1.4 General Principles of Sensing

Question 1

What are sensory receptors

Answer 1

the receive stimuli from the environment

Question 2

What are Neural pathways

Answer 2

that conduct information from receptors to CNS

Question 3

How does sensory processing proceed

Answer 3

Transduction of stimulus energy into receptor potentials and then APs in afferent neurones
o The pattern of APs in particular neurones is a code that provides information about the stimulus
 Intensity, location and specific type of energy
o Primary areas if CNS receive input and communicate with brain for processing
 May involve reflex responses, perception, storage, etc…

Question 4

what do sensory receptors do at peripheral ends of afferent neurones

Answer 4

they transduce information about the environment into graded potentials
- these potentials initiate APs that travel to CNS

Question 5

what can receptor refer to

Answer 5

either sensory or receptor proteins on their membrane

Question 6

what are different classes of receptors

Answer 6

o Mechanoreceptors 
o Thermoreceptors 
o Photoreceptors 
o Chemoreceptors 
o Nociceptors – pain

Question 7

What does the transduction process involve

Answer 7

involves the opening and closing of ion channels

- causes a change in membrane potential and causes a graded potential called a receptor potential

Question 8

what is not generated in the receptor region

Answer 8

AP

• local current flows to a trigger zone along the axon where there are channels
• if higher than the threshold then AP triggered

Question 9

How long does the APs fire for

Answer 9

as long as the receptor potential is sufficient

• increase in receptor potential (RP) causes increase in firing freq of AP
• magnitude of AP is not affected

Question 10

what controls the magnitude of RP

Answer 10

stimulus strength, rate of change of stimulus strength, temporal summation of RP and adaption

Question 11

what is primary sensory coding

Answer 11

Coding is the conversion of stimulus energy into a signal that conveys relevant sensory information to the CNS
o Type of energy, intensity, and location of body it affects

Question 12

single afferent neurone

Answer 12

with all its receptor endings makes up a sensory unit

o Generally the peripheral end divides into many fine branches each terminating with a receptor

Question 13

Receptive field

Answer 13

Area of body that when stimulated leads to activity in a particular neurone
o Receptive fields of defend sensory units often overlap so single activation rarely occurs

Question 14

what codes stimulus type (or modality)

Answer 14

coded by the type of sensory receptor that a stimulus activates

Question 15

are receptors of a single afferent neurone sensitive to the same or different stimulus

Answer 15

the same type of stimulus
o Adjacent sensory units may be sensitive to different modalities
o As receptive fields overlap, a single stimulus source can give rise to multiple sensations (i.e. ice cube –> touch and temperature)

Question 16

what does the frequency of APs convey

Answer 16

information about the magnitude of stimulus

larger stimulus is, larger receptor potential so a higher frequency of APs

Question 17

what happens if strength of local stimulus increases

Answer 17

receptors pf adjacent branches on an afferent neurone are activated
the summation of their local currents so greater AP reference

Question 18

whats recruitment

Answer 18

the process where stronger stimuli also affect a greater area so activate similar receptors on other afferent neurones

Question 19

what are labelled lines

Answer 19

Stimulus location is coded by the site of a stimulated receptor as well as the specific neural pathway the information is received on and these pathways are labelled lines

Question 20

whats acuity

Answer 20

the degree with which one stimulus can be located and differentiated from an adjacent one
o Depends upon the amount of convergence of the input in the ascending pathways
 The greater the convergence, the lower the acuity

Question 21

what does acuity depend on

Answer 21

depends on the size of the receptive field, the density of sensory units and the amount of overlap of receptive fields

Question 22

what allows better simulus location

Answer 22

smaller receptive fields

Receptive field overlaps aid stimulus location

Question 23

why do afferent neurones respond most vigorously to stimuli applied at the centre of its receptor field

Answer 23

This is due to greater receptor density

 More receptors activated and more APs generated

Question 24

w/ regards to stimulus location, and increase in AP rate

Answer 24

due to location or intensity
 Overlap means many sensory units will trigger so neurones at periphery of stimuli fire at lower frequency than central ones
 This provides better localisation and also intensity

Question 25

what does lateral inhibition enable

Answer 25

greater localisation of a stimulus iste for some sensory systems

Question 26

when does lateral information occur

Answer 26

at any stage, but usually at an early stage

Question 27

Lateral inhibition used in pathways provides…

Answer 27

… the most accurate localisation
o In the retina to create sharp visual acuity
o Skin hair movements

Question 28

what happens to information from afferent neurones whose receptors are at the edge of a stimulus

Answer 28

strongly inhibited compared to information from the stimulus centre
o Increases contrast between relevant and irrelevant information, thereby focussing on important messages

Question 29

afferent sensory pathways are formed by…

Answer 29

chains of three of more neuronesconnected by synapses
o These chains travel in bundles of parallel pathways to the CNS
o Also called ascending pathways as they go up to the brain

Question 30

where do central processes of afferent neurones enter

Answer 30

enter the brain or spinal cord and synapse upon many interneurons through divergence or convergence

Question 31

sensory pathways can be specific ascending pathways that…

Answer 31

carry information about one stimulus modality o Go to different cortices: somatosensory, visual, auditory, olfactory

(there can also be non-specific pathways)