Spinal cord, reflexes and muscle tone

Question 1

What are the 4 main types of movement?

Answer 1

• voluntary
• rhythmic
• reflexive
• postural

Question 2

Voluntary movement

Answer 2

• most complex, least automatic - originate from frontal lobe
• primary motor, premotor and prefrontal regions all contribute along with basal ganglia, cerebellum, brain stem and spinal cord

Question 3

Rhythmic movement

Answer 3

• Some basic patterns such as walking and breathing can be initiated and maintained by subcortical structures such as brain stem and spinal cord - coordinated by central pattern generators
• other more complex patterns require cerebral cortex input

Question 4

Reflexive movement

Answer 4

• simplest motor response is reflex arc
• consists of sensory and motor nerve, plus intervening synapse
• rapid and automatic responses to particular sensory stimuli - vary in speed, magnitude or direction
• some reflexes are more complex, involving multiple nerves and synapses extending over several spinal cord segments (limb withdrawal reflex)

Question 5

Postural

Answer 5

• small postural adjustments are continuously being made - detected by vestibular system and proprioceptors
• responses are mediated by descending projections from brain stem to spinal cord
• automated stereotyped responses to unexpected changes in body position (lunges on train) - help keep upright

Question 6

Central pattern generators

Answer 6

• coordinate some semi-automatic actions like walking and breathing
• can be selected and recruited by descending projections but can work independently and autonomously
• projections from brain also alter activity of CPGs to change patterns - e.g. different gait patterns in horses

Question 7

Direct and indirect cortical projections

Answer 7

• cerebral cortex can generate movements directly using corticospinal tract
• or indirectly using basal ganglia, brain stem, cerebellum and spinal cord - influence movements, posture and muscle tone
• projections to reticular formation influence descending pathways (reticulospinal pathways) - contribute to reg. of normal muscle tone

Question 8

Role of the cerebellum

Answer 8

• doesnt generate movements itself - advises motor areas of frontal lobe - gives smooth precise movements
• damage leads to clumsiness and poor coordination
• Frontal lobe > pons > contralateral cerebellum
• gives information about planned movements and receives from proprioceptors, inner ear, eyes etc.
• compares planned movements with actual movements and detects discrepancies - feedback to frontal lobe
• this process is only suitable for slow movements that have time for slower feedback - non-ballistic
• ballistic movements are much quicker - require on-line corrections

Question 9

Role of basal ganglia

Answer 9

• connections between motor cortex and basal ganglia are a set of loops - frontal lobe > basal ganglia > thalamus > cortical region of origin
• Controlled by DA
• involved in initiation of vol. actions, selection of a particular action among a range, learning and performance of habits
• disease is associated with movement disroders - parkinsons. However also involved in cognition, beahviour and emotion (non-motor loops)

Question 10

Spinal cord - overview and development

Answer 10

• derive from hollow, fluid-filled neural tube
• divides into anterior and posterior region
• anterior is basal plate - contains motor neurons - grows out to make ventral (motor) roots
• posterior is alar plate - sensory - grow out to form dorsal roots - cell bodies in dorsal root ganglia

Question 11

Internal structure of spinal cord

Answer 11

• H shaped grey matter (nerves and processes) - dorsal and ventral horns
• Surrounded by white matter (axons and myelin) - divided into 3 columns (funiculi) by horns. funiculi contain ascending and descending pathways
• cross sections - cervical has most white matter (all pathways present). cervival/lumbosacral most grey matter (varies in proportion to muscle)

Question 12

Spinal cord laminae

Answer 12

• grey matter divided into zones (laminae)
• I-VI = dorsal horn
• VII and X - mid region
• VII and IX = anterior horn
• LMNs (anterior horn) bodies arent in defined layers - in columns (IX)
• each column in IX supplies a particular functional muscle group (e.g. forearm flexors/extensors) - usually span across multiple spinal segments (e.g. biceps = C5/6)
• orgnaised in orderly manner, proximal muscles at midline, distal limbs are lateral, flexors at back and extensors at front

Question 13

Muscle spindle and stretch reflex

Answer 13

• muscle spindles are stretch detectors - are stretched whenever muscle belly is under tension
• sensory endings respond to velocity of change
• when muscle is stretched, spindles are excited, triggering reflex contraction of same muscle group, reisting changes in muscle length
• antagonist muscles are reciprocally inhibited

Question 14

Abnormal reflexes and tone

Answer 14

• stretch reflex is damped down by descending influences to ensure muscle tone isnt excessive
• lost in UMN lesions - stretch reflex very strong (hyperreflexia) and excessive tone (hypertonia)
• increased tone is velocity-dependent - more resistance felt when joints are flexed and extended rapidly
• in LMN lesions - flaccid paralysis of affected muscle with absence of tone (atonia) and loss of GTO reflexes (areflexia). followed by gradual wasting

Question 15

Causes of MN lesions

Answer 15

• UMN = stroke -> clonus, spasticity, clasp-knife rigidity and pyramidal posture
• LMN = peripheral neuropathy and anterior horn disease
• MN disease = both affected

Question 16

Withdrawal reflex

Answer 16

• automatic limb withdrawal from noxious stimuli
• flexor reflex = polysynaptic and extends over multiple segments
• cutaneous reflex is triggered by skin stimulation
• coupled to crossed extensor reflex - triggered simultaneously to support body and prevent falling if weight has to be transferred