The Spinal Cord

Question 1

Overview of the spinal cord

Answer 1

• Extends from the Atlas to L1
• spinal cord narrows at L1 to form conus medullaris
• the pia mater elongates to form the filum terminal which attaches to the coccyx
• there is Cervical and Lumbar enlargement of the spinal cord, where the cervical and lumbar plexus occurs
  
  • Cervical: C3-T2
  • Lumbar: L1-S3
• surrounded by the meninges (pia, arachnoid, dura)

Question 2

Regions of the Spinal Cord

Answer 2

• Cervical: C1-C7
  
  • ​the enlargement allows innervation to the upper limbs
• Thoracic: T1-T12
• Lumbar: L1-L5
  
  • ​the enlargement allows innervation to the lower limbs
• Sacral: S1-S5

Question 3

The spinal nerves of the spinal cord

Answer 3

• 31 pairs of spinal nerves attached to the anterior/motor roots/ efferent and the posterior/ sensory roots/ afferent

Question 4

Grey Matter of the spinal cord

Answer 4

• H-shaped, with anterior and posterior grey horns
  
  • horns are joined by a thin grey commissure which contains the central canal
  • ventral,
    
    • motor neurons
  • lateral
    
    • preganglionic synaptic neurons
  • dorsal horns
    
    • receiving sensory input
• amount of grey matter correlates with the amount of muscle innervated at that level (hence enlargements)

Question 5

White Matter of the Spinal cord

Answer 5

• Surrounds grey matter, made up of myelinated axons
• made up of white columns/tracts/funiculi
  
  • long ascending tracts carry afferent impulses to the brain
  • long descending tracts carry efferent impulses from the brain
• Dorsal column contains ascending tracts
• Lateral column contains descending and ascending tracts
• Ventral column contains mainly descending tracts

Question 6

What is the receptor type for touch, pressure and vibration sensations?

• describe the type of fibre present and the velocity of conduction

Answer 6

• Mechanoreceptors : they are A-Beta with a wide diameter, fast conduction velocity
  
  • Merkel’s cells
  • Ruffini end organs
  • Pacinian corpuscles
• Bare nerve endings: they are A-Theta with a medium diameter and speed

Question 7

What is the receptor type for pain and temperature sensations?

• describe the type of fibre present and the velocity of conduction

Answer 7

• Bare nerve endings: with A-theta fibres and a medium diameter and speed
  
  • for fast ‘picking’ pain
• Bare nerve endings: with C-fibres thin diameter, and slow conduction
  
  • for slow-burning pain, itch
  • also for non-discriminative touch

Question 8

Explain the different types of receptors seen in this diagram

Question 9

What factors allow somatosensory afferents to convey different functional information?

Answer 9

• Fibre type: axon diameter
• Their Receptive field
  
  • for cutaneous afferents, this is the area of the skin surface over which stimulation results in a significant change in the action potential
  • this along with innervation density allows for two-point discrimination
• Rapidly adapting afferents
  
  • afferents that fall silent on continued stimulation
  • good for conveying information about changes in ongoing stimulation
• Slowly adapting afferents
  
  • better at conveying spatial information such as size and shape
• the type of stimuli

Question 10

Explain lower motor neurons

Answer 10

• Lower motor neurons are collected in longitudinally organized columns
• Each column contains the larger, alpha (thick axon, high conductance velocity), and smaller, gamma (thin axon, low conductance velocity), motor neurons to one muscle (or a few functionally similar muscles).
• Each column extends through more than one segment of the cord.
• Each muscle receives motor fibres through more than one ventral root and spinal nerve
• Destruction of a single ventral root or a single spinal nerve will not produce paralysis, only weakness (paresis).

Question 11

What are Proprioceptive sensory inputs?

• give examples of organs

Answer 11

Proprioceptive sensory inputs: is to give detailed and continuous information about the limbs and other body parts in space. Proprioceptive sensory organs include:

• Muscle spindles- negative feedback regulation of muscle length
  
  • found in striated muscle
  • made up of intrafusal muscle fibres surrounded by a capsule of connective tissue and distributed among extrafusal fibres (produce force)
  • Innervation form group Ia afferent axons (rapidly adapting response) and group II afferents (sustained response to muscle length)
• Golgi tendon organs- negative feedback regulation of muscle tension
  
  • formed by branches of group Ib afferents

Question 12

Recreate this topographic map of the body musculature in the primary motor cortex

Question 13

What are the effects of a spinal cord lesion: Anterior cord syndrome?

Answer 13

• Bilateral lower motor neuron paralysis and muscular atrophy in the segment of the lesion (due to damage to lower motor neurons).
• Bilateral spastic paralysis below the level of the lesion (due to loss of anterior descending tracts).
• Bilateral loss of pain, temperature and light touch sensations below the level of the lesion (due to loss of anterior and lateral spinothalamic tracts).

Question 14

What are the effects of a spinal cord lesion: Brown-Séquard (cord hemisection) Syndrome

Answer 14

• Ipsilateral lower motor neuron paralysis and muscular atrophy in the segment of the lesion (due to damage to lower motor neurons).
• Ipsilateral spastic paralysis below the level of the lesion (due to loss of anterior descending tracts).
• Ipsilateral band of cutaneous anaesthesia in the segment of the lesion (due to loss of dorsal root).
• Ipsilateral loss of tactile discrimination and of vibratory and proprioceptive sensations below the level of the lesion (due to loss of ascending tracts in the dorsal white column on the side of the lesion).
• Contralateral loss of pain, temperature and light touch (due to loss of crossed lateral spinothalamic tracts on the side of the lesion).

Question 15

What are the effects of spinal cord lesion: Complete cord transection syndrome?

Answer 15

• Complete loss of sensation and voluntary movement below the level of the lesion.
• Bilateral lower motor neuron paralysis and muscular atrophy in the segment of the lesion.
• Bilateral spastic paralysis below the level of the lesion (due to loss of descending tracts).
• Bilateral loss of all sensations below the level of the lesion (due to loss of ascending tracts).
• Bladder and bowel function no longer under voluntary control (due to loss of descending autonomic fibres)

Question 16

Describe the circuit anatomy of the ascending tracts in the spinal cord

Answer 16

• First order (primary sensory) neuron
  
  • Enters spinal cord via dorsal root
• Second order neuron
  
  • Ascends spinal cord or brainstem
• Third order neuron
  
  • Projects to the cerebral cortex

Question 17

Give an overview of the Medial Lemniscus pathway (dorsal column)

Answer 17

• Fine touch: from cutaneous mechanoreceptors
• Proprioception: from muscle spindles, golgi tendon organs, joints
• Provides brain with positional information

Question 18

Describe the pathway of the Dorsal columns Medial Lemniscus Pathway

• first-order
• second-order
• third-order

Answer 18

First-order

• Enter the spinal cord and ascend the dorsal column on the same side within the
  
  • Fasciculus gracilis (medial) –> Nucleus gracilis in the medulla
    
    • information from lower limb
  • Fasciculus cuneatus (lateral) –> Nucleus cuneatus in the medulla
    
    • information from upper limb
• Fibres ascend dorsal column uncrossed and synapse on second-order neurons in the medulla

Second-orders neurons

• crosses in the medulla and ascends to the thalamus to form the medial lemniscus

Third-order neurons

• project from the thalamus to the somatosensory cortex (Brodmann areas 1, 2, 3)

Question 19

Explain the cause and effect of damage to the dorsal column?

Answer 19

• Lesion on one side of the spinal cord results in loss of tactile discrimination & proprioception on the same side__​
• Can be caused by multiple sclerosis
• Symptoms: Sensoy ataxia
  
  • ​loss of coordination and balance without visual cues (no positional information)
• Clinical Test: Romberg’s sign
  
  • sever swaying on standing with eyes closed/ feet together

Question 20

What is the role of the Spinothalamic tract

Answer 20

Carry information about

• Pain (from nociceptors)
• Temperature
• Crude touch

Question 21

Describe the pathway of the Spinothalamic tract

Answer 21

First-order

• Enters the dorsal horn and forms the tract of Lissauer
  
  • ​collateral branches are given off at the tip of the dorsal horn
  • runs up or down 1-2 spinal segments
• Synapses in the dorsal horn with second-order neurons

Second-order neurons

• Cross in the dorsal horn at each level
• Ascend in the anterolateral column to the thalamus
  
  • fibres from lower limb: lateral in the tact
  • fibres from the upper limb: medial in the tract

Third-order neurons

• project from the thalamus to the sensory cortex

Question 22

Explain the effects and cause of damage tot he anterolateral column

Answer 22

• Lesion on one side of the spinal cord causes loss of pain, temperature and crude touch on the opposite side
• Outer tract injury causes loss of lower limb pain first as the fibres sit laterally
  
  • cord compression due to herniated disk
• Inner tract injury causes loss of upper limb pain first as the fibres sit medially
  
  • grey matter tumour

Question 23

Give an overview of the Spinocerebellar tracts and its role?

Answer 23

• Unconscious muscle proprioception
  
  • information from muscle spindles, golgi tendon organs
  • for smooth motor coordination
• Two neurons in the pathway
• Comprise of three main tracts
  
  • Anterior and posterior spinocerebellar tracts: carries info from trunk and lower limb
• tracts terminate in the cerebellum on the same side
  
  • left cerebellum control left side of body

Question 24

Describe the pathway of the posterior Spinocerebellar tract and the effects of damage.

Answer 24

First-order neurons

• synapse in the dorsal horn

Second-order neurons

• Ascend in lateral column to cerebellum (very fast axons)
• Lesion on one’s side of the spinal cord leads to uncoordinated lower limb muscular activity on the same side
  • rarely damaged in isolation

Question 25

Give an overview of the Descending tracts in the spinal cord

Answer 25

* Control of muscular activity * groped into pyramidal or extrapyramidal

Question 26

Give an overview of the Ascending tracts

Answer 26

* carries two types of sensory information * **Proprioceptive:** the information originating from inside the body * from muscles, joints, tendons * **Exteroceptive:** information originating from outside the body * pain temperature touch

Question 27

Give an overview of the Corticospinal tract

Answer 27

* It's a great voluntary motor pathway * formed of a **pyramidal tract**, two neurons in the pathway circuit * **Upper motor (premotor) neurons** * **​**from the _cerebral cortex_ to the _ventral horn_ * **Lower motor neurons** * from the _ventral horn_ to _skeletal muscle_

Question 28

Describe the pathway of the Pyramidal tract to the Corticospinal tract

Answer 28

* Primary Motor cortex (BA 1,2,3) * Internal capsule * Cerebral peduncle * Pons * Pyramids of the Medulla: nerve fibres decussate within here * 80% cross midline to the Lateral corticospinal tract (axial & trunk muscles) * 20% on the same side to the Anterior corticospinal tract * Lateral/Anterior Corticospincal tract

Question 29

Explain the lower motor organisation in the ventral horn

Answer 29

* Neurons in the midline supply the trunk * Neurons in the anterior-lateral supply the proximal limb segments * Neurons in the posterior-lateral supply distal limb segments

Question 30

Explain Upper Motor Neuron Diseases and its impact

Answer 30

_Upper motor neurons disease_ * degeneration of upper motor neurons symptoms: * spastic paralysis * overactive tendon reflexes * no significant muscle atrophy * usually occurs following a stroke * **above pyramids:** affects the opposite side * **below pyramids:** same side

Question 31

Explain Lower Motor Neuron disease and its impact - give 2 examples

Answer 31

* degeneration of/damage to lower motor neurons in the ventral horn or periphery, symptoms: * Flaccid paralysis (no muscle tone) * No tendon reflexes * Muscle atrophy * ***Spinal Muscular Atrophy*** * ***​***effects cells in the anterior horn * ***Guillain-Barré syndrome*** * effects the axon and myelin

Question 32

Explain what Amyotrophic Lateral Scleroris is?

Answer 32

* a disease that selectively affects lower and upper motor neurons * causes progressive muscle weakness and atrophy but the minds intact * has a short life span ~5 years: due to respiratory failure _Symptoms_ * Symptoms initially in limbs or bulbar signs: speech and swallowing difficulties * Spasticity present when upper motor neurons affected

Question 33

Give an Overview of the Extrapyramidal tracts

Answer 33

* these are neurons that **don't** pass through the pyramids * important for maintaining posture and regulating involuntary movements * Rubrospinal: from red nucleus [midbrain] * Reticulospinal: from reticular formation [pons] * Vestibulospinal: from vestibular nuclei [medulla]

Question 34

What is the impact of lesions in the Extrapyramidal tracts

Answer 34

* Lesions to extrapyramidal tracts cause movement disorders * **Dyskinesia:** involuntary muscle movements * **Dystonia:** involuntary muscle contractions