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Lecture

Textbook notes-Chapter 10-Sensory Physiology


Department
Biological Sciences
Course Code
BIOD27H3
Professor
Ingrid L.Stefanovic

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MAMMILIAN PHYSIOLOGY
Chapter 10 ± Sensory Physiology
Special Senses: Include vision, hearing, smell, taste, equilibrium
Somatic Senses: include touch, temperature, pressure, pH, pain/itch, proprioception (the
awareness of body movement and position in space mediated by sensory
receptors in muscles and joints)
General Properties of Sensory Stimulation
All sensory pathways begin with a physical stimulus that acts on a receptor. The receptor acts as a
transducer and converts the stimulus to an intracellular signal. If it is above the threshold, it is
transferred to the CNS where it is integrated either consciously or unconsciously
Sensory receptors vary in complexity. The most basic, simplest receptor is the neuron with naked (free)
nerve endings (i.e. somatosensory receptors). In the complex ones, the nerve endings are encased in
connective tissue
The most complex type of sensory receptor is that of the special senses. All of them except for smell use
non-neuronal receptor cells that synapse onto secondary neurons (smell goes straight to the brain
through one of the cranial nerves)
When activated, they release neurotransmitters that initiate action potentials that send information to
the brain
Non-neuronal accessory structures are very important to help receptors. I.e. the cornea of the eyes help
focus light for photoreceptors; the hairs on ones arm help sense movement of air close to skin. These
attributes enhance information gathering
Receptors are divided into 4 major groups:
1. Chemoreceptors: respond to chemical ligands that bond to receptors. I.e. taste, smell
2. Mechanoreceptors: Respond to various types of mechanical energy. I.e. pressure, vibrations,
acceleration, sound (i.e. hearing)
3. Thermoreceptors: Respond to temperature
4. Photoreceptors: Respond to light
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Transduction
The first step of converting physical stimuli into an electrical one is transduction (conversion of stimulus
energy into information that can be processed by the nervous system)
Adequate Stimulus: Each type of receptor has a particular form of energy it is most responsive to
(Thermoreceptors t heat; Mechanoreceptors t pressure)
Although receptors may have adequate stimuli, they are also responsive to different forms of stimuli,
given it has enough intensity. (I.e. photoreceptors are sensitive to light but if you punch someone in the
eye, they will see stars (mechanoreceptors)
The threshold t the amount of stimulus required to activate a receptor t is much lower for their specific
adequate stimuli
Usually the physical stimulus causes the opening or closing of ion channels in the receptor membrane.
Opening causes an influx of Na or other cations causing depolarization whereas closing it or an influx of
K causes hyperpolarisation
The change in sensory receptor membrane potential is called graded potential or receptor potential. In
some cells, this causes an action potential to the nervous system. In other cells, it causes the release of
neurotransmitters which alters electrical activity in associated neurons
Receptive Fields
Somatic sensory receptors and visual neurons are activated if stimuli fall within a certain physical area t
called a receptive field
Each receptive field has a primary neuron which relays electrical info to the secondary neuron located in
the CNS
It is possible for receptive fields (and therefore their primary neurons) overlap and converge into the
same secondary neuron. This causes their individual receptive fields to merge into a single, large
secondary receptive field
The size of the secondary receptive field determines sensitivity: the large the secondary receptive field
(more convergence), the less sensitive
I.e. for skin cells, two-point discrimination is used to determine sensitivity on arms and legs. 2 pins
placed 20 mm away from each other are viewed as the same prick (high convergence, large secondary
receptive field). However in other areas, 2 pins placed 2mm away from each other can be interpreted as
2 different pricks (smaller secondary receptive field, little or no convergence)
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CNS Integration
Much of the information enters the spinal cord and through ascending pathways to the brain (some go
directly to the brain via cranial nerves)
The majority of information is sent to the thalamus, which relays it to the appropriate part of the brain
(only smell does not go through the thalamus, it goes straight to the brain)
Perceptual Threshold: The amount of stimulus intensity needed for someone to be aware of a
perception
It is possible to diminish sensory intensity until it is below the perceptual threshold (not noticeable). This
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modulation
If the modulated stimulus suddenly becomes important, it is possible to focus on it and overcome the
inhibitory modulation. This occurs in secondary and tertiary neurons
All sensory information is traduced into electrical action potentials; therefore the brain must determine
their 1. Nature (modality) 2. Location 3. Intensity 4. Duration
1. Nature (Modality)
This is determined by which sensory neuron was activated (sensory modality) and where the
pathway of the neuron ends in the brain (sensation).
Some neurons are most sensitive to touch; others are most sensitive to heat etc. Therefore by
knowing which receptor was activated, it is possible to know the modality
Also, activation of a certain group of receptors will always be interpreted as a certain modality
by the brain (called labelled line coding). I.e. if a group of receptors known as cold receptors are
activated, the brain will always interpret it as coldness whether real or artificial
2. Location
The location of the stimulus is coded according to which receptive fields are activated. Each area
of the body has a specific area on the cerebral cortex (the topographical map of the body on the
vertebral cortex
Auditory information is an exception. Neurons in the ear have no receptive fields; instead they
use timing of receptor activation to compute a location. I.e. a sound directly in front of a person
will reach both ears simultaneously whereas a sound coming from one side will hit the closer ear
several milliseconds faster than the farther ear. The difference in time is then computed in the
brain to determine a location
Lateral inhibition increases the contrast between activated receptive fields and their inactive
neighbours. The neurons where the stimulus is most intense and the ones around it (less
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