Lecture 4.1 Principles of Sensation Flashcards
What is Taste detected by?
- Chemoreceptors located in taste buds
- Taste buds contain gustatory cells with different
receptors - This allows detection of bitter, sweet, sour, umami,
and salt tastes
How do Eyes allow Sight?
- Photoreceptor cells (cones and rods) of the retina
are modified neurons, sensitive to photons - Cones contain photopigments that change shape as
they absorb light - Rods see in black & white
Where are Auditory Hair Cells located? What is their importance?
- Within the organ of Corti of the inner ear
- Mechanoreceptors located on the stereocilia of the
hair cells opened in response to tectorial membrane
resonance
What are Olfactory Receptor Neurones?
- Are bipolar neurones, with an apical dendrite
terminating in long cilia that detect dissolved chemical
odorants - A range of receptors detect different smells
Types of Sensory Receptors (5)
- Mechanoreceptors
- Thermoreceptors
- Nociceptors: pain (mechano, thermal, or poly)
- Chemoreceptors
- Photoreceptors
The outside world (somatic) is sensed by…?
Exteroreceptors
Internal stimuli (visceral) are sensed by…?
Interoceptors
Position and movement of muscles are sensed by
Proprioceptors
Where are Proprioceptors located?
Skeletal muscles, joints ligaments and associated
connective tissue
What is Sensory Transduction?
This is the transformation of a stimulus into an electrical signal
What is the strength of a stimulus is coded by? (2)
- Number of afferent fibres activated
- Frequency of action potential generation
How do Phasic Receptors work?
- They are rapidly adapting, with a maximal response at
the start and finish of the stimulus - They stop responding quickly, even if the stimulus
continues (for example tactile corpuscles or
photoreceptors)
How do Tonic Receptors work?
- They are slowly adapting
- Continue to respond as long as the stimulus is present
- For example nociceptors and proprioceptors
How are Mechanoreceptors activated?
They are are activated following mechanical deformity of the skin, muscles, tendons, ligaments etc
Examples of Mechanoreceptors: Meissner’s Corpuscles
- Peanut-shaped concentric stacks of Schwann cells * Found in the superficial dermis between the
rete ridges of fingertips, lips, forearm, palms & soles - Detect movement across the skin
Examples of Mechanoreceptors: Merkel’s Discs
- Nerve endings associated with specialised epithelial
cells called Merkel cells in glabrous skin (particularly
fingertips) - They are also associated with hair follicles.
- Tactile receptors that help distinguish texture and
shape of objects
What do Pacinian Corpuscles detect? Where are they found?
- Pressure, vibration and tickle
- Dermis, ligaments, joints
What do Ruffini’s End Organs detect? Where are they found?
- Proprioception, stretch and pressure
- Dermis and joints
What do Hair Follicle Receptors detect?
Light touch receptors that detect hair movement
What is a Receptor Field?
The region of skin within which a tactile stimulus evokes a sensory response in the cell or its axon
What happens in Lateral Inhibition?
Neurones inhibit weaker signals from nearby receptors to help pinpoint the strongest point of the stimulus, thereby improving acuity
What is Two–Point Touch Discrimination?
This is the ability to discriminate two discrete points in contact with a body area
Why is Two–Point Touch Discrimination important?
This is important in the assessment of tactile perception
and dorsal column testing
What do Nociceptors detect?
Slowly adapting and responsive to noxious or painful
stimuli
What fibres is pain transmitted through?
Pain is transmitted through Aδ or C fibres for acute and diffuse pain respectively
What are Thermoreceptors sensitive to?
- They are sensitive to hot, cold and extreme
temperatures - They are slowly adapting (tonic) receptors
Capsaicin, a chemical found in red peppers
activates what in heat-sensitive thermoreceptors
leading to their depolarisation?
It activates transient receptor potential cation channel subfamily V member 1 (TRVP1)
What does Menthol activate?
Menthol activates the transient receptor potential cation channel subfamily melastatin 8 (TRPM8) receptor
What are the 2 main ascending sensory tracts?
- Posterior/ dorsal column tract: fine touch,
proprioception, vibration - Spinothalamic tract: pain, temperature, crude touch
What does Proprioception mean?
It refers to the sense of limb position and movement
What pathway plays an important role in proprioception?
Spinocerebellar Tract
Pathways involve chains of neurones, what are 1st order neurones?
- 1st order neurones run from receptor to spinal cord
- These are sensory afferents with cell bodies in dorsal
root ganglia
Pathways involve chains of neurones, what are 2nd order neurones?
2nd order neurones are interneurons with their cell bodies in the spinal cord
Pathways involve chains of neurones, what are 3rd order neurones?
3rd order neurones transmit information from thalamus to the cerebral cortex
What is found in the White Matter of the Spinal Cord?
- Myelinated axons organised into funiculi (fibre tracts
running up and down the spinal cord) - Axons cross from one side of the spinal column to the
other (decussate) at the ventral white commissure
What is found in the Gray Matter of the Spinal Cord?
- Dorsal horn: interneuron cell bodies
- Lateral horn (seen from thoracic levels and above):
preganglionic sympathetic neurones - Ventral horn: interneurons and motor neurones
- Gray commissure: connects the gray columns
What is Somatotopy?
The spatial (topographical) arrangement results in what is called somatotopy for the touch system, which means that the map of the skin of the body is transmitted to the cortex in proper spatial order
By mapping the amount of the primary
somatosensory cortex dedicated to processing
information from the peripheries, we create a….?
Homonculus
Where is the conscious mind found?
Somatosensory Cortex
Each hemisphere processes sensory information and controls motor of what side of the body?
The opposite side of the body
Control of movement requires a number of regions of the brain, but the …. ….. …… produces
movement with minimum electrical stimulation
primary motor cortex
The axons of the large motor neurons in the precentral gyrus (pyramidal cells) travel down into the
spinal cord via the …..?
corticospinal (pyramidal tracts)
Posterior/Dorsal Column Tract (Medial Lemniscus Pathway)
- A 3 neurone pathway
- Propagates fine touch, proprioception and vibration
- Synapses occur in the medulla (where the path
also decussates) and the thalamus - The fasciculus gracilis carries information originating
below T6 (lower limb) - The fasciculus cuneatus carries information from T6
and above (upper limb)
What does to decussate mean?
2 or more things cross or intersect each other to form an X
Spinothalamic Tracts: What do the Anterior Spinothalamic Tracts conduct?
- Crude touch (poorly localised light pressure)
- 3 neurone pathway that decussates in the spinal cord
Spinothalamic Tracts: What do the Lateral Spinothalamic Tracts conduct? (2)
- Pain
- Temperature
- 3 neurone pathway that decussates in the spinal cord
Where do spinocerebellar tracts carry proprioceptive information from? (3)
Proprioceptive information from:
* Golgi tendon organs
* Muscle spindles
* Joint capsules
How many neurones involved in spinocerebellar tracts?
2-neurone pathways terminating in the cerebellum, where subconscious processing takes place
What can nerves be damaged by? (3)
- Axonal degeneration (toxic, metabolic or physical)
- Demyelination
- Vascular nerve damage
