PHGY 215 - Module 3

what are afferent division?

• Afferent neurons carry nerve impulses from PNS receptors and special sense organs to CNS 

  • Small round body with long dendrite and short axon 

  • Dendrite receives appropriate stimulus and conducts action potentials towards cell body 

• Afferent nerve cell bodies found in clusters (ganglia) extend into dorsal horn of the spinal cord

what is a threshold?

• Receptors located at peripheral of afferent neurons and respond to stimuli from both internal and external environments 

• Receptors require minimum threshold of stimulus before they are activated 

why do afferent neurons need a threshold?

Afferent neurons use action potentials to propagate signal → inform CNS that threshold has been reached and activate receptor

what is transduction?

conversion of environmental signal to electrical signal

what are the properties of receptors

1. modality

2. intensity

3. location

4. duration

what does modality refer to?

each type of receptor responds to different type of energy or stimulus

what are the different types of receptors?

1. photoreceptors

2. mechanoreceptors

3. thermoreceptors

4. chemoreceptors

what are photoreceptors?

responsive to visible wavelengths of light

what are mechanoreceptors?

responsive to mechanical energy, vibration, acceleration

what are thermoreceptors?

sensitive to heat

what are chemoreceptors?

sensitive to specific chemicals

what does receptor intensity refer to?

Strong signal can trigger increase in frequency of action potentials 

• Nerve cells code intensity of information by frequency of action potentials

how does location impact a receptor?

brain identifies site of sensory stimulation using location of activated afferent fibres 

what is a receptive field?

if stimulus appears in neuron’s receptive field, neuron will fire and location is communicated to the brain

what are multiple sensors?

compare inputs from more than one sensor

what are gradients?

with smell, determine location based on gradients

how does duration impact a receptor?

afferent neurons encode duration of stimulus and communicate this to brain for processing 

• Cells fire as long as stimulus is present and some fire briefly as stimulus goes on, stops, then fires when stimulus goes off

where does transduction occur?

ending of neuron

what are the two types of neuron endings?

1. specialized ending of afferent neuron

2. separate receptor cell closely with peripheral ending of neuron

what happens after a receptor is stimulated?

• alters membrane permeability causinng opening of non-selective cation channels 

• When cations enter neuron, depolarizes membrane

what happens to a receptor potential after a stimulus?

change in potential due to incoming signal in specialized receptor cells

what happens to a generator potential after a stimulus?

change in potential in ending of afferent neuron

what are graded potentials?

amplitude and duration vary depending on strength and duration of stimulus

what are the 2 types of graded potentials?

1. excitatory postsynaptic potentials (EPSPs)

2. inhibitory postsynaptic potentials (IPSPs)

what happens if graded potentials are of sufficient magnitude?

they initiate action potential in afferent neuron

what is the function of a specialized afferent ending?

receptor potential causes afferent nerve fibre to reach threshold and trigger action potential

what is the function of a separate receptor cell?

• when receptor potential is strong enough, it will release a chemical messenger that diffuses to afferent neuron and opens chemically gated sodium channels 

• If threshold achieved, afferent nerve fibre initiates and propagates action potential

How do afferent neurons encode intensity of a signal to transmit this information to the CNS?

Strong stimuli affects neighbouring receptors which communicates to CNS the intensity of the stimulus

what is receptor adaptation?

Adaptation: receptors regulate their responses

• Stimulus of same intensity does not always bring same magnitude so receptors adapt to signal by enhancing/lessening response

what are the 2 types of receptors?

1. tonic receptors

2. phasic receptors

what are tonic receptors?

slowly adapting or not adapting at all

• Used in situations where near constant signal from stimulus is needed

• Ex: pain receptors when something is hot

what are phasic receptors?

rapidly adapting such that initiation of stimulus action potentials are generated 

• Receptor stops generating action potentials rapidly with stimulus 

• Once stimulus is removed, phasic receptors respond with depolarization 

• Used to monitor changes in stimulus intensity

what is pain and how is it detected?

• Defence system which alerts CNS to immediate physical harm 

• Pain receptors (nociceptors) found in all parts of the body

what is nociception?

external and internal events and perceived events

• Nociceptors are specialized nerve endings of pain fibers

what are the 3 groups of nociceptors?

1. mechanical nociceptors

2. thermal nociceptors

3. chemical nociceptors

mechanical nociceptors

respond to physical damage → cutting, crushing

what are thermal nociceptors?

respond to temperature, especially heat

what are chemical nociceptors?

respond to noxious chemicals → external and internal to body

what are the 2 types of pain fibers?

1. fast pain fibers (A-delta diners)

2. slow pain fibers (C-Fibers)

what are fast pain fibers?

• Respond to temperature, chemical, and mechanical stimuli 

• Acute, sharp, or stabbing pain

what are slow pain fibers?

• Unmyelinated and responsible for responding to chemical, mechanical stimuli, and temperature 

• Activate poymodal receptors (receptors that can respond to more than stimuli) 

• Perceived sensoation of fibers causing burning, aching, throbbing 

• Bradykinin can directly stimulate nociceptors

how does the brain process pain?

• When action potential reaches end of afferent pain fibre axon, it triggers release of neurotransmitters 

• Substance P and glutamate activate ascending pathways and transmit pain signals for further processing

how does the cortex process pain?

cortical somatosensory processing localizes pain to discrete body region

how does the thalamus process pain?

processing here allows for perception of pain

how does the reticular formation process pain?

increases level of alertness and awareness of painful stimulus

how does the hypothalamus and limbic system process pain?

receives input from thalamus and reticular formation and allows for behavioural and emotional responses to pain stimuli

what is glutamate?

• amino acid that functions as neurotransmitter 

  • Released by nociceptive afferent nerve fibers to activate postsynnaptic glutamate receptors on neurons in dorsal horn of spinal corn

what are the receptors that receive glutamate?

1. AMPA Receptors

2. NMDA Receptors

what are AMPA receptors?

• Activation of AMPA leads to permeability changes 

• As sodium enters AMPA channel, causes depolarization 

• When depolarization is reached, Mg2+ ion in NMDA channel is dislodged and NMDA channel is activated

what are NMDA receptors?

• Once activated, allow calcium to enter neuron 

• Activates second messenger pathway that results in neuron being more excitable than normal cursing injured areas to be more sensitive to stimuli that would not normally cause pain

what is the endogenous analgesic system?

• Pain afferent fibres do not adapt 

• After initial painful stimulus, there is a decrease in the perception of pain as a result of CNS’s built-in pain suppressing system 

what is the impact of activating descending pathways that activate inhibitory neurons in the dorsal horn?

• Axons of interneurons terminate on afferent fiber nerve terminals 

• Release endogenous opiates (produce painkilling effects) that act on opiate receptors and cause suppression of neurotransmitter from afferent pain fibre

what are the functions of exogenous opioids

Exogenous opioids (not produced by body) activate opioid receptors to decrease pain perception

What happens if you step on a sharp piece of Lego?

• Pain perceived at source and travels up A-delta fibers 

• As action potential reaches the end of each A-delta fiber, glutamate is released

• Signal travels to dorsal horn where AMPA receptors are activated 

• Signal travels to reticular formation to increase alertness and localizes pain on foot

• Hypothalamus causes behavioural and emotional response

How is the perception of pain different if using an exogenous opioid?

• Morphine activates opioid receptors which result in suppression of neurotransmitter bring released from A-delta fibres 

• Decreases perception of pain

how does light enter the eye?

• Round opening in center of iris: pupil which allows light to enter eye 

• Size of opening adjusted by two sets of smooth muscle in iris 

  • Muscles under autonomic nervous system 

what is pupillary constriction?

one set of muscles organized in circular fashion and muscles constrict to make pupil smaller 

• Parasympathetic stimulation

what is pupillary dilation?

one set of muscles organized in radial fashion (pupil to iris) and these muscles contract to dilate pupil to allow more light to go through 

• Sympathetic stimulation

how are light rays formed?

• photons that travel in waves 

  • Waves vary in wavelength (distance between wave peaks) and intensity (amplitude of peak) 

how is light focused for the eye?

Light waves radiate outwards from source so they are bent to enter eye and focus them for processing 

what happens when light passes through transparent media with density different from air

1. Wavelength decreases 

2. Refraction: unless enters perpendicularly, direction will change

what is the cornea?

Contributes to refractive ability of eye because of large density difference at air-cornea

what is astigmatism?

surface of cornea is uneven resulting in uneven refraction of light

what are the lens?

• Convex structure behind pupil allowing to focus light rays on retina 

• Lens is adjustable

what is accommodation?

• eye’s ability to adjust the lens to maintain focus on something 

  • Controlled by ciliary muscle and suspensory ligaments

• If accommodation is not sufficient to create clear vision, use corrective lenses (glasses or contacts) 

what happens when the muscles are relaxed?

ligaments pull lens flatter

what happens when muscles contract?

• reduces tension of ligaments and lens becomes more convex

what causes contraction/stimulation?

Sympathetic simulation causes relaxation and parasypathetic stimulation causes contraction

what happens when there is a distant light source?

More than 6m away from lens, light rays are parallel to one another when they enter the eye

what happens when there is a near light source?

• Light rays diverge (move apart from each other) when they enter the eye 

• Eye changes shape of lens so it has a greater ability to bend light allowing eye to focus on the image

what is the retina?

• Retina is an extension of the CNS with direct connection to optic nerve 

  • 1 million fiberes bringing information 

• Goal of lens to convert energy into electrical signals sent to CNS

what are the 3 layers of the retina?

1. photoreceptor

2. bipolar

3. ganglion cells

what do the retina cells do?

• Retina visual pathway from photoreceptor to bipolar and then to ganglion cells 

• Direction of light is opposite direction of retinal visual processing

what are the rods and cones?

• outermost cones

• contain photopigments

function of cones

sensitive to colour

what are rods?

vision in low lighting

what are photopigments?

colour detecting molecules in cones → colour blind people have these photopigment defects

what are bipolar cells?

Middle layer involved in transmission of signals from rods and cones to ganglion cells

what are ganglion cells?

• Neurons at inner surface of tretina 

• Axons of ganglion cells make up optic nerve

where is the retina not found?

Retina is continuous throughout inner eye except optic disc

what is the optic disc?

• ganglion cells bundle together to form optic nerves (1/retina) and leave eye 

• Optic disc region has no rods or cones creating a blind spot 

  • Higher visual processing filled in this blindspot 

how is visual information processed?

• Information from optic nerve transmitted to visual pathway in thalamus 

  • Thalamus initial processing by separating different visual stimuli: colour, form, depth, movement and relays each to different zone from cortex 

• Visual cortex organized into alternating columns for left and right eyes 

  • Brain compares columns to allow depth perception to estimate distance 

• Vision takes up 30% of total cortex capacity

how does visual processing work?

• Each eye receives different visual input and sends different information to the brain 

• Having two eyes improves depth perception which is lost if a person loses their sight in one eye

what is the visual pathway

• Optic nerve made up of neurons from the right visual field (greeen) and left visual field (orange) 

• Neurons from left optic nerve cross over to right side 

  • Opposite happens with neurons from right optic nerve

what are sound waves?

vibrations of air that travel outwards from their source

function of sound waves

• Can travel to different mediums → denser the media, shorter the distance due to increased resistance 

• Sound wave energy converted into perceived hearing

what is pitch (tone)?

• Determined by frequency of vibrations 

• Greater the frequency, higher the pitch

what is intensity (loudness)?

• Depends on amplitude of sounds and waves 

• Greater the amplitude, louder the sound

what is timbre (quality)?

• Overtones that are superimposed on pitch 

• Allows one to locate source of sound as each sound produces different pattern of overtones

what are the 3 parts of the ear?

1. external ear

2. middle ear

3. inner ear

what is the function of the external ear?

channel sound waves to middle ear

components of the external ear

1. pinna

2. ear canal (fine hairs and wax)

3. tympanic membrane

what is the pinna?

external skin covered cartilage that collects sound waves → used to locate sound

what is the ear canal?

conducts sound waves towards typanic membrane 

• contains fine hairs and cells that secrete earwax whcih prevent airborne particles from entering canal

what is the tympanic membrane?

vibrates when hit by incoming sound waves

• requires air pressure on both sides to be similar (equalizes with atmospheric pressure)

what is the middle ear?

• Contains bones: malleus, incus, stapes which transfer movement of tympanic membrane and amplify sound as transmitted to fluid of inner ear 

• As tympanic membrane vibrates, malleus transfers vibration to incus to spaces 

what is the inner ear?

From oval window, sound waves converted to mechanical energy and transferred to inner ear

components of the inner ear

1. cochlea

2. organ of corti and basilar membrane

3. inner hair cells

4. outer hair cells

function of the cochlea

responsible for perception of hearing and determining pitch

function of the organ of corti and basilar membrane

• corti contains hair cells 

  • When fluid moves within inner ear, hair cells deformed and generate neuronal cells