2. General Principles of Sensory Physiology Flashcards
Interaction between sensory system and motor system
Sensory receptors in the PNS generate an impulse
The CNS interprets the impulse
The PNS stimulates a response
Sensory receptors
Some specialized structures activated by stimuli and convert a stimulus into neuronal activity
Types of sensory receptors
- Endings of afferent neurons (somatosensory and olfactory systems)
- Specialized epithelial cells adjacent to an afferent neuron (visual, taste, and auditory systems)
- Specialized structure associated with nerve terminals (Pacinian corpuscles)
Simple receptors
Neurons with free nerve endings, work on somatosensory and olfactory systems
Complex neural receptors
Nerve endings enclosed in connective tissue capsules, Pacinian corpuscles
Special senses receptors
Cells that release neurotransmitter onto sensory neurons, initiating an action potential
5 types sensory receptors
Mechanoreceptors, photoreceptors, chemoreceptors, thermoreceptors, nociceptors
Mechanoreceptors
For touch, is a pacinian corpuscle and located on the skin
For audition, is a hair cell and located in the organ of corti
For vestibular, is a hair cell locating on macula, semicircular canal
Photoreceptors
For vision, rods and cones are the receptors and located on the retina
Chemoreceptors
For olfaction: olfactory receptors on olfactory mucosa.
For arterial Po2 on carotid and aortic bodies.
Fo pH of CSF on ventrolateral medulla
Thermoreceptors
For temperture, receptors are cold receptors and warm receptors, on skin.
Nociceptors
Detect stimuli causing tissue damage, can be thermal, mechanical or polymodal, located on skin.
Receptor potential vs. action potential
Receptor potential = graded, non-propagated
Action potential = hyperpolarizing or depolarizing
Sensory transduction
- Stimulus -> ion channel opening
- Current –> receptor potential
- Depolarization -> voltage gated sodium channels, action potentials
- Propagation -> NT release
- Transmission to CNS through sensory afferent neurons (1st, 2nd, 3rd, and 4th order)
Sensory unit
primary sensory neuron with all sensory receptors (endings or associated sensory receptor cells).
The smallest unit of sensory response.
Receptive field
an area of the body surface that when stimulated results in a change in firing rate of a sensory neuron.
What determinates the size of receptive field of a sensory neuron?
Terminal size and associated cells; Convergence
Receptive field variation in size
The smaller the receptive field, the more precisely the sensation can be localized or identified.
The higher the order of the CNS neuron, the more complex the receptive field.
Receptive field firing rate
Receptive field can be excitatory or inhibitory depending on the change in firing rate.
Four aspects of a stimulus
Stimulus Modality: light, sound, temperature, taste, smell, pressure
Stimulus Intensity: strength of a stimulus
Stimulus Location: acuity, lateral inhibition
Stimulus Duration: adaptation
Modality of mechanoreceptors
Stretch and pressure
Modality of photoreceptors
Light
Modality of chemoreceptors
Chemicals
Modality of thermoreceptors
Cold, warm
Modality of nociceptors
Stimuli causing tissue damage
Adequate stimulus
Each sensory receptor is particularly sensitive to one stimulus type or modality
Labeled line
the distinct anatomical pathways (including ascending pathways, specific neuronal circuits, from sensory receptors to a specific region of the CNS) associated with a particular stimulus modality.
Do all receptors from same sensory unit respond to same modality?
Yes
What encodes stimulus intensity?
- The number of receptors that are activated. A stronger stimulus affects a larger area and recruit a larger number of receptors.
- Differences in firing rates of sensory neurons in the pathway. An increased stimulus results in a higher frequency of action potential.
- Activating different types of receptors. A light touch of the skin may activate only mechanoreceptors, while an intense damaging stimulus to the skin may activate mechanoreceptors and nociceptors.
Lateral inhibition
The capacity of an excited neuron to reduce the activity of its neighbors
Further enhance sensory acuity
Acuitity
Precision of stimulus location
Positively correlated with the density of sensory units
Negatively correlated with the receptive field size
Adaptation
sensory receptors decrease in sensitivity to stimulus of constant strength
⇒ action potential frequency decreases
Phasic sensory receptors
Phasic (rapidly adapting): generate a receptor potential and action potential at the onset (and offset) of a stimulus but very quickly cease responding; detect changes in the stimulus
Tonic sensory receptors
Tonic (slowly adapting): maintain a persistent or slowly decaying receptor potential and continue firing during a constant stimulus; encode duration and intensity of a stimulus
Microneurography
an invasive method used by neuroscientists to visualize and record the normal traffic of nerve impulses.
Relay nuclei
Integrate converging information
Prominent in thalamus.
Contain local interneurons and projection neurons
Topographic organiztaion
Neural maps
Somatosensory system: somatotopic map (sensory homunculus)
Visual system: retinotopic map
Auditory system: tonotopic map
Decussation
The crossings of sensory (and motor) pathways in the spinal cord or in the brain.
Commissure: e.g. corpus callosum
Optic chiasm
Classification of nerve fibres
Many! Look at table haha
Sensory (ascending) pathways
A series of sensory afferent neurons travel in bundles of parallel pathways to pass on certain sensory information to the brain.
First-order neuron is the primary sensory afferent neuron.
Second-order sensory afferent neurons is located in relay nuclei in the spinal cord or in the brain stem. They cross at the midline (decussation).
Third-order neurons typically reside in relay nuclei in the thalamus.
Fourth-order neurons reside in the appropriate sensory area of the cerebral cortex.
Divergence of Ascending Pathways
One primary sensory neuron synapses onto many higher-order neurons.
Convergence of Ascending Pathways
A higher-order sensory neuron may receive inputs from more than one primary sensory neuron (information processed rather than just relayed).
Where do ascending pathways terminate
Terminate in specific primary sensory areas Somatic receptors ⇒ Somatosensory cortex Eyes ⇒ visual cortex Ears ⇒ auditory cortex Nose (nasal cavity) ⇒ olfactory cortex
Further processing in associational cortical areas for complex integration.
Leads to perception of sensations
Perception vs Sensation
Perception is an active process and sensation is a passive process.
Influence of Desending Pathways
Descending pathways can inhibit (directly or indirectly) the transduction of sensory information to the brain. Not all sensory information reaches conciousness.
