Lecture 16, Sensory Physiology Flashcards
Sensory Physiology - Introduction
sensory physiology refers to the study of sensation and sensory systems
a sensory system has three primary components:
- one or more sensory receptors (1)
◦ can be responsive to stimuli in either the external
or internal environment
- regions of the CNS responsible for processing the information from the sensory receptors (2)
◦ information from one receptor can be sent to
more than one region
◦ may or may not be sent to regions of the brain
that allow for the conscious awareness of the
stimuli
◦ sensation (when the sensory information has
reached conscious awareness) vs sensory
information vs perception (ex. being aware that
something is painful)
- neural pathways that connect the receptors to the aforementioned regions (3)
Sensory Receptors
the first step of sensory processing occurs at the level of the sensory receptors
- transduction of the stimulus into a receptor (graded) potential (specific to sensory receptors) of an afferent neuron
- the receptor potential may turn into an action potential if the stimulus is significant enough to reach threshold potential
the stimulus opens ion channels, causing a change in the receptor potential
- if the receptors potential reaches threshold potential, an action potential will be initiated
- the first node of ranvier is usually the site of action potential initiation, due to the presence of voltage-gated ion channels (it matters that the action potential be at the first node of ranvier to reach threshold)
Sensory Transduction
stimulus detection
- if the stimulus acting on a sensory receptor is sufficient, it will cause a change in the permeability of the membrane
◦ opening of ion channels on the receptor
membrane
- change in the receptor potential
if the stimulus is strong enough, summation of the receptor potentials will lead to the initiation of an action potential
transduction: the process of translating a stimulus into an action potential
- not all stimulus received by the sensory receptor will result in an action potential
Sensory Transduction (magnitude of rp)
the magnitude of the receptor potential determines the frequency of action potentials in the afferent neuron, but does not determine the amplitude of the action potentials
- frequency = action potential firing rate
- the rate which is +30 mV will not change, but the rate at which we fire can be altered
factors that control the magnitude of the receptor potential include:
- stimulus strength
◦ example: amount of pressure felt by a
mechanoreceptor
- rate if change on stimulus strength
◦ example: a gradual vs sudden increase in
pressure
- temporal summation of successive receptor potentials
◦ more successive receptor potentials -> greater
cumulative receptor potential
- adaptation
◦ adaptation refers to a decrease in receptor
sensitivity to the same stimulus
◦ over time, action potential firing rate will decrease
despite the continuing presence of the stimulus
◦ receptors can be slow or fast adapting
Sensory Receptor Adaptation (phasic and tonic)
phasic receptors are rapidly adapting
* generate a high frequency burst of action potentials right at the onset of the stimulus
* the receptor quickly stops responding to the stimulus, despite is still being present
* these receptors are not always active; only active when stimulated
tonic receptors are slowly adapting
* are also quite responsive immediately at the onset of the stimulus
* maintain a persistent (or slowly decaying) action potential output while the stimulus is present
◦ background level of stimulation
* the receptor stops action potential output when the stimulus is removed
Sensory Receptor Adaptation (2)
tonic receptors will fire for the duration of the stimulus; often are tonically active
- for example: nociceptors are often tonic receptors, and will fire if a painful stimulus is present. if that stimulus is less severe, frequency of AP firing will be lower but still constant. the stimulus needs to be completely removed to stop AP firing
meanwhile, phasic receptors adapt very rapidly and are generally only fire APs in response to a change in the stimulus
Peripheral vs Central Adaptation
differences in adaptation at the level of the sensory receptors is peripheral adaptation
- adaptation can also occur at the level of the central nervous system = central adaptation
adaptation: reduction in the sensitivity of the nervous system to a constant stimulus
peripheral adaptation: phasic sensory receptors become less responsive to a stimulus
examples: becoming used to background noise, room temperature, the feeling of clothing
central adaptation: sensory receptors are still responsive to the stimulus, but adaptation occurs at the CNS due to inhibition of various nuclei
- sensory information is still present and may evoke reflex responses, but there is no sensation or conscious perception of that sensory information
- example: getting used to the odor of a place you visit frequently
Sensory Receptors (directly and indirectly activate)
sensory receptors monitor changes in specific variables, in either the external or internal environment
- usually are present at the peripheral (dendritic) end of an afferent neuron
◦ stimulus directly activates the afferent neuron
- can also be a separate receptor cell that sends a signal to the afferent neuron via a chemical messenger (neurotransmitter)
◦ stimulus indirectly activates the afferent neuron
◦ a separate receptor cell is a specialized neuron
sensory receptors exhibit specificity; they are only
responsive to a specific type of stimuli
- within one tissue, we may have several different types of receptors, each of which are sensitive to different stimuli
- this is called receptor specificity
sensory receptors (at the cell level) vs receptor proteins (at protein level) - know distinction
Sensory Receptor Specificity
there are many types of sensory receptors, with each one being responsive to a specific type of stimuli
* adequate stimulus: the stimulus that a sensory receptor is responsive to in normal functioning
some of the types of sensory receptors: nociceptors, mechanoreceptors, chemoreceptors, thermoreceptors and photoreceptors
*while sensory receptors are generally only activated by specific stimuli, secondary stimuli can sensitize or desensitize the sensory receptors
nociceptors
responsive to pain; responsive to tissue damage or potential tissue damage
◦ extreme temperature, chemicals, mechanical
damage
mechanoreceptors
responsive to mechanical stimuli
◦ pressure, touch, vibration
chemoreceptors
responsive to specific chemicals
◦ metaboreceptors = responsive to metabolites
thermoreceptors
responsive to cold or warmth; phasic receptors
photoreceptors
responsive to light stimuli (particular ranges of the electromagnetic spectrum)
Types of Sensory Receptors (classified by what two things?)
sensory receptors can be classified based on what they monitor:
- exteroreceptors – monitor the external environment
- interoreceptors – monitor visceral organs
- proprioreceptors – monitor position of skeletal muscles and joints
sensory receptors can also be classified based on the senses they contribute to:
general senses: (are usually the dendritic ends of the afferent neuron)
- pain, touch, temperature and proprioception
special senses: (tend to be more complex, and are located in specific sense organs)
- smell, sight, hearing, taste and balance
