Overview of central sensory and motor pathways Flashcards
explain this flow diagram
Arrows represent different stimuli e.g. temperature, pain etc.
Sensory afferents are carried by Ascending pathways, they start of from the lower part of the nervous system and then they ascend up the spinal cord until they reach the brain.
Somatic/Autonomic efferents are carried by descending pathways. They start up in higher centres (brain) and end up in lower centres.
what is the cortical representation of sensory and motor modalities?
The cerebral cortex is the highest centre for processing neuronal signals
Its main 2 functions are:
- Somatic sensation
- Voluntary movements
The cerebral cortex can be divided into the:
- Sensory brain
- Motor brain
The anatomical landmark that separates the sensory brain from the motor brain is known as the central sulcus. It results in the presentation of two structure:
The pre-central gyrus (commands motor control)
- In front of central sulcus
- Part of the frontal lobe
The post-central gyrus (commands sensory control)
- Behind the central sulcus
- Part of the parietal lobe
explain the organisation of sensory pathways of the brain
Sensory information from the body is relayed to the brain in a detailed and organised manner
The stimuli are kept separated throughout their ascension and at the level of the brain.
Sensory information is first processed in the spinal cord
It is then carried through the ascending tracts as follows:
Spinal cord (dorsal horn) –> medulla –> thalamus –> cerebral cortex
Sensation from the head and neck is conveyed to brain largely by CNV and the C2 and C3 spinal nerves
The first centre of the cerebral cortex to see sensory signals is the post-central gyrus (primary sensory cortex)
A lesion at the post central gyrus will eliminate sensation.
Somatosensory system: All neurones of the brain and spinal cord that act to convey somatic sensation
what are ascending tracts?
name the 3
Ascending tracts are fibre tracts of the sensory system by which sensory information is conveyed to the brain
There are 3 subtypes of ascending tracts:
LSTT: Lateral spinal thalamic tract - carries pain and temperature sensation
DC: Dorsal column
Fasciculus gracilis and the fasciculus cuneatus represent the dorsal columns (DC).
They carry proprioceptor signals.
D/VSCT: Dorsal/Ventral spinocerebellar tract.
what are the 2 categories of sensation?
There are 2 categories of sensations:
Conscious sensation (directly aware of it)
Pain, Temp. Crude Touch (largely carried by LSTT)
Non-conscious sensation (not directly aware of it)
Tactile sensation, Muscle length, Tension, Joint Position etc
Impulses giving rise to conscious sensation are conveyed differently from those giving rise to non-conscious sensation.
explain the first stage of spinal sensation
Primary sensory neurones have their cell bodies in the dorsal root ganglion
They have pseudo unipolar morphology (see below)
On reaching the spinal cord, the axon of the primary sensory neurone then divides into 3 branches.
Assuming the primary sensory neurone is carrying T6 sensory signals
1 axonal process remains in the dorsal horn of its respective spinal segment (e.g T6)
Another axonal process is sent to the dorsal horn of the spinal segment above (T5)
Another axonal process is sent to the dorsal horn of the spinal segment below (T7)
The Axonal segments travelling to the spinal segments below and above travel in the Posterolateral Tract of Lissauer
Sensory signals carried in these 3 branches are then processed by the spinal cord in the same way (see below)
explain variant #1 in which signals reach the post central gyrus (via the spinothalamic tract)
- The first sensory neurone carries signals to the dorsal horn where it terminates by synapsing on a follower cell (second order sensory neurone)
- A second-order sensory neurone picks up the sensory signal then passes under the central canal, to travel on the opposite side of the spinal cord.
- Here it then ascends the spinal cord until it reaches the thalamus, where it terminates by synapsing to the third order sensory neurone.
- Axons of third-order sensory neurones of the thalamus ascend further to terminate in the post-central gyrus of the Cerebral cortex.
- Therefore, you could say information is carried on the contralateral spinal cord, medulla, thalamus and cortex.
explain variant #2 in which signals reach the post central gyrus (via dorsal column)
- Primary sensory neurone enters the spinal cord.
- This neurone remains ipsilateral and ascends until the level of the medulla where it decussates to the contralateral side and then terminates within the medulla.
- The second-order neurone then travels in the contralateral medulla, pons, midbrain and thalamus. The thalamus is where it terminates.
- The third-order neurone then ascends and terminates in the contralateral sensory-motor cortex.
what is sensory decussation?
what sensations follow variant 1?
what sensations follow variant 2?
as a result of either variant 1 or 2, sensory information from the right side of the body is represented & perceived by the left Brain and vice versa.
This is called Sensory decussation.
Pain and Temperature information follows variant 1.
- Pain & Temperature from the right side is carried by the left spinal cord
Proprioceptive & Tactile information follows variant 2.
- Proprioceptive information from the right side is carried by the right spinal cord
All the sensation then come together on the same side at the medullary level and then travel together to the Primary somatosensory cortex.
what does the homunculus man represent?
The primary sensory cortex has full sensory representation of the body’s surface
The body surface map of the primary sensory cortex is not isometric with body’s surfaces’ size. The size of a body part does not equate to amount of primary sensory cortex that is dedicated to it. (homonculus man)
what is Syringomyelia?
The fibres that carry pain sensation come under the central canal to cross over to the other side.
Syringomyelia is a cyst that forms which cuts fibres that cross over the canal.
This results in anaesthesia – no pain reception at the level of certain parts.
what does the primary motor cortex do?
The Primary Motor Cortex occupies the Pre-Central Gyrus (Frontal Lobe)
Voluntary motor commands of muscles are initiated by the Primary Motor Cortex
what is the cortical representation of the motor system?
Motor System of the body: All neurones of the brain and spinal cord that act to produce movements.
E.g. the cerebral cortex and the sub cortical structures and the spinal cord and the tracts.
Voluntary motor commands are issued from the pre-central gyrus of the cortex (primary motor cortex)
These commands pass through the subcortical motor centres of the brain (thalamus, Midbrain, Brainstem, Pons, Medulla) and then descend the spinal cord via descending tracts, eventually stopping at a ventral horn.
They then exit the spinal cord and pass on to skeletal muscles.
what are the two classes of descending tracts?
There are two general classes of descending tracts:
cortical descending tracts
non-cortical descending tracts.
Both classes have their cell bodies in the brain
Cortical descending tracts originates in the primary motor complex of the pre-central gyrus. They are also known Pyramidal tracts.
Non-cortical descending tracts originates in subcortical areas of the brain e.g. thalamus, Midbrain, Brainstem, Pons, Medulla. They are known extra Pyramidal tracts.
what are the pyramidal and extra pyramidal tract fibres?
The pyramidal tract fibres are the:
lateral corticospinal tract
ventral corticospinal tract.
These have the same origin and endpoint.
The extra pyramidal fibres are the:
Rubrospinal tract
This starts at the level of the brain stem from a nucleus known as the red nucleus.
The medullar reticulospinal tract
The pontine reticular spinal tract
Medial longitudinal fasciculus
Tectospinal tract
what parts of the brain command skeletal muscle to contract?
how does that lead to the desired function?
clinical relevance?
The brain commands muscles to either rest or contract
The levels of contraction vary depending on the requirements of the body
Resting muscle Tone for Maintenance of posture (minimal contraction)
Displacement of joints for Movements (large forces may be required)
The brain’s motor commands of muscles are decussated (However there are exceptions to this rule):
Skeletal muscles of the left side of the body are commanded to move or rest by the right cerebral motor cortex
Skeletal muscles of the right side of the body are commanded to move or rest by the left cerebral motor cortex
Clinical relevance: Disorders of movement on the right side of the body tells you that the left portion of the brain is e.g. injured
