Special Senses Physiology Flashcards
Sensory System
Sensory receptors receive stimuli from the external or internal environment which is then carried by neural pathways to the brain or spinal cord
Somatosensory System
Part of the sensory system that is concerned with the perception of touch, pressure, pain, temperature, position, movement, vibration
Somatic Sensation
Defined as sensation from the skin, muscles, bones, tendons, and joints initiated by somatic receptors
Sensory Receptor
Specialized cells that generate graded potentials called receptor potentials in response to a stimulus
Somatosensation
The process that conveys information regarding the body surface and its interaction with the environment
-submodalities: touch, pressure, temperature, pain
Proprioception
Sense of posture and moment; a sensation of the position of your different body parts and muscle contraction in space
-different from somatosensation
Modality
A particular form of sensory perception
Meissner’s Corpuscles
Mechanoreceptors that respond to touch and pressure, rapidly adapting
Merkel’s Corpuscles
Mechanoreceptors that respond to touch and pressure, slowly adapting
Free Neuron Ending
Close to the surface of the skin
Include nociceptors, thermoceptors, mechanoreceptors,
Pacinian Corpuscle
Responds to vibration and deep pressure, rapidly adapting mechanoreceptor
Ruffini Corpuscle
Responds to skin stretch, slowly adapting mechnoreceptor
How are Afferents Activated?
When mechanoreceptors are activated, sodium channels open and sodium flows down its concentration gradient into the afferent neuron, resulting in a graded depolarization of the sensory receptor
2 Types of Sensory Receptor
- The sensory receptor is located directly on the afferent fiber
- The sensory receptors is located on a specialized receptor cell - releases a neurotransmitter that binds to the receptors of the afferent neuron
APs or EPSPS for Sensory Receptor Activation?
EPSP - activation of a sensory receptor generates a grade potential
Receptor Potential
The greater the stimulus, the more action potentials that are fired
Stimulus Intensity
Most receptors have multiple sensory endings
As more sensory endings are depolarized, more action potentials fire in the afferent neuron
Slow Adapting Receptors
Action potentials are generated the entire time that the stimulus is on
e.g. holding your arms out in front of you
Rapidly Adapting Receptors
Immediately generates a receptor potential with the initial stimulus; the receptor potential then quickly decays back to baseline
Another receptor potential is generated when the stimulus turns off
e.g. putting on and taking a shirt off
Stimulus Localization
Different mechanisms are responsible for stimulus localization so that we have the ability to localize where a stimulus is coming from
3 Factors for Stimulus Localization
- Receptive field size - the extent of the body which senses the poke
- Density of innervations - the number of sensory receptors within a certain area of the skin
- Multiple receptive fields exist and some overlap
Receptive Field
Different sensory neurons have different receptive field sizes
Each sensory neuron takes information back to the CNS
What Allows for a Better Localization of the Specific Site of Stimulation?
Smaller receptive fields
Density of Innervations
The more densely packed the sensory receptors are, the greater the ability to localize the stimulus
Overlapping Receptive Fields
Helps localize the site of a stimulus
Overlapping receptive fields allows the brain to compute the specific site of the stimulus based on the relative activation of different sensory neurons with the overlapping receptive fields
Lateral Inhibition
Occurs when there are overlapping receptive fields
Only in somatosensation and vision
Involves inhibitory interneurons
How Does Lateral Inhibition Work?
Information from afferent neurons whose receptors are at the edge of a stimulus is strongly inhibited compared to information from the stimulus’ center
Lateral inhibition enhances the contrast between the center and periphery of a stimulated region, thereby increasing the brains ability to localize a sensory input
Lateral inhibition removes the information from peripheral regions
Center Control of Somatosensory Information
Sensory signals are subject to extensive modification before they reach higher levels of the central nervous system
Modification comes from inhibition from collaterals from other ascending neurons, pathways descending from higher centers in the brain, by synapses on the axon terminals of the primary afferent neurons, or indirectly via interneurons that affect other neurons in the sensory pathways
2 Neural Pathways of the Somatosensory System
- Anterolateral system - pathway which carries pain, or hot/cold information up to the somatosensory cortex
- Dorsal column system - pathway which carries information on fine touch mechanoreception to the somatosensory cortex
Anterolateral System
Exposure to a painful stimulus activates free neuron endings
- first synapse is located in the dorsal horn of the grey matter of spinal cord on same side of body which was stimulated
- secondary neuron crosses over to the other side of the CNS at the level of the spinal cord
- secondary neuron synapses with projection neuron in the thalamus which travels to somatosensory cortex
- painful information crosses immediately and travels up the contralateral, or opposite, side of the body
Dorsal Column System
A tap should activate mechanoreceptors
- first synapse between the sensory neuron and the secondary neuron is in the brainstem
- secondary neuron crosses over to the other side of the CNS at the level of the brainstem
- secondary neuron synapses with projection neuron in the thalamus which travels to somatosensory cortex
- touch information travels up the spinal cord on the same side of the body as the stimulation
The Somatosensory Cortex
All sensory information goes from the thalamus to the somatosensory cortex
Located behind the motor cortex and the central sulcus
Activates motor cortex neurons
Each region in the body maps to a very specific region in the somatosensory cortex
-the smaller and more densely packed the sensory receptors are, the larger the region in the somatosensory cortex
Vision
Photoreceptors in the eye are depolarized at rest and hyperpolarized when activated
-contains an optical component and a neural component
Optical Component
responsible for focusing the visual image on the receptor cells, the front part of the eye
Neural Component
the back part of the eye where the photoreceptors are located, transforms the visual image into a pattern or graded potentials and action potentials
Sclera
white part of the eye, the membrane surrounding the eyeball
Extraocular Muscle
attached to the sclera, responsible for eye movements
Cornea
responsible for refracting light waves
Pupil
the hole that allows light to pass through into the back of the eye
Iris
regulates the size of the pupil and amount of light entering the eyeball, gives your eyes colour
-innervated by the ANS
Lens
behind the iris; works with the cornea to focus the visual image on the retina; the shape and size of the lens can change
Zonular Fibers
attached to the lens; attached to ciliary muscles
Ciliary Muscles
can contract/relax; change the shape of the lens
Retina
located behind the lens, back of the eye where the photoreceptors are found
Rods
activated in very light light conditions and are monochromatic
Cones
activated when there is more light present and are responsible for colour vision
Retinal Ganglion Cells
activated by the rods and cones; take information back towards the brain
Optic Nerve
leaves through the back of the eyeball towards the thalamus and the cortex; made of axons and retinal ganglion cells
Aqueous Humour
a gelatinous fluid that fills the space between the lens and the cornea
Vitreous Humour
a gelatinous fluid found behind the lens
Refraction
Light travels towards the eyeball and bends once they hit the cornea
Structure that is primarily responsible for refraction is the cornea
Lens focuses the visual image on the retina
Ciliary Muscles and Refraction
When an image comes very close to the eye, the ciliary muscle contracts which causes the lens to get fatter and shorter and increases the amount of refraction; allowing the visual image to focus on the back of the retina
Accomdation
The process of using ciliary muscles in order to focus on objects that are very close, lose this ability around 45 due to the breakdown of ciliary muscles
Prebyopia
loss of elasticity of the lens resulting in the inability to accommodate for near-vision; age-related
Myopia
Can focus on objects close-up but not far away
The eyeball is too long and too much refraction occurs
Corrected by wearing glasses or contact lenses with a concave shape to reduce the refraction
Hyperopia
Can focus on objects far away but not close up
The eyeball is too short and the visual image is reconstructed behind the retina as there is not enough refraction
Corrected by wearing classes or contact lenses with a convex shape to increase refraction
Astigmatism
Oblong shape of the eyeball
Glaucoma
Damage to the photoreceptors due to increased intraocular pressure
-buildup of aqueous humour which pushes on lens, lens then pushes back on the vitreous humour, which pushes back on the retina and photoreceptors causing damage
Cataracts
Clouding of the lens
Age-related
Interneurons in the Eye
horizontal
bipolar
amacrine
Bipolar Cells
interneurons which take information from the photoreceptors to the retinal ganglion cells
How are Cones Activated in the Dark?
- guanylyl cyclase converts GTP into cyclic GMP
- cyclic GMP-gated cation channels are in the membrane of the photoreceptor; the ligand which activates it is cGMP
- cGMP binds to its receptor on the cation channel; the cation channel opens, allowing sodium and calcium to flow into the cell
- the photoreceptor depolarizes as the positively charged ions enter the cell
- relatively depolarized when light is absent
How are Cones Activated when it is Light?
- the disk of the cones contains a photopigment which contains a chromophore called retinal
- when light hits the photopigment, retinal changes conformation from cis to trans conformation; this change in conformation activates a molecule called cyclic GMP phosphodiesterase
- cyclic GMP phosphodiesterase breaks down cGMP and into GMP
- cGMP is removed from the ion channel and the ion channel then closes
- sodium and calcium can no longer enter the cell and the photoreceptor is relatively hyperpolarized when light is present
OFF Pathway
- “no light is hitting me”
- relative depolarization of the photoreceptor
- graded potentials are generated in the photoreceptors and the result is glutamate release from this photoreceptor
- OFF bipolar cell is activated by glutamate and ON bipolar cell is inhibited by glutamate
- when no light is present: action potentials generated in the OFF pathway but not in the ON pathway
ON Pathway
- “light is hitting me”
- light is present: ON pathway is activated due to the release of the inhibition on the ON bipolar cell by glutamate
Effect of Light on ON/OFF Pathways
In both pathways, the photoreceptors is depolarized in the absence of light
When light strikes the photoreceptor cell, the photoreceptor hyperpolarizes
-cGMP in the photoreceptor is broken down by cGMP-dependent phosphodiesterase, decreasing cytoplasmic concentrations of cGMP and allowing cation channels to close -> cell hyperpolarizes -> when photoreceptor cell is hyperpolarized, the cell decreases its release of glutamate
Effect of Light on ON Pathway
When glutamate release from the photoreceptor cell is decreased, the glutamate inhibition onto the bipolar cell is decreased -> bipolar cell depolarizes and releases more glutamate -> ganglion cell depolarizes and generated action potentials
Effect of Light on OFF Pathway
There is reduced excitation of the OFF bipolar cell, as glutamate release from the photoreceptor has decreased -> bipolar cell hyperpolarizes and releases less glutamate -> the ganglion cell hyperpolarizes and generates fewer action potentials
Neural Pathways of Vision
The light signals are converted into action potentials through the interaction of the photoreceptors with bipolar cells and ganglion cells
- photoreceptor cells and bipolar cells undergo graded responses and lack voltage-gated sodium channels needed to generate an action potential
- ganglion cells have voltage-gated sodium channels and are the first cells in the pathway where action potentials can be initiated
Key Differences Between the ON and OFF Pathway
- Bipolar cells of the ON pathway spontaneously depolarize in the absence of input while bipolar cells of the OFF pathway hyperpolarize in the absence of input
- Glutamate receptors of ON pathway bipolar cells are inhibitory, while glutamate receptors of OFF pathway bipolar cells are excitatory
Neural Pathways of Vision: ON Pathway
- When glutamate is released onto ON bipolar cells, it binds to metabotrophic receptors that cause the breakdown of cGMP which hyperpolarizes the bipolar cell and prevents the release of glutamate onto the ganglion cells
- no light = no action potentials fired
- light = action potentials fired because ganglion cells are depolarized
Neural Pathways of Vision: OFF Pathway
- OFF pathway bipolar cells have ionotropic glutamate receptors that are non-selective cation channels, which depolarize the bipolar cells when glutamate binds
- depolarization of bipolar cells stimulates them to release excitatory neurotransmitters onto ganglion cells, firing action potentials
- OFF pathway generates action potentials in the absence of light (none in the presence of light)
What Does the Coexistence of ON and OFF pathways in Each Region Do?
improves image resolution by increasing the brain’s ability to perceive contrast at edges or borders
Where is the Visual Cortex
Occipital Lobe
Visual Pathways
Information from the lateral field - goes to the opposite side of the brain
Information from the medial field - goes to the same side of the brain
Pinna
The external ear on the side of the head
Where is the Auditory Cortex
Temporal Lobe
How is Sound Transmitted?
air is the most common medium in which we hear sound energy
Zones of Compression
Regions where air molecules are tightly packed or close together
Zones of Rarefaction
Regions where there are relatively few air molecules
How do Zones of Compression and Rarefaction Move?
Ripple outward, transmitting the sound wave over distance
Amplitude
The volume or loudness of sound
Determined by how many air molecules are located within one of the zones of compression or the difference between the pressure of molecules in the zones of compression and rarefaction
Frequency (Pitch)
Determined by the distance between the zones of compression, or the number of zones of compression or rarefaction in a given time
Tympanic Membrane
Ear drum
Vibrates in and out as molecules push against it
Cochlea
Inner ear
Contains the scale vestibuli, scala tympani, and the cochlear duct
Malleus, Incus, Stapes
The malleus is connected to the tympanic membrane
Bones act as levers and amplify sound
Skeletal muscles attached to the malleus and stapes contract to dampen movement of bones for loud sounds
Muscle Connected to Malleus
Tensor tympani muscle
Muscle Connected to Stapes
Stapedius muscle
Why do Muscles in the Ear Contract
Muscles contract to protect the ear from consistent, ongoing loud sounds
Muscles would not protect the ear from a loud sudden bang as they would not be contracted when the sudden loud noise occurred
Oval Window
The stapes pushes against the oval window and the cochlea is full of fluid, the stapes pushes fluid forward within the ear
Scala Vestibuli
has fluid called perilymph
Scala Tympani
has fluid called perilymph
Cochlear Duct
has fluid called endolymph; region of the inner ear where the sensory receptors for the auditory system are located
Sound Transmission in the Ear
Movement of fluid down from scale vestibuli to scala tympani results in the activation of the sensory receptors for the auditory system
Sensory Receptors in the Ear?
Hair cells
- located in the organ of Corti
- have stereocilia protruding from them at their tips
Organ of Corti
A specialized epithelium that allows for the transduction of sound vibration into neural signals
Stereocilia of the Single Row of Inner Hair Cells
extend into the endolymph and transduce pressure waves caused by fluid movement in the cochlear duct into receptor potentials
Stereocilia of the 3 Rows of Outer Hair Cells
At the bottom, each outer hair cell is attached to the basilar membrane
Different regions of the basilar membrane vibrate maximally at different frequencies
Vestibulocochlear Nerve
Takes auditory information from the ear towards the brain
Hair Cells of the Organ of Corti
The hair cells move back and forth when the basilar membrane moves
When stereocilia move, a mechanically-gated potassium channel opens
-how the auditory receptors are activated and how the depolarize
-when the basilar membrane moves, the hair cells move back and forth and the stereocilia bend
Neural Pathways in Hearing
Cochlear nerve fibers synapse with interneurons in the brainstem
-from the brainstem information is transmitted by a multineuron pathway to the thalamus and hten to the auditory cortex in the temporal lobe
Hearing Aids
An amplifier which is placed in the auditory canal which activates the existing auditory machinery
Amplifies the existing sounds
Restricted to the outer ear component and can be turned up and down in volume
Cochlear Implants
Machinery of the ear doesn’t work
A speaker on the outside of the head picks up noises and transduces them into electrical impulses
Electrodes go from the speaker to the vestibulocochlear nerve
The receiver takes auditory information, bypases the outer middle and inner ear, and directly stimulates the nerve
