Structure and function of the spinal cord Flashcards
Considering the anatomy of the spinal cord, describe the origin and ending of the spinal cord
Origin- cranial border of atlas
At L1 spinal cord narrows to form conus medullaris . Terminal flum attaches to coccyx
End- Cauda equna
How is the spinal cord protected?
Sits within vertebral column
Surrounded by meninges (pia, arachnoid and dura)
The spinal cord is divided into four regions. Describe them
Cervical (C1-8 spinal nerves)
Thoracic (T1-T12 spinal nerves)
Lumbar (L1-5 spinal nerves)
Sacral (S1-5 spina nerves)
What is meant by the 1. cervical enlargement and 2. lumbosacral enlargement?
- Innvervation to upper limb
Dura protects cord - Innervation to lower limb
Conus medullaris: tapered, lower end of the spinal cord between L1 and L2
Terminal filum: internal part - pia
Terminal filum: external part - dura
What do spinal nerves do?
Connect the periphery to the spinal cord
31 pairs, each formed by a dorsal (afferent fibres) and ventral root (efferent fibres)
Describe the internal anatomy of the spinal cord
Inner core- gray matter
- neuronal cell bodies
- H shaped
- Ventral, lateral and dorsal horn
- *** expanded at levels that supply the limbs
Outer, white matter
- myelinated axons
- white columns/tracts or funiculi
How is gray matter organised?
Dorsal horn= neurons receiving sensory input
Lateral horn= preganglionic symp neurons
Ventral horn= motor neurons
Also interneurons between/within levels
How is white matter organised?
Contain tracts
Long ascending tracts carry afferent (sensory) impulses to centres within the brain
Long descending tracts carry efferent (motor) impulses from centres within brain
**tracts to/from cerebral hemispheres- crossed (control of body)
Dorsal column = ascending tracts
Lateral column = descending and ascending tracts
Ventral column = mainly descending tracts
Explain the types of sensory information carried in ascending (sensory) tracts
Proprioceptive- information originating from inside the body (from muscles, joints and tendons)
Exteroceptive- information originating from outside the body (pain, temperature, touch)
Describe the anatomy of the ascending tracts
often three neurons in circuit:
- 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
Describe the function of the dorsal column-medial lemniscus pathway
- Fine touch (from cutaneous mechanoreceptors)
- Proprioception (from muscle spindles, golgi tendon organs, joints) - provides brain with positional information
Describe the role of first order neurons as part of the dorsal column-medial lemniscus pathway
Enter spinal cord and ascend dorsal column on same side within the:
- fasciculus gracili (medial): these terminate in nucleus gracilis (carrying info from lower limb)
- fasciculus cuneatus (lateral): terminate in nucleus cuneatus (carrying info from upper limb)
Fibres ascend dorsal column UNCROSSED
-longest neurons in body
Synapse on second-order neurons in the MEDULLA
Describe the role of second order neurons as part of the dorsal column-medial lemniscus pathway
CROSS in medulla and ascend to thalamus
-form medial lemniscus (ribbon)
Describe the role of third order neurons as part of the dorsal column-medial lemniscus pathway
Project from thalamus to somatosensory cortex
What happens when there is damage to the dorsal column?
Lesion on one side of spinal cord: e.g. in multiple sclerosis
Loss of tactile discrimination + proprioception on same side
Symptoms include
-sensory ataxia (loss of coordinationand balance without visual cues)
Clinical test: Romberg’s sign
- severe swaying on standing with eyes closed/feet together
What type of information is carried in the spinothalamic tract?
Pain, temperature, and crude touch
Describe the role of first, second and third order neurons as part of the spinothalamic tract
FO NEURONS
Enter dorsal horn and form tract of Lissauer
- Collateral branches given off at tip of dorsal horn
-Run up or down 1-2 segments
Synapse in dorsal horn with 2nd order neurons
SO NEURONS
CROSS in dorsal horn at each level
Ascend in anterolateral column to thalamus
TO NEURONS
Project from thalamus to somatosensory cor
What happens when there is damage to the anterolateral column?
Lesion on one side of spinal cord
- loss of pain, temperature and crude touch on opposite side
Outside compression of cord:
= loss of lower limb pain first (fibres sit laterally)
Inside gray matter tumour:
= loss of upper limb pain first (fibres sit medially)
What type of information is carried in the spinocerebellar tract and briefly describe it?
Unconscious muscle proprioception (from muscle spindles, golgi tendon organs)
For smooth motor coordination
Two neurons only, four tracts
UNCROSSED (Left cerebellum controls left body)
e.g. Anterior and posterior spinocerebellar tracts- carries proprioceptive information from trunk and lower limb
Describe the role of first and second order neurons as part of the posterior spinocerebellar tract
FO NEURONS
Synapse in dorsal horn
SO NEURONS
Ascend in lateral column to cerebellum
Very fast axons
(SO Neurons of anterior tract cross to the other side and then cross back)
What happens when there is damage posterior spinocerebellar column?
Lesion on one side of spinal cord
Uncoordinated lower limb muscular activity on SAME SIDE
(rarely damaged in isolation)
State 3 general facts about the descending (motor) tracts
Control of muscular activity
Run FROM the cortex
Many descending tracts
Describe the corticospinal tract
It is the great voluntary motor pathway
Contains 2 neurons in circuit:
- Upper motor (premotor) neuron
- from cerebral cortex to ventral horn - Lower motor neuron
- From ventral horn to skeletal muscle
Describe the tracjectory of the corticospinal tract
PYRAMIDAL TRACT
Primary motor cortex Posterior limb of internal capsule Cerebral peduncle Pons Pyramids of medulla Lateral/Anterior Corticospinal tract
Describe the pyramids of decussation that make up the corticospinal tract
The two pyramids contain the motor fibers that pass from the brain to the medulla oblongata and spinal cord.
Decussate (cross over) in the anterior median fissure of the medulla oblongata as the pyramidal 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 of the corticospinal tract in the ventral horn
Medial= Trunk Anterolateral= Proximal limb segments Posterolateral= Distal limb segments
When there is disruption to the corticospinal tract it can lead to motor neuron disease. Describe the difference between upper and lower MND?
UMND
- Degeneration of upper motor neurons
- causes spastic paralysis (increased muscle tone), overactive tendon reflexes and no significant muscle atrophy
e. g. following stroke
LMND
- Degeneration of lower motor neurons in ventral horn
- causes flaccid paralysis (no muscle tone), no tendon reflexes, muscle atrophy
e. g. spinal muscular atrophy caused by defects in the SMN1 gene
When there is disruption to the corticospinal tract it can lead to Amyotrophic lateral sclerosis, describe it.
-Selectively affects lower and upper 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 (approx 5 years due to resp failure)
- Affects 4-5/ 100,000
- Genetic mutation identified (SOD1 gene)
What are extrapyramidal tracts?
Tracts that do not pass through the pyramids
Descend from basal ganglia, cerebellum and pons
e. g. 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
