Sensory Processes

Josée L. Jarry, Ph.D., C.Psych.

Introduction to Psychology

Department of Psychology

University of Toronto

May 28, 2003

Sensation & Perception

• Sensation– Experience associated with stimuli – The initial steps by which the sense organs and

neural pathways take in stimulus information.

• Perception– Subsequent organizing of information – Meaningful interpretation of information

The Process of Sensation

physical stimulus

physiological response

sensory experience

The Process of Sensation• Physical stimulus

– Matter or energy impinging on the sense organs.

• Physiological response– Pattern of electrical activity that occurs in the sense

organs, the nerves, and the brain as a result of the stimulus.

• Sensory experience– The subjective, psychological sensation - sound,

taste, sight - that is experienced by the individual whose sense organs have been stimulated.

Basic Anatomy of Human Senses• Receptors

– Specialized structures that respond to physical stimulus by producing electrical changes that can initiate neural impulses

• Sensory neurons– Carry neural impulses from the receptors to the

central nervous system

• Sensory areas– Specific areas of the cerebral cortex devoted to

specific senses.

Transduction Common to All Senses

• Transduction– The process by which a receptor cell produces an

electrical change in response to a physical stimulus

• Receptor potential– In response to a stimulus, the membrane of the

receptor cell depolarizes which leads to action potentials in the axons of sensory neurons.

• Quantitative & Qualitative dimension

Stimulus Quantity and Quality

• Senses respond not only to a particular class of stimulus energy

• The also respond to variations in that energy

• All forms of energy vary along at least two dimensions

• Quantitative dimension– Concerns the amount or intensity of energy present

• Qualitative dimension– Concerns the precise kind of energy present

Coding of Stimulus Quantity & Quality

• Coding– Pattern of action potential sent to the brain that

preserves the quantity and quality of a stimulus.

• Coding of stimulus quantity– Stronger stimuli produce larger receptor potentials– Produce faster rates of action potentials in sensory

neurons.

• Coding of stimulus quality– Different receptors within any given sensory tissue

are tuned to different forms of energy.

Sensory Adaptation

• Change in sensitivity that occurs when a sensory system is either stimulated or not stimulated for a length of time.

• Absence of stimulation– Sensory system becomes temporarily more sensitive– Responds to weaker stimuli

• Presence of stimulation– Sensory system becomes temporarily less sensitive– Requires stronger stimuli to produce a response.

Pain• Other senses communicate information

about the external world

• The experience of pain comes from one’s own body

• Pain is not only a sense but it is also a drive

• People are motivate to avoid and/or reduce pain

• Pain has survival value

Neural Pathways of Pain

• Anatomically related to the cutaneous senses

• Free nerve endings– The sensitive terminals of pain neurons are not

surrounded by special capsules or end organs as are the endings of touch and temperature receptors

– Free nerve endings can be found in all body tissues from which pain is sensed, from the skin to the pulp of the teeth.

Two Types of Peripheral Pain Neurons

• A-delta fibers– Thick, myelinated, fast conducting neurons– Mediate the feeling of initial fast, sharp, highly

localized pain.

• C fibers– Very thin, unmyelinated, slow-conducting– Mediate slow, dull, more diffuse, often burning

pain.

Central Pain Pathways: Fast Pain

• Fast pain and A-delta fibres– A-delta fibres synapse on cells in the spinal

cord that lead to an area of the thalamus called the ventrobasal complex

– ventrobasal complex also receives neurons that mediate touch

– sends its output to the somatosensory cortex– allows us to localize where pain originates

Central Pain Pathways: Slow Pain

• Slow pain and C fibres– C fibres synapse on cells in the spinal cord – Relays to a midline nucleus in the thalamus and– to the limbic system– responsible for motivational and emotional

aspects of pain– Those connections are important for the

interpretation of pain.

Neural and Chemical Inhibition of Pain

• Gate control theory

• Ronald Melzack and Patrick Wall (1965, 1982)– For pain to be experienced, input from

peripheral pain neurons must pass through a gate located at the point where they enters the spinal cord and lower brain stem.

Pain-inhibiting System

• Periaqueductal gray (PAG)– PAG neurons have excitatory connections with

inhibitory interneurons in the spinal cord– These inhibitory interneurons prevent ascending

neurons to relay pain messages to the brain

• Endorphins– The spinal cord inhibitory interneurons releases

endorphins– Endorphins are inhibitory neurotransmiters