Biomedical Sciences BI 20 B 2 Sensory Systems

Biomedical Sciences BI 20 B 2 Sensory Systems Human Physiology - The basis of medicine Pocock & Richards, Chapter 8 Human Physiology - An integrated approach Silverthorn, 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 receptors

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 and strength of stimulus Usually sodium, remember direction of 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)

Transduction & Coding sensory receptor synapse synaptic integration 2 nd order neurone primary afferent neurone adequate stimulus related to stimulus intensity and duration frequency coded action potentials conducted down primary afferent neurone transduction and generation of graded receptor potential reduced frequency of action potentials conducted down 2 nd order neurone action potentials cause transmitter release & generate graded potentials (EPSPs) in 2 nd order neurone generated action potentials threshold 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 40 mv duration 4 ms exceeds threshold & generates action potentials conducted down sensory axon small amount transmitter released recording arrangement from sensory unit amplitude 65 mv duration 7 ms - note decay of receptor potential generates higher frequency of action potentials for longer period more action potentials conducted down sensory axon large amount transmitter released

Tonic and Phasic Receptors Amplitude Sensitive - Slowly adapting Ra p Velocity Sensitive - Rapidly adapting Ra dp/dt Acceleration Sensitive - Rapidly adapting Ra d 2 p/dt 2 Stimulus p t R= response, p = position, t = time

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.

transduction Simple Processing? Sensory units with overlapping receptive fields. Field size and receptor density equates to sensory discrimination. Axons of primary sensory neurones Axonal branches give rise to divergent outputs - diffuses input Second-order sensory neurones with convergent excitatory inputs Inhibitory interneurones give rise to lateral inhibition - refines input Axon projections to third-order sensory neurones Integration of sensory input Stimulus

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.