Neuro revision: Sensory pathways (VERY USEFUL) Flashcards
types of sensation
general sensation
- Somatic
- Visceral
special sesation
- hearing
- vision
- smell
two main sensory modalities
Spinothalamic pathway
Dorsal colum system
spinothalamic examples
o Temperature
o Pain
o Pressure/ crude touch
dorsal column examples
- Vibration
- Fine touch
- Proprioception (where our limbs are in space)
–> Detected by muscle spindles and receptors in joints - 2 point discrimination
–> Ability to resolve 2 simultaneous stimuli on the skin
–> E.g. how many fingers are being pressed on the body e.g. lips can resolve stimuli that are close together e.g. the back cannot resolve stimuli closely together
Different modalities travel along different
trajectories of the nervous system before they reach the brain
summarise what the somatosensory system does
System which carries conscious sensation from the body wall
e.g. skin, lining of pharynx and oral cavity, mucosa of the anus, parietal pleura and peritoneum
outline the pathway of the somatosensory system (general)
- First order neurone is a sensory neurone which detects stimuli using receptors (cell body in the dorsal root ganglion)
- It then projects into the CNS and synapses with second order neurone
- Either directly adjacent to where the first order neurone enters the spinal cord- spinothalamic tract and then ascends up the spinal cord (cell body in dorsal horn)
- Or the first order neurone will ascend up to the brainstem and synapse here- dorsal column-medial lemniscus system (cell body in medulla) - Second order neurones live entirely within the CNS and crosses the midline (decussates) into the opposite side of the CNS then synapses with the third order neurone found in the thalamus (thalamocortical neurones), projecting up to the somatosensory cortex -> perception occurs
Topographical receptor (somatotopy)
For every part of the body surface there is a corresponding part of the CNS
- Adjacent body parts are represented by adjacent parts of the CNS
- This trajectory is also involved in the conversion of the dermatomal pattern to the homulculus pattern
- At receptor level- dermatomal pattern
- At Sensory cortex- homunculus pattern
what are ascending pathways
Sensory pathways
- Dosal column-medial lemniscus system
- Spinothalamic system
The dorsal column-medial lemniscus system
The system the CNS uses to carry info to the brain concerned with:
- fine touch
- 2 point discrimination
- Proprioception
- vibration
Not as important for survival as the spinothalamic system
Dorsal column-medial lemniscus system example: Detecting vibration in a lumbar dermatome
- Receptor in lumbar dermatome communicates with first order sensory neurone (cell body in dorsal root ganglion) and projects into the spinal cord
- First order neurone ascends up the spinal cord to the medulla of the spinal cord, where it synapses onto a second order neurone (gracile nucleus- lower body)
- Second order neurone decussates (cross the midline) and ascends all the way up to the thalamus and synapses onto third order neurone
- Medial lemniscus- pathway connecting the gracile and cuneate nuclei with the thalamus - Third order neurone projects to the sensory cortex
- Lumbar region of the somatosensory cortex found medially
Dorsal column system: nuclei where the first order nuclei synapse
- Nucleus in the medulla where first order neurones from the lower half of the body synapse is called the gracile nucleus
- Nucleus in the medulla where the first order neurone from the upper half of the body synapse is called the cuneate nucleus
In the dorsal column system the higher up the first order neurone (e.g. cervical region) the more
laterally the axons will be found
- Clinical relevance- if there is a spinal cord lesion, as it grows it will affect the lumbar region first and then later the cervical region
The spinothalamic (anterolateral) system
Ancient system- crucial for the survival of the organism
- Temp
- Pain
- Pressure/ crude touch
The spinothalamic (anterolateral) system: e.g. detecting pain in the lumbar region
- Receptor in lumbar dermatome connects to first order neurone
- Synapses with second order neurone at approx. the level it enters (dorsal horn)
- Second order neurone decussates (crosses midline) and ascends up the cord (spinothalamic tract) into the brainstem and into the thalamus
- In the thalamus the second order neurone synapses with third order neurone which projects to the medial part of the primary sensory cortex (lower body)
The spinothalamic (anterolateral) system: e.g. detecting pain in the cervical region
- Receptor in cervical dermatome connects to first order neurone
- Synapses with second order neurone at approx. the level it enters (DH)
- Second order neurone decussates (crosses midline) and ascends up the cord (spinothalamic tract) into the brainstem and into the thalamus
- In the thalamus the second order neurone synapses with third order neurone which projects to the lateral part of the primary sensory cortex (upper body)
Spinothalamic tract projecting info from the upper and lower body (opposite to dorsal columns)
- The second order neurones from the** lower half of the body are found in the lateral region** of the spinothalamic tract
- The second order neurones from the **upper **half of the body are found in the medial region of the spinothalamic tract
Clinical relevance: expanding lesion in spinal cord will first destroy decussating fibres of the second order neurones (would be bilateral), then the lumbar part of dorsal columns will be affected- lower body and then the cervical regions of the spinothalamic tract- upper body
Differences between the spinothalamic vs dorsal column systems
Dorsal colum
- Decussates at level of the medulla
- Lumbar region most medial
Spinothalamic
- Decussates at level of entry
- Cervical region most medial
summary of the location of different tracts in the spinal cord: sensory and motor
somatic motor system
voluntary movement
components of the somatic motor system
upper motor neurones
lower motor neurones
relationship between UMN and LMN
- Upper motor neurones (UPM) projects onto the lower motor neurone (LMN)
- LMN- the final common path-> activates skeletal muscles
locations of the UMN
UMN= entirely within CNS
- In the primary motor cortex (precentral gyrus in frontal lobe)
- Not found in the basal ganglia or the cerebellum (won’t cause upper motor neurone signs- specifically related to neurones originating in primary motor cortex)
- Prognosis of damage to UMN worse than PNS
location of LMN
- LMN=both in the CNS (cell body and proximal axon) and PNS (distal axon synapsing with muscle)
- Most cell bodies found in ventral horn (some also in motor nuclei of the brainstem)
which tract is related to motor system
The corticospinal tract
- Major motor tract which descends through the spinal cord and controls activity of LMN
- Voluntary, precise movements
corticospinal tract also known as
descending tract
Upper motor neurone supplying the upper limb
- UMN of the upper limb originates in lateral aspect of the motor cortex (homunculus)
- Axon of the UMN passes down through the corona radiata and into the internal capsule (discrete white matter pathway running between the thalamus and lentiform nucleus)
- It then descends down through the midbrain via the cerebral peduncle, then through the pons and then to the medulla
- The medulla has medullary pyramids which carry corticospinal axons on their way down into the spinal cord
- Once the corticospinal axons have left the medullary pyramids the axons decussates and descends via the lateral corticospinal tract in the spinal cord where it synapses with a lower motor neurone and supplies muscles in the upper limb
Upper motor neurone supplying the lower limb
- UMN for the lower body originates in **medial aspect **of the motor cortex (homunculus)
- Axon of the UMN passes down through the corona radiata and into the internal capsule (discrete white matter pathway running between the thalamus and lentiform nucleus)
- It then descends down through the midbrain via the cerebral peduncle, then through the pons and then to the medulla
- The medulla has medullary pyramids which carry corticospinal axons on their way down into the spinal cord
- Once the corticospinal axons have left the medullary pyramids the axons decussates and descends via the** lateral corticospinal** tract in the spinal cord where it synapses with a lower motor neurone and supplies muscles in the upper limb
Lateral corticospinal tract organisation
- Axons destined for the upper limb are found more medially
o Need upper limbs axons more medially so they can peel off and synapse in the ventral horn earlier on (as to not waste axon and myelin) - Axons destined for the lower limb are found more laterally
Cortical spinal system is composed of 2 separate components:
- Lateral corticospinal tract
- Ventral corticospinal tract
Lateral corticospinal tract
- Much larger
- Function: innervating muscle of the distal parts of the extremities e.g. arm, hand and leg, foot
- E.g. fine movements
Ventral corticospinal tract
- Much smaller and crosses at the level of the lower motor neurone
- Function: innervating muscle of the proximal parts of the extremities e.g. limb girdle
Corticonuclear projections (cranial nerves)
- Cranial nerves can contain the axons lower motor fibres
- Corresponding upper motor neurones descend from the primary motor cortex and synapse upon on cranial nerve motor nuclei (e.g. trigeminal motor nucleus)
e.g. the trigeminal
- Contains the axons of lower motor neurones
- Cell bodies of lower motor neurones are found in the pons in the trigeminal motor nucleus
- These LMN follow the course of the trigeminal nerve to muscles of mastication (mandibular branch)
Bulbar cranial nerves and LMN
Medulla- lower motor neurones distributed along the IX, X, XI cranial nerves are found in elongated nucleus in the medulla
- Glossopharyngeal
- Vagus
- Abducens
- Hypoglossal
The internal capsule
White matter tract (not really a capsule) between the thalamus (medially) and the lentiform nucleus (laterally -globus pallidus and putamen)
- Bidirectional pathway
o Descending motor pathways (UMN)
o Ascending sensory pathways (third order sensory neurones)
- Basically the condensation of the corona radiata
Midbrain mickey mouse
Mickey mouse ears = Cerebral peduncles
o Connect the cerebral hemispheres to the midbrain
o Contain descending motor fibres which will enter the corticospinal tract
o Important motor structures
Eyes = red nucleus
o In fresh brain appears red
o Involved in motor control
o If damaged= course tremor
o Important landmark
Eyebrows= Substantia nigra
o Appears dark normally
o Produces dopamine
Tears = medial lemniscus (part of dorsal column / spinothalamic system)
o Parts of the sensory system
o Ascending sensory fibres on their way to the thalamus
Mouth=
cerebral aqueduct
o Channel connecting the third and fourth ventricles
Lips= periaqueductal grey matter
o Pain regulation
o Micturition
Lips= periaqueductal grey matter
o Pain regulation
o Micturition
- Nose= oculomotor nucleus
o Cell bodies of LMN
o Edwinger Westphal nucleus (cell bodies of parasympathetic neurones)
o Fibres coming off the nose= fibres of the oculomotor
Edinger–Westphal nucleus
The Edinger–Westphal nucleus supplies preganglionic parasympathetic fibers to the eye, constricting the pupil, accommodating the lens, and convergence of the eyes
Lesions to UMN
- Stroke
- Spinal cord injury
- Motor neurone disease
Lesions to LMN
- Peripheral neuropathy
- Motor neurone disease
- Spinal cord injury
- Disc prolapses
Fasciculations and fibrillations
Both features seen in LMN lesion (pathophysiology not fully understood)
Fasciculations
- Visible
- May be caused by aberrant activity in the distal portion of the LMN axon
Fibrillations
- Only detectable using electromyography
- May be caused by increased sensitivity of muscles fibres to ACh (changes in localisation of nicotinic receptors)
