PHY 3111
Sensory and Cognitive Neuroscience
Sensory Receptors in Human Skin
- Most receptors are on glabrous skin, not hairy skin - Pacinian are deep in dermis, Meissner’s and Merkel
are in the upper dermis
- Sense only becomes perception until third order neuron
Four Main Types of Mechanoreceptors - Slow Adaption Type I (SA I)
- Fast Adapting Type I (FA I) - Slow Adapting Type II (SA II) - Fast Adapting Type II (FAII) Pacinian Corpuscles (FA II)
- Located in the lower dermis
- Respond to pressure, vibration and stretch - Rapidly adapting due to corpuscle
structure, needs constant stimuli - In hairy and non-hairy skin
- Large receptive fields, so not suitable for dine detail - Fingers, mammary glands and genitals
- 92% is water, so is easily displaced Meissner’s Corpuscles (FA I)
- Located to the top of the dermis
- Response to light touch and low frequency vibration - Dense in the lips and fingers
- Smaller receptive field
- Needs constant changing stimuli Merkel’s Receptors (SA I)
- Unencapsulated mechanoreceptor → slowly adapting - Small receptive field
- In non-hairy skin: splits into two merkels discs
- Respond to vibration at low frequency, indentation and texture - High density in finger tips: braille
Ruffini’s Endings (SA II) - Located in the dermis - Both hair and non-hairy skin
- Large receptive fields and relatively sparse
- Slowly adapting and detect stretch in joints and skin Two Point Threshold
If two points touch in one receptive field, then only one touch will be felt. If you have two receptive fields touching each point, 2 points will be detected
Thermoreception: Perceiving Temperature Warm Fibers – 29-43 Degrees
- Both warm and cold receptors can respond between 23-40 degrees Celsius - Increase firing as temperature increases
- Do not detect heat when it gets warmer than 45 degree’s or over Cold Fibers 17-40 Degrees
- 30 times more numerous than warm receptors - Increase firing as temperature decreases
- They respond to cool temperatures between 10-20 degrees Celsius - Do not respond to very low temperatures below 10 degrees Celsius
- When skin temp is maintained between 30-36, a person does not notice thermal sensation Combined
- There is continuous firing of both cold and warm fibres at baseline of 34 degrees
- Receptors adapt and become relative to the environment, and the new warmer temperature will become the new base line
Proprioception Muscle Spindles
- Stretch (length) receptors with a fibrous, fluid filled capsule surround intrafusual muscle fibers
- Innervated by type Ia sensory neurons
- Spindles receives sensory and motor innervation - In parallel with muscle fibers
- Known as intrafusual fibres
- When a muscle lengthens, it fires and detects length Intrafusual vs. Extrafusual Muscle Fibres
- Two fiber types are in parallel - Extrafusual are for force generation
- Intrafusual are for changing spindle length and therefore, sensitivity
Gamma Co-activation – Maintain Sensitivity of Spindles
After contraction of Extrafusual fibers, muscle spindle is slack and loses ability to signal length
Alpha Motor Neurons target Extrafusual gibers, shortening bulk of the muscle
Gamma Motor Neurons target intrafusual fibers, shortening the muscle spindle
Opposing neural effects
- Alpha activation decreases Ia activity - Gamma activation increases Ia activity
Note: Gamma neuron picks up the slack, resets length and conveys length to original state
Golgi Tendon Organ
Tension force receptors in series with muscle fibres that are Innervated by Ib sensory axons, which synapse on interneurons in ventral horn, and some interneurons target alpha motor neurons on the same muscle
- They increase firing rate when detection tension and prevents injury if the load is too much
- No direct motor innervation
2ddddddd
Somatosensation II
Ascending Pathways
First-order neurons: Primary afferent neurons (sensory) that delivers sensations to the CNS
- Cell body is located in the ganglion and enters the dorsal root ganglion
- Travels to the dorsal horn
Second-order neurons: Axon of the 1st order sensory neuron synapses on the interneuron in the CNS → 2nd order neuron
- Decussates (crosses over) in the Medulla Oblongata on the nucleus gracilis and nucleus cuneatus
Third-order neurons: Second order neuron synapses with third order in the thalamus
- This step is needed for sensation to reach out awareness
Dorsal Column-Medial Lemniscus Tracts (Posterior Column) - Tactile perception and proprioception
- Carries sensations of highly localised fine touch, pressure, vibration, two point and proprioception
- Dorsal columns carry information from peripheral sensory neurons to the primary sensory cortex of the cerebral hemispheres
Pathway: Travel up spinal cord on ipsilateral side and crosses to contralateral side in the medulla, and goes through the ventral posterior nucleus of the thalamus and on to sensory cortex