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Jess Nervous > 4.1 Principles of sensation > Flashcards

4.1 Principles of sensation Flashcards

1
Q 
Name the sensory receptors responsible for the following 
A) Touch
B) temperature 
C) pain
D) chemical
E) eye
A 
A) mechanoreceptors 
B) thermoreceptors
C) nociceptors 
D) chemoreceptors
E) photoreceptors
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2
Q

Where are receptors for special senses (i.e vision, hearing and balance, taste, smell) located?

A

In complex sense organs

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3
Q

What comprises the majority of sensory receptors?

A

Dendritic endings of efferent neurons

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4
Q

Which types of mechanoreceptors are responsible for receiving info from the outside (somatic) world and the internal (visceral) world? What kinds of things do they detect>

A

Exteroceptor; touch, special senses

Interoceptor; chemical changes, tissue stretch, temperature, etc

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5
Q

What receptor senses position and movement of muscles and where are they?

A

Proprioceptors located in skeletal muscle, joints, ligaments and associated CT

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6
Q

Which type of receptor detects taste? How are different tastes (i.e bitter, sweet, sour, salt, etc) accounted for?

A

Chemoreceptors located in the taste buds detect taste. Taste buds also contain gustatory cells with different receptors which can detect different tastes.

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7
Q

Where are auditory hair cells located?

A

Within the organ of corti of the inner ear

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8
Q

What happens in response to tectorial membrane resonance?

A

Mechanoreceptors located on the stereocilia of auditory hair cells (in the inner ear) open

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9
Q

Describe the structure of olfactory receptor neurones and what they do. How are different smells accounted for?

A

They are bipolar neurons (meaning they have two extensions away from the soma) with an apical dendrite terminating in long cilia to detect dissolved chemical odorants (which are dissolved in the fluid produced but the bowman’s glands underneath)

A range of receptors detect different smells

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10
Q

What three forms can receptor nerve endings exist in?

A
  1. Free nerve endings (dendrites)
  2. Dendrites encapsulated in myelin
  3. Specialized (special sensory cells)
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11
Q

Define sensory transduction and briefly describe how it works

What does knowledge of the nature and location of the stimulus depend upon?

A

The transformation of a stimulus into an electrical signal

When a stimulus impinges on a receptor it induces a change in membrane potential proportional to its intensity -> this triggers the nerve to generate a series of APs that encode information about the intensity and duration of the stimulus

Knowledge of the nature and location of the stimulus depends on the connections afferent fibres make within the CNS

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12
Q

What determines the strength of a stimulus?

A

The number of afferent fibres activated and the frequency of APs

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13
Q

What is a phasic receptor? When is their maximum response to a stimulus?

A

A sensory receptor that rapidly adapts (and therefore stops responding quickly) to a stimulus even if it continues. Their maximal response is at the start and finish of the stimulus

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14
Q

Name three examples of phasic receptors

A

Tactile/Meissner corpuscle

Photoreceptors

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15
Q

What are tonic receptors? Name two examples

A

They are slow adapting and continue responding as long as the stimulus is present
I.e nociceptors and proprioceptors

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16
Q

Name the three ‘insults’ that can stimulate nociceptors

A

Mechanical (i.e pinching), thermal and chemical

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17
Q

Which fibres/primary afferents are responsible for transmitting acute and diffuse pain respectively and what are their associated mechanoreceptors? What kind of nerve ending do pain receptors have generally?

A

Pain receptors have free nerve endings

Acute: faster, large and myelinated A delta fibres
-associated with mechanoreceptors at sharp pain nerve endings

Diffuse: slower, small and unmyelinated C fibres
-associated with mechanoreceptors at “social” touch and diffuse pain receptors

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18
Q

What are thermoreceptors sensitive to and how do they respond to the stimuli?

A

Sensitive to hot, cold and extreme temperatures - they are tonic receptors (slow adapting)

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19
Q

What causes the sensation of spiciness and heat?

A

Capsaicin (a chemical found in red peppers) activates TRVP1, which are thermoreceptors that open and depolarize when heat >42 degrees C is detected

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20
Q

What receptor does menthol activate and what else does this receptor respond to? What is the final result?

A

Activates TRPM8, a thermoreceptor activated by a decrease in temperature and menthol (a chemical in mint). The signal is conducted by the primary afferents (A delta and C) to generate cold and cold pain

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21
Q

Which fibre types are associated with the following mechanoreceptors
A) proprioceptors
B) cutaneous mechanoreceptors
Which transmits a stimuli faster and what determines the speed of transmission?

A

A) A alpha - faster
B) A beta

Thicker, myelinated axons transmit faster signals

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22
Q

Describe the kind of axons which supply mechanoreceptors

A

Thickly-myelinated and highly sensitive

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23
Q

What are the four major types of mechanoreceptors in the body?

A
  1. Meissner’s corpuscles
  2. Merkel’s discs
  3. Ruffini’s end organs
  4. Hair follicle receptors
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24
Q

Describe the structure and location of Meissner’s corpuscles, what do they detect?

A

Peanut-shaped concentric stacks of Schwann cells (myelinated) found in the superficial dermis between the rete ridges/dermal papillae (epithelial extensions projecting into the underlying CT) in the fingertips, lips, forearm, palms and soles

They detect movement across the skin (light touch) and things slipping between fingertips

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25
Q

What are Merkel’s discs and which locations are they associated with? What type of nerve ending do they have?

A

Free nerve endings of specialized epithelial cells called Merkel cells. They are found in the basal layer and are associated with glabrous/smooth skin (especially fingertips) and hair follicles

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26
Q

What do Merkel’s discs detect? How is their receptive field and how quickly do they adapt to stimuli?

A

They are tactile receptors that help distinguish texture and shape of objects. They have a narrow receptive field and are slowly adapting

For example; capacity to read brail

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27
Q

Where are pacinian corpuscles and what do they detect? How is their receptive field and rate of adaptation?

A

Deeper in the dermis, ligaments and joints
They detect pressure, vibration and tickle

Not as important to localize the source of the vibration so receptive field tends to be broader, rapidly adapting

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28
Q

What do pacinian corpuscle looks like

A

Nerve ending wrapped in concentric layers of myelin, resembles Onions

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29
Q

Where are Ruffini corpuscles and what do they detect? Name one example of where they are found in the body

A

In the dermis and joints (i.e golgi tendon organs)

They detect stretch, proprioception (position and movement) and pressure deep in the fascia

30
Q

What do hair follicle receptors detect?

A

Light touch receptors that detect hair movement

31
Q

What is a receptor field?

A

A region of skin where a tactile stimulus evokes a sensory response in the cell or its axon

32
Q

What is two-point touch discrimination and when is it used? What does the discrimination depend on?

A

The ability to discriminate two discrete points in contact with a body area, used in assessing tactile perception and dorsal column testing

Therefore discrimination depends on the concentration of receptors in that region and the size of the receptive field

33
Q

What is the two point discrimination for the following body sites?
A) fingertips
B) palm
C) arm

A

A) 2 mm
B) 10 mm
C) 40 mm

34
Q

Define somatotopy

A

The point for point correlation between an area on the body and a specific point in the CNS

35
Q

What is the “homunculus” and how was it created?

A

It’s a distorted body figure mirrored in the primary motor cortex created by mapping the amount of the primary somatosensory cortex dedicated to processing info from certain peripheries.

36
Q

What are ascending/afferent pathways to the brain composed of, how do their differences influence their location of termination?

A

2 neurone chains (unconscious processes) terminate in the cerebellum

or 3 conscious neurone chains (conscious processes) terminate in the somatosensory cortex (postcentral gyrus)

37
Q

What are the three main ascending sensory tracts and what are they responsible for?

A
  1. Posterior/dorsal column tract: fine touch, proprioception, vibration
  2. Spinothalamic tract: pain, temperature, crude touch and pressure
  3. Spinocerebellar tract: proprioception
38
Q

Describe the three ‘chains of neurones’

A

First order neurones run from receptor -> spinal cord

Second order neurones are interneurones with their cell bodies in the spinal cord

Third order neurones transmit info from the thalamus to the cerebral cortex

39
Q

Which chain of neurones are sensory afferents with cell bodies in dorsal root ganglia?

A

First order neurones

40
Q

Describe the organization of white matter

A

Myelinated axons organized into fibre tracts which run up and down the spinal cord called funiculi

Axons cross from one side of the spinal column to the other (decussation) at the ventral white commissure

41
Q

Describe the structure of grey matter

*including the contents of all three horns and the grey commissure

A

Dorsal horn: interneurone cell bodies
Lateral horn: preganglionic sympathetic neurones
Ventral horn: Interneurons and motor neurones
Grey commissure: connects axons travelling from one side of the spinal column to the other (decussation)

42
Q

Which part of the spinal cord has lateral horns?

A

C8-L3

43
Q

Where does the posterior/dorsal column tract decussate?

*another question on this in detail

A

Where the synapses occur in the medulla and the thalamus

44
Q

Where does the fasciculus gracilis and the fasciculus cuneatus carry information from? Which ascending sensory pathway are they a part of?

A

Part of the posterior/dorsal column tract
Fasciculus gracilis: Info originating below T6 (lower limb)
Fasciculus cuneatus: info originating from T6 and above (upper limb)

45
Q

Describe the two “three neurone pathways” of the spinothalamic tracts and which sensations they transmit

A

Anterior spinothalamic tract: crude touch and pressure

Lateral spinothalamic tract: pain and temperature

46
Q

Name three sites on the body where spinocerebellar tracts carry proprioceptive information from and briefly describe each

A

Golgi tendon organs: proprioceptive receptors found within the tendons on each end of a muscle, senses changes in muscle tension/contraction as exerted by the tendon

Muscle spindles: stretch receptors in the body of muscle that detects changes in the length of the muscle

Joint capsules

47
Q

What kind of neurone pathway are spinocerebellar tracts? Where do they terminate and why is this significant?

A

Two-neurone pathways (subconscious)

They terminate in the cerebellum where subconscious processing takes place

48
Q

Which part of the brain is the conscious mind found?

A

Cerebral cortex

49
Q

Describe how the cerebral cortex hemispheres process sensory information and control motor functions

A

Each hemisphere processes sensory info and controls motor functions of the opposite side of the body

The precentral gyrus is responsible for communicating with neurones for motor movement

The postcentral gyrus is responsible for communicating with neurones for sensation

50
Q

Which region of the brain produces movement with minimal electrical stimulation?

A

Primary motor cortex (M1) in the frontal lobe along a bump called the precentral gurus

51
Q

How do pyramidal cells/neurones in the precentral gyrus of the cerebral cortex communicate with the spinal cord?

A

The axons travel down into the spinal cord via the corticospinal (pyramidal tracts)

52
Q

Name three ways nerves can be damaged by

A
  1. Vascular nerve damage
  2. Demyelination
  3. Axonal degeneration (toxic, metabolic or physical)
53
Q

How many modalities do receptors typically respond to? Name one exception

A

Typically one, although they may be activated by others

I.e The eye preferentially responds to light, but with a blow to the head (mechanical stimulus) we may “see stars”

54
Q

What determines the sensation felt?

A

The type of receptor activated

55
Q

Describe the principles of sensory acuity and how they relate to receptive fields

A

Each sensory neurone responds to a stimulus if the stimulus falls within its receptive field - and the size of the receptive field varies with receptor density.

The smaller the receptive field in a region, the higher the acuity, i.e our ability to locate the stimulus accurately and to distinguish between two closely applied stimuli (two-point discrimination)

56
Q

What enhances sensory acuity?

A

Lateral inhibition: The ability for interneurones to suppress the weaker signals from nearby (“Lateral”) receptors to help pinpoint the strongest point of the stimulus, thereby improving acuity

57
Q

Name one example on the body where there are large and small receptive fields

A

Large: trunk
Small: fingertips

58
Q

Which regions of the body, (according to the homunculus) have a greater allocation of space in the cortex and why?

Therefore, what determines the amount of cortex dedicated to a body area?

A

Areas where sensation is more important, such as the face, lips and palms have a higher density of receptors and neural circuitry - therefore more space in the cortex

Therefore, the amount of cortex dedicated to a body area is determined by how important sensation to that area is

59
Q

Describe in detail the three neurone pathway involved in the transmission of messages in the dorsal column pathway.

A

The first neurone is attached to a receptor, their cell bodies sit in the dorsal root ganglion (just outside and adjacent to the spinal cord)

Neuronal axons from the lower limb (below T6) travel in the gracile fasciculus and from the upper limb via the cuneate fasciculus. These pathways ascend to the medulla oblongata (lower half of the brain stem) where they synapse with with the second neurone (gracilis or cuneate nucleus). They then decussate in the medulla oblongata and ascend up the pons to an area of the thalamus called the ventral posterolateral nucleus. Here they synapse with the third neurone which projects from the thalamus to the primary cerebral cortex on the post central gyrus.

60
Q

Where generally does the ascending dorsal column pathway ascend and what does it ascend next to?

A

At the dorsum of the spinal cord next to the dorsal median sulcus

61
Q

Describe in detail the three neurone pathway involved in the transmission of messages in the anterolateral spinothalamic tracts

A

First order neurone has its cell body in the dorsal root ganglion (like dorsal column pathway) and enters the spinal cord and immediately ascends/descends 1-2 levels in an area called Lissauer’s fasciculus

They then synapse onto the second order neurone in the dorsal horn of grey matter in an area called the substantia gelatinosa

From here it decussates to the other side of the spinal cord and ascends via the spinothalamic fasciculus/tract up to the thalamus where it synapses with the third order neurone. This neurone projects onto the primary cerebral cortex on the post-central gyrus

62
Q

Describe the two neurone pathway involved in the transmission of messages in the spinocerebellar tracts, do all neurones decussate?

A

The first order neurone has its cell body in the dorsal root ganglion, enters the spinal cord and synapses onto the second order neurone in the dorsal horn of grey matter; in an area called Clark’s nucleus.

The second order neurone ascends (in the anterior OR posterior) spinocerebellar tract to the midbrain where it enters the cerebellum and synapses onto target Purkinje cells.

The Dorsal tracts ascent straight up and synapse on the same side of the body

The Ventral tracts cross over to the other side of the spinal cord as part of the anterior white commissure, ascends to the cerebellum and then cross back to their original place (“double-cross”)

63
Q

What other several minor ascending pathways are there in the spinocerebellar tracts?

A

Cuneocerebellar pathways

64
Q

What would damage to a single spinal nerve likely result in? How long is recovery and why?

A

Anaesthesia limited to the spinal nerve’s dermatome and no other symptoms, this is a minor lesion and sensation may turn within a shorter time (i.e 3-6 months) as peripheral sensory nerves can regenerate

65
Q

What would damage to a complete nerve to a limb (i.e sciatic nerve) result in?

A
  1. Anesthesia of the whole limb spanning a multitude of dermatomes
  2. Loss of sensory function of the limb (possibly damaging its protective reflexes, i.e burns)
  3. Motor function will be damaged as sensory feedback, movement induced sensory feedback (Re-afference), proprioception and kinaesthesia aren’t available to that limb
66
Q

What are the two types of photoreceptor of the retina and what do they detect to ultimately stimulate the optic nerve?

A

Rod cells: sensitive enough to respond to a single photon of light

Cone cells detect colour (contain photopigments which change shape as they absorb different lights)

Ultimately these depolarize the cell and stimulate the optic nerve

67
Q

What form do most nerve endings in the skin exist in?

A

Encapsulated in myelin (associated with Schwann cells) which reduce the sensitivity for firing but increase the specificity - therefore specific stimuli which results in a firing of AP depends on the structure of the nerve ending

68
Q

What might free nerve endings detect?

A

Crude touch and diffuse pain

69
Q

Describe a bipolar neurone and their major role in the body, what is one specific area where these are found?

A

Type of neurone with two extensions; one axon and one dendrite, many are specialized sensory neurones. Some can be found in the retina and communicate with the rod cells

70
Q

How does myelin influence the specificity, sensitivity and receptive field of an axon?

A

Decreases sensitivity and increases specificity of signals firing - small receptive field

71
Q

How does the concept of decussation relate to where a sensory deficit will be felt?

A

If the lesion occurs before the nerve has decussated (crossed to the other side of the spinal cord), the sensory deficit will be ipsilateral. If the lesion occurs (for example, higher up/above the medulla oblongata in the dorsal column tract) after the tract has decussated, the sensory deficit will be experienced contralateral.

Jess Nervous (16 decks)

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