4.1 Principles of sensation Flashcards
Name the sensory receptors responsible for the following
A) Touch
B) temperature
C) pain
D) chemical
E) eye
A) mechanoreceptors
B) thermoreceptors
C) nociceptors
D) chemoreceptors
E) photoreceptors
What comprises the majority of sensory receptors?
Dendritic endings of efferent neurons.
What are the 5 special senses?
Where are they located and how do they reach the somatosensory cortex?
Vision, hearing, balance, taste, smell located in complex sense organs and travel to SS cortex via cranial nerves
Which types of receptors receive info from the outside (somatic) world and the internal (visceral) world?
Exteroceptor: sense touch, special senses etc
Interoceptor: chemical changes, tissue stretch, temperature etc
What receptor senses position and movement of muscles?
Proprioceptors located in skeletal muscle, joints, ligaments and associated CT
Which type of receptor detects taste? How are different tastes (i.e bitter, sweet, sour, salt, etc) accounted for?
Taste buds contain gustatory cells which are chemoreceptors. These cells have different receptors which detect different tatse (bitter, sweet, sour, saltly)
What are Photoreceptor cells and how do they recieve sensory info?
Receptors located in the retina (cones and rods)
They are modified neurons that are sensitive to photons.
Eg. cones contain photopigments that change shape as they absorb light ultimately depolarising the cell.
Where are auditory hair cells located?
What receptor is found on these hair cells and how is it activated?
Within the organ of corti of the inner ear
Mechanoreceptors are located on the stereo cilia of the hair cells, they open in response to tectorial membrane resonance
Describe the structure of olfactory receptor neurones and state what they do.
How are different smells accounted for?
They are bipolar neurons, with an apical dendrite terminating in long cilia that detect dissolved chemical odorants.
A range of receptors detect different smells
What 3 forms can receptors exist in?
1) Free
2) Encapsulated in myelin
3) Specialized
What are the most common nerve endings in the skin?
What are they sensitive to?
Free nerve endings
These are sensitive to painful stimuli, to hot and cold, and to light touch
How does myelin influence the specificity, sensitivity and receptive field of an axon?
Decreases sensitivity
Increases specificity of signals firing
Small receptive field
What nerve endings detect fine touch?
Why is this the case?
Fine touch in skin are detected by encapsulated nerve endings (schwann cells).
These endings decrease sensitivity for firing but increase specificity.
Why are most neurones are multipolar?
Give an example of a bipolar neuron and describe its structure
So they can summate many signals at once
The retina contains bipolar neurons. These have two extensions; one axon and one dendrite.
Define sensory transduction
The transformation of a stimulus into an electrical signal
What 2 things determine the strength of the stimulus?
1) The number of afferent fibres activated
2) The frequency of action potential generation
Compare Phasic vs Tonic receptors
Give 2 examples for each
Phasic receptors are rapidly adapting, with a maximal response at the start and finish of the stimulus. They stop responding quickly, even if the stimulus continues
- Eg. tactile corpuscles or photoreceptors
Tonic receptors are slowly adapting, and continue to respond as long as the stimulus is present
- Eg nociceptors, proprioceptors, thermoreceptors
What do nociceptors respond to?
Name the three ‘insults’ that can stimulate them
Responsive to noxious or painful stimuli. Insults can be:
- mechanical (e.g. pinching)
- thermal
- chemical
Which fibres are responsible for transmitting acute vs diffuse pain?
What are their associated mechanoreceptors?
Acute:
- Aδ fibres (fast, large and myelinated)
- Associated with mechanoreceptors at sharp pain nerve endings
Diffuse:
- C fibres (slow, small and unmyelinated).
- Associated with mechanoreceptors for “social” touch and diffuse pain receptors
What are Thermoreceptors sensitive to?
Give 2 examples?
Sensitive to hot, cold and extreme temperatures
Capsaicin (chemical in red chilly) activates transient receptor potential cation channel subfamily V member 1 (TRVP1) in heat-sensitive thermoreceptors leading to their depolarisation.
Menthol activates the transient receptor potential cation channel subfamily melastatin 8 (TRPM8) receptor in cold sensitive thermoreceptors leading to their depolarisation.
State the associated mechano-receptors for the following fibres
- Aα
- Aβ
- Aδ
- C
State which is the fastest
- proprioception (fastest)
- cutaneous mechanoreceptors
- sharp pain nerve endings
- “Social” touch and diffuse pain receptors (slowest)
Where are Merkel’s discs located and what specialised epithelial cells are these associated with?
These are nerve endings associated with Merkel cells.
They are located in the dermis of glabrous skin, particularly fingertips.
They are also associated with hair follicles.
What is the function of Merkel’s discs/cells and how do they do this?
What can be said about their receptive field and what this allows
‘Merkel cells’ sit in the dermis. When they are deformed they depolarise and release NT’s at free nerve endings
These endings are known as ‘Merkel disc’.
These have a very narrow receptive field allowing fine tactile, discrimination and texture perception.
Allows shapes to be sensed and objects identified
What are Meissner’s corpuscles (shape and cell type) and where are they located?
What do they detect?
These are peanut-shaped concentric stacks of Schwann cells
They are located in the superficial dermis between the rete ridges of fingertips, lips, forearm, palms and soles
They detect movement across the skin - allows us to feel movable/slipping objects in our hands
Where are Pacinian corpuscles located and what do they detect?
Located in Dermis, ligaments and joints
Detetc Pressure, vibration and tickle
(Resemble an onion)
Where are Ruffini’s end organs located and what do they detect?
Located in the dermis and joints
Detect Proprioception, stretch and pressure
What are Hair follicle receptors and what do they detect?
Light touch receptors that detect hair movement
What are the 2 main ‘fine touch’ receptors we have in our fingertips?
Meissner’s corpuscles and Merkel discs
What is a receptor field?
Compare a small vs large receptor field and an example of where each is found
A region of skin within which a tactile stimulus evokes a sensory response in the cell or its axon
The smaller receptive field the more accurately the stimulus can be pinpointed
- Eg. fingertips
A larger receptor field with a lower density will be less accurate at pinpointing a stimulus
- Eg. Palm
What is two-point touch discrimination and when is it used?
What 2 things does discrimination depend on?
The ability to discriminate two discrete points in contact with a body area. This is important in the assessment of tactile perception and dorsal column testing.
Discrimination depends on:
1) The concentration of receptors in that region
2) The size of receptive fields. (smaller receptive fields means more accurate location)
What is the two point discrimination for the following body sites?
A) fingertips
B) palm
C) arm
Which organ has the greatest capacity to distinguish two closely placed points?
A) 2 mm
B) 10 mm
C) 40 mm
Tongue has the greatest capacity to distinguish two closely placed points
How does lateral inhibition improve discrimination?
In lateral inhibition, neurons inhibit weaker signals from nearby receptors that are not firing at such a high rate.
This helps to pinpoint the strongest point of the stimulus, thereby improving acuity and allowing localisation and sharpening of stimulus.
Define somatotopy
The point for point correlation between an area on the body and a specific point in the CNS
What is the “homunculus” and how was it created?
It’s a distorted body figure that mirrors the primary motor cortex. It is created by mapping the amount of the primary somatosensory cortex dedicated to processing info from the peripheries.
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?
Areas where sensation is more important (eg. face, lips and palms) have a higher density of receptors and neural circuits and have 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
How many neuron chains are involved in ascending pathways to the brain in:
a) unconscious processes
b) conscious processes
a) 2 neuron chains (unconscious processes)
b) 3 neurone chains (conscious processes)
What are the 3 main ascending sensory tracts?
What stimuli is each responsible for?
1) Posterior/dorsal column tract: fine touch, proprioception, vibration
2) Spinothalamic tract: pain, temperature, crude touch and pressure
3) Spinocerebellar tract: proprioception
Describe the pathway of first, second and thrid order neurons (three ‘chains of neurones’)
First order neurons run from receptor to spinal cord. These are sensory afferents with cell bodies in dorsal root ganglia
Second order neurons are interneurons with their cell bodies in the spinal cord
Third order neurons transmit information from thalamus to the cerebral cortex
Where are the dorsal root ganglion located?
Just outside and adjacent to the spinal cord
How can the posterior columns of the spinal cord be divided?
How do these divisions differ?
The fasciculus gracilis (inner) carries sensory information originating below T6 (lower limb)
The fasciculus cuneatus (outer) carries information from T6 and above (upper limb)
The Posterior/dorsal column tract is also known as what?
Medial lemniscus pathway
How many neurons are involved in the posterior column tract?
Describe the pathway
3 neurons
1) The 1st order neurone cell body sits in the dorsal root ganglion
- axons from upper limb (above T6) travel via the fasciculus cuneatus
- axons from lower limb (below T6) travel via fasciculus gracilis
2) These ascend directly to the medulla where they synapse with the 2nd order neurone
3) They cross at the medulla and ascend to the ventral posterolateral nucleus (in thalamus).
4) Here they synapse with the 3rd order neurone which projects from the thalamus to the primary cerebral cortex on the post central gyrus
How can the spinothalamic tract be divided and compare these
Anterior spinothalamic tract ➞ conducts crude touch and pressure
Lateral spinothalamic tract ➞ conducts pain and temperature
How many neurons are involved in the spinothalamic tract? (both anterior and lateral)
Describe the pathway
3 neuron pathway
1) The 1st order neurone has its cell body in the dorsal root ganglion
2) It enters the spinal cord and immediately ascends 1-2 levels to the Lissauer’s fasciculus and then synapses onto the 2nd order neuron in the substantia gelatinosa
3) Here it crosses to the other side of the spinal cord and ascends via the spinothalamic tract to the thalamus
4) In the thalamus it synapses with the 3rd order neurone which projects onto the primary cerebral cortex on the post-central gyrus
How many neurons are involved in the spinocerebellar tracts? (anterior and posterior)
Describe the pathway of the spinocerebellar tract
2 neurons
1) The 1st order neurone has its cell body in the dorsal root ganglion
2) It enters the spinal cord and travels up through the grey matter in the dorsal horn where it synapses with 2nd order neuron in Clarke’s nucleus
3) The 2nd order neurone ascends (in the anterior OR posterior) spinocerebellar tract to the midbrain where it enters the cerebellum and synapses onto target Purkinje cells.
What is meant by ‘double crossing’ of spinocerebellar tracts?
The anterior and posterior spinocerebellar tract both end up at the cerebellum
BUT, the anterior spinocerebellar tract will cross the spinal cord first as part of the anterior white commissure and then cross again to end in the cerebellum. The posterior does NOT cross at all.
Where does subconscious processing take place?
The cerebellum
Describe the organization of white matter
Myelinated axons organised into funiculi (fibre tracts running up and down the spinal cord)
What is the Gray commissure?
The Gray commissure is where axons cross from one side of the spinal column to the other (decussation)
There is also white commissure which has the same function
Describe the structure of grey matter including the contents of all three horns
Dorsal horn: interneuron cell bodies
Lateral horn (seen from thoracic levels and above): preganglionic sympathetic neurons
Ventral horn: interneurons and motor neurons
Which part of the spinal cord has lateral horns?
C8-L3
Name 3 sites in the body where spinocerebellar tracts carry proprioceptive information from and briefly describe each
1) Golgi tendon organs: proprioceptive receptors within tendons, senses changes in muscle tension/contraction
2) Muscle spindles: stretch receptors in the body of muscle, detects changes in the length of the muscle
3) Joint capsules
Which part of the brain is the conscious mind found?
Describe what is unique about how sensory information and motor function are controlled here
Cerebral cortex
Each hemisphere processes sensory info and controls motor functions of the opposite side of the body
What are pyramidal cells?
How do they travel down into the spinal cord?
Pyramidal cells are axons of the large motor neurons located in the precentral gyrus.
They travel down into the spinal cord via the corticospinal tract (pyramidal tracts)
Name three ways nerves can be damaged by
1) Vascular nerve damage
2) Demyelination
3) Axonal degeneration (toxic, metabolic or physical)
How does the concept of decussation (crossing over) relate to where a sensory deficit will be felt?
If the lesion occurs before the nerve has decussated (crossed to the other side of the spinal cord), the sensory deficit will be on the ipsilateral side.
If the lesion occurs after the tract has decussated, the sensory deficit will be experienced on the contralateral side.
