Flashcards in Movement & Descending Tracts Deck (50):
What is being tested during the stretch reflex?
Integrity of connections of neurones
Status of neurones of the reflex
Status of synapses of the reflex
Status of wider circuits built from stretch reflex
note: monosynaptic reflex (contraction of agonist muscle) is tested (the bisynaptic reflex which involves the relaxation of the antagonist muscle cannot be tested)
Contrast alpha and gamma-motoneurones.
Gamma-motoneurones resistant to descending inhibition ---> brief muscle contraction (muscle tone)
Alpha-motoneurones completely disabled by descending inhibition
What is the muscle stretch reflex? What are the different sub-types?
Muscle stretch reflex = stretch-activated reflex contraction of a skeletal muscle
- monosynaptic stretch reflex = direct connection of stretch receptor afferent neurone and lower motor neurone (one synapse)
- withdrawal reflex = move extremity away from noxious stimulus
- dampening and loading reflex = averages multiple nervous signals ---> smooth muscle contraction
Contrast the functions of UMNs and LMNs.
- supply skeletal muscle indirectly (exert actions via LMNs)
- includes all descending tracts
- includes all interneurones
- UMN signs can vary
- supplies skeletal muscles directly
- only one type, therefore only one variety of motor deficiency arise from LMN damage
- somatic motor efferent with cell body in lamina IX of spinal cord (spinal motor nucleus) or cranial nerve motor nucleus
- evoke voluntary movements when commanded by UMNs
- evoke reflex movements when recruited by spinal motor circuits
What is a motor unit?
One alpha-motoneurone + variable no. of extrafusal muscle fibres it supplies
Minimal functional unit of the motor system
In which laminae are the motor neurones located?
Laminae VIII & IX
What are template neural circuits? What are minimal neural circuits?
Template neural circuit = all motor circuits of the body are built from these; move connections added
Minimal neural circuit = underlies all movements of the body and sets all motor tone
Outline the sequence of the knee jerk reflex.
1. Vibration as tendon changes length of muscle detected by muscle spindle (increased length due to relaxation of muscle)
2. Sends sensory information to spinal cord
3. Command to contract muscle (interneurone also sends commands to LMN of antagonist muscle to relax)
What is a reflex? What are the components of the reflex?
Reflex = involuntary, unlearned, repeatable, automatic reaction to a specific stimulus that does not require the brain to be intact
- afferent fibre
- integration centre
- efferent fibre
note: not so much for preventing injury, but for allowing actions e.g. being able to hold a bowl whilst it is being filled
note: stretch reflex present in all muscles of the body
Contrast monosynaptic and polysynaptic stretch reflexes.
MONOSYNAPTIC: afferent ---> efferent
e.g. stretch reflex
note: effectively monosynaptic (cannot test component inhibiting the antagonist muscle)
POLYSYNAPTIC: afferent ---> interneurone ---> efferent
e.g. antagonist muscle inhibition during stretch reflex
e.g. withdrawal reflex
e.g. crossed extensor reflex
Outline the sequence of events in the withdrawal reflex.
1. Painful stimulus
2. Primary sensory neurone diverges:
3. Ascending pathway for sensation (pain) and postural adjustment (shift in centre of gravity)
4. Withdrawal reflex pulls limb away from painful stimulus (flexor motoneurone)
5. Crossed extensor reflex supports body as weight shifts away from painful stimulus
Contrast the muscle spindles and Golgi tendon organs.
Muscle spindle = receptor sensitive to stretch embedded between and parallel to fibres of striated muscles
Golgi tendon organ = receptor sensitive to stretch within tendons
Permanently connected to cell bodies of LMNs
Send information to:
- brain (via dorsal columns)
- cerebellum (via spinocerebellar tracts)
- spinal motoneurones
When is muscle tone inhibited?
Low in utero, suppressed in the newborn (allows birth)
Inhibited during REM sleep in all muscles EXCEPT:
- muscles of respiration
- extra-ocular muscles
- urinary sphincter
- anal sphincter
Give the spinal neural levels for the reflexes tested in the neurological exam.
Biceps = C5-C6
Supinator = C5-C6
Triceps = C7
Knee = L3-L4
Ankle = S1
Give some important spinal neural levels.
Diaphragm = C3-C5
Biceps = C5-C6
Wrist = C8-T1
Nipple = T4
Umbilicus = T10
Hip flexion = L1-L2
Quadriceps = L3-L4
Knee flexion = S1
Dorsiflexion = L5
Plantarflexion = S1-S2
Urinary &anal sphincter tone = S2-S4
Which part of the brain is responsible for movement? Differentiate the cortical and non-cortical descending tracts.
Pre-central gyrus of frontal cortex
Cortical descending tracts:
- cerebral cortex ---> brainstem
- cerebellum ---> brainstem
Non-cortical descending tracts:
- cerebral cortex ---> basal ganglia & cerebellum ---> thalamus ---> cerebral cortex
Neurones of pre-frontal, supplementary motor cortex, and somatosensory cortex terminate on basal ganglia or cerebellum (selection and feedback of motor regions)
Neurones of pre-central gyrus terminate on cell bodies of alpha-motoneurones of muscles on the contralateral side
Describe the path of the corticobulbar tracts.
Terminate in cranial nerve motor nuclei
Primary motor area in cerebral cortex supply:
-> contralateral and ipsilateral trigeminal motor nuclei (muscles of mastication)
-> contralateral facial motor nucleus (muscles of facial expression)
-> contralateral and ipsilateral nuclei for vocalisation
-> contralateral and ipsilateral nuclei for swallowing
-> contralateral hypoglossal nucleus (muscles of tongue movements)
Cingulate motor area ---> contralateral and ipsilateral facial motor nuclei (therefore muscles of brow have bilateral innervation)
Describe the path of the corticospinal tracts.
Terminate on cell bodies of spinal motor nuclei
Responsible for fractionation of finger movements on the contralateral side (not present in children as myelination has not occurred)
Divided in lateral and ventral corticospinal tracts:
LATERAL CORTICOSPINAL TRACT (75%-85%) = axons travel via internal capsule ---> decussate in medulla (medullary pyramids) ---> travel in lateral funiculus of spinal cord
VENTRAL CORTICOSPINAL TRACT (15%-25%) = axons travel via internal capsule ---> join medullary pyramids ---> decussate in spinal neural segment of target ---> travel in ventral funiculus of spinal cord ---> synapse directly with LMNs
What is a motoneurone?
Somatic efferent supplying skeletal muscles
- brings about displacement of limbs
- sets muscle tone (via background electrical impulses ---> minimal contraction) ---> maintain body posture and hold head upright
What are the bulbarspinal tracts? What are their general functions?
Synapse to interneurones of spinal reflex pathways of the spinal cord
Generally inhibit LMNs of reflex pathways
- rubrospinal tract
- reticulospinal tract
- tectospinal tract
- vestibulospinal tract
What are the functions of the extrapyramidal motor pathways?
RUBROSPINAL = flexor muscle tone (mostly rudimentary in humans)
TECTOSPINAL = turn head towards sights or sounds
RETICULOSPINAL = automatic movement e.g. locomotion
VESTIBULOSPINAL = balance and posture
Outline the path of the rubrospinal tract.
Originates from red nucleus of midbrain (rich in iron)
Fibres decussate in midbrain
Travel in lateral funiculus
Most fibres terminate in cervical spine (synapses with motoneurones of flexors of the upper limb)
Small and rudimentary in humans (overtaken by corticospinal tracts)
Outline the path of the reticulospinal tract.
Originate from reticular formations: pontine reticular formation (medial reticulospinal tract) and medullary reticular formation (lateral reticulospinal tract)
Partially decussates in the brainstem.
Terminates in spinal cord
Normally inhibited by corticospinal tract
When released from inhibition (brain damage at or below red nucleus) ---> decerebrate rigidity (unopposed extension of head and limbs)
Outline the path of the tectospinal tract.
Originates from tectum of mesencephalon: superior colliculus (visual tectum) and inferior colliculus (auditory tectum)
Decussates in brainstem.
Terminates in cervical and upper thoracic regions of the spinal cord on the contralateral side
Mediates postural reflex movements of the head using visual and auditory signals
Outline the path of the vestibulospinal tract.
Originates from vestibular nuclei e.g. Deiter's nuclei, nucleus of Bechterew
Mostly remain ipsilateral, some project bilaterally
Terminates in the spinal cord
Major influence on muscle tone
- upright posture
- maintenance of balance, posture of body, and head
Damage to causes:
- loss of righting reflex
- postural instability
Outline the cognitive and non-cognitive components of the initiation of movement. How does sensory and motor input and feedback affect this?
Plan ---> Execute ---> Move
Sensory input into planning
Sensory feedback into non-cognitive components
- how accurate the movement was
- any modifications needed?
---> schema for movement developed
Motor memory input into planning (schemas for specific movements)
What is the vestibulo-cochlear reflex? Which part of the brain governs this reflex?
Maintain fixed faze on object whilst head moves
How do the basal ganglia affect movement?
Regulate the amplitude and velocity of planned movements
Particularly in relation to use of proprioceptive information
Outline how the basal ganglia affect movements directly and indirectly.
Cerebral cortex ---> striatum
Striatum (D1 receptors - excitatory) ---> inhibits globus pallidus interna/para reticulata of sunstantia nigra ---> removes inhibition of thalamus (RELEASES BRAKE) ---> stimulates cerebral cortex & D1 receptors of striatum
Striatum (D2 receptors - inhibitory) ---> inhibits globus pallidus externa ---> removes inhibition of subthalamic nucleus, globus pallidus interna, and pars reticulata of substantia nigra
Cerebral cortex ---> subthalamic nucleus, brainstem, and spinal cord
(MAINTAINS BRAKES AT REST)
note: activation of D1 receptor facilitates movement via the direct pathway AND by switching off the indirect pathway (by inhibiting D2 receptor)
What neurotransmitters are used in the basal ganglia? How are these affected in disease states?
Globus pallidus and neostriatal connections = GABA
- loss of GABA ---> reduced inhibition ---> Huntington's
Substantia nigra = dopamine
- loss of fibres in pars compacta (Parkinson's) cause reduced inhibition of movement (resting tremor) and reduced excitement (rigidity)
Why are reflexes not permanently active?
Descending tonic inhibition (lifted to allow reflex to take place)
note: when a reflex circuit is active, other reflexes are inhibited by the spinal cord
Contrast the pathways and synapses of the upper and lower motor neurones.
UMNs = neurones of pyramidal and extrapyramidal tracts
- cell body in primary motor cortex
- synapses with LMNs
- long axons entirely within CNS
LMNs = peripheral motor nerves/cranial nerves
- cell body in ventral horn/brainstem nuclei
- axon exits CNS
How can hyporeflexia due to CNS depression be distinguished from hyporeflexia due to damage due to the motor efferent nerve?
Both due to increased desc. inhibition reducing reflexes
CNS depression e.g. drugs, alcohol
- same on both sides
- reflexes may be elicited using Jendrassik's manoeuvre
Damage to motor efferent nerve
- localised to one side
- will not be improved using Jendrassik's manoeuvre
Jendrassik's manoeuvre increases the afferent drive into the spinal cord to reinforce the stretch reflex and reduce desc. inhibition
How does desc. inhibition affect different motoneurones?
All alpha-motoneurones constantly inhibited (except during reflexes)
Larger cell body diameter ---> greater inhibition
- muscles of respiration (still able to breathe in deep sleep despite general paralysis of limbs)
- extra-ocular muscles (rapid eye movements in REM sleep)
Compare the functions of different types of motoneurone.
Alpha-motoneurone = innervate extrafusal muscle fibres (generate force of contraction; also responsible for baseline muscle tone and the stretch reflex)
Beta-motoneurone = innervate intrafusal muscle fibres (muscle spindles)
Gamma-motoneurone = innervate intrafusal muscle fibres (muscle spindles; also regulate the sensitivity of the muscle spindles to stretch)
Which types of motoneurones are activated first, and why? What is the advantage of this?
Ohm's law = defined excitatory input (current) to motor nucleus will produce a large voltage change on a cell body with a smaller diameter (lower cross-sectional resistance) than it would on a cell body with a larger diameter (higher cross-sectional resistance)
Therefore alpha-motoneurones are activated before gamma-motoneurones (alpha-motoneurones have a small cell body than gamma-motoneurones)
Henneman's size principle = motor units recruited in order of size, from smallest to largest
Slow twitch, low force, fatigue resistant muscle fibres ---> fast twitch, high force, less fatigue resistant muscle fibres
This minimises fatigue and allows fine control of force at all levels of output.
Which nerve fibres are affected first when gradually compressed? Give an example of a cause.
e.g. progressively growing ganglion (benign swelling of tendon sheath)
Heavily myelinated fibres affected first (proprioceptive fibres)
Initially presents as inability to carry out skilled fine movements
What is the MRC scale for grading muscle power?
5 = full and normal power against resistance
4 = can move against some resistance from examiner
3 = can overcome gravity but not resistance from examiner
2 = active movement possible with gravity eliminated
1 = contraction may be felt or seen but is ineffective (side-to-side)
0 = no muscle contraction
Reminder: what are the muscles and nerve roots for shoulder abduction?
Serratus anterior (C5)
Reminder: what are the muscles and nerve roots for shoulder adduction?
Pectoralis major (C5/C7)
Latissimus dorsi (C7)
Teres major (C5, C6, C7)
Coracobrachialis (C5, C6, C7)
Reminder: what are the muscles and nerve roots for elbow flexion?
Biceps brachii (C5/C6)
Reminder: what are the muscles and nerve roots for elbow extension?
Triceps brachii (C7)
Reminder: what are the muscles and nerve roots for wrist flexion?
Flexor carpi radialis (C6/C7)
Flexor carpi ulnaris (C8)
Reminder: what are the muscles and nerve roots for wrist extension?
Extensor carpi radialis (C6/C7)
Extensor carpi ulnaris (C7)
Reminder: what are the muscles and nerve roots for hip flexion?
Adductor magnus (L2/L3)
Adductor brevis (L2/L3)
Rectus femoris (L3/L4)
Reminder: what are the muscles and nerve roots for hip extension?
Gluteus maximus (L5/S1)
Adductor magnus (L2/L3)
Biceps femoris (S1)
Reminder: what are the muscles and nerve roots for knee flexion?
Biceps femoris (S1)
Reminder: what are the muscles and nerve roots for knee extension?
Rectus femoris (L3/L4)
Vastus medialis (L3/L4)
Vastus intermedius (L3/L4)
Vastus lateralis (L3/L4)
Reminder: what are the muscles and nerve roots for dorsiflexion?
Tibialis anterior (L4)
Extensor digitorum longus (L5/S1)
Extensor hallucis longus (L5/S1)