L9. Functional Organisation of the Spinal Cord

Question 1

What is the difference between a segmental reflex and an inter-segmental reflex?

Answer 1

A segmental reflex is when information enters the spinal cord, is processed and exits the spinal cord all that the same level.

A inter-segmental reflex involves a few regions of the spinal cord. These are usually for complex movements.

Question 2

Where does the spinal cord usually end for adults?

Why is this so?

Answer 2

L1/2

Because of the differential rates of growth of the spinal cord and the spinal column

Question 3

At what level are lumbar punctures usually performed and why?

Answer 3

At L3-4

Because it is after the termination of the spinal cord and thus without the danger of injurying neural tissue.

Question 4

How does white and grey matter appear histologically?

Answer 4

White matter stains dark while grey matter appears light

Question 5

Describe the distribution of white matter as you descend down the spinal cord

Answer 5

Progressing down to lower levels of the spinal cord there is less white matter as they all progressively terminate where they need to leaving only the lower distal limbs at the caudal end of the spinal cord.

by the end there is only a thin rim surrounding the grey matter.

Question 6

What are the main ways that white matter fibres travel through the spinal cord? [2]

What is the most common of them?

Answer 6

1. Running up and down the spinal cord in the same axis of it (most common)
2. Running across the spinal cord, transverse to the axis of the spinal cord

Question 7

What are the white matter tracts linking sections purely within the spinal cord (ie. not to/from the CNS) called?

Answer 7

Propriospinal cords

Question 8

How do motor neurons tend to lie in the spinal cord segments?

Answer 8

They tend to lie in clusters/pools in the anterior ventral horn projecting out through the ventral root to the periphery

Question 9

Other than the (ascension to the CNS) and the dorsal synaptic connections what other local synapse do primary sensory afferents make and why?

Answer 9

They make local synaptic connections with motor neurons in the ventral horn

This is to coordinate muscle movement and respond to the stimuli

Question 10

What is the definition final common pathway for movement?

Whay is it named this?

Answer 10

The common pathway that is shared by any kind of movement.

It is called this because all movements must occur via activity of this motor neuron pathway.

Question 11

What are the major descending/controlling inputs of the final common pathway for movement?

Which of the 2 is a major component of control?

Answer 11

1. Descending control from cortical areas for voluntary movement
2. Indirect interneuron control for reflex movement (most of the control)

Question 12

Describe the topographical mapping of the ventral horn of the spinal cord

Answer 12

Medially send axons to proximal muscles

Laterally have motor neurons that project out to control distal muscles.

(The ventral surface also tends to spread a superior and inferior mapping as well). However, they overlap significantly.

Question 13

What is a motor pool?

Answer 13

The collection of all motor neurons in the spinal cord that innervate one particular muscle.

Eg. the biceps muscle has a motor pool (collection) of thousands of motor neurons that contribute to its innervation.

Question 14

What is meant by the functional grouping/antagonism of muscles?

Answer 14

Because innervation of muscles is only excitatory (to contract) muscles and there are no neurons to inhibit and relax muscles, the limbs and neural supply to the limbs are organised in such a way that around a joint there is an organisation into push/pull or flexor/extensor pairs

Contraction of one muscle of the pair antagonises contraction of the other muscle

Question 15

Muscles have “muscle sense” or mechanoreception. What does this men?

Answer 15

There are specialised mechanoreceptors in the muscles (and tendons) that are important in providing sensory information regarding state of contraction, force and position to enable coordination and balance.

Question 16

What is a motor unit?

Answer 16

One neuron innervates a number of muscle fibres.

A motor unit is made up of a

• motor neuron
• the skeletal muscle fibers innervated by that motor neuron’s axonal terminals.

The more motor units active, the larger the force of contraction

Question 17

Describe the relationship between a motor unit and motor neuron pool

Answer 17

Groups of motor units often work together to coordinate the contractions of a single muscle

All of the motor units within a muscle are considered a motor pool.

Question 18

What is a muscle spindle?

Answer 18

A major component to motor control, present in all muscles of the body (except in the eye).

They are sensory receptors within the belly of a muscle that primarily detect changes in the length of this muscle.

Question 19

What is the main function of the muscle spindle?

Answer 19

To convey length information to the cCNS via sensory neurons. This information can be processed by the brain to determine the position of body parts.

Question 20

Describe the development (differentation) of the muscle spindles

Answer 20

During development nerves growing out of DRG grow into the muscle

The nerves induce normal muscle fibres to differentate: forming a CT sheath around it (intrafusal muscle fibres) so it becomes a sensory structure

They can still contract but its main job is to report back to the sensory nerve how much stretch is occuring.

Question 21

What is the Golgi Tendon Organ?

Answer 21

A proprioceptive sensory receptor organ that is at the origins and insertion of skeletal muscle fibers into the tendons of skeletal muscle

The neuron interdigitates within collagen and elastin matrix of the tendon (force transducer meshed in the structure)

Question 22

Compare the information gained from a muscle spindle with the information from a golgi tendon organ.

Answer 22

The muscle spindles are parallel with the muscle fibres so are able to give information about the length of the muscle.

The golgi tendon organ is part of the chain (series) (thus gives information about force - no knowledge of the length of the muscle)

Question 23

What is the main function of the motor reflexes (also known as the deep tendon reflexes)

Answer 23

They are the foundation for motor control (all behaviour relies on reflex control mechanisms to ensure the body is aware of limb position and movement at all times)

Question 24

Why is the Deep Tendon Reflex also known as the monosynaptic reflex?

Answer 24

Because only one synapse is involved in the entire reflex pathway.

(no interneurons)

Question 25

Describe how the deep tendon reflex elicits the response that it does

Answer 25

Tapping on a tendon causes a slight pulls in the muscle that it is attached to. **Stretch receptors** (highly sensitive) e**mbedded in muscles are activated as a result**. These stretch receptors release a burst of neural activity **via the muscle spindles** at about 12om/s (fastest in the PNS). This leads to a relay of information through the afferent neuron to the spinal cord where there is an excitatory synapse with the motor neuron to cause the **extensor muscle to contract further.**

Question 26

Describe the monosynaptic stretch reflex

Answer 26

1. Stretch receptors activated in the muscle by the tendon tap 2. Action potentials rapidly propagate through the primary afferent neuron to the level of the spinal cord associated with innervation 3. **One excitatory synapse** is made with a motor neuron in the ventral horn 4. This excites an action potential in the motor neuron to cause muscle contraction of the muscle. 5. The same afferent fibre that excited the motor neuron **also inhibits the activity of the anagonistic neuron through an inhibitory interneuron** to reduce activity of the flexor muscle

Question 27

Compare the threshold for activation of the muscle spindles and the golgi tendon organs

Answer 27

The muscle spindles (the ones involved in the monosynaptic stretch reflex) are a low threshold receptors. Golgi tendon organs are high threshold

Question 28

Imagine a hypothetical where a limb (eg. arm) is in a **static situation** **of supporting an unchanging load.** Draw a pathway of the **negative** feedback pathway to maintain this situation

Answer 28

The reflex acts to sense the length of the muscle in this static position. If it feels the muscle lengthening then it will send signals through the pathway to signal contraction. If it senses shortening then it will do the opposite The point is to know where in space, the arm is at all times without having to know about it consciouslly

Question 29

Now imagine that the load is increasing in weight. This is now a **dynamic situation.** Now draw what changes would occur through the reflex pathway

Answer 29

Adding load will force the flexor muscle to lengthen under the weight. This causes a large activation of the stretch receptors sending a larger action potential through the afferent neuron meaning an even larger stimulation to flex the muscle through the excitatory motor neuron and an even larger inhibition of the extensor muscle

Question 30

What are the main neurotransmitters used by the inhibitor interneurons in motor pathways?

Answer 30

GABA and glycine

Question 31

What about the muscle spindles in the extensor muscles? How do they contribute to the pathway?

Answer 31

They work in the exact same mechanism. Together the pathways work to **maintain position**

Question 32

Describe the golgi tendon organ receptors involvement in the pathway

Answer 32

Golgi Tendon organs detect the amount of **force generated by the muscle** and feedback to the spinal cord. The more active they are, the more force is being applied by the muscle and this causes a burst of activity that leads to an **inhibitory interneuron** that inhibits the same neuron. It also excites the antagonist muscle by an **excitatory interneuron.** This point is to **control force be reducing it when force increases.**

Question 33

Imagine the situations where the flexor muscle is holding a load but is **fatiguing** (ie. the force is reducing in the muscle). What receptor pathway is involved? Draw the pathway

Answer 33

The **golgi tendon organs** detect force The golgi tendon organs would detect a **reduction in force** as a result of muscle fatigue. This would reduce the action potential firing in the golgi afferent. Reducing the golgi afferent reduces the firing in the inhibitory interneuron that it innervates. This means that it inhibits the inhibition and reduces inhibition on the contraction of the flexor muscle (and reduces excitation of the extensor)

Question 34

What is an example of an inter-segmental reflex?

Answer 34

Withdrawal reflexes from noxious stimuli. Muscles involved in withdrawing a limb from the stimuli involves different levels of the spinal cord. This is because withdrawal of the limb requires support from other limbs as well (bilateral and multisegmental)

Question 35

Describe the pathway in removing a limb from a noxious stimuli

Answer 35

Flexion (withdrawl) reflex due to a nociceptor fibres. Nociceptors terminate as soon as they enter the dorsal limb. They activate flexors to remove the limb, at the same time extensors are inhibited. At the same time, the contralateral leg has to increase activity and support body weight thus a pathway excites extensors and inhibits the flexors.

Question 36

Does the removal of the limb from a noxious stimuli require CNS input?

Answer 36

No, it **is a reflex:** it is completely through the spinal cord (eg. a frog without a brain exhibited the ability to perform this reflex correctly)

Question 37

What is the function/reason why we have propriospinal fibres?

Answer 37

Propriospinal fibres (interneurons) connect different segments of the spinal cord for **coordination across large regions** (eg. walking pattern distribution to allow for arms to swing)

Question 38

Describe the distribution of the propriospinal fibres (short vs. long) through the cross section of the spinal cord Is there a reason for this distribution?

Answer 38

Short are located laterally through the spinal cord cross section. Long are located medially. Because lateral motor neurons are controlling the distal muscles (only interested in things close together). Digits are close together. Long fibres medially (muscles are long way away eg. postural control)