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Flashcards in Sensation & Ascending Tracts Deck (37):
1

What are some of the different types of sensory receptors of first order neurones?

Free nerve ending e.g. cold stimuli

Encapsulated nerve ending e.g. pressure

Specialised cell e.g. gustatory (taste) receptors sense glucose molecules

2

What is the difference between stimulus modality and the quality of a sensation? How specific are sensory receptors to stimulus modalities?

Stimulus modality e.g. light touch, temp., chemical changes

Quality of a sensation is a subdivision of stimulus modality
e.g. taste (modality) can be subdivided into sweet, sour, salt, etc.

Sensory receptors are modality specific up to a point (strong enough stimulus may activate a different specificity receptor e.g. seeing light when punched in the eye)

3

Contrast muscle spindles and golgi tendon organs.

Muscle spindle = receptor embedded between and parallel to fibres of striated muscle that is sensitive to stretch

Golgi tendon organ = receptor within tendon that responds to tension/stretching

Both are involved in proprioception

4

How is the strength of a stimulus determined? How can the strength of a stimulus affect sensory neurones?

Strength of stimulus determined by rate of action potential stimulus (frequency coding)

Stronger stimuli:
- activate sensory neurone with different sensory modalities
- activate neighbouring neurones

5

Contrast tonic and phasic receptors.

Tonic (slowly adapting receptors) = keep firing as long as a stimulus lasts
e.g. joint and pain receptors

Phasic (rapidly adapting receptors) = respond maximally and briefly to a stimulus e.g. light touch receptors (hence why we stop feeling our clothes whilst wearing them, unless movement causes a change in position of the clothing on our skin)

6

Define acuity. Briefly, what is it determined by?

Precision by which a stimulus can be located

Determined by:
- lateral inhibition (CNS)
- two-point discrimination
- convergence & divergence

7

How does lateral inhibition determine acuity of sensation?

Points of skin which are stimulated ---> first order neurone ---> second order neurone ---> stimulus detected by CNS

Points of the skin which are not stimulated ---> first order neurone ----> inhibitory interneurones inhibit second order neurone ---> no stimulus sensed by CNS

This allows precise points of stimulus to be sensed by contrasting areas of stimulation to areas not being stimulated

8

How does two-point discrimination determine acuity of sensation?

Two-point discrimination = minimal interstimulus distance required to perceive two simultaneously applied skin indentations

e.g. fingertips = 2mm apart, forearm = 40mm apart, trunk = 6cm apart

Determined by:
- density of sensory receptors
- size & overlap of neuronal receptive fields (the larger the field, the more likely it is to overlap with another field, therefore there is reduced two-point discrimination)

9

How does convergence and divergence of neurones determine acuity of sensation?

Convergence of multiple sensory first order neurones onto a single second order neurone ---> decreased acuity

Divergence of multiple sensory second order neurones from a single first order neurone ---> increased acuity (amplifies signal, as more axons are projecting to the CNS)

10

How is the thalamus involved in sensation? How can sensation be affected by thalamic lesions?

Thalamus involved in crude localisation and discrimination of stimuli via highly organised projections to the cortex

Thalamic lesions (e.g. stroke) ---> thalamic syndrome (severe chronic pain not in proportion to the stimulus)

11

What is synaesthesia?

Secondary subjective sensation (e.g. colour) is experienced at the same time as the sensory response normally evoked by the stimulus.

Caused by confusion between nerve bundles

12

What is the somatosensory cortex? Where is it? What can a lesion of the somatosensory cortex result in?

Post-central gyrus

Required for precise localisation and discrimination of stimuli (somatotrophic representation) - each body area has specific cortical representation (and specific modalities)

e.g. sensory homunculus

note: relative size of each area is reflective of the degree of sensory acuity associated with the body area

Relays to other cortical and subcortical areas ---> choice to respond to stimulus taken at cortical level (based on previous memories of stimulus, whether the stimulus is pleasant/unpleasant, whether action is needed, + input from limbic system & amygdala)

Lesion of somatosensory cortex (e.g. repeated epileptic events) =
- loss of two-point discrimination
- astereognosis = retains normal sensation but cannot recognise 3D objects by touch alone

13

What is ICU psychosis? Contrast this with institutionalised sensory deprivation and delirium.

ICU psychosis = patients in ICU experiencing anxiety, paranoia, auditory & visual hallucinations, disorientation, agitation etc. (contributed to by dehydration, hypoxia, heart failure, infection, drugs, etc.)

Delirium refers to an acute brain syndrome.

People who are institutionalised (e.g. in care homes) can be affected by the resultant sensory deprivation.

14

Define perception.

Awareness of a stimuli and our ability to discriminate between different types of stimuli

15

Contrast the central process and peripheral process of the ascending tracts.

CENTRAL PROCESS = conduct impulses to the spinal cord
- terminate in the grey matter OR
- ascend into the white matter without terminating

PERIPHERAL PROCESS = convert sensory signals to electrical impulses and conduct to main axonal segment of neurone

16

Give examples of conscious and non-conscious sensations which travel along the ascending tracts?

CONSCIOUS:
- tactile sensation
- pain
- temperature
- crude touch

NON-CONSCIOUS:
- tactile sensation
- muscle length
- muscle tension
- joint position
- joint angle
- proprioception

17

Describe the path of the first order sensory neurones of the lateral spinothalamic tract.

Primary sensory neurone divides on entering the spinal cord:

- one branch terminates in the dorsal horn of the respective spinal segment

- one branch travels 1-3 segments rostrally (via the posterolateral tract of Lissauer) before terminating

- one branch travels 1-3 segments caudally (via the posterolateral tract of Lissauer) before termianting

18

Describe the path of the second order neurones of the lateral spinothalamic tract.

Sends axon towards the midline (anterior to the central canal), crossing the midline to the opposite side (decussation) to form the anterior white commissure (therefore site of decussation is at the segmental level)

Subsequently joins the other fibres of the lateral spinothalamic tract which have already ascended.

Terminates in the ventral posterolateral nucleus of the thalamus

19

Describe the path of the third order neurones of the lateral spinothalamic tract.

Travel via the internal capsule of the ventral posterolateral nucleus of the thalamus

Terminate in the post-central gyrus of the cerebral cortex

20

What is the difference between the dorsolateral and ventromedial spinothalamic tracts?

Dorsolateral tracts = fibres arising from the lowest part of the body (level? below the cervical cord?)

Ventromedial tracts = fibres arising from the cervical cord

Any other differences???????

21

What are the sensory functions of the spinothalamic tracts?

Ant. = crude touch and pressure

Lateral = pain and temperature

22

Describe the path of the first order sensory neurones of the dorsal column.

Enters grey matter of the dorsal horn and divides into two branches:

- one branch terminates in the dorsal horn at the neural level of entry and synapses with the nucleus of Clarke

- one branch ascends through the dorsal funiculus to terminate in the medulla on the same side as its origin

Medial column = FASCICULUS GRACILIS
- transmits conscious proprioceptive signals from below T6

Lateral column = FASCICULUS CUNEATUS
- transmits conscious proprioceptive signals from above T6

23

Describe the path of the second order sensory neurones of the dorsal column.

- nucleus of Clarke sends axon towards cerebellum to join either the dorsal spinocerebellar tract or the ventral spinocerebellar tract

- dorsal column sends axons ventromedially in the medulla oblongata (as the internal arcuate fibres) to cross the midline (becoming the medial lemniscus), ascending on the contralateral side to terminate in the ventral posterolateral nucleus of the thalamus

24

Describe the path of the third order neurone of the dorsal column.

Travels through the internal capsule of the ventral posterolateral nucleus of the thalamus

Terminates in the sensori-motor cortex

note: does NOT terminate in the post-central gyrus

25

Describe the path of the first order sensory neurones of the spinocerebellar tracts.

Terminate in the grey matter of the dorsal cord either:

- at level of entry
- or ascend to synapse with Clarke's nuclei (nucleus dorsalis)

note: Clarke's column is between C8-L3, so information above C8 (the upper limbs) is conveyed along the cuneocerebellar tract (first order sensory neurones join the fasciculus cuneatus and terminate in the lateral cuneate nucleus)

26

What are the functions of the dorsal columns?

Joint position

Light touch

Vibration sense

Conscious proprioception

27

Describe the path of the second order neurones of the spinocerebellar tracts.

Either:

- ascend to join the dorsal spinocerebellar tract, which terminates in the vermis & anterior lobe of the cerebellum on the ipsilateral side

- cross the midline (decussate) anterior to the central canal to join the ventral spinocerebellar tract, which then crosses back over in the pons to terminate in the cerebellum (therefore still ipsilateral)

- the cuneocerebellar tract (information from arms) enters the ipsilateral inferior cerebellar peduncle and terminates in the spinocerebellum

28

Describe the path of the third order neurones of the spinocerebellar tracts. CHECK!!!

There are no third order neurones (check?)

29

What are the functions of the spinocerebellar tract?

Unconscious proprioception

Anterior spinocerebellar tract = proprioceptive info from lower limbs

Posterior spinocerebellar tract = proprioceptive info from lower limbs (decussates twice)

Rostral spinocerebellar tract = proprioceptive info from upper limbs

Cuneocerebellar tract = proprioceptive info from upper limbs

30

Define sensation.

Conscious or subconscious awareness of an external or internal stimulus

note: includes receptor activation

31

What parts of the body can touch stimuli be most accurately localised and why?

Fingertips, palms, lips

High density of sensory receptors

Small neuronal receptive fields (less overlap, therefore more precise)

32

Why are we not continuously aware of the touch of clothing as you sit still?

Touch receptors are phasic (rapidly adapting) and only respond to stimuli at initiation and cessation

Adapt quickly to maintained stimulus but sensitive to changing stimulus (responding to the rate of change of input)

33

What will the presentation of a patient with a lesion in the thalamus or internal capsule be?

Sensory agnosia (contralateral, includes face)

- ignore somatic sensation (inc. pain) from one side of the body

- associated with lesions of the parietal lobe ---> parietal lobe dysfunction

- often caused by stroke

34

What anatomical landmark determines whether sensation is lost in the face if it is involved in damage?

Trigeminal ganglion (pons)

Damage above this point - loss of sensation includes the face

Damage below this point - loss of sensation excludes the face

35

What will the presentation of a patient with a lesion in the thalamus or internal capsule be?

Sensory agnosia (contralateral, includes face)

- ignore somatic sensation (inc. pain) from one side of the body

- associated with lesions of the parietal lobe ---> parietal lobe dysfunction

- often caused by stroke

36

What anatomical landmark determines whether sensation is lost in the face if it is involved in damage?

Trigeminal ganglion (pons)

Damage above this point - loss of sensation includes the face

Damage below this point - loss of sensation excludes the face

37

List some important dermatomes.

Tip of shoulder = C3/C4
Middle finger = C7
Axilla = T2
Umbilicus = T10
Supra-pubic area = L1
Knee = L3
1st toe = L5
5th toe = S1