Sensory and motor pathways

Question 1

what is a receptive field?

Answer 1

the space occupied by a sensory receptor that can elicit a response to a stimulus.

Question 2

what is lateral inhibition?

Answer 2

the capacity of an excited neurone to reduce the activity of its neighbours. this helps to sharpen sense perception.
a stimulus will stimulate some neurones greater than others depending on the position of the stimulus in a receptive field. a highly stimulated neurone will activate interneurones that inhibit excitation of laterally positioned cells. this creates greater contrast between the receptive fields and allows the brain to recognise where the stimulus is coming from

Question 3

what do hair follicle receptors sense and what is their fibre type?

Answer 3

sensation- motion, direction

fibre type- type II (A beta)

Question 4

what do meissner corpuscles sense and what is there fibre type?

Answer 4

sensation- tap, flutter (5-40 Hz)

fibre type- type II (A beta)

Question 5

what do pacinian corpuscles sense and what is their fibre type?

Answer 5

sensation- vibration (60-300Hz)

fibre type- type II (A beta)

Question 6

what do Merkel cells sense and what is their fibre type?

Answer 6

sensation- touch- pressure

fibre type- type II (A beta)

Question 7

what do Ruffini corpuscles sense and what is their fibre type?

Answer 7

sensation- skin stretch/ slippage

fibre type- type II (A beta)

Question 8

what do free nerve endings sense and what is their fibre type?

Answer 8

sensation- nociception- pain, heat

fibre type- type III (A delta) or type IV (C)

Question 9

what is the difference between rapid adapting and slow adapting receptors?

Answer 9

rapid adapting- respond to the application/removal of a stimulus, transient, phasic, vibratory stimuli, fail to respond to maintained stimuli e.g meissners corpuscles, some hair follicle receptors, pacinian corpuscles
slow adapting- encode stimulus intensity, active for duration of stimulus, e.g Merkel cells, some hair follicle receptors and Ruffini corpuscles

Question 10

what are the areas of the brain involved in decision and planning of movement?

Answer 10

the association cortices (parietal, pre-frontal, pre-motor and frontal) and the basal ganglia

Question 11

what is the function of the basal ganglia in the planning of movement?

Answer 11

inhibits inappropriate movement s and allows appropriate movements

Question 12

what are the areas of the brain are involved in the execution of movement?

Answer 12

primary motor cortex and the corticobulbar and corticospinal tracts

Question 13

how is ongoing movement controlled?

Answer 13

movement leads to consequences which need to be adjusted and needs to be adjusted in near-real time speed. however ongoing sensory feedback is too slow to be helpful for this. therefore the brain feeds a copy of the motor output into an internal stimulation of the body in order to generate a stimulated outcome of movement (state estimate)quickly and accurately. the actual sensory outcome is compared to the stimulated sensory outcome to produce a discrepancy signal which indicates the difference between the actual and stimulated outcome. this signal is used to contribute to the fine tuning of ongoing movement and used to improve the accuracy of the internal stimulation. the cerebellum is responsible for this internal stimulation.

Question 14

what are the ascending and descending pathways of the spinal cord?

Answer 14

ascending- dorsal column, posterior and anterior spinocerebellar tract, anterolateral system (including the spinothalamic tract)
descending- lateral corticospinal tract, rubrospinal tract, reticulospinal tracts, anterior corticospinal tracts, vestibulospinal tract

Question 15

what is the pathway of the Dorsal column- medial lemniscus system?

Answer 15

the first order neurone enters through the dorsal root and accents in the ipsilateral dorsal column either in the cuneate fasiculus (from upper limb) or gracile fasiculus (lower limb). the neurone terminates in the dorsal column nuclei in the medulla (cuneate or gracile nuclei) and synapses with a second order neurone. this decussates to the contralateral side in the medulla and ascends in the medial lemniscus to synapse in the thalamus. the third order neurone projects from the thalamus to the sensory cortex

Question 16

what is the dorsal column medial lemniscus pathway responsible for sensing?

Answer 16

discriminative (fine touch), pressure, vibration, proprioception

Question 17

what is the pathway of the anterolateral system?

Answer 17

first order neurone enters through dorsal root and synapses with a second order neurone in the spinal cord which decussates to the contralateral side before ascending in the spinothalamic tract to synapse in the thalamus.
the third order neurone projects from the thalamus to the sensory cortex.

Question 18

what is the anterolateral system responsible for sensing?

Answer 18

non-discriminative tacile touch
pain
temperature

Question 19

what are the spinocerebellar tracts responsible for?

Answer 19

sub-conscious proprioceptive information. feedbacks to cerebellum to aid balance and fine movement adjustments

Question 20

what is a motor unit?

Answer 20

a motor neurone and the skeletal muscle fibres innervated by that motor neurones terminal axons

Question 21

what is the significance of small and large alpha motor neurones motor neurones?

Answer 21

small motor neurones innervate relatively few muscle fibres and form motor units that generate small forces.
large motor neurones innervate larger, more powerful motor units

Question 22

what are the types of motor unit?

Answer 22

slow motor unit- contract slowly and generate small forces. they are resistant to fatigue and are important for activities which require sustained contraction- maintaining posture
fast- fatiguable- large motor units the generate more force that are easily fatigued. important for brief exertions
fast fatigue-resistant- lie between the two, intermediate size, slower than FF but much more resistant to fatigue and produce much more force that slow.
slow motor units recruited first then larger ones

Question 23

what occurs at a neuromuscular junction?

Answer 23

axon terminal depolarises causing ca to enter the terminal causing vesicles containing neurotransmitter to fuse with the membrane and release neurotransmitter into the synaptic cleft. neurotransmitter binds to receptors on the muscle fibre causing Na/K channels to open and the fibre is depolarised causing contraction.

Question 24

what Are upper and lower motor neurones?

Answer 24

upper motor neurone- connects brain to lower motor neurone

lower motor neurone- connects LMN to muscle

Question 25

what are the pyramidal tracts?

Answer 25

pass through the medullary pyramids, responsible for voluntary control of face and body
corticospinal- musculature of the body
corticobulbar- musculature of head and neck

Question 26

what is the pathway of the corticospinal tract?

Answer 26

begin in the cerebral cortex.
neurones converge and descend through the internal capsule. they then pass through the crus cerebra of the midbrain, the pons and into the medulla. in the most inferior part of the medulla the tract divides into two:
1. lateral corticopinal tract- decussate at the pyramidal decussation and then descends into the spinal cord. the fibres terminate in the ventral horn and synapse on lower motor neurones which go on to supply the muscles
2. anterior corticospinal tract- remains ipsilateral to descend into the spinal cord. fibres decussate just before terminating and synapsing with lower motor neurones in the ventral horn (cervical and upper thoracic segmental levels)

Question 27

what is the pathway of the corricobulbar tracts?

Answer 27

arise from lateral aspect of the primary motor cortex. fibres converge and pass through the internal capsule to the brainstem. the neurones terminate on the motor nuclei of the cranial nerves and synapse with LMNs which carry signals to the muscles of the face and neck. many of these fibres innervate bilaterally except the UMNs of the facial nerve and hypoglossal nerve provide contralateral innervation

Question 28

what are the extrapyramidal tracts?

Answer 28

vestibulospinal tracts- arise in vestibular nuclei. convey balance information to the spinal cord ipsilaterally. they control balance and posture.
reticulospinal tracts- medial (arises from pons, facilitates voluntary movements, increases muscle tone), lateral (arises from medulla, inhibits voluntary movements, decreases muscle tone
rubrospinal tracts- originate in red nucleus decussate and descend into the spinal tract
tectospinal tracts- begin in superior colliculus, receives input from optic nerves. neurones decussate and enter the spinal cord. terminate at cervical levels and coordinate movements of the head in response to visual stimuli

Question 29

what is spasticity?

Answer 29

stiffened or tightened muscles, preventing normal movement. velocity dependant increase in muscle tone to passive movement