Basic Reflexes and Maintenance of Posture

Question 1

Describe the different levels of integration in the CNS.

Answer 1

Cortical level - highest level of neural activity
Subcortical - primitive actions (securing food + water, reproduction)
Spinal cord - homeostasis, reflex actions, simplest level

Question 2

Describe the typical reflex arc.

Answer 2

Sense organ - afferent neuron (via dorsal root) - control centre (spinal cord) - efferent neuron (via ventral root)

Question 3

Describe the reflex arc of muscle stretch. (stretch reflex).

Answer 3

• Depends on two variables: length and force
• Intramuscular receptors (length sensed by muscle spindle and force sensed by golgi tendon organ)
• Can be monosynaptic or more complicated
• Integrated by spinal cord
• Muscle contracts against the stretch
• Smooths and dampens muscle movement

Question 4

Describe how the body detects muscle stretch.

Answer 4

-Length transducer = muscle spindle (in parallel with muscle)
- Intrafusal fibres (lost most contractile proteins + wrapped in nerves)
- Afferent fibres from spindles = Type Ia (large, myelinated, fast-conducting)
- Type II fibres for tonic function of stretch reflex
- Gamma efferents for some contraction force (fusimotor system; efferents to dynamic nuclear bag, static nuclear bag, nuclear chain fibres; a + y co-activation; sets sensitivity of spindles; prevent spindle unloading; set baseline levels of alpha discharge and therefore muscle tone)
-Two main fibres in spindle
Sensing velocity (bag1 fibres, innervated by Type Ia nerves, fast-adapting to stimulus, phasic)
Sensing length (bag2 fibres, nuclear chain, innervated by slower-conducting Type II fibres, adapt slowly to stimulus, static response)

Question 5

Describe how force of muscle contraction is detected.

Answer 5

Force transducer = golgi tendon organ

• in series with muscle
• embedded within tendons
• fire in response to muscle contractile force - a lot of force needs to be put through muscle for tendon to stretch (collagenous fibres)
• synapse via inhibitory neuron on a fibre

Question 6

Describe the inverse stretch reflex.

Answer 6

• reflex inhibition of alpha output to muscle
• via an inhibitory interneuron, it reduces activity of motor neurons
• regulates force so it can be constant (e.g. grip)
• part of sophisticated control system sending info about force centrally
• enable us to control muscle force finely as well as length

Question 7

Give examples of some more complicated reflexes.

Answer 7

• Crossed extensor reflex
• Positive supporting reflex
• Spinal righting reflex
  (all function w/o supraspinal input)
• walking - central pattern generator in spine, alternate contraction of flexors + extensors

Question 8

How is standing possible lol

Answer 8

• Don’t need cortex
• Proprioception, pressure sensors in soles of feet, visual input, vestibular input
• Neck keeps head upright
• Trunk keeps C of G over the feet
• Leg muscle extensors active

Question 9

What happens to reflexes in UMN lesions?

Answer 9

increase