Boğaziçi University

SCIENCE 102:

Sensory Systems

Yrd.Doç.Dr. Burak GüçlüBiomedical Engineering Institute

MECHANICAL SENSES

CHEMICAL SENSES

HEARING

VISION

CORTICAL MAPS

Key concepts: Sensory receptor, sense organ, adequate stimulus

A classification of receptors:a) teleceptors: concerned with events at a distanceb) exteroceptors: concerned with external environment near at handc) interoceptors: concerned with internal environmentd) proprioceptors: information about the position of the body in space at any instant

SENSORY TRANSDUCTION:

Some sensory organs have separate receptor cells and synaptic junctions between receptors and afferent nerves (e.g. Vision, hearing, equilibrium, taste). In others, the receptors are specialized ends of nerve fibers (e.g. most cutaneous sense organs).

A nice example: Pacinian corpuscleMostly studied tactile receptor, large size, accessible from the mesentery of carnivores, lamellated capsule, a straight unmyelinated neurite in the middle.

SENSORY MODALITIES:

Generator potential vs. Action potential:

Adaptation:

If constant strength stimulus is maintained, sometimes the frequency of action potentials declines over time. Phasic receptors adapt rapidly, tonic receptors slowly.

Coding:

The action potentials are similar in all nerves. How do we distinguish information from different nerves? How do we determine a strong versus light stimulus?

a) Specific nerves, neurons, and networks

b) Frequency of action potentials

c) Number and type of neurons activated

SKIN:Meissner corpuscles

Scale bar: 0.5 mm (Bolanowski et al., 2000)

Pacinian corpuscle(Bell et al., 1994)

Four tactile-fiber classes:

Tactile pathways in the CNS :

Muscle receptors :

Four-channel model in psychophysics (based on thresholds):

Stimulus position, movement, direction.

Object texture and weight.

Body position, movement with respect to objects. Different forces and angles.

Imagery.

Memory.

Sub-modalities are integrated into a complex percept.

Example: wet object, mental rotation

Cognitive aspects of touch and proprioception:

Loss of touch and proprioception:

Trauma, stroke, infection, metabolic (e.g. Diabetic neuropathy), autoimmune (e.g. Guillain-Barré syndrome).

Local lesions: sensory nerves damaged, motor nerves conserved

Result: No information about the body, needs visual input, living in a virtual body...

• Can’t detect displacements/forces and discriminate weights without visual input

• Can’t learn new motor skills

• Problems in gait

BRAIN

EXT. WORLD

Introducing Mind&Brain, A. Gellatly and O. Zarate

Stange case: denial of loss

Mirror drawing: Difficult in normals due to sensory conflict, easy for patients with loss of touch/proprioception

Patients can’t explain what they’re doing !!

Philosophy:Awareness of physical existence by touch and proprioception

Mind-brain fusion (read about modern theories of neurophilosophy: Koch, Churchland et al.)

No special area for mind in the brain (contrast to brain surgeon Wilder Penfield)

No hard evidence for mind-brain duality (contrast to Cartesian philosophy)