Physiology 2130 Lecture Notes - Lecture 7: Lamellar Corpuscle, Low Frequency, Hair Follicle
21 Jun 2018
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Module 7 – Sensory System
Intro:
The human body has several sensory systems that allow it to detect changes in external conditions rapidly
Including:
oSomatosensory (touch) system
oVisual system
oAuditory and vestibular system
oOlfactory (smell) system
oGustatory (taste) system
The Transduction of Environmental Information:
The transduction of environmental information from the environment is how information from the external enviro is
turned into the language the brain understands – AP’s
In order for the brain to know what is happening outside the body, environmental stimuli – like light, heat, touch or
sound – must first be detected by sensory receptors which then convert this information into AP’s
Environmental Stimuli:
In order for the brain to consciously perceive aa stimulus that stimulus must be detected by a sensory receptor
Environmental stimuli come in a variety of forms, and therefore will require different receptors to detect and convert
the stimulus in to an AP
Examples:
oA mechanical stimulus will stretch sensory receptors in the skin and open ion channels, causing a
depolarization of the sensory neuron producing an AP
Mechanical stimuli: touch, pressure, vibration, proprioception, sound
oA chemical stimulus will bind with a receptor, causing a depolarization and then an AP
Chemical stimuli: taste, pain odours
oLight energy is absorbed by photoreceptors in the eye and eventually produce AP’s
Electromagnetic stimuli: light
oGravity and motion can be detected by hair cells in the vestibular system, which convert this stimulus into an
AP
Other stimuli: gravity, motion, acceleration, heat
Adequate Stimulus for the Receptor:
There are many types of stimuli and a corresponding detector – but some receptors can detect more than one type of
stimulus
An adequate stimulus is the particular form of environmental stimulus to which the sensory receptor is most sensitive
oEx. The adequate receptor for the rod and cone receptors is light
Sensory receptors do respond to other forms of energy but not in an optimal way
Receptor (Generator) Potentials:
Once the sensory receptor is stimulated by an environmental stimulus, a change in ion
permeability will occur, leading to localized depolarization
This localized depolarization is called a generator or receptor potential
The receptor potential must spread to an area on the sensory neuron that
contains voltage-gated channels
oBecause the receptor doesn’t have any
oThis is usually the first node of Ranvier on the axon
The AP will then be generated and propagated along the axon and into the spinal cord
In receptors with no axon the depolarization has to spread to the synapse to result in the release of nt
Shared Characteristics of Receptor (Generator) Potentials and EPSP’s/IPSP’s:
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Generally depolarizing, but can be hyperpolarizing
Caused by an increased in permeability to sodium (depolarizing) or potassium ions (hyperpolarizing)
Local and do not propagate down the neuron like an AP, but spread like an EPSP
Proportional to the strength of the stimulus – the stronger the stimulus the larger the receptor potential and more
likely to fire an AP
Receptor Potential and Neural Coding:
How does the body generate a larger number of AP’s to produce a larger receptor potential?
The heavier weight will produce a large receptor potential
This large potential will trigger many AP’s on the sensory neurons axon
This burst of high-frequency AP’s will eventually reach the brain where you will become consciously aware of the
heavier weight
The Somatosensory System:
The somatosensory system detects and processes the sensations of touch, vibrations, temperature and pain
The majority of these sensations originate on the skin
Detecting each sensation requires several different sensory receptors within the skin, each developed to detect its
adequate stimulus
The receptors in the skin are collectively referred to as cutaneous receptors and include the following
oHair follicle receptors – sensitive to fine touch and vibration
oFree nerve endings – respond to pain and temperature
oRuffini’s corpuscles – detect touch
oMeissner’s corpuscles – detect low frequency vibrations and touch
oPacinian corpuscles – detect high frequency vibrations and touch
Receptive Field:
Each receptor will only respond to a stimulus within a certain region on the skin
The receptive field is the area on the surface of the skin where an adequate stimulus will activate a particular receptor
to fire an AP in the neuron
oAny stimulus applied outside of the receptor field will not generate an AP
Somatosensory Pathways from the Periphery to the Brain
The Spinothalamic (Anterolateral) Tract:
The spinothalamic (anterolateral) tract transmits information dealing with very basic sensations
like pain, temperature and crude touch
The information from sensory neuron (first order neuron) enters the spinal
This neuron crosses to the opposite side or contralateral side of the spinal cord and ascends up
the spinal cord where it synapses with a second order neuron
The information now ascends up the spinal cord to the thalamus
The thalamus acts as a relay station for almost all sensory information (except smell)
A second synapse with a third order neuron occurs here and then travels to the somatosensory cortex
Dorsal Column, Medial Lemniscal System:
The dorsal column, medical leminiscal system transmits information associated
with the more advanced sensations of fine detailed touch, proprioception and vibration
The information from the sensory neuron (first order) enters the spinal cord and travels up
the spinal cord before crossing to the contralateral side (unlike previous pathway)
In the upper spinal cord, the sensory neuron thensynapses with a second order neuron
which then crosses to the opposite side
From here it continues to the thalamus where it synapses onto a third order neuron and travels to the somatosensory
cortex
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Primary Somatosensory Cortex:
Once the information has reached the brain it travels to the primary somatosensory
cortex, which is located in the parietal lobe on the precentral gyrus behind the central sulcus
The Somatosensory Homunculus:
The primary somatosensory cortex is arranged in a very specific manner
The sensory information arriving at this cortex is geographically preserved
oAs if the entire body was projected onto the surface of the brain like a map
oMedial to lateral - genitals, feet, legs, body, head, arms, hands, face, tongue
Some areas on the cortex receive more sensory information and require more of the brain to process that
information
oThe hands, tongue and lips are the most sensitive parts of the body
oContain more sensory receptors than other parts
The Visual System:
The visual system detects light, converts it into AP’s, and sends it to primary visual areas for processing
Once processed we become aware of our visual world and are able to distinguish and recognize features in
our external enviro
The visual system consists of
oThe eye – contains photoreceptors that convert light into AP’s
oThe visual pathway – transmits the AP
oThe primary visual area – in the occipital lobe, processes the incoming signals
The Eye:
The iris
oThe amount of light is regulated by the iris
oCan constrict with bright light or dilate with low light
The lens
oFlips the light upside-down and backwards
oFocuses the light onto the retina at the back of the eye
The retina
oContains photoreceptors called rods and cones
oThe rods and cones point toward the back of the head
The fovea
oThe center of your vision is focused here
oPart of the retina
oHas the highest concentration of cone cells
The Photoreceptors of the Eye:
Rods
oExtremely sensitive to light
oFunction best under low light
oContain one type of photopigment and do not detect colour
oLocated mostly in the region of the retina outside of the fovea
Cones
oFunction best in bright light
oIdeal for detecting detail
oThree different types of cone cells, each with a different photopigment and each sensitive to one primary
colour
oCones are mainly located in the fovea in large concentrations
Rod and cone cells do not have axons and therefore do not generate action potentials
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Document Summary
The human body has several sensory systems that allow it to detect changes in external conditions rapidly. Including: somatosensory (touch) system, visual system, auditory and vestibular system, olfactory (smell) system, gustatory (taste) system. The transduction of environmental information from the environment is how information from the external enviro is turned into the language the brain understands ap"s. In order for the brain to know what is happening outside the body, environmental stimuli like light, heat, touch or sound must first be detected by sensory receptors which then convert this information into ap"s. In order for the brain to consciously perceive aa stimulus that stimulus must be detected by a sensory receptor. Environmental stimuli come in a variety of forms, and therefore will require different receptors to detect and convert the stimulus in to an ap. Examples: a mechanical stimulus will stretch sensory receptors in the skin and open ion channels, causing a depolarization of the sensory neuron producing an ap.
