Chapter 15 - sensory pathways

3 types of sensory receptors

exteroreceptors - monitor external environment

interoreceptors - monitor visceral organs

proprioreceptors - monitor position of skeletal muscles & joints

special senses

smell, taste, vision, hearing, equilibrium

somatic senses

touch, pain, temperature

where in the brain does sensory info go?

cerebrum —> conscious

func. of sensory receptors

monitor changes in specific variables inside & outside the body

usually specific receptors are at the end of an afferent neuron

receptor specificity

each receptor responds to a specific type of stimulus

you have 2 types of senses : general & special

general senses

temperature, pain, touch, pressure, vibration & proprioception (muscle stretch)

receptors - usually on the dendrite ends of afferent (sensory) neurons

ex. tactile receptors

general senses (6 examples)

free nerve endings - pain, touch, pressure, temp

root hair plexus - hair movement

Merkel cells and tactile discs- fine touch, pressure

Tactile corpuscle (Meisner’s) - light touch

Laminated corpuscle - deep pressure

Ruffini corpuscle - distortion

special senses

olfaction, vision, gustation, equilibrium, hearing

• receptors are more complex & are located in sense organs (eye, ear,tounge)

Direction of the stimulus

an adequate stimulus acting on a sensory receptor causes a change in the membrane’s permeability

receptor potential

generation of the receptor cell’s graded depolarization/hyperpolarization

generator potential

a depolarization of the sensory neuron (=the receptor cell in general senses,2nd cell in special senses)

Transduction

process of translating stimulus into an action potential

range of detection

the number and type of sensory receptors we have limit what we can sense and become aware of

each of us is unique in our sensory abilities

Receptive field

area of body monitored by one particular afferent neuron and all it’s receptors

interpretation of sensory information

to generate conscious sensation , sensory information must be transmitted to the cerebral cortex

sensation

raw experience, may be unconscious

• activity in any & all sensory neurons

perception

SNS (1% of all sensation)

interpretation, meaning given to sensation, conscious awareness of sensation

• activity in cerebral cortex, and not just spinal cord & brain stem

not perceived - visceral sensory info delivered to dienchephalon, spinal cord & brain stem only

labelled line - pathway that conducts sensory info from a receptor to specific neurons in cortex

interpretation in cortex

modality (type) of stimulus (touch, pressure, temp, sound) is interpreted by labeled line

information about the type, strength, duration, and variation of the stimulus is coded by

a) type of sensory receptor cell activated

b) the rate of action potentials generated

eg. the harder/stronger the stimulus, the more action potentials generated (AP ARE MORE FREQUENT)

tonic receptors

always active

provide information about:

• background level of stimulation

• changes indicated by frequency of action potentials generated

• called slow-adapting receptors because they show little peripheral adaption

Ruffini corpuscle, Merkel tactile receptors, nociceptors (pain receptors). joint capsule proprioceptor, muscle spindle

phasic receptors

not always active fire only when stimulated

provide information about : intensity and rate of change of stimulus

called fast adapting receptors because they respond strongly at first & then show decline in activity + peripheral adaptation

pacinian corpuscles, meissner’s corpuscles, thermoreceptors,

root hair plexus

adaptation

reduction in sensitivity of nervous system to constant stimulus

higher centres can increase receptor sensitivity or facilitate transmission (eg, RAS heightens awareness - listen carefully)

peripheral adaptation

phasic sensory receptors become less responsive.

(Eg. become used to background noise, feeling of clothing, room temperature)

central adaptation

receptors still responding but central adaptation, involves inhibition at various nuclei

• sensory info may still generate reflex responses, but no conscious sensation or perception - awareness is reduced (eg. you get used to new odours in a room)

why does adaptation take place

so that the brain is not overwhelmed and is not using up resources paying attention to things that are not as important. Allows the brain to focus on important things