balance, posture and locomotion
What cell type comprises the Primary motor cortex?
What pathway do these travel down and where do they synaps?
Comprised of Betz cells which are are upper motor neurones with long axons
These travel via corticospinal tract to synapse with interneuron or alpha motor neurones.
What tract is responsible for conscious control of skeletal muscles?
How can this be split into 3 tracts and what does each specifically control?
Corticospinal tract, can be split into:
1) Corticobulbar: conscious control over eye and face muscles
2) Lateral corticospinal: conscious control over limb skeletal muscles
3) Anterior corticospinal: conscious control over axial skeletal muscles
Describe the pathway of the corticospinal tract
1) Originates in primary motor cortex
2) Descends via pyramids of the medulla
3) 75-90% decussate: mainly distal musculature
4) 10-25% don’t decussate: axial
5) Descend in anterior and lateral corticospinal tracts of the spinal cord
6) Synapse onto the ventral horn
Which tracts are ascending? (5)
Label these on the image below
Dorsal column (fasiculus cuneatus and gracilis)
Dorsal spinocerebellar
Ventral spinocerebellar
Lateral spinothalamic
Ventral spinothalamic
Which tracts are decending? (8)
Label these on the image below
Ventral white commissure
Lateral reticulospinal
Lateral corticospinal
Rubrospinal
Medial reticulospinal
Ventral corticospinal
Vestibulospinal
Tectospinal
What is the role of the Extrapyramidal motor pathways?
Label the 4 impt on the image below
Control involuntary actions, reflexes, locomotion, complex movements and posture
What is the specific function of the Rubrospinal tract?
Involuntary movements, specifically large muscle flexor movement and inhibiting extensor tone of upper limbs.
Describe the pathway of the Rubrospinal tract
1) Originates in the red nuclei of the brain stem
2) Descends in the lateral part of brainstem
3) Then in the lateral funiculus adjacent to the lateral corticospinal tract
What can be said about a lesion in the Rubrospinal tract?
Lesions produce minimal effects: demonstrating overlap
What is the specific function of the Reticulospinal tract?
Responsible for body posture and muscle tone
What 2 tracts comprise the Reticulospinal tract and Describe the origin and function of each
1) Pontine reticulospinal tract
- Medial pathway responsible for exciting extensor muscles
- Originates in the pontine reticular nucleus
2) Medullary reticulospinal tract
- Lateral pathway responsible for inhibiting excitatory axial extensor muscles and automatic breathing
- Originates in the medullary reticular nuclei
What is the specific function of the Vestibulospinal tract?
Involuntary control of head balance and body posture
Describe the pathway of the Vestibulospinal tract
1) Vestibular nuclei receives information from vestibulocochlear nerve about changes in the orientation of head.
2) Descends via vestibulospinal tract
3) Alters muscle tone, extension, maintaining posture
What is the specific function of the Tectospinal tract?
Orientation and positioning of the head for balance in response to stimuli.
Eye-head coordination
What is the origin of the Tectospinal tract?
Origin tectum of midbrain (tectum is the roof of the 4th ventricle comprising of superior and inferior colliculi)
What is a LMN?
a neuron whose cell body lies in either the ventral horn of the spinal cord or cranial nerve nuclei of brain stem and directly innervates a muscle with its axon
What is a Motor unit and how do these function?
What can be said about ‘number of fibres vs control of fine movement’?
Motor unit: the number of muscle fibres innervated by a single axon
Groups of motor units work together to coordinate the contractions of a single muscle
Smaller number = finer control
What are the 2 types of LMNs and what does each innervate?
1) α-MN: innervates the muscle itself
2) γ-MN: innervatew the muscle spindle and keeps it taught
What is a motor unit in terms of α-motor neuron?
How many are found in the eye vs the quads?
The α-motor neuron and the muscle fibres it innervates
- Extra-ocularmuscles = 10
- Quadriceps = 1000
What are Muscle spindles?
Encapsulated sense organ found within the muscle that detects muscle contraction and is responsible for tendon reflex
What is always activated alongside muscle spindles and why?
Coactivated with α-motor neurons so tighten as muscle contracts
What is the function of Muscle spindles?
What does it detect and how does it respond?
Allows fine motor control and provide limb positional information to the CNS
Detects extent of muscle tension (contraction/relaxation) using Proprioception, muscle tone, JPS and informs the CNS
Activate LMNs in response to muscle stretch
Describe the 3 main components of a muscle spindle
** Note there are extra-fusal fibres around the outside which contain the actin and myosin and contract
Describe how Muscle spindles respond to strech
When we want to contract our muscles stretch of the extrafusal fibres is sensed by afferent sensory neurones (1a axons) wrapped around intrafusal muscle
These send a signal to the spinal cord which stimulates the efferent alpha motor neuron causing contraction of the muscle
During this the intrafusal fibres become wobbly, so gamma motor neurons located ONLY at the ends are activated alongside ➞ keeps spindles taut during contraction
Where does the alpha motor neuron synaps?
Onto the muscle itself via the NMJ
Compare the activation of a single motor neuron vs multiple motor neurons at the NMJ
Activation of a single motor neuron leads to weak but distributed contraction
Activation of multiple motor neurons activates muscle fibres, increasing the force of contraction
What ion stimulates contraction of skeletal muscle?
Raised intracellular Ca2+
Explain how mucles contraction occurs in motor nerves
1) AP arrives at the motor end plate and releases ACh into synaptic cleft.
2) Activation of nicotinic receptors on postsynaptic membrane of NMJ opens ligand-gated Na+ channels on sarcolemma
3) This allows APs to propagated through invaginations in the sarcolemma (T-tubules)
4) depolarisation of T-tubules activates ryonidine receptors on SR, releasing stored Ca2+ into cytoplasm
5) Ca2+ binds troponin subunit (TnC), exposing myosin binding sites allowing muscle contraction to occur
6) Ca2+ is returned to the SR via ATPase pumps
Where are Golgi tendon organs located?
Found at the myotendinous junction between the muscle and tendon
What do Golgi tendon organs sense, how do they respond and what is the purpose of this?
Monitors degree of muscle contraction by sensing change of the collagen within the tendon when it stretches and straightens with muscle contraction
When activated it inhibits the muscle contraction and activates its antagonist
This prevents over contraction of muscles
What is muscle tone?
Background continuous muscle contraction found in all skeletal muscles of the body
What is meant by the ‘resting motor tone’ and what is it important in?
Resting tension in a skeletal muscle which occurs because our LMNs are continually activate when we are awake
This minimal contraction gives the muscle a small amount of force which allows body posture and rapid responses to environment
During ‘resting motor tone’ how is minimal contraction by LMN sustained?
LMN are under inhibition from descending extrapyramidal pathways
In what 2 instances is our ‘resting muscle tone’ poor
At birth and during REM sleep
