Flashcards in Ascending Tracts Deck (57):
What are the different types of sensory neurones? What are they best for?
Free nerve endings - pain and thermal stimuli
Encapsulated nerve ending - touch, vibration (Pacinian)
Nerve ending with specialised cell - taste
What are stimulus modalities?
The types of stimuli detected eg light, touch, temperature, chemical changes (taste)
What are qualities of modality?
Eg for taste - sweet, sour, salty etc.
Give an example of when a receptor is not modality specific
If it is hit hard enough with a different modality eg when hit in the eye, see lights
What and where are proprioceptors?
Found in joints and muscles
Tell position of limbs etc.
List the types of proprioceptors and what they do
Muscle spindles - length of the muscle
Golgi tendon organs - power the muscle is exerting
How is a sensation detected and sent to the CNS?
Stimulus evokes change in permeability to ions of the receptor membrane
Creates a receptor potential - depolarisation
If depolarisation/generator potential is large enough, triggers an action potential which is propagated to the CNS
How is the strength of a stimulus detected?
The rate/frequency of action potentials produced
A stronger stimulus can activate adjacent cells
What is the difference between slowly adapting (tonic) receptors and rapidly adapting (phasic) receptors?
Tonic - keep firing as long as the stimulus lasts eg joint and pain receptors
Phasic - respond maximally and briefly to a stimulus eg touch receptors. Initially die action potentials and then stop. Some can fire when stimulus is taken away
What is acuity?
The precision by which a stimulus can be located
What is acuity determined by?
Lateral inhibition of the CNS - increases the ability to locate a stimulus. As a potential passes from a first order to second order neurone, get inhibition of lateral second order neurones
What is two-point discrimination?
The minimal interstimulus distance required to perceive two simultaneously applied skin indentations
What is two point discrimination determined by?
The density of sensory receptors (higher density, greater discrimination)
Size of neuronal receptive fields (the smaller they are, the better the discrimination)
Psychological factors such as stress and tiredness can reduce it
How does synaptic convergence and divergence affect acuity?
Convergence decreases acuity
Divergence amplifies the signal, making it harder to locate
Where is convergence seen?
When visceral pain fibres merge with somatic pain fibres
What happens at the thalamic level in the somatosensory system?
There is localisation and discrimination of stimuli
Gives highly organised projections to to cortex
Where in the cortex is the somatosensory cortex?
In the post-dental gyrus
What is the structure of the somatosensory cortex?
Cortical columns of information dedicated to precise areas, modalities and qualities
What can lead to loss of two-point discrimination?
A lesion to the sensory cortex eg in repeated epileptic events
What is astereognosis?
Inability to identify an object by active touch of the hands
What makes up the dorsal column of the spinal cord?
Fasciculi gracilis - contains fibres from the lower limbs and located medially
Fasciculi cuneatus - found only from C1-T6 - fibres from the upper limb
What information do the dorsal columns carry?
Where are the cell bodies of sensory neurones located?
In the dorsal root ganglion
On which side do neurones in the dorsal column travel on?
The ipsilateral side
In the dorsal tract, where does each neurone synapse?
First and second - nucleus gracilis or cuneatus in the medulla on the ipsilateral side
In the dorsal tract, when do neurones decussate?
Second order neurones decussate at the medulla
In the dorsal tract, where do the second order neurones go?
Run in the medial lemniscus
Ascends through the pons and midbrain
Synapse with third order neurones in the synapse
What does damage to the dorsal tract result in?
Loss of fine touch and proprioception in the ipsilateral side below the level of the spinal cord lesion
Symptoms of damage to the dorsal column?
Loss of tactile sense, proprioception and an inability to identify objects placed in the hand
What information does the lateral spinothalamic tract carry?
Pain and temperature sensations
When does the lateral spinothalamic tract fibres decussate?
At the level they enter the spinal cord
Via the ventral white commissure
Where do first order neurones of the lateral spinothalamic tract synapse with second order neurones?
In the dorsal horn of the spinal cord
In which side do neurones in the lateral spinothalamic tract ascend?
The contralateral side
What is the order (from medial to lateral) that the fibres of the lateral spinothalamic tract ascend in?
Where do axons of the lateral spinothalamic tract synapse with the third order neurones?
In the thalamus
Symptoms of damage to the lateral spinothalamic tract?
Loss of pain and temperature sensation on the contralateral side of the body - at and below damage
Why does loss of pain sensation happen a few levels below the location of damage to the lateral spinothalamic tract?
The first order afferent fibres in the zone of Lissauer ascend/descend one or two levels before making their synaptic link with the second order neurone
Why does sacral sparing occur?
Sacral and lumbar fibres are located more laterally in the lateral spinothalamic tract than cervical and thoracic fibres
Any expanding tumour/lesion in the grey matter will affect the thoracic and cervical fibres first, while lumbar and sacral fibres will still have intact pain and temperature fibres (initially)
What information does the anterior spinothalamic tract carry?
Crude touch and pressure
What is the course of the anterior spinothalamic tract?
Same as the lateral spinothalamic tracts
Known collectively as the anterolateral ascending tracts
What information do the anterior and posterior spinocerebellar tracts carry?
-muscle spindle fibres and Golgi spindle organs
Convey information about whole limb movements and postural adjustments to the cerebellum
Where do the anterior and posterior spinocerebellar tract first order neurones synapse with second order neurones?
In the dorsal horn
Where do fibres of the anterior spinocerebellar tracts decussate?
At the level they enter in the tract
Again in the pons
Where do the second order neurones synapse with third order neurones in the anterior and posterior spinocerebellar tract?
Second order neurones terminate in the cerebellum
What does damage to the anterior spinocerebellar tract result in?
Loss of proprioception and co-ordination to the contra-lateral lower limb
What does damage to the posterior spinocerebellar tract result in?
Loss of proprioception and co-ordinated movement on the ipsilateral side
Where do fibres of the posterior spinocerebellar tract decussate?
What information does the cuneocerebellar tract transmit?
What is the course of fibres in the cuneocerebellar tract?
Ascend on the ipsilateral side
Synapse in the brain stem
Second order neurones terminate in the cerebellum
Where are the cell bodies of first order neurones located?
In the trigeminal ganglion
Where do first order neurones of the trigeminal nerve synapse
In the trigeminal nucleus of the brain stem
Where do second order neurones and third order neurones of the trigeminal nerve travel?
Second order from the trigeminal nucleus of the brain stem to the thalamus
Third order travel to the cerebral cortex
What is tabes dorsalis caused by?
What happens in tabes dorsalis?
Central processes of the dorsal root ganglion degenerate - therefore affects the dorsal columns (fasciculus gracilis and cuneatus)
Signs and symptoms of tabes dorsalis
Dorsal columns affected
-loss of fine touch - loss of two-point discrimination
-loss of conscious proprioception - ataxia
What is sensation?
The conscious/sub-conscious awareness of an external or internal stimulus