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Sensory Systems

Human Physiology - The basis of medicinePocock & Richards,Chapter 8

Human Physiology - An integrated approachSilverthorn, Chapter 10

Lecture 1 General features of the sensory systems

Introduction

• Environmental awareness is limited to those forms of energy that sensory receptors are designed to detect.

• Sensory receptors may convey information to the cortex with awareness or sensation and may lead to cerebrally controlled responses.

• Sensory receptors also serve as afferent pathways for reflex action with or without conscious awareness.

• Perception is awareness of the source of the stimulus where the sensory input is correlated with past or present information.

• High level processing leads to recognition or identification of the input and depends on learnt experience.

Which receptor?

General Principles

• Information about the external & internal environment reaches the CNS via a range of sensory receptors.

• Each type of receptor is activated by only one type of environmental energy. (Classification)

• Sensory receptors convert or transduce various forms of environmental energy into action potentials in sensory neurones. (Principles of Transduction)

• Action potentials in the sensory neurones encode the quality of the environmental stimulus. (Coding of Stimulus Intensity & Duration)

• Specific sensory receptors are associated with specific CNS sensory pathways. (Organisation)

Classification

• Receptors may be either specialised endings of afferent neurones or separate specialised cells at the end of the afferent neurones.

• Receptors respond best to one form of stimulus energy, the adequate stimulus, but they may respond to other energy forms if the stimulus intensity is abnormally high.

• Regardless of how a specific receptor is stimulated, activation of that receptor always leads to perception of one sensation (the doctrine of specific nerve energies). Not all receptor activations lead to conscious sensations.

• Sensory receptors are principally classified by their stimulus modality.

Typical Sensory Receptors

Classification of sensory receptorsReceptor type bystimulus modality

Generalclassification

Class based onlocation

Example

Mechanoreceptor Special senses

Muscle & Joints

Skin & viscera

Cardiovascular

TelereceptorsInteroreceptorsProprioreceptors

Exteroreceptors

Interoreceptors

Cochlear hair cellsVestibular system hair cellsMuscle spindlesGolgi tendon organsPacinian corpuscleBare nerve endingsArterial baroreceptorsAtrial volume receptors

Chemoreceptor Special senses

Skin & viscera

TelereceptorsExteroreceptorsExteroreceptorsInteroreceptors

Olfactory receptorsTaste receptorsNociceptorsNociceptorsGlomus cells (carotid body PO2)Hypothalamic osmoreceptors &glucose receptors

Photoreceptor Special senses Telereceptors Retinal rods & cones

Thermoreceptor SkinCNS

ExteroreceptorsInteroreceptors

Warm and cold receptorsTemperature–sensinghypothalamic neurones

Principles of Transduction

• The process by which an environmental stimulus becomes encoded as a sequence of nerve impulses in an afferent nerve fiber is called sensory transduction.

• Different kinds of receptor are activated in different ways but the first stage in sensory transduction is the generation of a graded receptor potential.

• The magnitude of the stimulus is related to that of the receptor potential which in turn is related to either a) the sequence or frequency of all-or-nothing action potentials generated in the afferent nerve fiber; or b) modulated release of transmitter from the receptor cell generating a sequence of action potentials in a second order neurone.

Adequate Stimulus

Activated only by specific mode andstrength of stimulus

Usually sodium, remember directionof the flux is determined by the gradient

The graded potential can either be depolarising or hyperpolarising

Alternatively - modulated release of transmitter from receptor cell

Generation of graded or action potentials in second order neurones

Can allow more local integration (eg retina of the eye)

In teg ra tion o fin fo rm ation b y C N S

P rop og a tion o fac tion p o ten tia ls

to C N S

G en era tion o f ac tion p o ten tia lsin a ffe ren t n e rve te rm in a l

C h an g e inm em b ran e p o ten tia l

o f n e rve en d in g

C h an g e in th eion ic p e rm eab ility

o f a ffe ren t n e rve en d in g

S en sory R ecep to r

S tim u lu s

adequatestimulus

sensory receptor

primary afferent neurone

synapse

2nd order neurone

graded receptor potential

threshold

generated action potentials

frequency coded action potentials conducted down

primary afferent neurone

synaptic integration

action potentials cause transmitter release & generate graded potentials

(EPSPs) in 2nd order neurone

reduced frequency ofaction potentials conducted

down 2nd order neurone

Transduction & Coding

related tostimulus intensity

and duration

transduction and generationof graded receptor potential

EPSPs

Coding of the Stimulus

• Different sensory receptors exhibit differing degrees of adaptation in response to an adequate stimulus.

• Slowly adapting receptors continuously signal the intensity and the duration of the stimulus (Tonic).

• Rapidly adapting receptors signal the onset and offset of a stimulus (Phasic).

• The quality of the stimulus is encoded in the frequency of the action potentials transmitted down the afferent fibre and the number of sensory receptors activated.

• Adaptive ability is a property of the sensory receptor and is usually associated with its structure or the morphology of the surrounding tissue.

Sensory Coding for Intensity & Duration

amplitude 40mvduration 4ms

amplitude 65mvduration 7ms- note decay ofreceptor potential

small amounttransmitterreleased

large amounttransmitterreleased

exceeds threshold& generatesaction potentials

action potentialsconducted downsensory axon

generates higherfrequency of actionpotentials for longerperiod

more action potentialsconducted downsensory axon

recording arrangement from sensory unit

Tonic and Phasic Receptors

Amplitude Sensitive - Slowly adapting R p

R= response, p = position, t = time

Stimulus

p

t

Velocity Sensitive - Rapidly adapting R dp/dt

Rd2p/dt2Acceleration Sensitive - Rapidly adapting

Peripheral Organisation

• A single afferent neurone with all its receptor endings is a sensory unit.

• The area of the body that, when stimulated, causes activity in a sensory unit or other neurone in the afferent pathway is called the receptive field for that neurone.

• The size of the receptive field varies inversely with the density of receptors. High receptor density gives rise to small receptive fields, which lead to greater acuity or discriminative ability of the input.

• Overlapping receptive fields (of identical sensory receptors) allows interactions between sensory inputs and refines sensory discrimination.

Inhibitory interneurones give rise tolateral inhibition - refines input

Axon projections to third-order sensory neurones

Second-order sensory neuroneswith convergent excitatory inputs

Axonal branches give rise todivergent outputs - diffuses input

Axons of primary sensory neurones

Sensory units with overlappingreceptive fields. Field size and receptor density equates to sensory discrimination.

Inte

gra

tion o

f se

nso

ry input

Stimulus Simple Processing? transduction

Central Organisation

• Specific sensory pathways (primary afferent) relay information from only one type of sensory receptor to specific primary receiving areas of the cerebral cortex about only a single type of stimulus.

• Non-specific pathways convey information from more than one type of sensory unit to the brainstem reticular formation and regions of the thalamus that are not part of the specific ascending pathways.

• The arrangement of the sensory pathways gives rise to convergence or divergence of the sensory input.

• This influences the quality of the sensation at the conscious or subconscious level within the CNS.

Sensory Pathways

• Olfactory pathways from the nose project directly to the cortex

• Equilibrium pathways project to the cerebellum with a branch to the cortex via the thalamus

• All other pathways pass through the thalamus before they project to their relevant cortical area

Summary

• The external & internal environments are monitored by sensory receptors.

• Each type of receptor is excited most effectively by only one modality of stimulus known as the adequate stimulus.

• The stimulus is converted into an electrical potential.

• Stimuli are detected as either static or dynamic events.

• The intensity & duration of the stimulus is frequency coded as bursts of action potentials in the primary afferent nerve.

• Primary afferent nerve fibres convey information from the sensory receptors to specific areas of the CNS.

• Sensory input is processed at both the sub-conscious and the conscious levels within the CNS.