Lecture 21: Motor Circuits and Spinal Reflexes

Question 1

Reflexes are not reactions

Answer 1

Catching a cricket ball = reaction, not a reflex

Question 2

Examples of reflexes =5

Answer 2

1 * Knee jerk/ patella tendon reflex

2 * Pupillary light response

3 * Blink reflex

4 * Middle ear muscle reflex

5 * Foot withdrawal

Question 3

Spinal reflex pathway

Answer 3

1. Stimulus energy
2. Sensory Transduction
3. Neural Code: (sensory afferent AP firing rate)

2 way:

4. Reflex pathway through spinal cord
5. Muscle Activity
6. Behaviour

or
4. Sensory cortex
5. Sense of limb position etc.
6. Motor cortex
7. Voluntary movement pathway
8. Muscle Activity
9. Behaviour

Question 4

Synapses of sensory neurons

Answer 4

Sensory neurons synapse with motor neurons and interneurons in ventral horn

Also relayed to cortex via
ascending pathways

Question 5

Relationship between muscle
spindles and golgi tendon organs

Answer 5

Relationship between muscle
spindles and golgi tendon organs

together reflexively work to regulate MUSCLE STIFFNESS

The function of the
GTO can be considered opposite of the muscle spindle, which serves to produce muscle contraction.

When people lift weights, the golgi tendon organ is the sense organ that tells how much tension the muscle is exerting. If there is too much muscle tension the golgi tendon organ will inhibit the muscle from creating any force (via a reflex arc), thus protecting the you from injuring itself.

Question 6

Muscle spindle =3

Answer 6

1. Intrafusal fibres wrapped in stretch
   sensitive type
   - Ia and type II afferent
   nerve endings
2. Parallel to larger extrafusal muscle fibres
3. Small fibres – don’t contribute significantly to muscle force

Question 7

The monosynaptic stretch reflex

Answer 7

• Two neuron circuit: single excitatory
  synaptic connection between Ia afferent from a muscle spindle and an
  alpha motor neuron
• Provides a NEGATIVE FEEDBACK
  mechanism for regulating muscle length
• Ia is a large diameter nerve, fast conduction velocity
• Single synapse means a SHORT SYNAPTIC DELAY

Ia afferent muscle spindle to alpha motor neuron

Question 8

Muscles don’t act in isolation

Answer 8

1. Reflex pathways involve agonist muscles and
   antagonist muscles
2. Synergistic muscles are often involved also
3. Involvement of inhibitory interneurons is common

Question 9

Muscle spindle sensitivity to stretch is modulated by gamma motor neuron… increase or decrease in ap firing and why

Answer 9

Increase in firing rate due to stimulation of gamma motor neuron

Question 10

Muscle spindle sensitivity to stretch is modulated by gamma motor neurons

Answer 10

Gamma motor neuron will prevent complete SLACKENING of spindle so that the afferent is sensitive to length change
even at a short muscle length

Question 11

Muscle spindle sensitivity to stretch is
modulated by gamma motor neurons

explain the 9 steps

Answer 11

1. descending facilitation and inhibition
2. Alpha motor neuron
3. motor neuron firing
4. muscle
5. force
6. Load <— disturbance
7. Length change —>
8. Spindle
   (GAMMA MOTORNEURON)
9. Spindle afferent discharge

Question 12

Proprioceptive receptors: Golgi tendon organ (3)
-LOOKS, CONTRACTS, RESPOND TO

Answer 12

1 * Nerve endings are interwoven between collagen fibres

2 * Muscle contracts → collagen fibres pulled → nerve endings compressed → ACTIVATES MECHANOSENSITIVE RECEPTORS

• Respond to MUSLCE TENSION (FORCE)

Question 13

The inverse myotactic reflex (2)

Answer 13

• Occurs when a high load is unexpectantly applied to the muscle, causing relaxation of the agonist muscle
• Involves an inhibitory
  interneuron

Question 14

The inverse myotactic reflex…WHAT, FEEDBACK, WHY

Answer 14

1. The Golgi tendon organ mediates an important negative feedback mechanism for regulating muscle tension
2. Excessively high muscle force development can cause damage to muscle tissue

Question 15

Multiple synaptic connections to one neuron is common

Answer 15

Remember: MULTIPLE NEURONS can SYNAPSE onto A SINGLE NEURON

The COMBINATION of INFLUENCE from OTHER NEURONS SYNAPSING ONTO onto the ‘1b INHIBITORY’ INTERNEURON will DETERMINE if ACTION POTENTIALS are GENERATED IN THE NEURON

Question 16

Renshaw Cell =5

Answer 16

1. Involved in REGULATION of MOTOR ACTIVATION
2. Is an iINHIBITORY INTERNEURON
3. Has COONECTIONS to MOTOR neurons and INTERNEURONS
   thus REGULATING MUSCULAR ACTIVTY
4. STABILISES MOTOR NEURON activity by COUNTERACTING LARGE CHANGES IN FIRING RATE
5. The ACTIVITY of Renshaw cells can be MODIFIED by DESCENDING INPUTS.

Question 17

Spinal cord circuitry can generate coordinated movement patterns

Answer 17

REFLEXPATHWAYS can be COMPLEX with MULTIPLE SYNAPTIC CONNECTIONS ….Spinal cord circuitry can generate coordinated movement patterns

Foot withdrawal reflex:
* Interneurons underly response in the opposite limb
* Response is opposite in the contralateral limb

Similar to stepping reflex present in newborn babies

Question 18

Foot withdrawal reflex:

Answer 18

• Interneurons underly response in the opposite limb
• Response is opposite in the contralateral limb

Question 19

Descending input from higher motor centres can modify sensitivity of reflexes

Answer 19

1. Neurons receive descending EXCITATORY and INHIBITORY INPUTS that can ALTER MOTOR NEURON RESPONSE
2. Need to DE-SENSITISE RELEXES in some instances e.g.
   HOLDING SOMETHING FRAGILE
3. DAMAGE to these neurons can lead to HYPERSENSITIVITYof RELEXES RESPONSES

Question 20

Summary
- What
- Complex or simple?
- interneurons?
- complex circuits?
- Spinal cord?

Answer 20

1 * Reflex pathways are formed by SENSORYNEURONS MAKING DIRECT OR INDIRECT CONNECTIONS WITH MOTOR NEURONS.

2 * Reflexes can be SIMPLE MONOSYNAPTIC pathways or COMPLEX MULTISYNAPTIC PATHWAYS

3 * INHIBITORY INTERNEURONs are COMMONLY INVOLVED in reflex pathways

4 * COMPLEX CIRCUITS can PRODUCE RECIPROCAL ACTIONS ON OPPOSINGMUSCLE GROUPS RESULTING IN COMPLEX MOVEMENT PATTERNS

5 * SPINAL CORD CIRCUITARY is MODULATED by HIGHER BRAIN CENTRES