Somatic Sensations
Col. Asst.Prof. Dangjai Souvannakitti MD. PhD. Department of Physiology Phramongkutklao College of Medicine
1 Objective : Be able to…
Describe classification of somatic of sensation Pathways of each somatic sensation Describe two point discrimination threshold Describe vibration pathway Describe itching/tickling pathway Describe position senses Describe Temperature (cold/warmth) sensation Describe cortical representation and plasticity
2
Outline Somatic sensation classification Pathways of somatic sensation : Free (or naked or bare) nerve endings/Merkel disk receptors/Meissner’s corpuscles/Ruffini endings/Pacinian corpuscles/Hair end-organ (or hair follicle)/Field receptor Receptive field and Two point discrimination Vibration Itching/tickling Position senses Temperature 3 Cortical representation and plasticity
Somatic Sensations Classified by receptor location : – Exteroceptive sensation (Cutaneous sense) : from skin – Deep sensation : from deep tissues fasciae, muscles, bones (deep pressure, pain, vibration) – Proprioceptive sensation from physical state of body posture – *Visceral sensation from internal organs
* Some text may not include in somatic sensation
4 Classified by stimulus energy: – Mechanoreceptive somatic senses • Tactile sense: touch, pressure, vibration, tickle senses • Position sense – Thermoreceptive senses – warmth & cold – Pain senses – chemical releasing from tissues damage
5 Classified by neurological classification – Epicritic sensation • Fine aspects of touch – encapsulated receptors – Gentle contact & localize touching position (topognosis) – Detection vibration (frequency & amplitude) – Touch spatial detail (e.g. texture, two-point discrimination) – Detection shape of object (stereognosis) – Protopathic sensation • Pain & temperature sense including itch & tickle) – bare nerve ending
6 Tactile Sensations (Touch, Pressure, and Vibration ) Same receptors Touch sensation : Stimulation of tactile receptors underneath skin (superficial) Pressure sensation : Deformation of deep tissue Vibration sensation: Rapidly repetitive sensory signals
7 Tactile Receptors Hairy skin Glabrous skin
Epidermis Epidermal – dermal junction Free nerve endings Ruffini ending Meissner’s corpuscles Dermis Merkel disk receptors Hair receptors *Field receptor Pacinian Peripheral n. bundle corpuscle
8 1. Free nerve endings .Skin and tissues (esp. cornea : only free nerve ending) .small type Ad myelinated fibers (5-30 m/s) or small type C unmyelinated fibers (<1-2 m/s) (tickle&itch) . Detect touch & pressure
9 2. Merkel disk receptors . Superficial layer, large type of myelinated fibers large type Ab (5-12 mm) myelinated fibers (30-70 m/s) . Slow adapt . Continuous touch, compressing strain on skin . localize touch sensation & object texture
10 3. Meissner’s corpuscles
.Superficial layer, Ab myelinated .Fast adapt .Sensitive to object motion & low-frequency vibration .Fingertips, lips (fine sensation) .Adapt เร็ว จึงไวต่อการเคลอื่ นไหวของวตั ถุบนผิวหนัง และไวต่อ
11 4. Ruffini endings .Deeper subcutaneous tissue, Aβ .Slow Adapt .continuous state of deformation of skin in deeper tissues (e.g. heavy & prolonged touch & pressure), stretch of skin, bending of fingernails .joint capsule : degree of joint rotation
12 5. Pacinian corpuscles .Deeper subcutaneous tissue, Aβ .Super fast Adapt .Rapid movement of tissues, rapid indentation of skin, tissue vibration or other rapid changes in mechanical state of tissues .High-frequency vibration
13 6. Hair end-organs (Hair follicles) .Slight movement of any hair, Aβ .Fast adapt : detect motion of object on skin .Touch receptor 7. Field receptors .Fast adapt .Skin stretch/ joint flex .Joint of fingers/elbows 14 Different Size of Receptive Field Merkel disk receptor & Meissner’s corpuscle Small Receptive field . Fine spatial difference (fine tactile discrimination) Ruffini ending &Pacinian corpuscle Large receptive field . Coarse spatial difference, Can’t accurate spatial localization /fine spatial detail
15 Receptive field in different body area Larger receptive field Less mechanoreceptor density – E.g. Body trunk Smaller receptive field More mechanoreceptor density – E.g. distal part of fingers
16 Two-point Discrimination Two-point threshold : Minimal distance by which two touch stimuli must be separated ~ diameter of receptive field in each area (at least one silent nerve fibers) Less two-point threshold More density &Smaller size of receptive field e.g finger 3 mm, back ≥ 65 mm 17 Vibration Tactile receptors
– High-frequency vibration (30-800 cps):
• Pacinian corpuscles (Ab nerve fibers1000 impulses/s) – Low-frequency vibration (2-80 cps):
• Other tactile receptors esp. Meissner’s corpuscles (slower adaptation than pacinian corpuscles)
18 Tickling & Itch Relatively mild stimulation (e.g.gental motion on skin) Specific receptors: rapidly adapting mechanoreceptive free nerve endings in superficial layers of skin very small type C, unmyelinated fibers (slow pain) Scratch reflex : mild surface stimuli remove irritant Repetitive local mechanical stimulation & some chemical agents – Plasma concentration of bile itching – Histamine / Kinins intense itching
19
Position Senses (Proprioceptive senses) 1. Static position sense (or limb-position sense) • conscious perception of orientation of different parts of the body with respect to one another 2. Rate of movement sense (limb-movement sense, kinesthesia, dynamic proprioception)
20 Position sense receptors – Muscle spindle receptor – specialized stretch receptors in muscles – Golgi tendon organs – receptors in tendon that sense contractile force or effort exerted by a group of muscle fibers – Receptors in joint capsules (Pacinian corpusles, Ruffini endings, and receptors (~Golgi tendon organs) flexion/ extension of joints – Stretch-sensitive receptors in skin (Ruffini endings, Merkel disk receptors in hairy skin, field receptor) postural information for controlling lip movements in speech & facial expression 21 Temperature
Naked nerve endings : respond to temp. of subcut. tissues
Thermal receptor :
– cation channel
– Transient Receptor Potential (TRP) protein family
22
Temperature Receptors 2 types : 1. Warmth receptors: .Response to temp > Body temp. (30 - 45 oC) .C fibers 2. Cold receptors: .Response to temp < Body temp. (10 - 38 oC) .Ad & C fibers Cold-sensitive:warmth-sensitive spots ~ (4 -10):1 Adaptation: between 20 - 40 oC o o > 45 C or < 15 C : pain sensation (pain receptors23)
Temperature
Discharge frequencies at different skin temp .
C ooling the skin below the resting level
24 Primary afferent fibers :3 types of for cutaneous senses:
1. large myelinated Ab fibers : mechanical stimulus 2. small myelinated Ad fibers : cold receptor / nociceptor (fast pain) / mechanoreceptor 3. unmyelinated C fibers : pain [slow pain]) / temperature
25 Touch Pain Temp pathway Sensation from Extremities & body trunk)Dorsal root ganglion Fine touch and proprioception : dorsal column-medial lemniscal system Crude touch, temperature and pain: anterolateral system – touch : ventral spinothalamic tract
– pain + temperature : lateral spinothalamic tract 26 Sensation from head &face trigeminal sensory neuron – Fibersdorsal column-medial lemniscal & anterolateral systems in brain stem • Pain & temperature impulses spinal nucleus of the trigeminal nerve • Touch & proprioception the main sensory & mesencephalic nuclei of the trigeminal nerve
27 Cortical representation Specific sensory nuclei of thalamus 2 somatic sensory areas of cortex:
– sensory area I (S-I) – sensory area II (S-II) • representation of body parts is not as complete or detailed as in S-I
28 Size of the cortical receiving area : proportionate to the number of receptors in the part. Stimulation of the various parts of the postcentral gyrus sensations projected to
appropriate parts of the body. 29 Cortical Plasticity
Extensive neuronal connections in brain will complete in early childhood Neural networks : dynamic & continuously modified by experiences e.g. – Little finger was cut cortical represent of neighbor finger – Damaging of cortical represent of little finger nearby somatosensory map will be newly created
30 Questions?
31