Lecture 2: Somatosensory System Flashcards
What is somatosensation?
sensation arising from the body; includes touch, pressure, vibration, temperature, limb position and pain.
What detects somatosensation?
Detected by sensory cells whose somi are in the dorsal root ganglion (limbs & trunk) and trigeminal ganglion (face)
Single process from DRG cells extends to the periphery and to the CNS (spinal cord)
Note that mechanosensory fibers (touch, pressure, vibration) project up the ipsilateral side of the spinal cord.
Pain/temperature fibers cross immediately to the contralateral side of the spinal cord before projecting up towards the brain.
What is sensory transduction?
Sensory transduction – the conversion of environmental stimuli into an neuro-electric signal
picture is ex of pacinian corpuscle
What is sensory modality?
Refers to what environmental information a receptor responds to.
Labeled Line reception
Labeled line - a receptor only responds to specific kind of stimulus
This is the primary method of encoding
For example, the body uses different receptors to detect mechanical stimuli (somatosensory receptors in the skin) as opposed to visual stimuli (rods & cones in the retina).
Frequency encoding reception
Frequency coding - the pattern of activity communicates sensory information
Is used to detect differences between stimuli that are part of the same sensory modality.
For example the same thermal receptor can detect whether the temperature is 15° or 25°C; it distinguishes between the two by being more active for the warmer temperature.
What are the 4 afferent axon types?
Describe the receptive field. What is two point discrimination?
region of skin that is innervate by a given sensory neuron (or the region of skin that produces a response in a sensory neuron)
Response of neuron decreases as you move away from the center of the receptive field
Receptive field size varies in different regions of the body; smaller fields in more sensitive regions of the body (fingers & toes); smaller fields are also intrinsically more sensitive
Two-point discrimination – minimum distance between 2 stimuli that allows them to be distinguished as 2 stimuli
What is adaptation? What are the two types we’re focusing on?
Adaptation – the capacity of a neuron to reduce its’ firing rate or even cease firing even though a stimulus is still being applied.
Rapidly-adapting afferents tend to encode temporal or dynamic properties of a stimulus (e.g. a vibration.)
Slowly-adapting afferents tend to encode information of a more static nature (e.g. the magnitude of the stimulus).
What is the difference between primary and secondary afferents
Responses by primary & secondary afferents to (A) onset of a linear stretch, (B) a tap, (C) a vibration, and (D) release from a linear stretch.
The top traces show the type of stimulus applied. The middle traces show the response of the primary (type I) afferents. The bottom traces show the response of the secondary (type II) afferents.
What are accessory structures and what do they do?
These are specializations at the endings of mechanosensory cells that help determine
[1] what kind of stimulus a sensory cell responds (temperature, pressure, vibrations, etc.) and
[2] how a sensory cell responds to a given stimulus. An example of this latter function is adaptation.
Free nerve ending cutaneous mechanoreceptors
pain (nociception)
Meissner corpuscle cutaneous mechanoreceptors
Rapidly adapting, detect low freq vibrations (< 40 Hz), good for detecting texture (feedback for grip control)
Consist of a capsule (Schwann cells) that contains multiple disc-shape nerve endings
Very sensitive, small receptive fields
Merkel cells (discs)
Slow-adapting, very high spatial resolution, appear to be involved detecting curves, points and edges
Ruffini endings
Slow adapting, appear to respond to cutaneous stretch (especially in regions where there is limb movement)
pacinian corpuscles
Rapidly adapting; even more sensitive than Meissner corpuscles (10 nm displacement), but the corpuscle acts as a filter so that only high frequency stimuli reach the afferent fiber inside (≥ 250 Hz)
Likely provide critical sensory feedback for fine motor control, e.g. tool use.
How is braille read?
all inputs fro merkel discs, meissner’s corpuscle, ruffini endings, and pacinian corpuscle at once
Haptics
interpretation of complex spatiotemporal patterns that is the result of input from multiple types of cutaneous mechano-sensitive neurons.
Stereognosis
the ability to identify an object by touch
Describe the mechanosensory pathways from limb stimulation to brain.
1st order sensory neurons (mechanosensory reception from body)
Travel ipsolateral up dorsal column.
Synapse on dorsal column nuclei in the Medulla
crossing over
Travels up medial lemniscus on contralateral side
through pons, midbrain
Synapse in VPL of thalamus
Project to primary somatic sensory cortex (S1)
Describe the mechanosensory pathway from the face to the brain
Mechanosensory receptors from face to trigeminal ganglion, to principle nucleus of trigeminal complex.
Contralateral projection to medial lemniscus
syanpses in VPM of thalamus
Projects to SI
Where is S1 (somatic sensory cortex)
Parietal lobe; posterior to central sulcus
in postcentral gyrus
Somatotopic order in the human primary somatic sensory cortex
Not all of the regions (represented by Brodmann’s areas) receive the same input
Part of this separation of function reflects different sets of inputs from the ventral posterior complex of the thalamus (VPL + VPM) that carry distinct somatosensory information.
However, not all regions of S1 receive the same level of thalamic input
Neurons in the primary somatosensory cortex form functionally distinct columns
Neurons with similar response properties are clustered together to form functional columns (in this case similar response properties refers to position on the body and adaptation)
These columns extend through the depth of the cortex so that so that all of the neurons in the column (100,000s neurons in each) have the same functional properties, such as responding to slowly-adapting vs. rapidly-adapting sensory inputs.
The columns are organized so that they have a topographic organization (close proximity on the body translates to close proximity in the brain), but also that similar response properties are close to each other (i.e. adaptation).
Descibe functional expansion of a cortical representation by a repetitive behavioral task
Following a task that required increase use if digits 2, 3 and 4, the corresponding receptive fields in S1 expanded.
Other examples of experience-dependent changes in S1 receptive field organization
Application of anesthetic to a region of skin causes the effected region of S1 to re-organize so that it receives input from the still-functioning skin around the effected area. When the anesthetic wears off “expanded” region of S1 returns to its original size, but there is a period when that region of skin feels larger than normal.
Loss of a digit will be reflected by a remapping of S1, so that neurons that use to receive input from lost digit now receive input from the remaining digits.
