1. Neuro physiology Sensory receptors Somatic sensation 1 and 2 References : Guyton and Hall chapters 46, 47, 48 Done by : Dr. Reham Al-Dwairi
2. Types of Sensory Receptors and the Sensory stimuli they detect there are five basic types of sensory receptors: (1) mechanoreceptors: which detect mechanical compression or stretching of the receptor or of tissues adjacent to the receptor. 2) thermoreceptors: detect change in temperature. (3) Nociceptors pain receptors : detect physical and chemical damage to the tissues. ( 4) electromagnetic receptors : detect light in the retina. (5) chemoreceptor's: detect taste, smell, oxygen level in the arterial blood ,ets.
3. Differential sensitivity of receptors: mean that is each type of receptor is highly sensitive to one type of stimulus for which it is designed. What ever the type of stimulus that excites the receptor its immediate effect is to change the membrane electrical potential we call this( receptor potential). Mechanisms of receptor potential: all receptor acts By opening of ion channels either by stretching the receptor membrane, or by chemicals , temperature , light. Maximum amplitude of most sensory receptor potentials is about 100 millivolts.
4. The frequency of repetitive action potentials transmitted from sensory receptors increase approximately in proportion to the increase in receptor potential. This allows the receptor to have an extreme range of response , from very weak to very intense. Adaptation of receptors: All receptor responds at high impulse rate at first and then at a progressively slower rate until finally the rate of action potentials decreased to very few or often to none at all. All mechanoreceptors adapt completely within seconds or more. Baroreceptors need hours or days to do so ( adaptation time for many carotid and aortic Baroreceptors is about 2 days) called nonadapting. Chemoreceptor's and pain receptors never adapt completely
5. Slowly adapting receptors (tonic receptors): detect continuous stimulus strength can continue to transmit information for many hours. Rapidly adapting receptors (rate, movement, or phasic receptors) detect change of stimulus strength , they react rapidly while the change is actually taking place.
6. Nerve fibers come in all size between 0,5-20 micrometers in diameter. The larger the diameter the greater the conducting velocity. The range of conducting velocities is between 0,5 -120m/sec. Types of nerve fibers: Type A large myelinated fibers subdivided into , , and Conduct impulses at fast velocities. Type C fiber small unmyelinated conduct impulses at low velocities
7. Transmission of signals of different intensity in nerve tracts in two ways: 1. Spatial summation: by increasing signal strength using greater numbers of fibers
8. 2. Temporal summation: increasing the strength by increasing the frequency of nerve impulses in each fiber.
9. The central nervous system is composed of 100s-1000s or even millions of neuronal pools, each pool has its own special characteristics of organization that cause it to process signals in its own special way. We should know that discharge of a single exitatory presynaptic terminal almost never causes an action potential in a postsynaptic neuron.
10. We need a large number of input terminals must discharge on the same neuron either simultaneously or in rapid succession to cause excitation. When the stimulus is enough to cause excitation of a neuron called excitatory stimulus or suprathreshold. When the stimulus is not enough to cause excitation in a neuron called subthreshold stimulus and the neuron facilitated neuron.
11. Discharge zone /Excitatory zone / liminal zone. Facilitated zone/ subliminal zone / subthreshold zone
12. Divergance of signals passing through neuronal pools: Two major type Divergence in same tract ( ex. corticospinal pathway in its control to the skeletal muscles). Divergence into multiple tracts ( information transmitted in the dorsal columns of the spinal cord takes two course in the lower part of the brain a) into the cerebellum. b)through the lower regions of the brain in to the thalamus and cerebral cortex.
13. Convergence of signals: Signals from multiple inputs uniting to excite a single neuron this provide enough spatial summation. Ex: 1. peripheral nerve fiber entering the cord. 2. corticospinal fibers from the cerebral cortex. 3. Propriospinal fibers passing from one segment of the cord to another.
14. reciprocal inhibition circuit : Some times an incoming signal to a neuronal pool causes an output excitatory signal going in one direction and at the same time an inhibitory signal going elsewhere. This type of circuits is for controlling the antagonistic pairs of muscles (help in preventing over activity in many parts of the brain).
15. Reverberatory (oscillatory) circuits as a cause of signal prolongation: caused by positive feedback to re-excite the input of the same circuit. all part of the brain is connected either directly or in directly mean that an excitation in one part will re-excite the other and like sothis will lead to continues cycle of re-excitation the nervous system prevent this from happening by two ways: 1. inhibitory circuits. 2. fatigue of synapses. (mean simply that synaptic transmition becomes progressively weaker the more prolonged and more intense the period of excitation.
16. Means of stabilizing the nervous system Automatic short-term adjustment of pathway sensitivity by the fatigue mechanism. Long term changes in synaptic sensitivity caused by automatic downgrading or upgrading of synaptic receptors.
17. Somatic sensation 1:general organization ,the tactile and position senses Somatic senses are the nervous mechanisms that collect sensory information from the body associated with special senses (vision, smell, hearing, taste and equilibrium) Classified into three physiologic types: 1.Mechanoreceptive somatic senses respond to( tactile and position 2.thermoreceptive sense respond to ( cold and heat). 3. pain sense respond to( tissue damage).
18. Other classification of somatic sensation: 1. exteroreceptive sensation on the (surface of the body). 2. proprioceptive sensations to detect (physical state of the body). 3. visceral sensation in ( internal organs). 4. deep sensation in ( fasciae, muscle and bone). .
19. Tactile receptors are six type: 1. free nerve endings, 2. meissners corpuscle . 3. meissners corpuscle with expanded tip tactile receptors. 4. hair end-organ. 5.ruffinis end-organs. 6. pacinian corpuscles
20. Transmission of tactile sensation: All the specialized sensory receptor transmits their signal in type A nerve fiber with velocity from 30-70 m/sec. Free nerve ending tactile receptors transmitted by small type A myelinated fiber velocity of 5-30 m/sec. Some tactile free nerve ending transmitted by C unmyelinated fibers velocity from a fraction of meter up to 2 m/sec. All the different tactile receptor involve in detection of vibration (rapidly repetitive sensory signals. Tickling and itch: Transmitted by very small type C unmyelinated fiber (similar to those transmits aching pain, slow type of pain).
21. Sensory path ways for transmitting somatic signals into the central nervous system: Anterolateral systemDorsal column-medial lemniscal system Small myelinated fiber few meter per second up to 40 m/sec Large myelinated fiber velocity 30-110 m/sec Signals interring the spinal cord from the dorsal spinal root synapse in the dorsal horn of gray matter then cross to the opposite side throw the lateral white column then terminate at all level of the brain stem and in the thalamus Carries signals in the dorsal columns of the cord then signals synapse cross to the opposite side in the medulla then pass up word throw the brain stem to the thalamus
22. Somatosensory cortex: Cerebral cortex divided into 50 areas called brodmanns areas based on histological structural differences. The large central fissure (central sulcus) extend horizontally across the brain, signals from all modalities of sensation terminate in the cerebral cortex posterior to the central fissure and anterior half of the parietal lobe is concerned with reception and interpretation of somatosensory signals. EX.. Visual signals terminate in the occipital lobe. Auditory signals in the temporal lobe.
23. Somatosensory area I has a high degree of localization of the different area of the body, while its poorly localized in somatosensory area II. Each side of the cortex receive sensory information exclusively from the opposite side of the body (with the exception of a very small amount of sensory information from the same side of the face).
24. Bilateral excision of somatosensory area I lead to loss of following types of sensory judgment: 1. The person is unable to localize discretely the different sensation in the different parts of the body. 2. The person is unable to judge critical degree of pressure against the body. 3. The person is unable to judge the weight of objects. 4. The person is unable to judge shapes or forms of objects (astereognosis). 5. The persons is unable to judge texture of materials.
25. Somatosensory association area : Brodmanns area 5 and 7 of the cerebral cortex, play important role in deciphering the sensory information that enter the somatosensory area. Effect of removing the somatosensory associations area (Amorphosynthesis): The person is loses the ability to recognize complex object and complex form by the process of felling them on the opposite side of the body. The person is mainly oblivious to the opposite side of the body-that is forget that its th