Transcript Sensory Processes - University of Toronto

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
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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