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.
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.
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
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 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.
