Yuste C11 - Reflexes

Question 1

Difference between the lower and upper nervous system

Answer 1

Voluntary (cortically initiated) vs involuntary (spinal reflexes) movement

Question 2

Purpose of the nervous system

Answer 2

To make the animal move. And to generate and control movements in a way that is intelligent.

Question 3

How does our brain generate most movements?

Answer 3

Top down - motor cortex is involved in planning, initiating, and directing voluntary movements. The basal ganglia and cerebellum help with this. Motor commands are then sent through he brainstem into the spinal cord.

Question 4

Extrapolating the logic of lower and upper NS

Answer 4

Motor cortical areas are involved in decision making and awareness and must generate some idea of the self.

Question 5

What does the upper motor system involve?

Answer 5

The cortex, basal ganglia, and cerebellum

Question 6

What does the lower motor system involve?

Answer 6

The spinal cord and the brain stem. Focuses on reflex and locomotion.

Question 7

What is a reflex?

Answer 7

Involuntary response to a sensory stimulus.

Question 8

Role of the spinal cord in locomotion

Answer 8

S.C has sensory inputs and motor neurons; these sensory-motor loops generate motor reflexes locally in S.C.
S.C is partly reflexive and partly top-down-controlled.

Question 9

How do spinal reflexes implement control theory?

Answer 9

Use feedback.

Question 10

Locomotion is mostly build by what?

Answer 10

by circuits => Central Pattern Generators

Question 11

What do skeletal muscles do?

Answer 11

Move the bones and different parts of our body by contracting or relaxing.

Question 12

How are muscles activated

Answer 12

By the firing of motor neurons in the ventral horn of the S.C.

Question 13

How are muscles divided

Answer 13

They are divided into motor units, which are all the muscle fibres that a particular motor neuron innervates.

Question 14

Muscle fibres

Answer 14

In a motor unit, not next to each other because axons from a motor neuron branch in the muscle. This results in force exerted by the contraction of all the muscle fibres being spread out through the entire muscle, generating a smoother movement.

Question 15

Duration and the strength of the contraction of a muscle

Answer 15

Depends on the motor neuron you activate

Question 16

Three types of muscle fibres

Answer 16

• Slow
• Fast fatigue-resistant
• Fast fatigable

Question 17

Slow fibres

Answer 17

Slow to respond, generate relatively little force, do not get tired.

Question 18

Fast fatigue-resistant fibres

Answer 18

Fast to act, generate more force, resist fatigue.

Question 19

Fast-fatigable fibres

Answer 19

Fast to act, generate a lot more force, but tire out quickly.

Question 20

Why do we need all these types of muscle fibres?

Answer 20

For different types of behaviour, need to recruit different proportions of muscle fibres.

Question 21

Which fibres we use when we exercise

Answer 21

We first engage the slower fibres, then the fast fatigue-resistant, leaving the muscle fibres that fatigue quickly until the end.

Question 22

Ordered recruitment

Answer 22

Idea that motor neurons and their motor units are recruited in order, only using the fast and fatiguable fibres for the most strenuous behaviours. This idea is known as the size principle because it correlates with the size of the motor neuron and the motor unit.

Question 23

When the human body evolved to adapt to hunter-gatherer lifestyles

Answer 23

Paleolithic period. Humans have ability to ride on our slow or fatigue-resistant motor units for trekking - outlast prey.

Question 24

Sheerington’s contribution to reflexes/spinal cord

Answer 24

Argued that the motor neurons in the spinal cord were the “final common path” - all behaviour gets funnelled through the activity of these neurons.
Alpha motor neurons.

Question 25

Alpha motor neurons

Answer 25

Motor neurons in the S.C. Largest and located in the ventral horn.

Question 26

How are ventral motor neurons related to columns?

Answer 26

The ventral horn motor neurons associated with a. particular muscle are in columns that run up and down the spinal cord. The columns are elongated clusters of neurons that innervate the same muscle.

Question 27

Topography of muscle innervation in the spinal cord

Answer 27

Neurons innervating the musculature that is axial (closer to the centre of the body) are located on the medial part of the ventral horn on the S.C. Muscles that control the limbs (more lateral) are innervated by neurons that are located on the lateral ventral horn.

Question 28

Local connectivity within the spinal cord

Answer 28

Projections on the ipsilateral side, but motor neurons are decussate to innervate the contralateral side. Local connectivity builds motor reflexes (among other unknowns).

Question 29

Proprioceptive system

Answer 29

Monitors the position and tension of all of our muscles and tendons and provides real time info on the state of our skeleton and body.

Question 30

Axons that carry proprioceptive info

Answer 30

The thickest and most myelinated of axons = the fastest.

Question 31

Muscle spindles

Answer 31

Sit in the middle of muscle fibres. Have intrafusal muscle fibres; group A axons wrap themselves around them with stretch receptors in the membrane of their nerve endings. Monitored by proprioceptive sensory axons.

Question 32

How are APs in the group A axons sent to the S.C

Answer 32

When the muscle contracts, the intrafusal fibres also contract, generating APs in the group A axons.

Question 33

Some implications of axons monitoring stretch, length etc.

Answer 33

Your NS keeps careful track of the position, length, and forces of all of your muscles

Question 34

Sherrington’s major contribution to neuroscience

Answer 34

Explaining how reflexes work.

Question 35

Monosynaptic reflexes

Answer 35

Involve a single synaptic connection in the spinal cord, a connection from a sensory neuron to a motor neuron. AKA “stretch,” “deep tendon,” or “myotonic”.

Question 36

When the tendon is hit (monosynaptic reflex) what happens?

Answer 36

The patellar tendon pulls and stretches the quadriceps. which activates and fire the proprioceptive axons in its muscle spindles. These axons enter the dorsal horn of the spinal cord and go down to the ventral horn, where they contact the motor neurons that activate the quadriceps, causing the muscle to contract and your leg to pull up. The proprioceptive axons also contact inhibitory neurones act inhibit the antagonist leg muscle, the biceps.

Question 37

Why do we have stretch reflexes? Like the patellar tendon reflex

Answer 37

These reflexes are some of the many built-in mechanisms to ensure that muscle function is smooth and coordinated. The knee-jerk reflex probably avoids body imbalances created by sudden changes in posture or in terrain.

Question 38

What is the neural circuitry underlying the knee-jerk reflex?

Answer 38

Feedback loop. Control theory = a set point (desired length of the muscle) and a feedback signal (proprioceptive input). New input generates a change in the system that compensates for it and maintains the system state. Cf. homeostasis.

Question 39

Flexion-extension reflex

Answer 39

if you step on a pin, you immediately pull back (flex) your leg and straighten/extend your other leg so that you don’t fall.

Question 40

Pathway of flexion-extension reflex

Answer 40

Afferent sensory neurons carrying the pain info from the foot enter the SC via the dorsal root to synapse onto interneurons, which project to motor neurons that activate the quads. You then flex and lift your leg. Simultaneously, these interneurons make synapses on the contralateral side of the SC, contacting the motor neurones that activate the biceps on the other leg. So you extend and straighten that other leg.
=> Excitatory connections.

Question 41

Inhibitory connections of the flexion-extension reflex

Answer 41

Ipsilatoral and contralateral SC segment have inhibitory connections that inhibit the antagonist muscles (biceps of the leg you’re lifting and quads of the leg you’re straightening).

Question 42

What is the problem with Sherrington’s reflex model

Answer 42

It doesn’t account for behaviour that occurs spontaneously, without sensory inputs.

Question 43

Locomotion

Answer 43

A result of organising muscle activity in terms of space and time.
The flexor and extensor are activated antagonistically in different limbs at different times, and this pattern changes depending on whether the animal is walking, trotting, pacing or galloping.

Question 44

Brown’s experiments

Answer 44

Cut all the sensory inputs into the SC by sectioning the dorsal root ganglion nerves. Cut the connection from the brain to the SC, leaving the SC completely isolated, with motor neurons coming out of the SC and no sensory neurons coming in.

Question 45

Brown findings

Answer 45

His cats were still able to walk even though they had no sensory inputs. Proposed that the SC itself can generate a pattern of firing that is spontaneous and cyclical and occurs regardless of the sensory input. Generates locomotion.

Question 46

Connection between motor neurones and interneurons

Answer 46

Motor neurons that correspond to a given muscle on one side fo the body connect with interneurons hat inhibit the same muscle on the contralateral side.

Question 47

What happens when inhibition ends?

Answer 47

There is rebound excitation that happens automatically.

Question 48

What is rebound excitation

Answer 48

Has to do with a property of sodium channels (release from their inactivation) which brings the neuron back to AP threshold. This can be aided if there are background excitatory inputs depolarising the neuron. These can be unspecific.

Question 49

What happens when inhibition ends?

Answer 49

The contralateral flexor motoneuron fires, which inhibits the ipsilateral flexor motorneuron, which then fires later, etc. Crossed reciprocal inhibition within the SC and the small delay caused by the inhibition until the rebound excitation kicks in => movement to alternate between sides.

Question 50

What is central pattern generator

Answer 50

A neural circuit that generates spontaneous rhythmic activity.

Question 51

Hans Berger experiemnt

Answer 51

Using electroencephalography on his young son, discovered that when his eyes were closed, there was strong oscillatory activity in his visual cortex => the alpha rhythm. Oscillation = CPG.

Question 52

Why are CPGs so widespread?

Answer 52

Generate spațio-temporal patterns of activity.

Question 53

Llinas

Answer 53

Argues that CPGs can be cajoled for more abstract computations, where locomotor states now correspond to symbolic states in a model of the world such as memories, perceptions, ideas, emotions etc.

Question 54

What is the relationship between CPG and attractors

Answer 54

The states of the CPG are homologous to the attractors of feedback neural networks.

Question 55

What could the encephalisation of CPGs be responsible for

Answer 55

The evolution of the mind. Cf. Llinas.