PSL300H1 Lecture Notes - Lecture 8: Lateral Inhibition, Thalamus, Sensory Processing
PSL300
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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