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Sensation and Perception

Chapter 13: The Body Senses

Lecture Outline

• The somatosensory systems • Skin

• Touch – Mechanoreceptors

• Slow vs. fast adapting • Receptive field size • SA I – pattern, texture and shape • FA I – slip and grip • SA II – skin stretching and hand

conformation • FA II – fine textures

– Pleasant touch

• Nociception – Categories of pain

– Nociceptors and fibers

• Thermoreception

• Proprioception • Reflex arc

• Pathways to the cortex – Lemnsical – Spinothalamic

• Somatosensory cortex – S1 – 3a, 3b, 1, 2 – S2 – Somatotopic maps – Cortical magnification

– Representations – Orientation and direction tuning – Dorsal and ventral pathways – Discriminative vs. affective

dimensions of pain

• Pain – Top-down pain reduction – Gate control theory

• Cortical plasticity

• Haptic perception • The vestibular system

A Weird Collection of Objects…

Touch / The Somatosenses

• What is classically called “touch” is technically called the somatosenses and is actually composed of several separate sensory systems:

– Tactile perception – the deformation of the skin in response to pressure

– Nociception – pain from stimuli that either may or are causing tissue damage

– Thermoreception – temperature of objects touching the skin

– Proprioception – stretching of muscles and information about the angle of the joints

– Haptic – information about an object’s shape perceived through tactile perception and proprioception

Skin

Mechanorecepters

SAI: Pattern, Texture, Shape

FAI: Slip and Grip

SAII: Skin Stretching and Hand Conformation

• Hand conformation – how the hand is arranged

• Information from how various parts of the skin on the hand are stretched tells us about the conformation of the hand

– Tight fist

– Flat hand with fingers spread apart

• Movement across skin

FAII: Fine Textures

Pleasant Touch

• C-tactile (CT) Mechanoreceptors

– Signals pleasant skin sensations

– Respond to slow, gentle touch

– Only in hairy skin

– Unmyelinated

Proprioception

• Muscle spindles

• Golgi tendon organs

• Joint receptors

Nociception

• Categories of pain – Nociceptive – pain from damage to tissue due to physical trauma

(cutting, pinching, burning, freezing, acids, etc.)

– Inflammatory – arises after tissue damage and often lasts until tissue is healed

– Neuropathic – pain that arises from damage to the peripheral or central nervous system

• Nociceptors – one type of free nerve endings found in the skin – Aδ (A-delta) fibers – rapidly carry information from nociceptors

• Potentially damaging mechanical stimuli and excessive heat

• Sharp, immediate pain

– C fibers – slowly (unmyelinated) carry information • Respond to wide range of painful stimulation

• Dull, throbbing, long-lasting pain

Thermoreception

• Thermoreceptors

– Warm fibers

– Cold fibers

– Adapt after a couple of seconds

Reflex Arc

Pathways to Cortex

• Dorsal column- medial lemniscal pathway (tactile, proprioception)

• Spinothalamic pathway (nociception and thermoreception)

• Medulla

• Thalamus (ventral posterior nucleus)

• Somatosensory cortex

Somatosensory Cortex

• Anterior parietal lobe

• Contralateral organization

• S1 – primary somatosensory cortex

– Four areas called 3a, 3b, 1 and 2 (in that order)

• S2 – secondary somatosensory cortex

Somatotopic Maps

• 3a, 3b, 1 and 2 each have a somatotopic map

• Cortical magnification

Somatosensory Representations

• Tactile information goes to areas 3b and 1

• Proprioceptive information goes to areas 3a and 2

• Dorsal pathway – tactile information that supports touch based action

• Ventral pathway – tactile and proprioceptive used for somatosensory object recognition and formation of somatosensory memories

• Nociceptive information goes to areas 3a and 3b and then to S2

Somatosensory Responses

• Sustained vs. transient responses

• 3b sustained and transient response neurons as good as mechanoreceptor response in distinguishing the letters

• Area 1 responses not so good at distinguishing the letters

• Area 1 – orientation selectivity; direction selectivity

Orientation Tuning in S2

• Neurons in S2 show orientation tuning – responding best to edges on the skin of a particular orientation and less and less well as the orientation increasingly departs from the preferred orientation

• Some S2 neurons integrate information from large regions of fingers and hand

Dorsal and Ventral Pathways

• Dorsal – guide actions that require tactile / proprioceptive information

• Ventral – determine shape and identity of objects through touch and proprioception

Pathways for Discriminative and Affective Dimensions of Pain

• Discriminative vs affective dimensions

• Spinothalamic pathway

• Discriminative: thalamus → 3a, 3b → S2 → posterior insular cortex

• Affective: thalamus → anterior cingulate cortex → amygdala → anterior insular cortex

Top-Down Pain Reduction

• Analgesia – modulation of pain signals via top-down processing

– Endogenous opioids / endorphins

– Released by pituitary gland, the hypothalamus, and the descending nerve fibers in the spinal cord.

• Cognitive processes also influence the perception of pain

– Directing attention away from pain reduces perceived intensity

– Placebo effect

Gate Control Theory

Touch (Aβ)

Pain (Aδ, C)

Cortical Plasticity / Phantom Limbs

• Cortical re- mapping

• Practice / experience

Haptic Perception

The Vestibular System

• Perception of balance and acceleration

• Inner ear

• Semicircular canals – Ampulla

– Crista

• Otolith organs – Utricle and saccule

• Macula

• Otoconia

• Vestibular nerve

• Vestibular complex

• Parietal insular vestibular cortex