PHGY 215 Module 3: The Peripheral Nervous System

afferent neuron structure

small, round cell body with a single long dendrite and short axon

afferent nerve cell body location

clustered into ganglia immediately external to the spine

afferent nerve axon location

extends into the dorsal horn on the spinal cord

transduction

conversion of environmental signal to electrical signal

when does transduction occur in receptors

when the stimulus reaches threshold and an AP is triggered

receptor properties

modality, intensity, location, duration

modality

each receptor responds to a different type of energy/stimulus

photoreceptors

responsive to visible light

mechanoreceptors

responsive to mechanical energy

i.e. stretch receptors, hair cells

thermoreceptors

responsive to heat

chemoreceptors

sensitive to chemicals

intensity

coded by nerve cells by frequency of APs

location

brain can identify site of sensory stimulation by location of activated afferent fibres

receptive field

region where a neuron is sensitive

how can neurons encode location

- receptive field

- multiple sensors

- gradients

gradients

when we smell something and move in a certain direction we know where the source is based on the change in intensity

duration

some cells fire for as long as stimulus is present while others fire when stimulus starts and stops

2 receptor types

specialized ending of an afferent neuron or a separate receptor cells closely associated with the peripheral ending of a neuron

receptor potential

change in potential due to an incoming signal in specialized receptor cells

generator potential

change in potential due to an incoming signal in the ending of an afferent neuron

separate receptor cell grade potential

chemical messenger is released if potential is strong enough which will open chemically gated Na+ channels on afferent neurons

adaptation

receptor's ability to regulate their own responses

tonic receptors adaptation

slowly adapt or dont adapt at all

tonic receptor examples

muscle stretch receptors require constant knowledge of contraction to maintain balance

pain receptors

nocireceptors

pain receptors

convey duration and are permanently active

phasic receptors adaptation

rapidly adapt upon initiation of APs

helps monitor changes in stimulus intensity

pacinian corpuscles

mechanoreceptors in the skin that rapidly adapt to continued stimulus

how are nociceptors classified

based on modality: mechanical, thermal, and chemical nociceptors

pain fibres

afferent nerve fibres with nociceptors as their specialized endings

why do we have sharp and duller/diffuse pain to the same stimulus

due to firing of both slow and fast pain fibres

A-delta fibres

fast pain fibres

acute, sharp, stabbing pain

c fibres

slow pain fibres

burning, aching, throbbing sensation

polymodel receptors

receptors that respond to more than one stimuli

found in slow pain fibres

bradykinin

stimulates a slow pain pathway, causing long lasting persistent pain

a chemical activated by enzymes released by damaged cells

pain neurotransmitters

substance P and glutamate

reticular formation pain processing

increases alertness and awareness of painful stimulus

thalamus pain processing

processing here allows for pain perception

hypothalamus/limbic system pain processing

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

cortex pain processing

somatosensory processing finds pain location

how do cells use glutamate to communicate pain

pain fibres release glutamate to activate postsynaptic glutamate receptors on neurons in the dorsal horn of the spinal cord

glutamate's two actions

AMPA and NMDA receptors

AMPA receptor function

- allows sodium to enter and depolarize the dorsal horn neuron

- activates NMDA

NMDA receptor activation

MG2+ is dislodged at a certain depolarization, activating NMDA

NMDA receptor function

allows Ca2+ to enter, activating a second messenger pathway that makes the neuron more excitable

makes injured areas more sensitive to stimuli than normal

why does our perception of pain decrease when pain fibres don't adapt

endogenous analgesic system

endogenous analgesic system

central processing of pain -> activation of descending pathways -> activation of dorsal horn inhibitory neurons -> inhibitory neurons release endogenous opiates -> opiate receptors stimulated -> afferent pain fibre neurotransmitter release is suppressed

pupil function

lets a certain amount of light in

pupillary constriction

- parasympathetic stimulation

- done by a circular set of muscles

pupillary dilation

- sympathetic stimulation

- a set of radial muscles

refractive structures of the eye

cornea and lens

what happens to light when it passes through an object of different density

- wavelength decreases

- refraction, direction change

cornea

has a constant refractive ability

astigmatism

uneven cornea surface, causes uneven light refraction

lens

convex structure behind pupil that adjustably focuses light rays

accomodation

eye's ability to adjust the lens to maintain focus, controlled by the ciliary muscle and suspensory ligaments

relaxed ciliary muscle

ligaments pull lens flatter

contracted ciliary muscle

reduces ligament tension to make lens more convex

how is ciliary muscle contraction controlled

autonomic nervous system

sympathetic -> relaxation

parasympathetic -> contraction

distant light source rays

light rays enter parallel at the eye

near light source rays

closer than 6 meters

light rays diverge and lens becomes more convex to accomofate

retina layers

photoreceptors, bipolar, and ganglion cells

photoreceptor layer

contains rods and cones

rods function

vision in low lighting

cones function

colour sensitive

bipolar layer

transmits signals from photoreceptors to ganglion cells

ganglion cells

axons make us the optic nerve

optic disc

where ganglion cell axons bundle together and leave the eye, leaving a blind spot

central processing of vision

optic nerve transmits to the thalamus where it does initial processing of visual stimuli and relays them to different zones in the cortex

visual cortex structure and function

structure: alternating columns devoted to left and right eye

function: compares with neighbouring columns to allow depth perception

visual pathway

the right visual field is processed in the left hemisphere and vice versa

sound waves

vibrations in the air

pitch/tone

frequency of vibration

intensity (loudness)

amplitude of sound waves

timbre (quality)

overtones that are superimposed on the pitch which allows us to distinguish between voices and instruments that have the same tone and loudness

external ear structures

pinna, ear canal, tympanic membrane

pinna

external cartilage that allows for precise pinpointing of sounds

ear canal

conducts sounds waves to tympanic membrane

ear canal protection

fine hairs and special cells that secrete earwax that make the environment acidic

tympanic membrane

vibrates with incoming sound waves

needs equal pressure on both sides which is modulated by the middle ear

structures of the middle ear

malleus, incus, stapes, oval window

bones of the middle ear order

malleus, incus, stapes

why do the bones of the middle ear need to amplify sound

the middle ear is air while the inner ear is fluid so a stronger signal is required to propagate the signal

how are sounds dampened

muscles contract to restrict movement of ear bones to protect the inner ear

structures of the inner ear

cochlea, the organ of corti and the basilar membrane, inner hair cells, and outer hair cells

cochlea function

hearing perception

spiral shapes helps determine pitch

organ of corti

contains hair cells and is the actual sense organ

supported by basilar membrane

how many rows of hair cells do we have

1 row of inner cells and 3 rows of outer cells

inner hair cells

transform fluid vibrations into action potentials to propagate auditory messages

fires faster at higher frequencies

outer hair cells

don't transmit to the brain

modifies inner hair cell electrical signalling to make them more sensitive to sound intensity and pitch

basilar membrane structure

narrow and stiff near the oval window and wide and flexible at the end of the spiral shape

basilar membrane function

higher frequencies are detected by hair cells near the oval and window and lower frequencies are detected near the end

auditory nerve pathway

brainstem (alertness and arousal)

thalamus (sorts and sends for higher processing)

cortex

vestibular apparatus

provides info on equilibrium and coordination

motion sickness cause

very sensitive vestibular apparatus that causes them to be dizzy and nauseous

vestibular apparatus functions

1. maintains balance and posture

2. allow eyes to remain fixed when turning the head

3. perceive motion and orientation

what processes are triggered by taste and smell chemoreceptors

appetite and release of digestive juices

taste chemoreceptor location

tongue, oral cavity, throat

papillae

small bumps on the tongue that contain hundreds of taste buds

taste buds structure

clusters of nerve endings with ~50 taste receptors and supporting cells with a small opening that allows fluids to contact the receptors

taste bud life span

10 days