Yuste C9 Flashcards
Touch
The most direct way we interact with our environment, Ising physical contact to extract info about the position, shape, surface, movement, consistency and temp of objects.
Somatosensory system - general
Has many parallel channels, carefully measures amplitude, duration, and phase of the stimulus. Uses this info to build a representation of the object, constructed by our internal model of the object and our memories of past experiences.
The two somatosensory systems
Dorsal column system and anterolateral system.
They run parallel to each other from the skin to the spinal cord, thalamus and end in the somatosensory cortex.
Dorsal column system
Runs along the dorsal white matter of the spinal cord, evolutionarily modern, uses thick, fast myelinated axons.
Has many sub channels that carry info about touch, vibration, pressure and proprioception.
Anterolateral system
Runs along the anterolateral white matter of the spinal cord. Evolutionarily older, has thinner and slower axons, carried nociceptive pain, temperature, and coarse or sensual touch.
Dorsal pathway
Touch pathway. Starts the skin, in the dermis, in receptors specialised for different modalities of touch.
Axons from the dorsal pathway, which come from first order DRG neurons, either make synapses on neurons in the dorsal horn of the spinal cord (creating reflex arc) or continue upward through the dorsal column of the spinal cord to the medulla, where they synapse on second order neurons. Axons from their second order neuron then decussate to the other side of the brain and proceed to the thalamus where they synapse on a third order neuron. This last neuron in the pathway extends into the cortex.
Dermis
Where we find the majority of nerve endings; has receptors specialised for different modalities of touch. These nerve terminals are loaded with mechanosensory channels that transduce pressure or vibration in electrical signals.
Three basic types of mechanosensory channels
- Channels that stretch open as a result of membrane tension.
- Channels that have an EC anchor (like tip links) which is elastic and attached to a lid that keep the channel closed.
- Channels where elastic spring is connected to a membrane protein that turns on a 2º messenger pathway that opens the channel.
All three types flux Na and other cations, so depolarise the cell.
Types 1 and 2 are fast; type 3 have slower kinetics but can amplify the signal.
Two classifications of touch receptors
By the temporal properties of their responses.
- Slow adapting = respond to steady skin indentation with sustained electrical discharges.
- Rapidly adapting = stop firing as soon as the indentation becomes stationary.
SA receptors
Spiking frequency is directly proportional to the Toal amount of pressure; better suited to monitoring the strength, duration and shape of the stimulus.
RA receptors
detect temporal onsets and offsets of discrete stimuli, like changes in temporal and spatial patterns of stimulation.
Four major subtypes of RA and SA touch receptors in skin
Near the surface:
- Messner corpuscles (RA)
- Merkel disks (SA)
Deeper in the dermis:
- Pacinian corpuscles (RA)
- Ruffini corpuscles (SA)
Pacinian corpuscles
Have an axon terminal enveloped in onion-like layers of fluid filled connective tissue that works to dampen the applied pressure. These layers absorb and redistribute the pressure, act as mechanical shock absorber. APs are only triggered when the stimulus is first applied, when the initial pressure wave propagates and stretches the membrane and opens the channel. APs are also triggers when the stimulus is removed, because a negative pressure propagates and also stretches the membrane.
Takes the derivative of the stimulus, to detect virabtions – changes of the touch stimulus as a function of time. Tells you about the structure underneath the surface of an object.
Respond to vibration frequencies as high as 500 Hz, firing impulses every 2 ms.
Brain and these receptors
Keeps tack of the intensity of the stimulus by the number of active neurons in each receptor population; this population code depends on the intrinsic sensitivity of each neuron to a stimulus.
Dorsal root ganglia
Axons of the nerve terminals from the skin, muscles, and joints of the limbs and trunk are generated by neurons that are clustered rogether in the DRGs. Its neurons are part of the PNS, pseudo unipolar in shape = have a T shape w/ two branches that stem from the same cell body branch.
The peripheral branch terminates on the skin/muscles as free nerve endings or with specialised receptors (their axons are more like dendrites receiving sensory inputs).
The central process, the actual axon, enters the spinal cord and either terminates within the grey matter or ascends to nuclei in the medulla and into the brain.
DRGs location
At every level of the spinal cord: cervical, thoracic, lumbar, sacral.
Receptive field of sensory neurons
If you stick an electrode into your hand and record from the axons that come from that spot, will find that APs occur only if you touch a particular patch of skin.
RF: the skin area in which a stimulus can activate a sensory neuron.
Small RFs in more sensitive areas like our fingertips and larger ones in our back. Size of the stimulus determines the total number of neurons activated: neural representation of an object in a collective mosaic of individual receptive fields.
Somatosensory axons
Fast axons are thicker, slow axons are thinner. Current flowing thru a cable is directly proportional to the width of the cable and indirectly proportional to the membrane resistance. Fast axons are thicker and also heavily myelinated whereas slow axons are thin and unmyelinated. Myelin acts to insult the axon by decreasing the denominator.
Group 1 = innervate skeletal muscles and carry proprioceptive info.
Group 2 = innervate the skin mechanoreceptors.
Group 3 = thinner and innervate nociceptors and thermoreceptors.
Group 4 = carry pain, temperature info, and coarse touch and itch via anterolateral pathway.
Dorsal pathway vs anterolateral pathway
due to the nature of the width of the axons and the myelination, messages from the dorsal pathway arrive to the CNS faster than those from the anterolateral pathway.
Main difference between them is the point of decussation. In DP - decussates in the midrabin; AP - decussates in the spinal cord.
Somites
Our bodies develop out of many bilaterally-paired repeated block of mesoderm. Transient structures that alter differentiate.
The first somite appears on the 20th day of pregnancy; more somites develop continuously and quickly along both sides of the neural tube.
As neurogenesis begins, the NS develops a nerve that innervates all of the tissues derived from a particular somite.
Dermatome
Sensory and motor info from a specific segment of the skin - dermatome - travels through a dedicated spinal nerve and is fed to diff regions of the spinal cord. The nerve bifurcates into the dorsal root ganglion for afferent sensory signals and ventral root for afferent motor signals. = dermatome maps.
Decussate
Cross the midline
Anterolateral pathway
Pain pathway. The first order neuron terminates in the dorsal horn of the spinal cord, synapsing onto a second order neuron whose axons decussate straight from there and travel all of the way to the thalamus through he anterolateral column of the opposite side of the spinal cord. The third neuron receives the input from the thalamus and delivers it to the cortex.
Somatotopic maps
Maps of the body that correspond to a point on the body and its representation in the brain, structured dermatome by dermatome.
Medulla
Some basic processing of sensory info organised in somatotopic maps takes place here.
Somatosensory cortex
Somatosensory info from the thalamus arrives to the 1º somatosensory cortex in the post-central gyrus of the parietal lobe.
1º somatosensory cortex comprises areas 3a, 3b, 1, and 2. The bulk of thalamic neurons project to areas 3a and 3b, which project to areas 1 and 2. Areas 3a and 1 receive their inputs from receptors in the skin whereas 3b and 2 receive proprioceptive info from receptors in the muscles, joints and skin. Neurons in S1 are at least 3 neurons away from the stimulus.
Cortical neuron also has a larger receptive field.
Can generate a hallucinatory experience of touch by directly stimulating a specific area of the cortex.
Neocortex
Larger and evolutionarily more recent. Six layers which run from the outer surface to the inner white matter.
Mountcastle
Discovered that neurons in the 1º somatosensory cortex have similar receptive fields as you go down in depth. Found that S1 is arranged in vertical columns or slabs, traversing all 6 cortical layers. These columns receive input from a specific body part, so that all together they form a somatotropin map. And each column is divided into smaller ones that receive inputs from SA and RA receptors.
Somatosensory map
Cortical homunculus. Represents each area of the body surface. The parts more densely innervated by the NS occupy larger parts of the brain.
Cortical plasticity
Sensory maps change depending on experience. The size of the different body regions enlarges or shrinks depending on how much we use them.
S2, posterior parietal cortex, 1º motor cortex
Receptive fields of neurons increase in size. SA and RA receptive fields are small dots on the hand, and receptive fields of 1º somatosensory cortical neurons cover the entire fingertip or several adjacent fingers, receptive fields in higher cortical areas cover an even larger area - spanning whole functional regions of skin.
Unimodal vs multimodal association areas
Cortical areas involved in the early stages of sensory processing concerned with only one sensory modality = unimodal. These areas then converge on multimodal association areas, which combine sensory modalities.
Descending control
In some sensory systems there are 10 times more axons going down than up. Attention is one example of top down process. If you pay attention to your hand in anticipation of touch, that will cause a greater activation of higher cortical areas once the touch actually occurs.
Somatosensory system and principles of sensory processing
Parallel processing (labelled lines), measurements of phase and amplitude, receptive fields, topographic maps, hierarchical processing for abstraction of stimulus properties, descending control and construction of sensory percepts, and experience-based plasticity.
Somatosensory system and principles of neuroscience
Hierarchical processing, wiring, maps, plasticity.
