Lecture 9 - proprioception and spinal reflexes

Question 1

Proprioception

Answer 1

• Sense of body position including kinaesthesia = sense of motion
• Based on mechanoreceptors (activated by touch and stretching) in joints, muscles and tendons
• Rarely viral infection leads to auto-immune attack against sensory nerves
• Pins and needles = lose sensation and when release blood starts again = overwhelmed response of sensory feedback

Question 2

Joint receptors

Answer 2

• Free endings in joint capsule and surrounding tissue
• Many signal the extremes of joint angles
• Minimally important = lots of people with artificial joints who have no receptors
• Dangerous extremes of motion

Question 3

Golgi tendon organ

Answer 3

• In tendon
• Lie in series within muscle
• Detects tension in tendon/muscle
• Di-synaptic reflex arc
• Activated by passive stretch or by active muscle contraction
• Relaxes muscles
• Made up of collagen fibres

Question 4

Muscle spindles

Answer 4

• Within muscle
• Parallel to muscle
• Myelin sheath
• Own muscle fibres = weaker, measure stretch of muscle
• Fusiform = spindle shapes
• Spindle contains intrafusal muscle fibres and associated with nerve endings:
• ->“1a” sensory axon = different axon to CNS
• -> Specialised motor neuron = gamma motor neuron (efferent axon from CNS)
• Intrafusal muscle fibres are intermixed between extrafusal muscle fibres

Question 5

Length feedback

Answer 5

• Reflexes stabilise posture and maintain active position
• Can over ride them through motor cortex (and other centres) to produce fluid movement
• Monosynaptic reflex arc
• Reflex has latency circa of 40ms = too quick for brain involvement

Question 6

Tendon tap reflex

Answer 6

• When tap tendon in lag = causes knee to flex upward
• When tap causes muscle fibres to contract = spindle sends signal to spinal chord = tell motor neuron leg being stretched = extrafusal muscle fibres contracts

Question 7

Gamma regulation of spindle sensitivity

Answer 7

• Gamma motor neuron controls spindle length
• Sets the “reference” value for feedback control
• Changes the sensitivity of the spindle
• Determines position

Question 8

Gamma motor neuron

Answer 8

• Alter the length of the intrafusal muscle fibres and there by their resting tension
• Can bias spindle towards static or dynamic sensitivity
• Regulate sensitivity during contraction
• Important in the regulation of muscle tone
• Spasticity (spinal cord injury, cerebral palsy)
• -> Pathological increased muscle tone (rigidity) because of excessive stretch reflex = cerebral palsy

Question 9

Reciprocal innervation

Answer 9

• The signal doesn’t just innovate motor neuron on that muscle, goes elsewhere = some motor neurones will activate antagonist
• When activate muscle want to relax antagonist

Question 10

Supraspinal pathways

Answer 10

• Spinally organised reflexes control a lot of posture etc
• But almost all spinal inputs are also sent up to brain and stem
• Spinal reflexes and CPG lie on a hierarchy of descending control

Question 11

Central pattern generators

Answer 11

• Surgically detach cortex from stem and spinal chord = thought that…
• …By having interconnected circuits that alternate between extensor and flexor muscles = can generate walking pattern
• Found it happened
• Spinal chord can drive walking
• If step on pin = withdraw foot = counterbalance weight and change posture