Structure and function of the spinal cord Flashcards
Where does the spinal cord extend from and to?
Atlas - C1
L1
The spinal cord narrows at L1 to form what?
Conus medullaris
What attaches to the coccyx?
Terminal filum
What is found in the lumbar cistern?
Cauda equina (lumbar/sacral, dorsal/ventral roots)
What surrounds the spinal cord?
Sits within vertebral canal and surrounded by meninges and fat
What 4 regions are the spinal cord divided into?
Cervical region (C1-8) Thoracic region (T1-12) Lumbar region (L1-5) Sacral nerve (S1-5)
What is the cervical enlargement?
Innervation to upper limb
What is the lumbosacral enlargement?
Innervation to lower limb
How many spinal nerves are there?
31 pairs formed by dorsal and ventral roots
What do spinal nerves do?
Connect the periphery to the spinal cord
What fibres are ventral roots?
Efferent
What fibres are dorsal roots?
Afferent
Describe the inner core of the spinal cord
gray matter - neuronal cell bodies, H shaped, ventral,lateral and dorsal horn
Describe the outer anatomy of the spinal cord
Myelinated axons
White columns/tracts or funiculi
What happens to the gray matter at levels that supply the limbs?
Expands
Where are lateral horns found?
Only on thoracic vertebrae T1-12
Where are sensory inputs received?
Dorsal horn
Where are preganglionic sympathetic neurons found?
Lateral horn
Where are motor neurons found?
Ventral horns
Where are interneurons found?
In or between levels
What does the white matter contain?
Tracts
What are long ascending tracts?
Carry afferent impulses to centres within the brain
What are long descending tracts?
Long descending tracts carry efferent (motor) impulses from centres within brain.
What happens to the tracts to and from the cerebral hemispheres?
Tracts to/from cerebral hemispheres – crossed
(i.e. left cerebral hemisphere controls right side of body)
What is found in dorsal column?
Ascending tracts
What is found in lateral column?
Descending and ascending tracts
What is found in the ventral column?
Mainly descending tracts
What two types of sensory information is carried in the ascending tracts?
Proprioceptive
- Information originating from inside the body (from muscles, joints, tendons)
Exteroceptive
- Information originating from outside the body (pain, temperature, touch)
Describe the 3 neurons in circuit in ascending tract
- 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
What information does the dorsal column-medial lemniscus pathway carry?
Fine touch (From cutaneous mechanoreceptors)
Proprioception (From muscle spindles, golgi tendon organs, joints)
- Provides brain with positional information
Describe the first order neurons in the dorsal column medial lemniscus pathway
Enter spinal cord and ascend dorsal column on same side within the:
- Fasciculus gracilis (medial) - Fasciculus cuneatus (lateral)
Fibres ascend dorsal column uncrossed
Longest neurons in the body
Where do the first order neurons synapse on second order neurons in the dorsal column-medial lemniscus pathway?
In the medulla
Where does fasciculus gracilis terminate?
Nucleus gracilis (gracile)
What does fasciculus gracilis give information about?
Information of lower limb
Where does Fasciculus cuneatus terminate?
nucleus cuneatus (cuneate)
What does fasciculus cuneatus give information about?
Information of upper limb
Describe the second order neurons of the dorsal column-medial lemniscus pathway
Cross in medulla and ascend to thalamus
- Form medial lemniscus (ribbon)
What do the third order neurons of the dorsal column medial lemniscus pathway do?
Project from thalamus to somatosensory cortex
Describe what happens when there is a lesion on one side of the spinal cord in the dorsal column
E.g. in multiple sclerosis
- Loss of tactile discrimination + proprioception on same side
- Symptoms include sensory ataxia - Loss of coordination and balance without visual cues
(i. e. no positional information)
What is a clinical test for a lesion on one side of spinal cord in the dorsal column
Romberg’s sign - severe swaying on standing with eyes closed and feet together
What does the spinothalamic tract give information about?
Pain, temperature and crude touch (from nociceptors)
Describe the first order neurons of the spinothalamic tract
Enter dorsal horn and form tract of Lissauer- collateral branches given off at tip of dorsal horn. Run up and down 1-2 segments.
Where do the 1st and 2nd order neurons synapse in the spinothalamic tract?
Synapse in dorsal horn with second order neurons
Describe the second order neurons of the spinothalamic tract
Cross in dorsal horn at each level
Ascend in anterolateral column to thalamus
Where do the third order neurons of the spinothalamic tract project?
From thalamus to somatosensory cortex
Describe a lesion on one side of spinal cord to the anterolateral column
- Loss of pain, temperature and crude touch on opposite side
What does an outside compression of the cord cause?
loss of lower limb pain first (fibres sit laterally)
What does an inside grey matter tumour cause?
loss of upper limb pain first (fibres sit medially)
What information do spinocerebellar tracts carry?
Unconscious muscle proprioception
- For smooth motor coordination
What is special about the spinocerebellar ascending tract#?
Two neurons only
Four tracts
Uncrossed - left cerebellum controls left side of the body
What do anterior and posterior spinocerebellar tracts carry?
Proprioceptive information from trunk and lower limb
Describe the first order neuron of the spinocerebellar tract
Synapse in dorsal horn
Describe the second order neuron of the spinocerebellar tract
Ascend in lateral column to cerebellum - very fast axons
What does a lesion on one side of the spinal cord in the spinocerebellar tract cause?
Uncoordinated lower limb muscular activity on same time although rarely damaged in isolation
What are descending (motor) tracts?
Control of muscular activity
From the cortex
Many descending tracts
What is the purpose of the corticospinal tract?
Great voluntary motor pathway
Describe the upper motor (premotor) neuron of the corticospinal tract
From cerebral cortex to ventral horn
Describe the lower motor neuron
From ventral horn to skeletal muscle
Describe the pyramidal tract of the corticospinal tract
Primary motor cortex Posterior limb of internal capsule Cerebral peduncle Pons Pyramids of medulla Lateral/anterior corticospinal tract
What are the pyramids of decussation?
80% cross - lateral corticospinal tract
20% on same side - anterior corticospinal tract
(cross in spinal cord - supply deep neck muscles)
Describe the topographical organisation in the ventral horn
Medial = Trunk
Anterolateral = Proximal limb segments
Posterolateral = Distal limb segments
What is motor neuron disease?
Disruption of the corticospinal tract
Describe upper motor neuron disease
Degeneration of upper motor neurons
- Spastic paralysis (increased muscle tone)
- Overactive tendon reflexes
- No significant muscle atrophy
Above pyramids: Opposite side
Below pyramids: Same side
E.g. Following a stroke
Describe lower motor neuron disease
Degeneration of lower motor neurons in ventral horn - Flaccid paralysis (no muscle tone) - No tendon reflexes - Muscle atrophy E.g. Spinal muscular atrophy - caused by defects in the SMN1 gene
What is amyotrophic lateral sclerosis?
Selectively affects upper and lower motor neurons
- Progressive muscle weakness and atrophy but mind intact
- Symptoms initially in limbs or bulbar signs (speech and swallowing difficulties)
- Spasticity present when upper motor neurons affected
- Short life span (~5 years – due to respiratory failure)
- Affects 4-5 in 100,000
- Genetic mutations identified (e.g. SOD1 gene)
What are extrapyramidal tracts?
Do not pass through the pyramids
Descending tracts from basal ganglia, cerebellum and pons
E.g. Reticulospinal tract
Describe the reticulospinal tract
- From reticular formation (pons) to spinal cord
Regulates ventral horn motor activity/reflexes
Facilitates and inhibits lower motor neurons
Important for posture and locomotion
