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
