Spinal Cord, Reflexes, Muscle Tone

Question 1

What is the groove that divides the neural tube into ventral and dorsal regions?

What do the alar and basal plates become?

What is the thick layer of white matter in the spinal cord formed of?

Which spinal region has the most white matter? Why?

How is the spinal grey matter divided?

How is the spinal white matter divided?

How does the spinal grey matter vary along the cord?

Answer 1

• Sulcus Limitans
• Alar plate into the Dorsal horn, Basal plate into the Ventral horn
• Myelinated axons
• Cervical region; all the pathways and tracts are present at this level
• Left and right dorsal/ventral horns
• Divided by the grey matter into 3 longitudinal columns (Funiculi); Posterior, Lateral, Anterior
• Varies in proportion to the amount of muscle at each level; cervical and lumbosacral regions have the most grey matter (to supply the limbs) while the thoracic region has the least

Question 2

How is the grey matter arranged?

What’s an example of an important group of spinal grey matter with discrete functional roles?

How are the LMNs arranged?

Answer 2

• In 10 zones = Laminae of Rexed;
  o Laminae I – VI = Dorsal Horn
  o Laminae VII and X = Mid-region
  o Laminae VIII and IX = Ventral Horn; each column in Lamina IX supplies a particular muscle group
• Phrenic Nucleus
• • Those innervating proximal muscles are more medial, Those innervating distal limb muscles are more lateral
  • Those innervating flexor groups are more posterior, Those innervating extensors are more anterior

Question 3

Types of movement:
1. Voluntary:
Where do they originate from?

What does it include?

Which parts of the CNS are used for it?

2. Rhythmic:
   What are they?

What are they maintained by?

What are they coordinated by?

3. Reflexive:
   What are they?

What do the reflex arcs consist of?

4. Postural:
   What are they?

What are they in response to? What is the reason for this?

What is mediated by?

Answer 3

• Frontal lobe
• Speaking, which consists of coordinated muscle contractions of larynx, tongue, rib cage, diaphragm
• Primary motor, Premotor, Prefrontal regions, basal ganglia, cerebellum, brain stem, spinal cord

• Semi-automatic, consciously-initiated movements e.g. walking, breathing
• By subcortical structures (brain stem, spinal cord); little/no contribution from cortex
• Central Pattern Generators

• Automatic, quick, simple reflex arcs
• Sensory, Motor

• Automatic, small postural adjustments
• Made in response to changes in position; keeps an optimum centre of gravity to prevent falls
• Descending (reticulospinal) projections from brain stem to spinal cord

Question 4

Central Pattern Generators (CPGs):
What are they? What are they important for?

How do these relate to gait patterns?

Direct and Indirect Cortical projections:
What can the cortex do to movement directly and indirectly? Via what?

Answer 4

• Networks of neurons within the spinal grey matter or brain stem; important for rhythmic movement
• Projections from brain can modify activity in the neural network to alter the patterns of movement
• Can generate movements directly (via corticospinal tract), Can influence movements indirectly (via projections to basal ganglia, brain stem, cerebellum, spinal cord)

Question 5

Role of Cerebellum:
What’s its role?

What can damage here lead to?

What does it receive?

Which movements does it compare? Why?

How is an incorrect movement corrected by the cerebellum?

When is this corrective process most useful/suitable? What is this type of movement referred to as?

Answer 5

• Collect sensory to advise frontal motor areas; helps to ensure smooth, precise, and coordinated movement
• CEREBELLAR ATAXIA (poor coordination)
• A continuously updated load of sensory information from numerous sources about the precise relative positions of the head, limbs, and trunk
• Planned movements compared with Actual; can detect any discrepancies
• Cerebellum projects information back to frontal lobe to represent an “error signal” that’s used to correct on-going movements
• Slow-enough movements; enables moment-by-moment feedback and correction whilst the action is still being performed
  o Non-ballistic; ballistic movements are rapid/explosive

Question 6

Role of Basal Ganglia:
How are the connections between it and the motor cortex arranged? Where do these arise and travel?

Which chemical are these loops controlled by?

What are the loops involved in?

What is damage here commonly associate with? Give an example

Answer 6

• As a set of “Loops”; loops arise in the frontal lobe and pass through the basal ganglia before projecting back to its origin
• DOPAMINE
• • Initiation of voluntary actions
  • Selection of particular action among a range of possible actions
  • Learning and performance of various repetitive, semi-automatic behaviours
  • Cognition, behaviour and emotion due to the presence of non-motor loops
• Movement disorders; e.g. PARKINSONS

Question 7

What do muscle spindles respond best to?

What happens in the Stretch Reflex?

What happens to the opposing muscle during the same reflex?

Answer 7

• Short, Sharp stretches e.g. tendon hammer impact
• When a muscle is stretched, its spindles are excited, and a reflex is triggered to contract the same muscle group; keeps muscle at a constant length
• Are inhibited at the same time (Reciprocal Inhibition) by inhibitory interneurons

Question 8

Abnormal Reflexes and Tone:
What does the brainstem normally do to the stretch reflex? Why? How is this affected with UMN Lesions?

What’s a common cause of UMN-type pattern of weakness?

What are common causes of LMN-type pattern of weakness?

Why are the symptoms of a LMN Lesion the way they are?

Why are there mixed symptoms seen in Motor Neuron Disease?

Answer 8

• Normally tones down the reflex by descending influences from brain stem to ensure that muscle tone isn’t excessive; this influence tends to be lost with an UMN Lesion
• Stroke
• Peripheral Neuropathy, Anterior Horn Cell Disease
• Loss of motor nerve supply to muscle = Flaccid Paralysis, Atonia, Areflexia, Gradual, and Atrophy
• Both UMNs and LMNs are affected

Question 9

Withdrawal reflex:
What are the 2 reflexes involved in this?

What does it do?

What is the Flexor reflex? What is it stimulated by? What does its stimulation cause?

When is the Crossed Extensor reflex triggered? What does its stimulation cause?

Answer 9

• 2 spinal reflexes; Flexor, Crossed Extensor Reflex
• Coordinate automatic limb withdrawal from a noxious stimulus
• Polysynaptic, extending over sever spinal cord segments; stimulated by Nociceptors in skin = Ipsilateral Limb Flexion (Withdrawal)
• Triggered at same time = Contralateral Limb Extension