Skeletal Muscle Reflexes

Question 1

What does control of the skeletal muscle relie on?

Answer 1

• contractile apparatus
• efferent signals to the muscles to cause contraction
• Afferent sensory feedback to help coordinate motor tasks = reflexes

Question 2

What does continuous feedback of sensory signals give information about?

Answer 2

• length of the muscle
• instantaneous tension
• speed of change & length of tension

Question 3

The reflex arc: unconscious

Answer 3

1. Sensory receptor
2. Sensory nerve (Afferent) = carries action potential
3. Sensory nerve cell body = dorsal root ganglion
4. Alpha-motor neurone
5. Motor nerve (efferent) = carries action potential
6. Muscle contracts

Question 4

Why is the stretch reflex the simplest in the body?

Answer 4

• only involves one nerve-nerve connection
  = one synapse
  = monosynaptic reflex arc

Question 5

What is the stretch reflex activated by?

Answer 5

• increases in muscle length
• by passive stretch

Question 6

What does the stretch reflex lead to?

Answer 6

• contraction in the stretched muscle

Question 7

What is the stretch receptor important for?

Answer 7

• important in controlling resting tone in muscles
• specifically extensor muscles of the lower limb, as they resist gravity

Question 8

What is the sensory receptor for the stretch reflex?

Answer 8

• muscle spindle

Question 9

What is a muscle spindle?

Answer 9

• specialised muscle cell which runs parallel to the normal fibres

Question 10

The process of the stretch reflex:

Answer 10

• action potentials are conducted to the spinal cord via an Afferent nerve
• they synapse with the alpha-motor neurone, to the stretched muscle
• excitatory neurotransmitter is released
• this produces an action potential in the motor axon
• which conducts it back to the neuromuscular junction
• muscle contraction results
• also influences muscles other than those initially stretched
• the sensory nerve has branches in the spinal cord, they synapse with another set of neurones = renshaw cells
• these are connecting cells, synapse with alpha-motor neurones which supply muscles antagonistic in action to the stretched muscle
• interneurons = inhibitory = contraction in these antagonists is inhibited
• this reduces the degree of stretch in the muscle where the reflex starts from

Question 11

How the muscle spindle retains the sensory output:

Answer 11

1. Alpha motor-neurone causes an extrafusal muscle contraction
2. Muscle shortens v, the spindle slackens
3. Intrafusal fibres contract via y-motor neurones
4. The sensory output is maintained

Question 12

What are the Golgi tendon organs?

Answer 12

• another set of stretch receptors attached in series with the muscle fibres of the muscle, located within the tendon

Question 13

What activates the Golgi tendon organs?

Answer 13

• increasing muscle tension, by a stretch or contraction

Question 14

Where is the sensory Afferent connected to in Golgi tendon organs?

Answer 14

• connected by an inhibitory interneurons within the spinal cord to the alpha-motor neurones, that supply the same muscle

Question 15

What is the response of the reflex?

Answer 15

• protective
• reducing muscle tension by relaxing it, before permanent damage to the muscle or tendon results

Question 16

Process of the Golgi tendon organ reflex:

Answer 16

1. Stretch / contraction stimulates the Golgi tendon organ
2. Afferent signal to the spine
3. Synapses with 1. Inhibitory interneurone 2. Excitatory interneurone
4. Inhibits motor activity to stimulated muscle, excites the antagonist muscle
5. The stretched muscle relaxes

Question 17

What is the crossed extensor reflex?

Answer 17

• induced by painful stimuli, experienced at a conscious level
• a protective polysynaptic spinal reflex

Question 18

What is the stimulus for the crossed extensor reflex?

Answer 18

• pain affecting a limb

Question 19

What is the response of the cross extensor reflex?

Answer 19

• flexion of affected limb
• extension of opposite limb

Question 20

What does the stimulus cause in the affected limb?

Answer 20

• contraction of flexors
• relaxation of extensors

Question 21

In the opposite limb what does the stimulus cause?

Answer 21

• relaxation of flexors
• contraction of extensors

Question 22

What does the crossed extensor reflex involve?

Answer 22

• interneurons crossing the midline of the spinal cord
• can be modified consciously

Question 23

Example of the withdrawal reflex:

Answer 23

• if you touch a hot dish, you pull your hand away reflexly
• due to the excitation of the flexor muscles in that arm by polysynpatic connections between the sensory nerves from the pain receptors
• and the alpha-motor neurones
• other branches of sensory nerves = connect to inhibitory interneurons to the alpha-motor neurones supplying the extensor muscles in the arm
• these relax, leaving flexion to happen

Question 24

What connections are made to the opposite arm?

Answer 24

• further connections
• give rise to the crossed extensor reflex

Question 25

Crossed extensor reflex is a mirror image to what?

Answer 25

- withdrawal reflex - extensors are activated and flexors are inhibited

Question 26

What does the crossed extensor reflex to after the withdrawal reflex?

Answer 26

- follows half a second later - causes a rapid strengthening of the opposite arm - pushes the whole body away from the source of trouble

Question 27

How does overall motor control work? THE SUMMARY:

Answer 27

- cortex plans the desired movement - transmitted down the descending tract - after the synapse, the alpha-motor neurones which supply carries the signal to the effector - the muscle contracts - changes in muscle tension or length stimulate receptors - sensory nerves transmit the information to the spine - they synapse with local circuitrary and ascending tracts - in the brain, the information is perceived, compared, evaulated and integrated - original command is modified over & over again