S3: Spinal Cord, Reflexes and Muscles Flashcards
Describe the motor system as a hierachy
- The cerebral cortex is at the apex (highest point) and is responsible for spontaneous, novel and adaptive behaviours.
- The cortex, basal ganglia and cerebellum are involved in movement in different ways.
- At the other extreme, the most basic type of involuntary motor response is the monosynaptic spinal reflex arc.
- So, at the bottom of the hierachy, there is the simple reflex arc and CPG involved in semi-automatic actions like walking, chewing and breathing.
- Movements can be initiated at various ‘levels’ in the motor hierachy but all structures and pathways are under the authority of the cerebral cortex.
How are motor neurones embryologically developed?
The anterior part to the sulcus limitans is called the basal plate. Motor neurones develop in the basal plate region and their axons grow out of the anterior part of the spinal cord to form the ventral (motor) spinal nerve roots.
How are sensory neurones embryologically developed?
Sensory neurones grow into the posterior aspect of the cord (alar plate) forming the dorsal sensory spinal nerve roots. The cell bodies of the sensory neurones are contained in the dorsal root ganglia.
What do the ventral roots and dorsal roots form?
The ventral roots carry the motor neurones and the dorsal root carrys the sensory neurone (first order) which then mix to become a mixed spinal nerve.
Describe the cross section of spinal cord
Spinal cord grey matter forms a ‘H’ shape which consists of nerve cells (bodies). This is surrounded by a thick layer of white matter which is composed of myelinated axons (stains blue with dye).
Spinal cord grey matter is divided into left and right dorsal horns which belong to the alar plate and therefore sensory in function. The ventral horns derive from the basal plate and contain motor neurones (anterior horn cells).
What is the spinal cord laminae of rexed (1-X)?
The grey matter of the spinal cord is arranged into ‘zones’ or ‘laminae’ which are orignally described by Bror Rexed - laminae means thin layer/plate. The laminae of Rexed are labelled from one to ten using roman numerals.
- Laminae I- VI make up the dorsal horn.
- Laminae VII-X are the intermediate zone (mid-region of cord including the area around the fluid filled central canal.
- Laminae VII-IX correspond to the anterior horn.
Describe laminar IX
- The cell bodies of anterior horn cells (lower motor neurones) are not contained within well defined areas but are found in discrete longitudinal columns.
- The seperate columns of motor neurones are referred to collectively as lamina IX.
- Each column in lamina IX supplies a particular functional muscle group (e.g. forearm flexors, knee extensors, thigh adductors).
- These columns usually span more than one spinal segment.
How are lower motor neurones arranged in laminae?
- Lower motor neurones are arranged in an orderly manner.
- Those innervating proximal (axial and limb girdle) muscles are found close to the midline whilst motor neurones supplying the distal limb muscles are placed laterally (a medial-lateral gradient).
- There is also a ventral-dorsal pattern arrangement.
Neurones supplying flexor groups are nearer the back of the cord while extensors are more towards the front. - So an anterior-medial group will be likely supplying proximal extensors e.g. erector spinae.
- A posterior-lateral group will likely be supplying distal flexors e.g. hand flexors
Three examples of groups of motor neurone in grey matter which have discrete functional roles and are given specific names.
- Phrenic nucleus (C3,C4,C5) which supplies the diaphragm via the phrenic nerve and is involved in breathing.
- Spinal accessory nucleus (C5, C6) which supplies the trapezius and SCM for head turning and shoulder shrugging.
- Onuf’s nucleus (S2,S3, S4) which is important for continence as it supplies the external urethral and anal sphincters.
How is the white matter in the spinal cord divided?
The white matter of the spinal cord is divided by the lines of attachment of the dorsal and ventral nerve roots into three longitudinal columns known as funiculi:
There are the ventral, dorsal and lateral columns that can be used to describe where lesions and pathways are.
Describe the columns in white matter
These columns contain descending pathways from the brain (e.g. those carrying instructions related to movements) and numerous ascending pathways (e.g. tracts mediating sensations such as touch, vibration, pain and temperature).
Describe the ascending pathway in spinal white matter. Include role of cerebellum.
The ascending pathways carry information from the cord to the brain.
- The dorsal columns bring vibration, proprioception and discriminative touch to the brain.
- The spinothalamic tract brings pain and temperature information to the brain.
- The spinocererbeller tract carries information from the lower limbs to the cerebellum. It tells the cerebellum what the lower limbs are doing. The cerebellum also receives information from the frontal lobe on what it wants the lower limbs to be doing and the cerebellum makes a comparison between the two.
If the lower limbs are doing something different there is a mismatch and the cerebellum generates an error signal sent to the motor cortex which then makes the correction.
Describe the descending pathway in spinal white matter.
- Corticospinal tract which controls movement in the limbs and trunk
- The lateral corticospinal tract occupies the lateral column of the cord. Lateral corticospinal tract contains about 90% of the fibres and these are the ones that crossed over at the level of the foramen magnum.
It deals with distal flexors mainly. - Anterior corticospinal tract occupies anterior column of the cord. It is a direct continuation of the pyramids of the medulla. This is why it is situated either side of the midline at the front and doesn’t cross. It deals with the proximal extensors.
Pattern of white matter in spinal cord for different vertebrae
Cross sections at different spinal cord levels show that the cervical region (top of spinal cord) contains the most white matter because of all the descending and ascending pathways present at this level that haven’t reached their area of termination. In the lowermost part of the spinal cord, most the descending pathways are no longer present and most of the ascending pathways have yet to joint the cord so they have less white matter (sacral level).
Pattern of grey matter in spinal cord for different vertebrae
The pattern for grey matter varies in proportion to the amount of muscle tissue at each level. For this reason the cervical and lumbosacral regions have the largest amount of grey matter (to supply the limbs) whilst the thoracic cord has the least.
