Ascending Tracts & Sensation

Question 1

Name some stimulus modalities and understand what is meant by their quality

Answer 1

Light
Touch
Temperature
Taste (chemical changes) 
Quality - subdivision of a modality e.g. taste = sweet/sour/salty

Question 2

Describe the role of sensory receptors

Answer 2

Sensory receptors of muscles and joints provide information on body position (proprioception)
Sensory receptors are modality specific unless increased stimulus

Question 3

Explain how we detect changes

Answer 3

Stimulus evokes change in membrane permeability to ions of the receptor –> receptor potential –> generator potential triggers AP –> AP a propagates into CNS

Question 4

Explain how we detect the strength of a stimulus

Answer 4

Strength is determined by the rate of AP stimulus (frequency coding)
Stronger stimuli can also activate neighbouring cells

Question 5

Differentiate between slowly adapting (tonic) and rapidly adapting receptors

Answer 5

Detect how long the stimulus lasts 
Slowly adapting (tonic) - keep firing as long as stimulus lasts e.g. joint, pain receptors, respond continuously
Rapidly adapting (phasic) - respond maximally and briefly to a stimulus e.g. light touch receptors, sensitive to change

Question 6

Understand how information about the localisation of a stimulus reaches the CNS

Answer 6

Acuity - precision by which a stimulus can be located
Convergence decreases acuity, divergence amplifies acuity
Lateral inhibition - due to presence of inhibitory interneurones
Two point discrimination - minimum interstimulus distance required to perceive two simultaneously applied skin indentations, depends on size of receptive fields, psychological, fingertips = 2mm, forearm = 40mm

Question 7

Understand the concept of the sensory homunculus

Answer 7

The more receptors in a given area of skin, the larger that areas region on the homonculus.
The size of each body region in the homonculus is related to the density of sensory receptors
Large face and hands compared to torso

Question 8

With reference to cutaneous receptors, understand the effect of the distribution of sensory endings upon tactile discrimination

Answer 8

Increased density of receptors = decreased size of receptive field
Fingertips = 2mm
Forearm = 40mm

Question 9

Name the ascending tracts and their association with the somatic senses

Answer 9

DCML - light touch, proprioception, vibration
Anterior spinothalamic - crude touch, pressure
Lateral spinothalamic - pain, temperature

Question 10

Describe the DCML tract including function, cell bodies of 1st, 2nd, 3rd order neurones, decussation and termination

Answer 10

Function - light touch, vibration, proprioception
1st order neurone - dorsal root ganglion
2nd order neurone - nucleus gracilis (lower limb), nucleus cuneate (upper limb)
3rd order neurone - thalamus
Decussation - medulla
Termination - primary sensory cortex in post central gurus

Question 11

Describe the anterior spinothalamic tract including function, cell bodies of 1st, 2nd, 3rd order neurones, decussation and termination

Answer 11

Function - crude touch, pressure
1st order neurone - dorsal root ganglion
2nd order neurone - dorsal horn (substantia gelatinosa)
3rd order neurone - thalamus 
Decussation - spinal cord 
Termination - sensory cortex

Question 12

Describe the lateral spinothalamic tract including function, cell bodies of 1st, 2nd, 3rd order neurones, decussation and termination

Answer 12

Function - temperature, pain
1st order neurone - dorsal root ganglion
2nd order neurone - dorsal horn
3rd order neurone - thalamus 
Decussation - spinal cord
Termination - sensory cortex

Question 13

Describe the anterior and posterior spinocerebellar tract including function, cell bodies of 1st, 2nd, 3rd order neurones, decussation and termination

Answer 13

Function - unconscious proprioception
1st order neurone - dorsal root ganglion
2nd order neurone - spinal grey matter
3rd order neurone - none
Decussation - spinal cord (anterior), none (posterior)
Termination - cerebellum

Question 14

Describe the cuneocerebellar tract including function, cell bodies of 1st, 2nd, 3rd order neurones, decussation and termination

Answer 14

Function - unconscious proprioception 
1st order neurone - dorsal root ganglion
2nd order neurone - nucleus cuneate 
3rd order neurone - none
Decussation - none
Termination - cerebellum

Question 15

Understand and explain Brown-Sequard syndrome

Answer 15

Hemisection (one sided lesion) of spinal cord
Involves DCML, spinothalamic, corticospinal tracts
Loss of tactile sensation and proprioception ipsilateral
Loss of pain and temperature sensation contralateral
Ipsilateral hemiparesis
Causes - trauma, tumour, ischaemia/infarction, MS
Diagnosis - MRI
Treatment - methylprednisolone (reduce swelling), surgical management

Question 16

Use your knowledge of the ascending tracts to understand the sensory effects of lesions in the CNS

Answer 16

Injury to DCML tract:
Loss of fine touch and proprioception ipsilateral (if lesion is within spinal cord)
Injury to spinothalamic tract:
Loss of pain and temperature contralateral
Injury to spinocerebellar tracts:
Loss of muscle coordination ipsilateral

Question 17

What is somatic sensation?

Answer 17

Perception, awareness and ability to discriminate different types of stimuli (e.g. tactile, thermal, pain, proprioception)
Conscious or subconscious awareness of an external or internal stimulus

Question 18

Describe how we detect modality, rate of change, location and intensity of a stimulus

Answer 18

Modality - type of receptor stimulated, specific sensory pathway to brain
Rate of change - frequency of APs, receptor adaptations
Location - size of receptive field, lateral inhibition, projection to particular area of cortex
Intensity - frequency of APs, number of receptors activated

Question 19

Understand and explain Syringomyelia

Answer 19

Development of a cyst (syrinx) in the central canal of spinal cord
Loss of pain and temperature (lateral spinothalamic)
Affects cervical and upper thoracic cord (cape-like sensation)
Causes - post trauma, neoplasia, meningitis complication, congenital malformation (Arnold Chiari)
Diagnosis - MRI
Treatment - surgical decompression