Module 6 - PNS Afferent Division Flashcards
Describe the sensation of pain
• Uses nociceptors
• Activated by intense thermal, mechanical, or chemical stimuli
o Accompanied by behavioural and emotional responses unlike other stimuli
• Located everywhere but the brain
• Free nerve endings that do not adapt to sustained or repeated stimuli
• Sensations may be out of proportion to damage or appear for no obvious reason
• Influenced by prostaglandins, histamine, potassium, serotonin, and substance P (peptides)
Describe fast pain
- Uses myelinated A-delta (δ) fibres
- Responds to cold, warmth, and mechanical stimuli
- Occurs immediately after stimulus applied
- Not felt in deeper tissues of the body
- Precisely located to stimulation
Describe slow pain. Why does this pain last even after the stimulus is removed?
• Also called chronic, burning, aching, or throbbing pain
• Uses unmyelinated C fibres
• Responds to pain, heat, cold, and mechanical stimulation
• Activated by bradykinin released to the ECF from damaged tissue
o Contributes to pain and inflammatory response
o These may be the reason the pain sensation lasts after removal of stimuli
• Polymodal nociceptors
o Respond to more than one type of noxious stimuli
• Begins a second or more after stimulus is applied
• Gradually increases in intensity over seconds or minutes
• Can be felt in skin and deeper tissues
• Felt in affected area but more diffuse
Describe afferent neurons
Receive stimuli from the environment and transmit the impulse to the CNS
Single long dendrite that is functionally similar to an axon
Short axon
Smooth rounded cell body
Describe the two types of neurons in the afferent division of the autonomic nervous system
Visceral Afferent Neurons
• Convey information to the CNS from receptors
• Located in the smooth muscle of the visceral organs (mainly thorax, abdomen, and pelvis)
• Usually happens no conscious awareness except pain signals coming from viscera
Somatosensory Neurons • Consists of receptors in the PNS and processing centers in the CNS • Person is consciously aware of this input • 4 primary types of receptors are o Thermoreceptors o Mechanoreceptors o Photoreceptors o Chemoreceptors • Input from the body surface o Somaesthetic sensation from the skin o Proprioception from the muscles, joints, skin, and inner ear • Includes input from special senses
What is perception?
Conscious interpretation of the external world
What is transduction?
o Energy conversion process that converts mechanical or chemical stimulation into an electrical signal
What is adequate stimulus?
o Property of a sensory receoptor that determines the type of energy to which a sensory receptor responds with the intention of sensory transduction
Ex. Receptors in the eye are most sensitive to light etc.
o Some receptors can respond weakly to stimuli other than their adequate stimuli
Receptors are activated to a lesser degree
Same sensation detected
Ex. The eye sees “stars” in response to mechanical pressure
What is modality?
o A particular way of sensing the stimulus
o Determined by the type of sensory neuron that’s activated and its point of termination in the brain
What stimulus do photoreceptors respond to?
Visible wavelengths of light
What stimulus do mechanoreceptors respond to? What are some examples of these receptors and their specific stimulus?
Mechanical energy
Skeletal muscle receptors - stretch
Hair cells in the ear - bend to sound waves
Baroreceptors - blood pressure
Osmoreceptors - change in osmotic pressure
What stimulus do thermoreceptors respond to?
Varying amounts of heat
What receptors are responsible for the sensation of wetness?
A combination of touch, pressure, and thermal input at the central integration area
What stimulus do chemoreceptors respond to?
Specific chemicals
May include receptors for smell and taste
Chemical content of digestive tract
Oxygen and CO2 levels in the blood
What may the structure of the sensory neurons be and what is the depolarization of each type called?
A specialized ending of the afferent neuron - generator potential
A separate receptor cell closely associated with the peripheral ending of the neuron - receptor potential
Where are action potentials created in afferent neurons?
• All receptors have graded potential with action potentials initiated at the peripheral end of the afferent neuron next to the receptor
What is stimulus intensity determined by?
o Frequency of action potentials generated
o Number of receptors activated in the area
What is adaptation?
- Decrease in sensation during prolonged stimulus
* Frequency of action potentials generated in afferent neuron decreases
Describe the 2 types of receptors based on their speed of adaptation?
Phasic Receptors
• Rapidly adapting receptors
• Activated when stimulus is first encountered
• Important to signal a change in stimulus intensity rather than relay status quo information
o Ex. Tactile (touch) receptors so you can wear clothes
Off Response
• Slight depolarization when stimulus is removed
Tonic Receptors
• Slow to adapt or do not adapt at all
• Will maintain sending signals even with a sustained stimulus to allow to tell duration of the sensation
• Located where it is valuable to maintain information about stimulus
o Ex. Proprioceptors
Describe labelled lines of afferent neurons.
• Chains of neurons that create the somatosensory pathways
• Sends specialised information to specific area in the somatosensory cortex
o Allows brain to interpret specific sensations
First Order (Primary) Neurons
• Sensory neurons in PNS carrying information to CNS
• Axons synapse with second order neurons
Second Order (Secondary) Neurons
• Usually located in brain or spinal cord
• Axons ascend to the thalamus and synapse with third order neurons
Third Order (Tertiary) Neurons • Synapse located in the thalamus • Project axons into primary somatosensory cortex
What is the receptive field?
The region of skin surface surrounding a somatosensory neuron in which it will respond to a stimulus
Size is inversely related to number of receptors in the area
What is discriminative ability?
The greater acuity created by a small receptive field the more a neuron is able to sense and discriminate between different sensations (compare sensation of the same object between fingertips and elbows)
What is lateral inhibition?
Occurs within the CNS, the most activated stimulus area inhibits the less exited pathways. It’s what allows us to feel a pin point sensations
Most lateral inhibition occurs with touch and vision, allowing the most accurate localization
Describe Pacinian corpuscles. What do they respond to? What is their appearance? How quickly do they adapt?
Lamellar Corpuscle
• Also called a Pacinian corpuscle
• Respond to
o High frequency vibration
o Touch
o Deep pressure
o Allows for sensation from texture
• Located in the dermis, subcutaneous tissue, and other tissues of the body
• Myelinated neuron ending surrounded by a multilayered connective tissue capsule that resembles a sliced onion
o Onion structure causes constant pressure to slip, resulting in adaptation
• Phasic receptor so is rapidly adapting
o Respond to transient touch rather than sustained pressure
Describe Meissner’s corpuscles. What do they respond to? What is their appearance? How quickly do they adapt?
Tactile Corpuscles
• Also called Meissner’s corpuscles
• Respond to
o Low-frequency vibrations
Sensation of texture and if an object is sliding past the skin
o Light touch
• Located in dermal papillae of hairless skin and concentrated in areas sensitive to light touch such as fingers, lips, and nipples
• Elongated oval structure with a core of modified Schwann cells and neuronal endings enclosed by a capsule of connective tissue
• Afferent neuron is myelinated
• Phasic receptor so is rapidly adapting
• Small receptive fields
Describe Merkel’s discs. What do they respond to? What is their appearance? How quickly do they adapt?
Merkel’s Discs • Also called a tactile disc • Respond to o Low frequency vibrations o Tissue displacement (very sensitive) Provides touch information • Located in superficial layers of skin, mammary glands, and mucosa, clustered beneath fingerprints • Myelinated afferent neurons with endings clustered in specialized epithelial structures called touch domes or hair disks • Slow to adapt
Describe Ruffini corpuscles . What do they respond to? What is their appearance? How quickly do they adapt?
Bulbous Corpuscles
• Also called Ruffini corpuscles
• Highly sensitive to stretching and pressure
• Located in the dermis, subcutaneous tissue, and joints
• Branched neuronal endings that are distributed between collagen bundles and Schwann cells enclosed in a fibrocellular capsule with a myelinated nerve
• Slowly adapting
• Large receptive fields
Describe free nerve endings. What do they respond to? What is their appearance? How quickly do they adapt?
Free Nerve Endings
• Bare dendrites
• Respond to
o Touch and pressure
o Temperature
o Nociception (pain)
o Tickling and itching
• Certain chemicals can result in an itch sensation (such as mosquito saliva)
• Located in skin, hair roots, around the eyes, and many others
• Bare dendrites and neuron may be myelinated or unmyelinated
Describe the basics of pain sensation
• Uses nociceptors
• Activated by intense thermal, mechanical, or chemical stimuli
o Accompanied by behavioural and emotional responses unlike other stimuli
• Located everywhere but the brain
• Free nerve endings that do not adapt to sustained or repeated stimuli
• Sensations may be out of proportion to damage or appear for no obvious reason
• Influenced by prostaglandins, histamine, potassium, serotonin, and substance P (peptides)
What are the 2 main neurotransmitters used along pain pathways?
Substance P and Glutamate
Describe substance P’s role along the pain pathways
• Activates ascending pain pathways • Signals sent to o Cortex Somatosensory area localizes the pain o Thalamus Can perceive pain o Reticular formation Increases level of alertness o Interconnections with hypothalamus and limbic system Elicit behavioural and emotional response
Describe glutames role along the pain pathways
• Major excitatory neurotransmitter
• AMPA receptors
o Results in permeability changes that initiate action potentials in dorsal horn
o These action potential head up to higher levels for processing
• NDMA receptors
o Calcium entry into dorsal horn cell
o Not involved in pain pathway but instead activates a second-messenger system that makes the dorsal horn neuron more excitable than normal
o Contributes to exaggerated sensitivity that resolves with healing
Describe how fast pain is experienced
- Uses myelinated A-delta (δ) fibres with a free nerve ending receptor
- Responds to cold, warmth, and mechanical stimuli
- Feels sharp, stabbing, or acute
- Occurs immediately after stimulus applied
- Not felt in deeper tissues of the body
- Precisely located to stimulation
Describe how slow pain is experienced
• Also called chronic, burning, aching, or throbbing pain
• Uses unmyelinated C fibres with a free nerve ending receptor
• Responds to pain, heat, cold, and mechanical stimulation
• Activated by bradykinin released to the ECF from damaged tissue
o Contributes to pain and inflammatory response
o These may be the reason the pain sensation lasts after removal of stimuli
• Polymodal nociceptors
o Respond to more than one type of noxious stimuli
• Begins a second or more after stimulus is applied
• Gradually increases in intensity over seconds or minutes
• Can be felt in skin and deeper tissues
• Felt in affected area but more diffuse
Describe the bodies internal analgesic system including what endogenous opiates are and how they function in the body
• Suppresses transmission in pain pathways at entrance to spinal cord
• Two regions provide profound analgesia
o Periaqueductal grey matter
o Reticular formation
o Stimulation here blocks release of substance P from afferent pain-fiber terminals
• Endogenous opiates
o Internal substances that bind to opioid receptors on afferent pain-fiber terminals and dull pain
o Include
Endorphins
Enkephalins
Dynorphins
o Released from descending analgesic pathway
o Binding blocks release of substance P
What are the protective mechanisms for the eye?
Bony socket
Eyelids
Tears
Eyelashes
What are the 3 layers of the eye and their primary structure(s)?
Outer - fibrous tunic - sclera and cornea
Middle - vascular tunic - choroid, ciliary body, and iris
Inner - retina
How does the eye prevent any reflection or scattering of light within the eye?
Highly pigmented choroid and outer layer of the retina
What is glaucoma, what is it caused by, and what are the results?
Excess pressure within the anterior chamber of the eye
Caused by aqueous humor not draining as quickly as it is created (eg blockage of drainage canal)
Can push lens back into vitreous humor, results in pressure against the inner neural layer of retina which can lead to retinal and optic nerve damage and ultimately blindness
Describe the sclera of the eye
- White of the eye
- Made of tough connective tissue
- Maintains shape and protects inner structures
- Provides attachment points for extraocular muscles
Describe the cornea of the eye
- Transparent
- Avascular connective tissue with regularly spaced collagen fibers
- Allows light to enter the eye
- Curved surface a refractory structure for incoming light
Describe the choroid of the eye
- Posterior portion of vascular tunic
- Lines most of the internal surface of the sclera
- Contains blood vessels that also supply the retina
- Contains melanocytes to produce pigment that minimize light scattering
Describe the ciliary body. Where is the aqueous humor secreted and how does it attach to the lens?
• Anterior portion of vascular tunic
• Extends from anterior margin of retina to just posterior of the corneoscleral junction
• Contains melanocytes producing melanin
• Ciliary processes
o Protrusions or folds on internal surface of the ciliary body
o Contain capillaries secreting aqueous humor
• Zonular fibers
o Also called suspensory ligaments
o Extend from the ciliary process
o Attach to the lens
o Composed of thin, hollow fibrils resembling elastic connective tissue fibers
• Accommodation
o Contraction or relaxation of ciliary muscles changes tension of zonular fibers
o This changes the shape of the lens adapting it for near or far vision
Describe the iris and the 2 muscles that make it work
• Muscular, coloured portion of the eye
o Colour determined by amount of melanin produced
• Contraction and relaxation change the size of the pupil regulating the amount of light that enters
• Suspended between cornea and lens as an anterior projection of the ciliary body
• Sphincter pupillae muscle
o Also called the circular (constrictor) muscle
o Flat, thin band of circularly oriented muscle fibers
o Contraction causes constriction of the pupil
o Stimulated by parasympathetic fibers of oculomotor (III) nerve
• Dilator pupillae muscle
o Also called the radial (dilator) muscle
o Attaches to the outer circumference of the sphincter pupillae
o Projects like spokes towards the base of the iris
o Stimulated by sympathetic neurons causing dilation
What is light?
A form of electromagnetic radiation composed of particle-like individual packets of energy called photons that travel in a wavelike fashion
What is a wavelength and what are the wavelengths of visible light? How do these wavelengths correspond to the colours we see?
The distance between two wave peaks
400-700 nm
Shorter wavelengths are violet and blue
Longer wavelengths are orange and red
What changes to the waveform of light in a bright light vs a dim light?
Light energy changes in intensity, so the amplitude or height of the wave is changed but the length between two wave peaks remain the same
What is a light ray?
The forward movement of a light wave in a particular direction
How does the body take divergent (radiating) lights and change it to an accurate image of the light source?
By bending the light rays inward and focusing them back to a point on the retina called the focal point
What two structures contribute to the eyes refractive ability? Which ones contributes the most?
Cornea and lens; the cornea contributes the most due to the curve and the difference in densities between the air and the cornea
What is astigmatism?
Uneven curvature of the cornea resulting in unequal refraction
Describe the accommodation used for far vision
Ciliary muscle is relaxed and suspensory ligaments (zonular fibers) are taut. this pulls lens into flattened, weakly refractive shape
Describe the accommodation for near vision
Ciliary muscle is contracted and suspensory ligaments (zonular fibers) are slack. This lens becomes more convex and stronger allowing for near vision
How does the autonomic nervous system adjust for far/near sightedness?
Sympathetic system causes relaxation of ciliary muscle, resulting in far vision
Parasympathetic stimulation causes contraction of ciliary muscle for near vision
What is presbyopia?
Loss of elasticity in the lens so it does not accommodate for near vision well, resulting in people needing reading glasses. Age related, and affects most people by middle age (45-50)
What is a cataract?
A now opaque elastic fiber in the lens that prevents light rays from passing through; can be surgically replaced with an artificial lens to correct vision
What is emmetropia?
Regular vision where incoming light can be focused on the retina without assistance
What is myopia?
Nearsightedness
Far light lands in front of the retina and is blurry. Close objects can be brought to retina without accommodation (even though this is normally needed)
What is hyperopia?
Farsightedness
Far objects focused on the retina with accommodation (which isn’t usually needed for far objects). Close objects focus behind the retina and remains blurry
What are the 3 layers of excitable cells (called the neural layer) in the retina?
- Photoreceptor cell layer (rods and cones)
- Bipolar cell layer (see activating action potentials below)
- Ganglion cell layer (see activating action potentials below)
Are the rods and cones a peripheral organ?
No, they are a part of the CNS; during embryonic development the retinal cells back out of the nervous system so the retinal layers are facing backwards
In the eye, what do the axons of ganglion cells join to form?
Optic nerve
Describe the point in retina where optic nerve and blood vessels pass through
Optic Disk
• Also called the blind spot
• Optic (II) nerve creates blind spot as no photoreceptors here
• Your brain patches in the information with what’s in the surroundings
Describe the fovea centralis
- Small depression in the center of the macula
- Vision is sharpest
- Full of cones, no rods
- Bipolar and ganglion cells displaced to periphery as they scatter some light
Describe the fovea centralis
- Small depression in the center of the macula
- Vision is sharpest
- Full of cones, no rods
- Bipolar and ganglion cells displaced to periphery as they scatter some light
Describe the macula lutea?
- Also called the yellow spot
- Exact center in posterior portion of the retina
- Large concentration of cones
- High acuity that is slightly less than the fovea because the ganglion and bipolar cells are overlying the rods and cones here
Describe macular degeneration
• Also called age-related macular disease (AMD)
o Occurring in those 50 years or older
o Most common cause of blindness in those over 75
• Leading cause of blindness in western hemisphere
• Degenerative disorder of retina and pigmented layer
• Abnormalities of the macula occur
• Symptoms
o Blurring and distorting vision at center of visual field
o Gradual vision loss as pigmented layer atrophies and degenerates (dry AMD)
o Loss of ability to see straight ahead
o Maintain peripheral vision
• wet AMD causes new blood vessels to form in choroid and leak plasma or blood under the retina
o Vision loss can be slowed by laser surgery destroying leaking vessels
• Smoking increases risk
• No effective treatment
Describe the 3 main parts of rods and cones
• Outer segment o Detects light stimulus o Face away from incoming light o Either rod or cone shaped o Contains stacked, flattened, membranous discs containing photopigment • Inner segment o Forms mid-region of the cell o Contains metabolic machinery • Synaptic terminal Closest to eyes interior
Describe photopigments
• Undergoes chemical alterations when activated by light initiating the action potential
• Opsin
o Protein integral to disc membrane of the outer segment of the rod or cone
• Retinene
o Bound in interior of opsin molecule
o Light-absorbing part of the photopigment
o Derivative of vitamin A
• Rhodopsin used in rods
• Photopsin used in cones
Describe the location, functions, and photopigments of rods
• Highest concentration in the periphery of the retina
• Low light
• Rhodopsin is the photopigment
o Absorbs all wavelengths of light but brain cannot distinguish them
o Interpreted only in shades of grey
o Detects different intensities rather than different colours
• More sensitive but less acuity than cones
• Over 100 rods may converge via bipolar cells to a single ganglion cell
Describe the location, functions, and photopigments of cones
• • Highest concentration in the fovea and macula
• For bright light
• 3 slightly different photopigments, giving blue, green, red cones
• Photopigment is photopsin
• Brain interprets the various receptors to determine colour
• Generally, 1 cone to 1 ganglion cell
Found in the fovea
What is phototransduction?
• Converting light stimuli into electrical signals
What is phototransduction and how to light and dark effect it?
- Converting light stimuli into electrical signals
- Photoreceptors inhibited by adequate stimulus (hyperpolarized by light)
- Photoreceptors excited in absence of stimulation (depolarized by darkness)
