Somatosensory System PDF

Somatosensory system Where opportunity creates success Learning objectives Describe the principles of the somatic sensation Understand the role of the receptor cells Compare the different signal transduction pathways Explain how the information is integrated Contents ❖ Somatic sensati...

Somatosensory system Where opportunity creates success Learning objectives Describe the principles of the somatic sensation Understand the role of the receptor cells Compare the different signal transduction pathways Explain how the information is integrated Contents ❖ Somatic sensation ❖ Touch Mechanoreceptors Primary afferent axons Spinal cord Somatosensory cortex ❖ Pain Nociceptors Spinal cord Anterolateral system Regulation of the pain ❖ Temperature Thermoreceptors Temperature pathways  Somatic sensation Somatic sensation – Enables body to feel, ache, sense temperature and pressure – Responsible for touch and pain Somatic sensory system→ different from other systems – Receptors: broadly distributed – Responds to many kinds of stimuli (at least four senses rather than one) Touch  Types and layers of skin – Hairy and glabrous→ hairless – Epidermis (outer) and dermis (inner) Functions of skin – Protects – Prevents evaporation of body fluids – Provides direct contact with world Mechanoreceptors – Most somatosensory receptors are mechanoreceptors  Mechanoreceptor of the skin Sensitive to physical distortion 1. Pacinian corpuscles – Highest density in the fingers 2. Ruffini's endings – Found in hairy and glabrous skin 3. Meissner's corpuscles – Located in the ridges of the glabrous skin 4. Merkel's disks – Nerve terminal + flattened non-neuronal epithelial cell 5. Krause end bulbs – Borders of dry skin and mucous membrane Receptive field and persistence in the response  Mechanosensitive ion channels Located in the unmyelinated axon terminals of the mechanoreceptors Convert mechanical force into a change of ionic current Force can be applied to a channel by – the membrane itself when it is stretched or bent – connections between the channels and extracellular proteins or intracellular cytoskeletal components Mechanical stimuli may somehow trigger the release of second messengers  Two points discrimination Ability to discriminate fine details of the stimulus Varies at least 20-fold across the body Fingertips have the highest resolution – More mechanoreceptors – Receptor types with small receptive fields – More brain tissue dedicated to the fingers Nature vs nurture Class experiment→ Two-point discrimination test Paper clip bent into the shape of a U One person testing and the other being tested without looking Start with the ends about an inch apart, and touch them to the tip of a finger Bring the points closer together, and touch them to your fingertip again How close the points have to be before they feel like a single point? Try it on the back of your hand, on your lips, on your leg, and any other place that interests you  Primary afferent axons Information from the somatic sensory receptors to the spinal cord Enter through the dorsal roots of spinal cord Diameter correlate with the type of the sensory receptor Just group C or IV are unmyelinated  Spinal cord Spinal segments→ dermatomes Residual somatic sensation→ the adjacent dorsal roots innervate overlapping areas Cauda equina – Filled with CSF – Lumbar puncture→ collect CSF for medical diagnostic tests Second-order sensory neurons – Dorsal horns of the spinal gray matter – One branch synapses in deep part of the dorsal horn→ unconscious reflexes – Another straight to the brain→ perception of the stimulus  Dorsal Column-Medial Lemniscal pathway Process the information of touch and vibration 1. Aβ axons enters the dorsal column of the spinal cord (ipsilateral) 2. Dorsal column nuclei (ipsilateral) 3. Medial lemniscus (contralateral ↓) 4. Ventral posterior nucleus of the thalamus 5. Primary somatosensory cortex (S1) In each set of synapses the information is transformed – Lateral inhibition from adjacent sets of inputs in the dorsal column–medial lemniscal → enhance the responses – Output of the cortex even can influence their own input  Trigeminal pathway Sensation of the face and neck – Trigeminal nerves (V) – Additional information from the face→ Facial (VII), glossopharyngeal (IX) and vagus (X) 1. Trigeminal nerve 2. Trigeminal nucleus (contralateral) 3. Ventral posterior nucleus of the thalamus 4. Primary somatosensory cortex (S1)  Somatosensory cortex Primary somatosensory cortex (S1)→ parietal lobe Brodmann’s area 3b – Receives dense inputs from VP nucleus of the thalamus – Very responsive to somatosensory stimulus – Lesions related with somatic sensation Area 3a→ sense of body position Area 1 and 2 – Receive inputs from 3b area Area 1→ texture Area 2→ shape and size Columnar organization of S1 Cortical columns→ neurons with similar response – Rapidly adapting – Slowly adapting Cortical somatosensory layers – Thalamic inputs→ layer IV – From layer IV→ projections to other layers  Cortical somatotopy Somatotopy→ orderly map of the body on the cortex (homunculus) Inverse to real position of the body It is not always continous Disproportional representation – Mouth – Fingers Multiple somatotopic maps – Different areas – Mirror image maps Similar across the species? Cortical map plasticity Reorganization of cortical circuits 1. Lesion Functional remapping Adjacent areas Seen in thalamus and brainstem Phantom limbs 2. Experience and learning Task repetition Local anaesthetic Phantom Limbs and Phantom Pain  Beyond S1: corticocortical and descending pathways From primary somatosensory cortex to secondary somatosensory cortex (S2) – From all subdivision of S1 – Lesion in S1 eliminate response of S2 – Amygdala and hippocampus projections (memory and learning) From area 2 to posterior parietal cortex – 5a and 7b – Motor cortical areas (integration motor and sensory) Descending projection→ thalamus, brainstem and spinal cord (modulation in the response) Posterior parietal cortex Neural representations of the objects – Integration of the different aspect of the stimuli – Visual stimuli, movement planning, attentiveness… Lesion in posterior parietal cortex – Agnosia→ the inability to recognize objects even though simple sensory skills seem to be normal. – Astereoagnosia→people cannot recognize common objects by feeling them – Prosopagnosia – Neglect syndrome Pain  Painful stimulus→ mechanical, temperatures, chemicals, oxygen deprivation… Pain and nociception – Pain→ feeling of sore, aching, throbbing sensations – Nociception→ sensory process, provides signals that trigger pain  Pain perception First pain – rapid and sharp – Aδ fibers Second pain – more delayed, diffuse and long-lasting – C-fibers Respond to all nociceptive stimuli Sensitivity profiles  Nociceptors Present in the most body tissue Located in the free nerve endings of Aδ and C fibers – Aδ fibers Type I→ respond to low mechanical and chemical stimulation/ high heat threshold Type II→ High mechanical and chemical stimulation/ low heat – C-fibers Respond to all nociceptive stimuli Sensitivity profiles Transduction – Painful stimulus activate gated ion channels – Depolarization of the cell and generation of action potential  Capsaicin Nociceptive C fibers TRPV1 Influx Na⁺ and Ca²⁺ Analgesia→ desensibilization processes Itch Disagreeable sensation that induces desire or reflex to scratch Usually brief, minor annoyance—can become chronic, debilitating condition Triggered by skin conditions or non-skin disorders Similarities to and differences from pain Signaling molecules and receptors mediating itch not yet identified  Spinal cord Dorsal root Contralateral Synaptic transmission by glutamate and Substance P Dorsolateral tract of Lissauer→ nociceptive nerves branch with second-order neurons – Rexed’s laminae I and II→ C fibers – Rexed’s laminae I and V→ Aδ fibers Multimodal lamina V neurons→ wide-dynamic range neurons (referred pain) Referred pain Wide-dynamic range neuron→ specialized in internal pain Perceived internal pain as a cutaneous pain – Confusing phenomenon – The pain source located at another site – Diagnosis Examples: – Anginal pain→ upper chest wall and the left arm – Gallbladder pain – Esophageal pain  Anterolateral system Sensory-discriminative pathway – Spinothalamic pathway – Location, intensity information – First pain Affective-emotional pathway – Unpleasant felling, fear, anxiety – Second pain  Nociceptive and mechanosensory pathways Both pathways travels in different parts of the spinal cord – Somatosensory→ ipsilateral/ Contralateral – Nociceptive→ contralateral Clinically relevant signs – Spinal cord lesion (determinate level of the lesion) – Dissociated sensory loss Ipsilateral touch and pressure Contralateral pain and temperature  Spinothalamic pain pathway Process the information discriminate aspects of pain 1. Aδ and C axons enters the dorsal column of the spinal cord (ipsilateral) 2. Anterolateral system to medulla (contralateral) 3. Ventral posterior lateral nucleus of the thalamus 4. Primary somatosensory cortex (S1)  Trigeminal pain pathway Process the information discriminate aspects of pain in the face and neck – Trigeminal nerves (VII, IX and X) 1. Trigeminal nerve 2. Enter the pons and descend to medulla (ipsilateral) 3. Spinal thalamic tract – Par interpolaris – Par caudalis 4. Second-order neurons (contralateral) 5. Ventral posterior medial nucleus of the thalamus 6. Primary somatosensory cortex (S1)  Regulation of the pain Pain is highly variable (stimuli and behavioural context) 20% of the population is affected by chronic pain – Sensitization (peripheral and central) – Afferent regulation – Descending regulation – Placebo effect – Endogenous opioids  Peripheral sensitization Primary and secondary hyperalgesia ↓ threshold for pain ↑intensity painful stimuli Damaged tissue can release substance to depolarize the cell (proteases, ATP, bradykinin, lactic acid, histamine…) Peripheral receptors and some neurotransmitters – Can trigger inflammation – Can modulate the excitability of nociceptors Central sensitization Allodynia→ innocuous stimulus induce pain Activity-dependent increase in excitability of neurons Dorsal horn neurons by activity in nociceptors Subthreshold in nociceptive afferent become enough to generate AP Mechanism central sensitization 1. Windup – progressive increase of pain in response to repeated low-painful stimulus – From the summation of the slow synaptic potential 2. LTP-like enhancement of post-synaptic potential – Persistent pain syndromes – Neuropathic pain  Afferent regulation Reduction of pain by simultaneous activity in mechanoreceptors (Aβ fibers) Gate theory of the pain – Melzack and Wall (1960) – Inhibitory interneuron can supress or allow the nociceptive signal  Descending regulation Horrible injuries but not pain feeling Strong emotion, stress or determination can suppress feeling of pain Periaqueductal gray matter – Receive inputs from limbic system – Send outputs to medullary nucleus which are connected with dorsal horn  Placebo effect Expectation effect Clinical trials Inert substance can work as an effective treatment Explanation for alternative treatments Opioid receptors  Endogenous opioids Opioids vs endogenous opioids Widely distributed in the CNS Classification – Endorphins – Enkephalins – Dynorphins Modulate the transmission nociceptive information – Suppressing the release of glutamate – Inhibiting neurons by hyperpolarizing their postsynaptic membranes  Headache Disorders & Neurologic Pain Syndromes Primary headache syndromes 1. Migraine Genetic predisposition (CACNA1A, SCN1A and ATP1A2) and environmental factors Cortical spreading depression Migraine attack divided into 4 phases: – the premonitory phase – the aura phase – the headache phase – the postdrome 2. Tension-Type Headache Most common primary headache disorder in the population Tension-type headache subdivided – infrequent episodic (once per month; 180 days per year) 3. Trigeminal Autonomic Cephalgia Disorders characterized by unilateral headache associated with ipsilateral autonomic features Cluster headache – severe headaches that occur in bouts – usually 0.5 to 8 per day – cause unilateral orbital, supraorbital, and/or temporal pain lasting 15 to 180 minutes – often referred to as “suicide headaches” Neurologic pain syndromes 1. Fibromyalgia chronic widespread musculoskeletal pain and tenderness often a comorbid condition in patients with chronic headaches diagnosis is based on clinical symptoms of widespread pain and neuropsychological symptoms counselling on improvement of quality of life and antidepressants 2. Trigeminal Neuralgia the most common of all neuralgias severe, paroxysmal, sharp and stabbing facial pain without numbness second and third divisions of the trigeminal nerve are more commonly affected 3. Phantom Limb Pain Pain sensation is coming from a body part that no longer exists Tricyclic antidepressants, anticonvulsants and memantine Transcutaneous nerve stimulation, mirror therapy, biofeedback, and cognitive-behavioural therapy Temperature  Thermoreceptors Temperature sensitivity is not uniform across the skin Neurons are sensitive to temperature Varying sensitivities – depend of the ion channel type – Hot receptor→ TRPV1 – Cold receptor→ TRPM8  Temperature pathway Organization of temperature pathway – Identical to pain pathway Cold receptors coupled to Ad and C fibers Hot receptors coupled to C fibers Axons of second-order neurons decussate