Lecture 13 - Somatic sensation Flashcards
Note about somatovisceral
Note - sometimes see the term somatovisceral system to emphasise that we also get sensory information from the gut
Primary afferent neuron
Primary afferents are sensory neurons (axons or nerve fibers) in the peripheral nervous system that transduce information about mechanical, thermal, and chemical states of the body and transmit it to sites in the central nervous system.
Line/barrier indicates entry of this axon into the level of the spinal cord
Area that this axon is responsive to is called the receptive field
Receptive field
Area that an axon is responsive to is called the receptive field
Pathways
Axons cross over low in spinal cord (anterolateral, or ventral spinothalamic pathway), for pain, temperature, or cross high, in medulla (dorsal column/medial lemniscal pathway), for fine touch, proprioception.
Branches also in brainstem, to ‘reticular activating system’ (RAS), and for proprioception and touch to cerebellum.
Receptive fields vary in size and density on different parts of the body
Anterolateral pathway
Anterolateral pathway - directly after entering in through the dorsal root ganglion in the spinal cord, the anterolateral fibres pass to the front of the spinal cord and to the other side up to the brain stem and this transports general information such as brain and temperature (broad sensations)
Dorsal column pathway
Dorsal column pathway - enters through the dorsal root and it stays at the back of the spinal cord on the dorsal side of it and goes up on the same side that it comes in and crosses over higher up
Sensory receptors of the skin
Convert physical stimulus to action potentials in primary sensory neuron (transduction).
Sensation or stimuli are transduced into action potentials
Temperature receptors are called
Temperature (Thermoreceptors)
Different types more active at warm or cold temperature ranges, especially responsive to changing temperature
Proprioceptors
Position of limbs (proprioceptors)
Muscle spindles, golgi tendon organs, joint capsule) skin mechanoreceptors
Alerts to changes in the environment for safety
Touch receptors
Touch receptors
Sensitive to mechanical deformation (mechanoreceptors)
Location corresponds to folds in the skin so they have a specialised location that is involved with that interaction of texture with that body surface with something we are touching
Various types, with different sensitivities (sub-modalities) and locations
Nociceptors
Pain (nociceptors)
Respond to extreme mechanical, temperature, and/or chemical stimuli
Skin touch receptors
Non hairy skin is called glabrous skin and includes the palms and the soles of the foot
Pain fibres have quite wide receptive fields and they are found everywhere in the body except the brain and the spinal cord
A = Tactile (Meissner’s) corpuscle - light touch
Close to the finger print i.e. on the surface of the skin
B = Tactile (Merkle’s) corpuscle - touch
C = Free nerve ending - pain
D = Lamellated (Pacinian) corpuscle - deep pressure
Tends to be deep in the skin and has a mechanical structure outside of the axon
E = Ruffini corpuscle - stretch, directional, it is stretch receptor and it is sensitive to the orientation at which the skin is being pulled
Hair receptor
Tactile (Meissner’s) corpuscle
Tactile (Meissner’s) corpuscle - light touch
Close to the finger print i.e. on the surface of the skin
Tactile (Merkle’s) corpuscle
touch
Free nerve ending
pain
Lamellated (Pacinian) corpuscle
deep pressure
Tends to be deep in the skin and has a mechanical structure outside of the axon
Ruffini corpuscle
Ruffini corpuscle - stretch, directional, it is stretch receptor and it is sensitive to the orientation at which the skin is being pulled
Hair receptor
associated with a hair follicle
Pacinian corpuscles
Associated with myelinated fibres therefore there is precise real time sensation in terms of our interaction with the environment
If the receptor potential reaches threshold then it will generate an action potential
Influx of ions such as sodium or calcium which depolarises the membrane and causes a receptor potential to form
Adaption results from receptor mechanical properties, and axon ion channel properties
Pacinian corpuscles steps
mechanosenstive channels
Mechanical stimulus - influx of ions (Na+, Ca2+) - receptor potential - action potential generated at first node - potential decays, and APs stop = adaptation. Adaptation results from receptor mechanical properties and axon ion channel properties
Sensory coding list
Modality (e.g. touch vs temp)
Intensity
Location
Duration
Sensory coding - modality
Modality (e.g. touch vs temperature)
Specificity of receptors, ‘labelled line’
The basic sensory modalities include: light, sound, taste, temperature, pressure, and smell.
Sensory coding - intensity
Frequency of action potentials in each axon, umber of axons activated
Sensory coding - location
Mapping of receptive fields of individual primary afferents to specific cortical locations. [Somatotopic representation in somatosensory cortex].
Sensory coding - duration
“Rapidly adapting” receptors respond briefly, even if stimulus is sustained. Detect movement, changing pressure. “Slowly adapting” receptors can signal true duration of stimulus.
Vibrations for humans
Vibration sensors are very useful for spiders - in humans they contribute to sensation of surface texture
Sensory pathways in the CNS
Anterolateral pathway and dorsal column pathway
Anterolateral pathway is for
Pain, temperature, some touch (broad surface area sensation)
Dorsal column pathway is for
Fine touch, position (proprioception)
Anterolateral pathway order
primary afferent neuron (pain or temperature receptor) - synapse in spinal cord - anterolateral position of axons - secondary axon through spinal cord and synapse in thalamus, secondary axon also has branches in reticular formation - 3rd axon projects from the thalamus to the somatosensory cortex
Dorsal column pathway order
Primary afferent (touch and proprioceptive endings) - primary axon in dorsal column - synapse in dorsal column nucleus (spinal cord) - 2nd axon from spinal cord to thalamus where it synapses, 2nd axon has branches in brainstem - 3rd axon from the thalamus to the sensory cortex
Lateral inhibition
Lateral inhibition is a fundamental mechanism to increase accuracy of sensory information.
Mediated by inhibitory interneurons
In somatic sensation lateral inhibition localizes sensation to a restricted area of skin.
Lateral inhibition is used wherever accurate location of a stimulus is required – or where a pattern of input needs to be discriminated [e.g. vision, smell…]
Lateral inhibition results in centre- surround inhibition
Information from separate icons coming from the skin can have overlapping receptive fields - the overlap allows them to interact such that we get extra precision on this information
In the image, pressure is activating all of these neurons because the receptors overlap. Red lines are the inhibitory interneurons and they cross connect these separate pathways that are all carrying information from the same region of the skin but the central neuron is going to be more strongly activated because it is getting more direct input which activates the inhibitory interneuron to shut down ones either side of it and without this happening the sensory cortex would see information coming from a bigger area of skin but with this system we get more contrast between centre of the field and the outside of the field
Yellow circle is the receptive field of the central neuron
Primary somatosensory cortex
Post central gyrus
Information on sensation on the body is being mapped on the primary somatosensory cortex
Note the parietal-lobe association cortex is where different forms of information are bought together to produce a coherent picture of the environment
Somatotopic representation in primary somatosensory cortex
Ordered representation of body
Area proportional density of receptors (fineness of discrimination) and use
Margins of representations are modifiable (“plastic”) - you are not stuck with the same mapping for life
Homunculus
Example - Claudia
Could stimulate sensory axons that used to carry information into her missing arm and then could get sensation as though her arm was still there
