Neuroscience 1B Spinal Cord Dysfunction

Neuroscience 1b – Spinal Cord Dysfunction Anil Chopra

1. Draw a diagram of a cross-section through the spinal cord, labelling the main areas of grey matter and main ascending and descending tracts. 2. Define the function of the cells in the dorsal, ventral and intermediate horns of the spinal cord. 3. Distinguish between the terms nerve, root and ramus in relation to the spinal cord. 4. Explain the relationship between spinal and vertebral levels and its clinical significance. 5. Define the cervical and lumbar enlargements and state the spinal segments involved in them. 6. Describe the meningeal layers of the spinal cord and explain how they differ from those of the brain. 7. State at what level a lumbar puncture is usually performed and explain why. Why is it hazardous to perform lumbar puncture in the presence of raised intracranial pressure? 8. Explain the sensory and/or motor deficits which would follow: a. complete section at mid-thoracic level b. complete section at mid-cervical level c. complete section at C2 level d. hemi section at mid-thoracic level 9. Demonstrate in diagrams the course of the following main ascending and descending tracts through the CNS and define their function: a. corticospinal tract b. corticobulbar tract c. spinothalamic tract d. dorsal columns-medial lemniscus e. reticulospinal tract f. vestibulospinal tract

Spinal Cord: Grey matter (cell bodies) surrounded by white matter (axons).

Dorsal Horns: receive sensory information from spinal nerves and dorsal roots. This is then either taken to the brain for processing, or used in a reflex. Ventral Horns: contain the axons of the motor neurons used in muscle contraction via the ventral roots and spinal nerves.

White matter: contains shorter axons which connect the different segments of the spinal cord, and longer pathways which run to and from the brain.

Nerve is the nerve fibre coming from a single cell body

The root, ventral or dorsal is the nerve root in which a nerve is contained when it leaves the spinal column, dorsal roots contain afferent sensory fibres, the ventral roots contain efferent motor fibres. Roots contain more than one nerve

A ramus is a smaller structure which a nerve divides into. Therefore near a target a nerve, originating from a single cell body can divide into a number of rami to innervate different targets

Each vertebral level gives of a pair of spinal nerves below the inferior border, except C1-7 which are above the superior border. Therefore there are 31 pairs of spinal nerves (29 vertebrae – C1-7, T1-T12, L1 – L5, S1-5, 1 Coccyx) and the coccygeal nerve above the coccyx Number of Number of spinal Relationship of nerve to vertebrae nerves vertebra Cervical 7 8 Above, except C8 Thoracic 12 12 Below Lumbar 5 5 Below Sacral 5 5 Below Coccyx 1 fused (4 unfused) 1 Below Total 30 31

The level of injury to the vertebral column will therefore predict the neuronal damage that occurs. Generally the higher the injury, the more severe the disability

To perform a lumbar puncture, a needle is inserted between vertebrae L4 and L5. This is because the spinal cord stops but there is still CSF in the meningeal column. It should never be performed in the presence of raised intracranial pressure because it may lead to herniation of the cerebral hemispheres through the foramen magnum.

Meninges

 CSF in Sub-arachnoid space similar to that of the brain.  Dentate ligaments in the pia mater extend to the dura which stabilise the spinal cord holding it in place.  The filum terminale connects the inferior end of the spinal cord to the coccygeal vertebrae.  The lumbar cistern (Subarachnoid space below the end of the spinal cord) contains the lumbar and sacral spinal roots. Factors that Affect Severity of Spinal Lesion • Loss of neural tissue • Vertical level • Transverse plane It is important to note how the tracts of the white matter are affected. Generally, the higher the lesion, the greater the disability.

Important Points: - Phrenic nerve C3-C5 (diaphragm innervation) - Lumbosacral level (loss of bowel and bladder control). - Mid-thoracic (paraplegia – unable to use lower limbs as well as bowel and bladder) - Cervical (quadriplegia)

Damage of the spinal cord can lead to both positive and negative effects:

 “Negative” effects can include paralysis, anaesthesia.  “Positive” Effect of Spinal cord Injury can lead to spacicity. Hyper-reflexia can also result. They may not occur until after spinal shock subsides.

CNS tracts cannot regenerate.

Descending Tract Ascending Tract As a general rule in the white matter ascending sensory tracts from the dorsal horns run in the periphery, and descending motor tracts run centrally

Different sensory inputs innervate different laminae of the central grey matter, e.g. skin terminates in laminae I-IV

Lateral Corticospinal Tract Dorsal Columns Spinothalamic Tract

3 Factors: Loss of neural tissue Vertical Level Transverse plane

oComplete section at mid-thoracic level oComplete section at mid-cervical level: Quadriplegia: no-use of all 4limbs. oComplete section at C2 level: Breathing problems. oHemi section at mid-thoracic level

Main ascending and descending tracts through the CNS Corticospinal tract oCorticobulbar tract oSpinothalamic tract oDorsal columns-medial leminiscus oReticulospinal tract oVestibulospinal tract Ascending  The major difference between the main ascending sensory tracts is that fine touch information in the dorsal column tract is conveyed on the same side as it enters, whereas pain, temperature and crude touch via the spinothalamic tract is portrayed on the opposite side  The point at which the tract crosses to the contralateral side is known as the decussation  Sensory tracts are both arranged segmentally, i.e. the fibres from the same level run together in the tract

The Dorsal Column Pathway  One function is to re-arrange the input from dermatomes of the primary sensory fibres into the grossly distorted map of the body surface seen in the primary sensory cortex – the sensory homunculus  It segregates information into modality-specific pathways for touch, hair movement, pressure and joint rotation  Contains feedback mechanisms to gate the amount of incoming information to the cortex  Decussation occurs in the medulla  The functions are carried out in the areas where the pathway is interrupted by synapses – medulla, thalamus, cortex  The primary sensory neurons synapse in the dorsal column nuclei:  Gracile nucleus – leg and lower trunk  Cuneate nucleus – arm and upper trunk  Trigeminal nucleus – face via the trigeminal nerve (V)  Thalamic nucleus = contralateral ventroposterolateral nucleus (VPL) for cuneate and gracile fibres, and in the contralateral ventroposteromedial nucleus (VPM) for trigeminal inputs  Travels through the medial lemniscus  VPL and VPM project to the cortex via thalamocortical radiations

The Spinothalamic Tract:  Carries pain, crude touch and thermal information  Pain is also carried in the spinoreticular and spinomesencephalic tracts  Noxious and thermal information carried into the dorsal horn via fast myelinated Aδ fibres (sharp, stabbing pain), and slower un-myelinated C fibres (dull, nagging pain and thermal information)  Aδ fibres terminate in laminae I and V, the axons decussate and ascend in the anterolateral white matter forming the spinothalamic tract  C fibres terminate in lamina II and use interneurons to influence the firing of dorsal spinothalamic cells  Spinothalamic fibres join the medial leminiscus in the medulla and project to the thalamus  The thalamic termination of the tract is in the ventroposterior nuclei and intralaminar nuclei, from which there is a relay to the cortex  Nociceptive afferents from face carried in the trigeminal (V) nerve to the trigeminal nucleus, then they cross over the medulla to join the other inputs in the thalamus The Dorsal Column Pathway

The Spinothalamic Tract

The Spinocerebellar Tract:  Deals with proprioceptive information and is divided into 2 parts  The dorsal spinocerebellar tract is formed by the axons of cell bodies in the base of the dorsal horn (Clarke’s column) running from T1 – L2. It conveys information about body movement from the trunk and lower limbs to the cerebellum via the inferior cerebellar peduncle. The same kind of information from the upper limb is transferred via the external cuneate nucleus located laterally in the medulla  The ventral spinocerebellar tract receives its inputs from cell bodies in lamina VII (the spinal interneuron layer). This primarily sends inhibitory interneuron signals to the cerebellum via the superior cerebellar peduncle Descending: The Corticospinal Tract: » Also known as the pyramidal tract is the major controller of skeletal muscle activity It has two branches: » The decussating lateral tract controls precision movements of the limbs, innervating lateral motor neuron pools » The uncrossed anterior tract controls the less precise movements of the trunk, innervating medial motor neuron pools The fibres that influence motor neurons innervating muscles in the head, e.g. tongue and facial muscles run in the corticobulbar tracts at the appropriate cranial nerve nuclei.  The somatotopic (specific arrangement) arrangement of the descending motor fibres from the cortex includes the head in the cerebral peduncles (narrow structure, acts as support or connection), but not at the level of decussation in the medulla  The tract carries signals for highly skilled movements and so is highly somatotopic and there are very few collaterals so the excitation from one fibre is not transmitted to another  There are also fibres which regulate spinal reflexes in the tract and feedback to the dorsal horn sensory circuits from the sensory cortex The corticospinal and corticobulbar tracts Other descending tracts:  Mainly involved in autonomic and involuntary control of muscles  Largely deal with axial and proximal muscles which control posture  Tectospinal tract deals with head and neck posture  Vestibulospinal tract is involved in keeping the head balanced on the shoulders as the body moves through space to turn the head  The reticulospinal tract controls posture and helps control crude imprecise movements. The reticular formation describes a diffuse network of cells in the brainstem which receives information from a large part of the central nervous system and is involved in the control of many of the body’s automatic processes. The pontine reticulospinal tract projects to neurons innervating axial muscles. The medullary reticulospinal tract projects to neurons innervating distal muscles.