Biomedical Sciences BI20B2
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
Receptor type by General
stimulus modality classification
Mechanoreceptor Special senses
Class based on
location
Telereceptors
Interoreceptors
Muscle & Joints Proprioreceptors
Chemoreceptor
Skin & viscera
Exteroreceptors
Cardiovascular
Interoreceptors
Special senses
Telereceptors
Exteroreceptors
Exteroreceptors
Interoreceptors
Skin & viscera
Photoreceptor
Special senses
Telereceptors
Thermoreceptor
Skin
CNS
Exteroreceptors
Interoreceptors
Example
Cochlear hair cells
Vestibular system hair cells
Muscle spindles
Golgi tendon organs
Pacinian corpuscle
Bare nerve endings
Arterial baroreceptors
Atrial volume receptors
Olfactory receptors
Taste receptors
Nociceptors
Nociceptors
Glomus cells (carotid body PO2)
Hypothalamic osmoreceptors &
glucose receptors
Retinal rods & cones
Warm and cold receptors
Temperature–sensing
hypothalamic 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.
Stimulus
Sensory Receptor
Change in the
ionic permeability
of afferent nerve ending
Change in
membrane potential
of nerve ending
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
Generation of action potentials
in afferent nerve terminal
Alternatively - modulated release of
transmitter from receptor cell
Propogation of
action potentials
to CNS
Generation of graded or action
potentials in second order neurones
Integration of
information by CNS
Can allow more local integration
(eg retina of the eye)
Transduction & Coding
sensory receptor
synapse
synaptic integration
2nd 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 2nd order neurone
action potentials cause transmitter
release & generate graded potentials
(EPSPs) in 2nd 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 40mv
duration 4ms
exceeds threshold
& generates
action potentials
action potentials
conducted down
sensory axon
small amount
transmitter
released
recording arrangement from sensory unit
amplitude 65mv
duration 7ms
- 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 d2p/dt2
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.