Lec 10 Touch

What is the skin

sensory organ for touch and the largest sensory organ of the body

How large is skin

about 1.8 m² (19 ft²)

How much does skin weigh

about 4 kg (9 pounds)

What are the layers of skin

epidermis dermis and subcutis

What are the four main tactile receptors

Meissner corpuscles Merkel cell neurite complexes Pacinian corpuscles Ruffini endings

What are key attributes of touch receptors

type of stimulation receptive field size and adaptation rate

What does receptive field size affect

precision of touch (smaller means more detailed touch)

What does adaptation rate mean

how quickly a receptor stops responding to a constant stimulus

Where are Merkel cell neurite complexes located

at the boundary between epidermis and dermis

What is the function of Merkel cells (SA I)

detect steady pressure and fine spatial detail

Merkel receptor response type

slowly adapting (SA I)

Merkel receptive field size

small

Merkel sensitivity frequency

very low frequency vibrations (<5 Hz)

What is Merkel used for

texture pattern perception Braille and object detail

What do people feel when Merkel receptors are electrically stimulated

pressure sensation

Where are Meissner corpuscles located

boundary between epidermis and dermis

Meissner function

detect light touch and low-frequency vibration

Meissner adaptation type

fast adapting (FA I)

Meissner receptive field size

small

Meissner frequency sensitivity

5–50 Hz

What is Meissner useful for

detecting object slip to adjust grip

What do Meissner stimulation feel like

wobble or flutter

Where are Ruffini endings located

deep in dermis

Ruffini function

sense skin stretch and sustained pressure

Ruffini adaptation type

slow adapting (SA II)

Ruffini receptive field size

large

What is Ruffini useful for

finger position and grasp control

What is felt when Ruffini is stimulated

usually no distinct sensation requires multiple fibres activation

Where are Pacinian corpuscles located

subcutaneous tissue

Pacinian function

detect high-frequency vibration

Pacinian adaptation type

fast adapting (FA II)

Pacinian receptive field size

large

Pacinian frequency sensitivity

50–700 Hz

What is Pacinian useful for

detecting first contact with objects (e.g. mosquito keyboard press)

What do Pacinian stimulation feel like

buzz sensation

What are kinesthetic receptors

receptors that provide sense of limb position and movement

What do muscle spindles detect

rate of change in muscle fibre length

What do tendon receptors detect

muscle tension

What do joint receptors detect

extreme joint angles

Why are kinesthetic receptors important

they allow awareness of body position and movement

What happened to Ian Waterman

he lost kinesthetic sensation due to nerve damage from viral infection

How did Ian Waterman compensate

he relied on vision to control movement

What did his condition show

kinesthetic senses are essential for movement control

What are thermoreceptors

receptors that detect changes in skin temperature

What are the two types of thermoreceptors

warm fibers and cold fibers

What is normal skin temperature range

30–36°C

When do thermoreceptors activate strongly

when contacting objects hotter or colder than skin

What are nociceptors

receptors that detect painful or damaging stimuli

What are A-delta fibers

fast pain fibers that detect sharp immediate pain

What do A-delta fibers respond to

strong pressure and heat

What type of pain do A-delta fibers produce

sharp fast initial pain

What are C fibers

slow pain fibers that produce sustained pain

What type of pain do C fibers produce

throbbing long-lasting pain

Why is pain perception important

it protects body from injury and harmful stimuli

What happens in pain-insensitive individuals (e.g. Miss C)

they cannot detect injuries and may die from untreated damage

Why is pain loss dangerous in diabetes

minor injuries go unnoticed and can become severe

What are C tactile afferents

unmyelinated fibers that mediate pleasant touch

What do C tactile afferents respond to

slow gentle touch like petting

Are C tactile afferents related to pain

no

Where does pleasant touch processing occur

orbitofrontal cortex (reward and pleasure)

What is tickle sensation processed by

neurons in somatosensory cortex

What is the role of tickle sensation

social and sensory interaction signal

How far can touch signals travel to the brain

up to ~2 meters from feet to brain

What is the first step in touch neural pathway

synapse in spinal cord

How are tactile signals transmitted

through axons forming nerve trunks

Why are nerve trunks important

they combine signals from different body regions to reach brain

What are the two main neural pathways of touch in the spinal cord

spinothalamic pathway and dorsal-column medial lemniscal (DCML) pathway

What is the spinothalamic pathway

evolutionarily older pathway for pain and temperature that is slower and has multiple synapses

What does the spinothalamic pathway carry

heat and pain information

Is the spinothalamic pathway fast or slow

slow with multiple synapses

What is a function of the spinothalamic pathway besides pain transmission

pain inhibition mechanisms

What are the first synapses in the spinothalamic pathway

multiple synapses in the spinal cord

Where is the pain gate located

substantia gelatinosa of the dorsal horn

What activates gate control inhibition

cold pressure itching or stimulation at distant sites

What is the DCML pathway

evolutionarily newer pathway for touch and proprioception

What does DCML carry

tactile and proprioceptive information

Is DCML fast or slow

faster with fewer synapses

Why is DCML faster than spinothalamic

wider axons and fewer synapses

Where does DCML first synapse

cuneate and gracile nuclei

Where does DCML go after brainstem nuclei

ventral posterior nucleus of the thalamus

Where does DCML project in cortex

somatosensory areas S1 and S2

What is DCML used for

planning and execution of fast movements

What are primary somatosensory areas

cortical areas (S1 and S2) that process touch information

What is the sensory homunculus

a distorted body map showing cortical representation of body parts