Neural control of animal locomotion

Question 1

What does our nervous system do (in terms of locomotion)?

Answer 1

Most basic structure of a controller:

Initiate movement 
           ↓
motion programme → activate muscles 
               ↑                               ↓
           check if everything okay

Question 2

How do the mechanics of locomotion provide a intergrative perspective?

Answer 2

Mechanics and sensing & control
Whole body dynamics - physical environment and joint forces
Limb skeletal function - muscle length & velocity
Muscle tendon interactions - muscle tendon force
Muscle contraction - Neuromuscular activation

Question 3

What are the special senses?

Answer 3

Vision

Vestibular/balance

Question 4

What are the functions of the brain in the central nervous system?

Answer 4

Volition
Internal model “map”
Planning
Initiation

Question 5

What are the functions/where are the functions programmed in the spinal cord?

Answer 5

Functions:
- GPG's 
- Muscle synergies 
Where:
- motor neurons 
- sensory neurons 
- interneurons
- reflexes 
- ascending tracts

Question 6

What are the functions/where are the functions programmed in the peripheral/neuromuscular nervous system?

Answer 6

Functions:
- motor neurones - motor units 
- excitation-contraction dynamics 
- intrinsic muscle dynamics 
Where:
- sensory nerves from:
- muscle spindles 
- golgi tendon organs 
- proprioceptors

Question 7

What are the mechanical functions produced by the feedback loop?

Answer 7

Skeletal dynamics
Muscle - tendons interactions
Limb - substrate interactions

Question 8

What are reflexes at the spinal level?

Answer 8

Reflexes are simple sensori-motor loops

Question 9

What is a mono-synaptic reflex?

Answer 9

Only one synapse produces a response in one efferent fibre (muscle)
- i.e. the afferent fibre only has one branch

Question 10

What is a poly-synaptic reflex?

Answer 10

Multiple synapses from one afferent fibre which can produce responses in many branches of the efferent fibre
- i.e. from a sense organ to excite a muscle/to inhibit an antagonistic muscle

Question 11

What is a motor unit?

Answer 11

Functional units of muscle activation

• group of muscle fibres
• certain group of muscle fibres only activates one cell body and can only produce a certain response

Question 12

What delays can slow down the response of a control process?

Answer 12

Sensing delay 
Force generation delay 
Electromechanical delay 
Neuromuscular junction delay 
Nerve conduction delay synaptic delay

Question 13

What are the largest and most varied delays?

Answer 13

Conduction and force generation

Question 14

What is the time of the delay caused by sensorimotor loop and wingbeat frequency in a moth?

Answer 14

Sensorimotor loop = 7-8Hz (130ms)

Wingbeat frequency = 28Hz (36 ms)

Question 15

What is the transmission/conduction delay?

Answer 15

Time for neural signal to travel from the CNS to the motor end plate

Question 16

How to calculate the transmission delay?

Answer 16

Transmission distance/conduction velocity

Question 17

What factors affect conduction velocity?

Answer 17

Myelination 
Nerve diameter (d)
Excitation-coupling delay (electromechanical delay)

Question 18

How to calculate conduction velocity?

Answer 18

Conduction velocity (cv) = k√d

Question 19

What is the excitation-coupling delay (electromechanical delay)?

Answer 19

Time taken from the nerve signal reaching the muscle to development of force
- or from cessation of the neutral signal to the relaxation of force

Question 20

What are the functional consequences of delay?

Answer 20

Must anticipate mechanical events and activate muscles in advance

• important consequences for:
• mechanisms of control
• changes with speed of locomotion
• changes with body size

Question 21

What is feedforward control?

Answer 21

Predictive:
- like playing a record of the motor pattern, requires no sensory feedback
- Central Pattern Generators (CPG’s), rhythmic spinal circuits, can generate rhythmic locomotor commands without sensory inputs
Brain - CGP - motor neuron - extensor/flexor

Question 22

What is feedback control?

Answer 22

Reactive:
- use sensory inputs to shape and trigger motor outputs
- reflex pathways from muscle spindles, golgi tendon organs, cutaneous, joint receptors
- sensory input from vision, vestibular system and other special senses
Brain - CGP - motor neuron - extensor/flexor (feeds back to the CPG)

Question 23

Why does feedback control become slow at high speeds, what results because of this?

Answer 23

Less time to react to a change e.g. a false floor as an animal runs over it and therefore the animal must rely on its intrinsic mechanics of the musculoskeletal system to react to this change, i.e. extend the leg further than normal to prevent a fall

Question 24

What mechanism do larger animals rely on?

Answer 24

Feedforward and intrinsic mechanical mechanisms of control