Lecture 22 - motor control

Question 1

Reflexes

Answer 1

Reflexes are organised neuronal circuits contained within the spinal cord - neurons do not necessarily need to travel to the brain in order to trigger actions
Produces reproducible, automatic responses to a particular stimulus
e.g. stretch reflex and withdrawal reflex

Reflex is all happening at the level of the spinal cord with a simple synaptic connection reporting to the brain, simple processing in terms of excitation or inhibition of muscles on opposite sides of a joint

Question 2

Reflex arc

Answer 2

A reflex arc is the nerve impulse conduction route to and from the CNS. The smallest portion of the nervous system that can receive a stimulus and produce a response

Question 3

Withdrawl reflex

Answer 3

1. Painful stimuli action potential up afferent pain fibres which synapse in the spinal cord
2. Signals go up to the brain to alert it of the pain, to other levels of the spinal cord to coordinate movement of other joints (e.g. movement away from the stimulus) and onto efferent motor fibres within that spinal level
3. Stimulation of flexors and inhibition of extensors allows for withdrawal away from the painful stimulus

Question 4

Stretch reflex

Answer 4

1. Muscle spindles/stretch receptors are stimulated when the muscle is stretched
2. This results in activation of nerve fibres and impulses are sent from muscle spindles to the spinal cord along afferent fibres
3. Information is processed at the level of the spinal cord synapsing onto an interneuron and then onto a motor neuron
4. Activation of the motor neurons causes innervation/contraction of the muscle in order to prevent overstretching

One function of the stretch reflex is to automatically hold a muscle at a desired length in the face of external forces.

Question 5

Clinical significance of stretch reflex

Answer 5

The stretch reflex Is important clinically because to is affected in specific ways in various neurological conditions. For example, stretch reflexes, are generally diminished in demyelinating diseases.

Question 6

Muscle spindle

Answer 6

Muscle spindles - located within muscles and are responsible for sensing the position of muscles and maintaining posture. Functions by sensing changes in stretch, especially abrupt changes like in the stretch reflex

Muscles have stretch sensors called muscle spindles. These are receptors that are activated (depolarised) by an increase in muscle length i.e. a stretch

Question 7

How does a muscle spindle send information about a stretch?

Answer 7

A muscle spindle send information about a stretch by initiating action potentials. These action potentials are conducted via sensory neuron icons in peripheral nerves through the dorsal root to the spinal cord. The sensory nerve axons are myelinated. The axons of these sensory neurons branch to send information to several other neurons including interneurons(which project up the spinal cord to higher centres, for conscious sensation and voluntary movement planning) and motor neurones in the ventral spinal cord. Information is passed to the interneurons and motor neurons via a synapse

The motor neuron axon projects to skeletal muscle via the ventral root and peripheral nerve. The motor neuron axons are myelinated. The specialised synapse between the motor neuron axon and the skeletal muscle = NMJ.

Question 8

What happens when a skeletal muscle is stretched?

Answer 8

Stretching the skeletal muscle activates the sensory axon endings in the muscle spindles, generating action potentials in the sensory axon. This leads to activation of the motor neuron, which in turn activates the skeletal muscle

Question 9

How is a muscle relaxed?

Answer 9

A muscle can be inhibited, you can stop the motor neurons that innervate that muscle from being active and that how it is done in the spinal cord to cause a muscle to relax

You make a muscle relax by turning off its motor neurons (it is not the brain that directly inhibits a muscle)

Question 10

Voluntary movement

Answer 10

Genrally these movements have a purpose and are conscious movements

Question 11

Control of muscle tension

Answer 11

Muscle tension is proportional to the amount of motor units being activated
Increasing the number of motor units activated will increase the tension
Increasing action potential frequency will increase duration that the tension is sustained
Each of the motor units can take turns being activated resulting maintenance of tension/tension modulation

Question 12

Preparing for voluntary movement

Answer 12

A decision is made in the frontal lobes when a movement is to occur
The message is then sent to the premotor cortex which communicates between the cerebellum and basal nuclei (if it is a goal directed behaviour then we consider it to arise most likely from the frontal cortex)(sent to the premotor cortex to start organising the times of muscle contractions, movements and specific orders of things we are going to do
The cerebellum is responsible for preparing and performing movement, coordinating muscles and is guided by sensory feedback (informed of goals wanting to be achieved by the premotor cortex to translate into motor output)
It compares intended movement with actual movement being carried out, maintains posture and gaze. The cerebellum learns and automates movement

Question 13

Cerebellum

Answer 13

The cerebellum - “little brain” - in preparing for and performing movement …
Coordinates muscles guided by sensory feedback
Compares intended movement with actual result
Helps maintain posture and gaze
Helps learn and automate movements

Question 14

Performing voluntary movement

Answer 14

The basal ganglia initiates patterns of movement
Cerebellum monitors balance and adjusts upper motor neuron activity depending on sensory input-comparing expectations with reality
Sensory input and feedback from muscle spindles provide info about body motion and position
Cerebellar deficits can cause ataxia (drunken gait) (also other symptoms characterised by drunk people such as slurred speech, unable to judge distance etc)

Motor cortex sends output to low motor neurons to recruit motor units as required either side of the joint, how many motor units are recruited on each side of a joint is determined by the cerebellum before this behaviour is generated.

Cerebellum monitors balance and equilibrium and adjusts upper motor neurons activity. Cerebellum compares sensory feedback about the actual movement to the intended movement, and signals adjustment to primary motor cortex.

Question 15

Describe the direct effect of the tap on the achilles tendon on the soleus muscle

Answer 15

The soleus muscle is stretched by the tap on the achilles tendon. If the muscle gets stretched to much, damage or imbalance could occur. To prevent/minimise this, it contracts reflexively. This reflex is initiated as the stretch receptors (muscle spindles) within soleus are stretched,

Question 16

Describe the direction of the foot movement that occurs as a result of the tendon tap. What must the soleus muscle have done to cause this movement?

Answer 16

Plantarflexion occurs.

Soleus contracts concentrically to cause this movement (the muscle shortens)

Question 17

Latency

Answer 17

Time delay of stimulus to response

Question 18

Reflex latency versus voluntary movement latency

Answer 18

For any given person their reflex latency should remain same across all trials. This is because the same neurons and number of synapses, essentially the same path are involved. However, the latency for the voluntary reactions will vary depending on how focused or distracted they are, how motivated they are to respond as fast as they can, or if they don’t really care. These variations will alter the number of neurons and synapses involved, thereby making the latency shorter or longer.

Question 19

Difference between ankle jerk pathway and the voluntary movement pathway

Answer 19

The ankle jerk reflex pathway includes the structures – muscle spindle, sensory neuron, spinal-cord, alpha motor neuron, soleus muscle, with a single synapse in the spinal-cord hence why it is called a monosynaptic reflex. There is also the neuromuscular junction which is a synapse and a part of the reflex. The length is measured from the slightest muscle, up the limb to the spinal-cord then back down to the soleus

The voluntary path includes all of the above except not the muscle spindle but instead a sensory touch receptor, plus extra sensory neurons, any number of neurons in the brain including those in the somatosensory and motor cortes, and primary motor neuron. Any number of synapses could be included depending on the participants focus and motivation. The length is measured from the point of tap, up the limb, the back, and neck to the top of the head and back down to the Soleus muscle.

Question 20

Outline at least two reasons for any difference in latency between reflex and voluntary responses…

Answer 20

Persons reflex will always be faster as there is a shorter latency than the voluntary movement because the path of the action potentials will be shorter. And reflects of the action potential is initiated in the sensory neuron attached to the stretched muscle spindle then conducts up the sensory axon onto the spinal-cord, where it synapses with a motor neuron. Action potential is initiated in the motor neuron, then conducts down that axon to the soleus muscle.

In voluntary reaction movements the action potential is initiated in the sensory touch neuron at the foot or where ever the person is tapped, then conducts up the sensory axon into the spinal-cord and up to the medulla we are at synapses with a secondary sensory neuron, initiating another action potential to conduct along that axon, to synapse with a tertiary sensory neuron in the brain. Can you synapses and other neurons can be involved, from here depending on the focus or motivation of the participant. Eventually an action potential will be initiated in a primary motor neuron, then conduct down the axon to synapse with an alpha motor neuron in the spinal cord. Potential initiated in the Alpha motor neuron will conduct down the axon to the soleus muscle to initiate a contraction

Question 21

Common reflex examples

Answer 21

Coughing 
Shivering 
Withdrawl reflex 
Startle response 
Tendon reflex 
(a reflex is reproducible)

Question 22

Path length

Answer 22

The distance that an action potential has to travel to get the stimulus to the brain

Question 23

Voluntary vs reflex - path length, latency and scatterplot would show…

Answer 23

Reflex
Shorter path lengths (soleus to spinal cord and back again)
Shorter/faster latencies
Much more densely clusters = less variation for both path length and latency

Voluntary movement
Longer path length (foot to brain and back again)
Longer/slower latencies (takes more time because longer distance and more synapses)
Less densely clustered = more variation for both path length and latency

Question 24

Shorter vs longer person in terms of ankle jerk reflex

Answer 24

Shorter leg means faster ankle jerk reflex, shorter latency compared to a taller person. Consider the path length, from the activated stretch receptors in the soleus muscl, along the sensory axon, into the spinal cord and back along the motor icon to the gastrocnemius muscle. Shorter legs have a shorter distance for action potentials to travel and therefore have a faster ankle jerk reflex due to a shorter latency