PSL300H1 Lecture Notes - Lecture 8: Lateral Inhibition, Thalamus, Sensory Processing

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5 Jun 2018
Lecture 8: Introduction to the Senses
The senses carry information about the body and surroundings to the CNS
We will focus on the 5 special senses (vision, hearing, equilibrium, taste, and smell) and 4 somatic senses (touch,
temperature, proprioception, and nociception, i.e. pain and itch)
o Equilibrium bodies position in space
Vestibular apparatus in the ear tells the brain at what orientation the head is
Angular acceleration and linear acceleration
o Proprioception involves knowing how the joints are orientated relative to the rest of the body
o Nociception noxious stimuli that invoke a sense of pain or itch (sensations that we don’t like)
We are usually at least partly conscious of data from these 9 senses
We are largely unconscious of other sense data such as blood pressure, lung inflation, blood-glucose
concentration, internal body temperature, pH, etc.
Receptors and Neurons
Every sensory system begins with receptors
Receptors are cells, which may or may not be neurons, and which convert stimuli (e.g. light, sound) into electrical
signals transduction
A receptor converts stimulus energy into a graded change in membrane potential called a receptor potential.
o The receptor may then release neurotransmitters to affect a neuron
o If the receptor itself is a neuron, it may fire action potentials
Each receptor has an adequate stimulus: the form of energy to which it is most responsive (though it usually
responds to other forms as well)
o Thermoreceptors are more sensitive to temperature than to pressure
o Photoreceptors are sensitive to light, but if you press down on the eye you can see spots
Receptors are classed according to their stimuli
o Chemoreceptors respond to specific molecules or ions (e.g. glucose, or oxygen or H)
o Mechanoreceptors respond to mechanical energy such as pressure, vibration, gravity, and sound
o Thermoreceptors respond to temperature
o Photoreceptors respond to light in the retina
Some receptors are very sensitive
o Any receptor has a threshold the weakest stimulus it can detect
o Some photoreceptors can detect a single photon of light (rods in the eyes)
o Chemoreceptors for smell may respond to a single odorant molecule
o The perceptual threshold is the weakest stimulus you can consciously perceive
Higher stimulus than the threshold for a single receptor
Sensory systems involve a series of neurons
The first neurons in a system (either the receptors or the cells immediately
downstream) are called the primary sensory neurons
o Primary sensory neurons synapse onto secondary sensory neurons, and these
synapse onto tertiaries, and so on
o At each stage, many presynaptic cells may contact any one postsynaptic cell
convergence allows secondary and higher neurons to combine data from many receptors
Sensory neurons carry information about many aspects of the stimulus
o Once aspect is the stimulus modality, whether it is a light, a sound, a touch, etc.
o Sensory systems indicate modality by labeled lines, meaning that the modality is revealed by which axons
carry the signal
Activity on neurons in the visual pathway means light
Activity on neurons in the auditory pathway means sound
Groups of neurons can represent intensity in 2 ways
o Stronger stimuli may activate more neurons
Population coding
o Stronger stimuli may make the individual neurons fire at a faster rate
Frequency coding
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