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CNS anatomy lec 6
General sensory pathways of the trunk and limbs
| Term | Definition |
|---|---|
| general (common) sensations | includes pain, temp, tactile, vibrations, & proprioception |
| proprioception | the awareness of the position of the body parts, particularly the skeletal parts, through muscle spindles and golgi tendons during passive or active movements |
| special sensations | include vision, hearing, and taste |
| ascending pathway | pathways through which general sensations from receptors in the trunk and limbs enter the spinal cord via the dorsal root and travel upwards to reach the cerebral cortex |
| 1st order neuron- in tactile, vibration & proprioception | a pseudounipolar neuron located in the dorsal root ganglion. divides into peripheral and central processes. the peripheral process terminates as a somatosensory receptor, while the central process enters the spinal cord and synapses w/ 2nd order neuron |
| 2nd order neuron - in tactile, vibration & proprioception | present in the dorsal gray horn (gracilis and cuneatus nuclei). Their axons cross to the opposite side and form a fasciculus that ascends in the contralateral white column, terminating in a synapse w/ 3rd order neurons in the VPL nucleus of the thalamus. |
| 3rd order neuron- in tactile, vibration & proprioception | Neurons present in VPL nucleus of the thalamus. They project fibers that pass through the posterior limb of the internal capsule and corona radiata to terminate in the primary somatosensory area. Considered as projection fibers |
| sensory fibers | Fibers that enter the spinal cord, varying in size and conducting different sensations |
| 1st order neuron- in pain & temp pathway | Neurons with cell bodies in the dorsal root ganglion, terminating peripherally at nociceptors or thermoreceptors |
| 2nd order neuron- in pain & temp pathway | Neurons present in the dorsal gray column, crossing to the opposite side and ascending in the contralateral white column |
| lateral spinothalamic tract | Pathway followed by the second-order neurons in pain sensory pathway, ascending through the spinal cord and continuing through the medulla and pons |
| spinal lemniscus | lateral & ant spinothalamic tracts in addition to spinotectal tract continue to ascend in the post part of pons & tegmentum of medulla oblongata |
| 3rd order neuron- in pain & temp pathway | Neurons present in the VPL nucleus of the thalamus, projecting fibers to the primary somatosensory area in the postcentral gyrus |
| fast pain | Acute, sharp, and pricking pain that is experienced shortly after the stimulus (within 0.1 sec). Impulses travel directly to the VPL nucleus of the thalamus |
| delta A fibers | large, myelinated fibers that conduct fast pain |
| postcentral gyrus (role in fast pain impulses) | interpretation of pain in relation to past experiences |
| cingulate gyrus (role in fast pain impulses) | interpreting the emotional aspect of pain |
| insular gyrus (role in fast pain impulses) | interprets pain stimuli from the internal organs and triggers an autonomic response |
| slow pain | Burning, aching, and throbbing pain that is experienced later after the pain stimulus (1 sec or later). impulses terminate in the reticular formation |
| C fibers | small, unmyelinated, fibers that conduct slow pain |
| reticular formation (role in slow pain impulses) | alerts the entire nervous system to the throbbing and aching pain |
| temperature sensation | conducted by both delta A and C fibers |
| What is the Gating Theory of pain modulation? | suggests that at the level of the dorsal gray horn, inhibition of pain can occur through the activation of connector neurons by large, myelinated afferent fibers (A beta fibers) carrying nonpainful touch and pressure information |
| How do tactile fibers contribute to pain modulation according to the Gating Theory? | enter the spinal cord and give collateral branches into the dorsal gray horn. These collateral branches stimulate connector neurons that inhibit the second-order neuron of pain, contributing to pain modulation |
| How does excess tactile stimulation affect pain according to the Gating Theory? | It inhibits the transmission of pain signals. However, when nonpainful tactile stimulation ceases, the "gate of pain is open," allowing information on painful stimuli to ascend the lateral spinothalamic tract |
| What are some clinical implications of the Gating Theory? | massage, acupuncture, and transcutaneous electrical nerve stimulation (TENS) as modalities that can modulate pain |
| What is the Analgesia System in pain modulation? | an endogenous pain modulation system involving the periventricular area of the diencephalon and periaqueductal gray matter of the midbrain |
| Which brain areas are involved in the Analgesia System? | periventricular area of the diencephalon and periaqueductal gray matter of the midbrain |
| How does the Analgesia System modulate pain? | projects fibers to reach the nuclei of reticular formation (ex: nucleus raphe magnus). serotenergic neurons at these nuclei release serotonin that descends to the dorsal gray horn. neurons are then stimulated to release enkephalins |
| What is a clinical implication of the Analgesia System? | the use of surgically implanted electrodes in the areas of the analgesia system. Patients can stimulate these electrodes to relieve pain |
| What type of sensations are poorly localized with little discrimination of intensity? | Light touch and pressure sensations |
| Describe the characteristics of first-order neurons involved in light touch and pressure sensations | They are pseudounipolar neurons with a peripheral process terminating as receptors for light touch and pressure, and their central process enters the spinal cord from the posterior root ganglion |
| Where do fibers of the first-order neurons terminate in light touch pathway? | They terminate by synapsing with second-order neurons in the substantia gelatinosa in the posterior gray column |
| Where do fibers of the anterior spinothalamic tract in light touch pathway terminate? | They terminate by synapsing with the third-order neuron in the VPL (ventral posterolateral) nucleus of the thalamus |
| Where do axons of the third-order neurons pass through to reach the cerebral cortex? | They pass through the posterior limb of the internal capsule and corona radiata. |
| What happens in the thalamus regarding light touch and pressure sensations? | Crude sensations are appreciated, and an emotional response is initiated |
| What is discriminative (fine) touch? | It is the awareness of being touched, and the intensity of touch is precisely localized |
| What are the three phenomena of discriminative touch? | 1. Two-point discrimination 2. Stereognosis 3. Graphesthesia |
| Define two-point discrimination | It is the awareness of being touched at two different points in the body simultaneously, even if they are close together |
| What is stereognosis? | It is the ability to recognize an object based on tactile information alone, without auditory or visual input |
| What is graphesthesia? | It is the recognition of letters and numbers drawn on the skin |
| Where are the cell bodies of first-order neurons in descrimitive touch pathway located? | they are located in the dorsal root ganglia |
| what are the types of somatosensory receptors associated with 1st order neurons in descrimitive touch pathway? | 1. missner corpuscle: fine touch 2. pacinian corpuscle: pressure 3. muscle and golgi tendons: proprioception |
| what are the two ascending fasciculi in the dorsal white column of spinal cord? | gracile and cuneate fasciculus |
| fasciculus gracile | contains fibers of the sacral, lumbar, and lower 6 thoracic spinal nerves |
| fasciculus cuneate | contain fibers of the upper thoracic and cervical spinal nerves |
| where are the 2nd order neurons in descrimitive touch pathway? | gracile and cuneate nuclei of medulla oblongata |
| What is the pathway of the second-order neurons in descrimitive touch pathway? | They synapse in the Gracile and Cuneate nuclei, give rise to internal arcuate fibers, and then ascend contralaterally in the medial lemniscus |
| Where do the third-order neurons synapse in descrimitive touch pathway? | They synapse in the VPL (ventral posterolateral) nucleus of the thalamus |
| What is interpreted in the postcentral gyrus? | The quality of the sensation, precise location, intensity, sharpness, etc. |
| What is a homunculus? | It is a representation of the body in an upside-down fashion in the primary somatosensory area based on neurological mapping |
| How is the representation in the homunculus? | The representation is inverted, contralateral, and disproportionate |
| How are the face and arms represented in the homunculus? | They are represented ventrally |
| How are the legs and feet represented in the homunculus? | They are represented more dorsally |
Created by:
dima.saadeh03
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