Theme of Sensory Systems
- Our perceptions are abstractions based upon the biasing of sensory information processing to extract
meaningful information - By meaningful, we mean information that is relevant for the individual, or that which has evolved to promote fitness, or optimize adaptation
Our sensory systems distort information to make it more meaningful
- Examples:
-sensory adaptation (olfactory, tactile) Example= your seams in your clothes
-input can be ignored that is not relevant for survival
-information can sometimes be masked, such as pain
input is non-linear ….
– the output (perception or neuronal
response) is not proportional to the input stimulus, e.g., the
visual system enhances edges to be sharper than they
actually are
Law of specific nerve energies –
specific receptors and neural
channels for each sense are independent, and can only produce
that one sensation
Only respond to that specific type of info - photoreceptors only perceive photons
adequate stimulus –
the specific type of energy that a sensory organ/ receptor is most sensitive to
- e.g., photon of light for eye, soundwave for the ear; chemical for
taste/ smell
Hearing range in mammals -
Make it easier for hunting/evolution, cats hear frequencies that humans don’t
Transduction –
conversion of physical energy by a receptor into an electrochemical pattern in neurons
- Reduced the pressure into electrical impulse
Common themes in ALL sensory systems (3)
- They have sensory receptors that transduce physical/ chemical information from the environment
- They involve activation of sensory receptors (i.e., adequate stimulus) that
produce generator potentials (sensory receptors themselves - these are very similar to EPSPs/ IPSPs) that
trigger an AP in the sensory neuron - All sensory neurons encode and represent sensory information, e.g.,
spatially – this is present in how info for each sense is organized in the brain (info for each sense is encoded in brain - where info is processed)
receptive fields –
area or range of sensory space in which the presence of a stimulus will alter the firing rate for that sensory neuron (neurons have receptive fields - activity can be increased or decreased, i.e field of vision)
coding –
neural activity patterns that simplify and represent the information for efficient transmission
Examples of coding strategies in sensory systems - Rate coding –
firing rate of a sensory neuron encodes stimulus intensity as a function of strength – more APs = strong stimulus
Examples of coding strategies in sensory systems - Temporal coding –
timing of APs corresponds to what type of information it conveys
Examples of coding strategies in sensory systems -Labeled line –
idea that individual sensory receptors will respond to only a single quality for a particular sense
-information about each quality is then transmitted via separate afferent
pathways to the CNS
Examples of coding strategies in sensory systems - Across-fiber –
individual sensory receptors respond to different qualities
for a particular sense
-information about each quality is then is transmitted to the brain by
afferent fibers that have broadly overlapping responses, i.e., with
information from other sensory receptors
More common themes in ALL sensory systems
- Sensory neurons are often tonically active to give a broader dynamic
range of responses; sensory receptor activation then modulates AP
frequency - Hierarchical processing – Sensory information is represented at
multiples levels of the CNS - Cortical representation – specific parts of the cortex process each
sensory modality - Integration – sensory systems combine information – usually in
association cortical regions; these are sites that can bind together
features from multiple sensory modalities
somatosensation
sense of touch – a.k.a. somatosensation;
detection of body sensations, including touch and pain
pacinian corpuscle
The pacinian corpuscle is an exemplar
touch receptor; detects sudden
displacement or high-frequency vibrations
on the skin
– Onion-like outer structure resists
gradual or constant pressure
– Sudden or vibrating stimulus bends the membrane and increases the flow of Na+ to trigger an AP
steps in somatosensation at receptor:
- pacinian corpuscle changes shape when pressure is applied, thus opening Na+ channels
- the corpuscle produces a generator potential (very similar to an EPSP)
- When the potential is big enough, the receptor reaches threshold and
generates an AP
Generator potential =
EPSP
Adaptation –
the progressive loss of response to a maintained stimulus
Tonic receptors -
show slow or no decline in AP frequency
Phasic receptors -
display adaptation and decrease frequency of APs
Adaptation is key for ….
representing CHANGES in one’s environment…with no adaptation we are constantly bombarded with meaningless information
Somatosensory pathways - from receptor to cortex
Info from below the head enters the
spinal cord from dorsal spinal nerves
and up the cord to the CNS
Body: Somatosensory information
ascends ipsilaterally (same side), in
the dorsal column system AND makes
synapses with neurons in medulla AND
crosses to contralateral side to
thalamus AND continues on to the
primary somatosensory cortex (S1)
Head: Information from touch
receptors enters the CNS through the
cranial nerves
