Week 4 Somatosensory Flashcards
Touch is important to survival
-Body information – posture, movement, pain
-Use of objects – food, objects, tools
-Communication – conveying messages, in the way you touch someone
Proper development – body growth: Don’t develop the rest of sensory system without touch information, also don’t develop hormonally
Touch is a diverse modality, incorporating
Cutaneous sensation: Pressure - mechano, Vibration – mechano, A-beta fibres – myelinated, fast conduction, Temperature – thermo, Pain – noci
Proprioception: Body information – where it is positioned relative to itself, Are you moving?, A-alpha fibres – myelinated, fast conduction
Kinaesthesia
Pain: nociception
Itch: Pain and itch are parts touch that are qualitatively different, Different stimuli, different pathways
Cutaneous sensation
Often what we think of when we think of touch - Arises from any surface of the body - Submodalities o Pressure o Vibration o Temperature o Pain - Stimuli o All are physical stimuli – although pain can involve chemical stimuli as well o Mechanical compression o Vibration o Thermal energy transfer o These can all be in the pain submodality if intense enough
Sensory system, 4 criteria
- Specialized to receive particular stimulus – i.e. has specific receptors for specific physical energy/chemical molecules
- Performs signal transduction (stimulus → neuronal potential)
- Relays the neural signal to the brain via certain pathway (synapse 1 → synapse 2 → synapse 3 …)
- Has its own cortical region for processing (sensory cortices + association cortices)
System for non-painful sensation
-Activate receptor – going all the way to the spinal cord using only one cell
o Signal transduction
- To spinal cord from cell body and up to brainstem – massive distance before relay
- Long neurons – axons forming peripheral nerves and cranial nerve V for the face
Process of touch – non-painful cutaneous sensation
- Sensory receptors are activated under the skin, this causes signal transduction in the
sensory neuron - Neural signal travels along axons (= peripheral nerve) into spinal cord.
- Signal travels up spinal cord and relayed to brainstem neurons – medial-lemniscal Signal relayed to interneuron, then motor neuron – exits spinal cord – reflex arc
- Signal relays to contralateral thalamus – medial-lemniscal
Motor nerve activation may cause muscular response – reflex arc - Signal relays to somatosensory cortex – medial-lemniscal
Mechanoreceptors receptive to pressure/vibration stimuli; 4 types of receptors
Meissners corpuscle
Merkels disk
Ruffinis corpuscle
Pacinians corpuscle
2 types of touch fibres
Rapidly adapting (RA) -Turn receptor on – firing stops quickly even if pressure continues Slowly adapting (SA) -Continually fires as long as there is pressure
2 types of receptive fields
Diffuse
- Larger field – could push anywhere in the field and feel it Punctate
- Smaller – only in that small circle directly to activate cell
Meissner corpuscles
- Shallow, long axis perpendicular to skin
- RA – punctate innervation
- Transient stimulation – rapidly adapting
- tells you something has touched you, not for how long
Merkel disks
- Shallow, small, little branches
- SA – punctate innervation
- Steady pressure – slowly adapting
Ruffini endings
- Deeper, long axis parallel to skin
- SA – diffuse innervation
- Steady pressure and stretching – slowly adapting
- Get stretching information
Pacinian corpuscles
- Deepest, fewest in number but most sensitive
- RA–diffuse innervation
- Transient stimulation for even lightest touches
Free nerve endings
- Nociceptors and thermoreceptors here
- Feel pain and thermal information
Reflex arc
- Within spinal cord
- Sensory neuron – afferent information from skin
- Interneuron
- Motor neuron – efferent information to muscles
Ascending pathway
- Non-painful information is relayed via medial-lemniscal pathway
- Sensory neuron
- 2’ neuron in brainstem – decussates at brainstem
- 3’ neuron in thalamus
- somatosensory cortex
Somatosensory cortex
-2 major subdivisions:
S-I – receives input from thalamus: Broken into Brodmann’s 3a, 3b, 1, 2, S1 = postcentral gyrus, Output to motor cortex
o S-II – receives input from thalamus and S-I: Output to motor cortex and limbic system, Limbic for tactile learning and memory
-The judge of whether the visual info or somatosensory info is correct
-2 important features of somatosensory cortex: Somatotopic organisation, Hierarchical processing
Somatotopic arrangement
-The amount of space on the somatosensory cortex devoted to each body part is proportional to the sensitivity of that part: Areas with the most afferents have the most cortical space
-Generally neighbouring regions of body map to neighbouring regions of
cortex
Experience-dependent plasticity
-Topographical representation is not permanent – Somatotopic organisation is
malleable
-S1 cortical representations can change depending on amount of sensory input being received
Somatosensory processing
- Sensory information undergoes hierarchical processing (area 3- 1, 2, 5- SII)
- At each stage more and more info is integrated
Proprioceptive and kinaesthetic sensation
- The ability to sense the position and movement of our own body: Proprioception = position of body relative to itself, Kinaesthesia = sensing movement of your joints and muscles
- Often not what we think of when we think of touch
- Arises from inside the body: requires a physical stimulus from the internal environment
- Sensory info that largely goes unnoticed, yet essential to daily function: Posture, Large scale movement
Process of touch – proprioception and kinaesthesia
- Sensory receptors are activated within the body, this causes signal transduction in
the sensory neuron - Neural signal is relayed towards the brain
- Cortical processing?: Brainstem- thalamus and cerebellum, Thalamus-cortex(motor cortex?), Cerebellum-online parietal cortex/cerebellum processing loop
Three types of receptor neurons – all do signal transduction
- Muscle spindles – proprioceptors: Respond to change in muscle length, High density in hand, neck, ocular muscles, In skeletal muscle
- Golgi tendon organs: Respond to change in muscle tension, Within tendons
- Joint receptor neurons: Respond to joint movement, At joints
Functions of touch
- Body information
- Identification and use of things
- Communication
- Development
