Sensory Receptors and Exteroception Flashcards
Understand that the basis for the receptor (generator) potential is the opening or closing of different ion channels
Receptor potential: detection of adequate stimulus by receptor proteins directly or indirectly to open ion channels in order to elicit a depolarization in the receptor neuron.
What is a transduction channel, and how does it differ from
voltage-dependent ion channels?
Process of detection and transformation of information into a neural signal. This involves a sensory receptor (directly gated channel) that is sensitive to physical stimuli such as light, heat, cold, mechanical deformation, or to chemicals dissolved in solution.
Stimulus interacts with receptor proteins to elicit a change in membrane potential.
Stimulus-elicited change in membrane potential involves an ion gradient and leads to depolarization or hyper potential.
Differs from voltage-depended ion channels because transduction channels are NOT voltage-dependent, they are sensitive only to the adequate stimulus. They do not responds to open to cell depolarization!!!
Why are action potentials necessary to transmit
information in long sensory receptors?
Action potentials employ regenerative action potentials to carry info from the receptive ending to the synaptic release site (spinal cord). The receptor potential only affects a limited part of the cell by the receptive ending.
•Ex: Skin Mechanoreceptors
Sensory endings in the skin, cell bodies located ft away in the dorsal root ganglia and long axons that travel along the spinal cord. These make synaptic connections with 2nd order neurons in brainstem
Understand that in different sensory systems stimulation by the sensory stimulus results in either depolarization, hyperpolarization or oscillatory changes in membrane potential.
Thus in mechanoreceptors pushing on the membrane results in depolarization, in the photoreceptor light elicits hyperpolarization and in the auditory system sound elicits
oscillatory changes in membrane potential
Mechanoreceptors: depolarized by stretch
•Increase in nonspecific cation conductance in the receptive area membrane
•Conductance increase causes the membrane potential to move towards 0mV.
•Cation conductance increase (depolarization) increases in a graded fashion with the intensity of the stimulus.
•Ex: muscle mechanoreceptors in sensory ending that dorsal root ganglion neurons extend into (muscle spindle). These open in response to touch.
Photoreceptors: hyperpolarized by light
•Hyperpolarizing Response: to adequate stimulus arises when it causes some of receptive area cation channels to close
•Ex: rod photoreceptor.
Understand the concept of labeled lines: How do we perceive the modality of a stimulus?
What is the route generally taken by sensory information that will reach sensory cortex to become conscious?
•Conscious appreciation of sensory modality determined by specific neuronal connections from sensory organs through thalamus to cerebral cortex.
•Convert one particular type of physical energy to a change in membrane potential to transmit info to a second-order nerve cell
•For 3 main funtcions:
1). Conscious sensation
2). Control of movement
3). Maintaining arousal
Route taken by sensory info that will become conscious
•it all goes through the thalamus (except olfactory)!!!
•Light – through lateral geniculute of thalamus – visual cortex of occipital lobe
•Auditory – through medial geniculute nucleus – auditory cortex of temporal lobe
Understand that, with the exception of the olfactory system, sensory systems send axonal input into the thalamus where they convey information, through
synaptic input, into thalamic neurons. Thalamic neurons then convey information to sensory cortex
Okey dokey
Explain how stimulus intensity is encoded by sensory receptors. How is encoding of stimulus intensity different between short and long receptors?
The magnitude of the generator potential increases as the intensity of the stimulus is increased. The fraction of time the transduction channels spend open depends on stimulus intensity. Helps to maintain a proportional relationship between stimulus and response.
Long sensory receptor cells code stimulus intensity as an increase in action potential fining frequency.
How are peripheral nerves innervating the skeletal muscles and the skin classified in terms of conduction velocity and size?
Muscle: A-alpha nerve fibers. Largest, fastest (10-20um and 60-120 m/s)
Skin: A-beta nerve fibers. Second largest, Second fastest (5-10um, 30-60 m/s)
What is the relationship between size/myelination and conduction velocity?
Myelinated = faster
Larger diameter = faster
What kind of information do different size peripheral nerves
carry?
A-alpha (10-20um) are muscle spindle and tendon organ afferent
A-beta (5-10um) are mechanorecptors of skin and secondary muscle spindle afferents
A-delta (1-5um) are sharp pain and cold temp
C nerve fibers (.2-1.5um) are warm temp, burning pain, itch and crude touch
What is understood by receptive field?
•Receptive field refers to the area over which a receptor can be stimulated. Deep receptors have large receptive fields and respond to changes in wide areas of the skin. More superficial receptors support finer tactile discrimination
Understand the flow of information (anatomical pathway) along the medial lemniscal system.
Mechanoreceptors on first-order neurons out in the periphery house their cell bodies in the DRG→from the DRG the first-order neuron extends a process through the dorsal horn of the spinal cord→the first-order neuron ascends the spinal cord in the dorsal column (ipsilaterally) which is divided into the fasciculus cuneatus (upper extremity) and fasciculus gracilis (lower extremity)→the first order neuron synapses on a second-order neuron in the dorsal column nuclei (cuneatus or fasciculus) in the caudal medulla→the second-order neuron crosses the midline at the level of the medulla and ascends in the medial lemniscus→second-order neuron synapses in the ventro-posterior-lateral nucleus in the thalamus→cells from the VPL synapse in the primary somatosensory area
What is somatotopy? Why are some regions (e.g. mouth and hands) enlarged compared to others in the somatotopic map in primary somatosensory cortex? Understand that there are parallel somatotopic maps in different Brodmann’s areas in somatosensory cortex.
Map of brain relating the extent of innervation of the body part to the size of the body part on the map
The Broadmann’s areas of the somatosensory cortex and their function:
•3a: deep tissue muscle stretch receptors (proprioception)
•3b: skin slow and fast-adapting receptors (touch)
•1: orientation and direction
•2: shape, orientation and direction
What are cortical barrels or columns?
As you move vertically down through the cortex there are groups of functionally related cells. A cortical barrel is a vertical slice through the cortex that contains all of the different groups. Each group is distinct in its modality and receptive field location and there are a total of six different groups or layers, with Layer 1 being the most superficial:
•Layer 1: a few scattered neurons and tufts of dendrites (no specific function that I could find)
•Layer 2: projects to ipsilateral secondary somatosensory area; contralateral primary somatosensory area, posterior parietal cortex and motor cortex
•Layer 3: same as 2
•Layer 4: receives neurons from thalamus
•Layer 5: sends neurons to basal ganglia, brainstem and spinal cord
•Layer 6: sends neurons to thalamus
What kind of stimulation do cells in different Brodmann’s areas of primary somatosensory cortex respond to?
The somatosensory cortex is made up of Broadmann’s areas 3a, 3b, 1 and 2.
- 3a responds to stretch in deep muscle receptors that govern proprioception
- 3b responds to touch at the surface of the skin
- 1 responds to changes in orientation and direction and integrates info. from other areas of the somatosensory cortex
- 2 responds to changes in SHAPE, orientation and direction and integrates info. from other areas of the somatosensory cortex
