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Sensory Neurophysiology
“One does not see anything until
one sees its beauty. Then, and then
only, does it come into existence”
Oscar Wilde
•In this lecture, we will deal
with stimuli which reach the
conscious level of
perception.
Sensory Receptors
•Some organs such as the eye and ear are complex, containing over 126 million
and 16,000 receptors, respectively.
•Internal and external stimuli are converted into a graded potential upon
activation of sensory receptors. If graded potentials exceed the threshold for
firing, they generate APs along the axon, which in turn elicit neurotransmitter
release from axon terminals onto postsynaptic neurons in the CNS.
•The specificity of a receptor for a particular type of stimulus is called the law of
specific nerve energies.
•At each synapse along the way to the cerebral cortex, sensory information can
be modified.
•Some stimuli come into our conscious perception, and others are acted upon
without our awareness. Perceptual threshold refers to the level of stimulus
intensity needed to become aware of a particular sensation. Our CNS avoids
processing information below the perception threshold to avoid becoming
overwhelmed with information. Eg. turning down the radio when studying sets
the stimulus below perception threshold. Primary sensory neurons may perceive
the sound, but higher order neurons dampen the intensity by the time it reaches
the auditory cortex.
Sensory Pathways
•Sensory projections ascend from the spinal cord into the brain through the
brain stem. They travel through the thalamus, which acts as a relay
processing station of signals to other brain regions.
•Olfactory signals project directly to the olfactory cortex. The olfactory
cortex is close to the temporal lobes which process memory. Thus, particular
smells trigger a long-stored memory.
Sensory Neuron Receptor Fields
•The receptor of each primary neuron can pick up information from a
specific area called the receptive field of the receptor.
•Receptive fields can be irregular in shape and overlap with receptive
fields of other neurons.
•If receptive fields from primary neurons converge onto a single
secondary neuron, the individual receptive fields merge into a single
receptive field. This allows subthreshold stimuli to be summed by the
secondary neuron.
Two-point Discrimination Helps to Determine
The Size of the Secondary Receptive Field
•The sensitivity to touch is demonstrated by the two-point discrimination
test. The smaller the distance between two points of a divider, the smaller the
size of the receptive field, and greater the touch sensitivity.
The Amount of Space Devoted to Different Body
Parts In The Primary Somatosensory Cortex
The somatotopic map (homunculus)
is not scaled like the human body.
The Body Needs to Distinguish Four Properties of
a Stimulus
1. Modality (nature) of the stimulus.
•
The brain associates a signal coming from a specific group of
receptors with a specific modality. Eg. stimulation of a cold
receptor is always perceived as cold. The 1:1 association of a
receptor to a sensation is called labeled line coding.
2. Localization of the stimulus.
•
The organization of receptors in the periphery is preserved in
the brain. Eg. Touch-pressure receptors on the hand project to a
specific area on the cortex. Stimulation of the specific area of
that cortex would be interpreted as a touch to the hand,
eventhough there was no actual touch.
3. Magnitude of the stimulus.
4. Duration of the stimulus.
The Brain Uses Timing Differences to Localize
Sound and Smell
•Auditory and olfactory stimuli
are exceptions to the localization
rule.
•Neurons in the ear and nose are
sensitive different sound
frequencies and different odors,
respectively. However, their
activation provides no
information about the location of
the stimulus.
•Instead, the brain uses timing
differences in receptor activation
to compute the localization of
sound or odors.
Localization of a Stimulus Can Be Isolated
Through Lateral Inhibition
•Receptors close to the
sensation activate their
respective primary neurons.
This in turn would normally
activate their respective
secondary neurons.
•However, the primary neuron
whose receptors are closest to
the stimulus will signal the
secondary neuron, which in
turn suppresses activity of
adjacent secondary neurons.
•The inhibition of neurons farthest from the stimulus enhances the
contrast between the centre and side of the receptive field. This
aids to localize the site of the sensation more easily. This is termed
lateral inhibition.
Intensity of the Stimulus is Coded in the AP Frequency
•Stimulus intensity
is coded by the
number of
activated receptors,
and the resulting
frequency of AP
propagation along
the axon.
•The amount of
released transmitter
from axon
terminals is
proportional to the
frequency of APs.
Stimulus Duration Affects The Speed of Adaptation
of the Receptors
•The duration of the stimulus is
coded by the duration of the APs. As
the stimulus persists, some receptors
turn off and cease to respond.
•Tonic receptors are slowly adapting
receptors which continue to transmit
signals to the CNS as long as they
are stimulated. Stimuli which
activate tonic receptors are those
which need to be continually
monitored by the body. Eg. pain
receptors are tonic receptors.
•Phasic receptors are rapidly adapting receptors which fire when they first receive a
stimulus, but cease firing if the stimulus strength remains the same. Once a stimulus
reaches a steady intensity, phasic receptors adapt to a new steady state and turn off.
These type of responses allows the body to ignore information which is not a threat to
its well-being or homeostasis. Eg. the smell of cologne.
Touch-pressure Receptors
Touch receptors are the most
common receptors in the body.
They are found in the
superficial layer of the skin and
deep layers of the body such as
viscera.
Sensory Projections to the Somatosensory Cortex
•Neurons from one side of the body project to the opposite part of the brain.
Summary
•Sensory pathways begin with a stimulus which is converted by the receptor into
an electrical potential.
•There are 5 kinds of sensory receptors: chemoreceptors; mechanoreceptors,
thermoreceptors, photoreceptors & nociceptors.
•Primary sensory neurons carry the stimulus to higher order neurons in the CNS.
Sensory projections through the spinal cord input into the thalamus.
•The perceptual threshold is the level of stimulus needed for awareness of a
particular sensation.
•The area over which a receptor picks up information is called a receptive field.
Primary neurons projecting to a single secondary neuron can create a single
large receptive field.
•The body needs to distinguish four properties of a stimulus: modality, location,
magnitude and duration.
•Sensory projections carrying touch-pressure signals are processed in the
primary somatosensory cortex.
•Dermatomes map the area of skin innervated by a single dorsal root ganglion.
References
1. Tortora, G.J. & Grabowski, S.R (2003). Principles of
Anatomy & Physiology.New Jersey: John Wiley & Sons.
Ch.10, pp..
2. Silverthorn, D.U (1998). Human Physiology: An Integrated
Approach. New Jersey: Prentice Hall. Ch.12, pp.263-276.