Sensory systems
• Sensation - is how the senses deliver signals to
the CNS about the current state of the world
• Perception - is how an individual interprets these
signals in terms of previous experience,
knowledge of the world and expectation
• sensory systems: sensory receptor, afferent
pathway, central projection
Components of sensory pathways
Stimulus
• any change in the external environment or
internal milieu
• a stimulus is only registered to the extent that we
possess the machinery to transduce that stimulus
into a neural signal
• sensory organs steer stimuli to sensory receptors
Stimulus
• transduction –
conversion of
stimulus energy
into a receptor
potentials (mostly
depolarization)
• transformation –
conversion of
receptor potential
into action
potential
All sensory systems convey
types of information:
1. modality (what it is)
2. location (where)
1 and 2 - labelled line coding
3. intensity (how much)
4. timing (when)
3 and 4 - frequency coding
Modality
- is a property of sensory nerve fiber that is activated
primarily by a certain type of stimulus
- the axon of the receptor functions as a modalityspecific
line of communication; activity in the axon
necessarily conveys information about a particular
type of stimulus
- each sensory nerve fiber makes specific
connections to structures in the CNS whose activity
give rise to specific sensation
Sensory receptors - types of energy
mechanical - touch, pressure, sound ...
chemical - taste, olfaction, osmoreceptors...
thermal - warm and cold receptors (skin, hypothalamus)
electromagnetic - photoreceptors
Sensory receptors - structure
• encapsulated receptors - touch, pressure..
• free nerve endings – myelinated and
unmyelinated – nociceptors, thermoreceptors
Receptors
• slowly adapting - tonic (carotid bodies, nociceptors ..)
• rapidly adapting - phasic (cones, muscle spindle,
olfactory receptors.....)
Receptive field
• the range of locations
where stimulation will
excite a sensory receptor
eg. skin sensation, vision
Stimulus intensity coding
Frequency of AP and a number of receptors activated
Perception and stimulus intensity
psychophysical law:
R= K × SA
R...sensation felt
S...stimulus intensity
K,A...constants
Stimulus duration coding - pattern of generated APs
Sensory systems have a common plan
• populations of sensory neurons convey sensory
information + somatotopic organization
• hierarchy: cortex - thalamus
• parallel and serial processing
Somatosensory system
• different types of receptors
1) touch, vibratory sense, proprioception
2) crude sensation, temperature, pain
• primary somatosensory cortex - postcentral gyrus
Pain
• an unpleasant sensory and emotional experience
associated with actual or potential tissue damage,
or described in terms of such damage.”
• subjective
• stimuli:
– thermic above 45°C or below 5°C (thermoreceptors)
– intensive mechanical - sharp, localized pain
(mechanoreceptors)
– mechanic, thermic and chemical - diffuse pain
(polymodal receptors)
Pain
• acute (physiologic) x chronic (pathologic)
• surface, deep, visceral
• referred pain
• modulation of pain:
– endogenous opioids
– spinal cord mechanisms
(rubbing close to the
wound decrease pain)
– brainstem mechanisms:
periaqueductal grey,
serotoninergic and
noradrenergic
descending pathway
The ear has three functional
parts
• capturing mechanical energy
• transmission to the receptor organ
• transduction into electrical signals
Optical apparatus
• refractive power (app. 60 D): cornea and lens
• accommodation: changes of refractive power of the lens
to focus on near point or far point
Myopia (nearsightedness)
Presbyopia
Hypermetropia (farsightedness)
Pupils
• 2-8 mm
• regulation of light intensity
• miosis (constriction, parasympathetic NS, m.
sphincter)
• mydriasis (dilatation, sympathetic NS, m.
dilatator)
• pupillary light reflex: direct and consensual