The Nervous System, Somatic Sensations, the tactile and Position senses

1. General Organisation; the tactile and Position senses
1. The somatic senses can be divided into three main components
1. Mechanoreceptors 
2. Thermoreceptors
3. Nociception - pain and damage
2. discriminative - precisely localised touch 
3. Crude - poorly localised - proprioception -
1. touch, pressure, vibration and the senses of static body position and movement
4. Exterioceptive sensation 
5. Visceral sensations
2. Detection and Transmission of Tactile Sensation
1. Even though touch, pressure and vibration are often classified as separate and distinct sensations, they are each detected by the same general class of tactile receptors: the Mechanoreceptors
1. Free nerve endings - skin and in the cornea of the eye
2. Meissner's corpuscule - 
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2. The tactile corpuscles of Wagner and Meissner (Fig. 936) are oval-shaped bodies. These tactile corpuscles occur in the papillæ of the corium of the hand and foot, the front of the forearm, the skin of the lips, the mucous membrane of the tip of the tongue, the palpebral conjunctiva, and the skin of the mammary papilla.



FIG. 936– Papilla of the hand, treated with acetic acid. Magnified 350 times. A. Side view of a papilla of the hand. a. Cortical layer. b. Tactile corpuscle. c. Small nerve of the papilla, with neurolemma. d. Its two nervous fibers running with spiral coils around the tactile corpuscle. e. Apparent termination of one of these fibers. B. A tactile papilla seen from above so as to show its transverse section. a. Cortical layer. b. Nerve fiber. c. Outer layer of the tactile body, with nuclei. d. Clear interior substance.
3. Merkel's Discs - expanded tip receptors - found in both glabrous (non-hairy) and hairy skin - continuous touch
4. Hair end-organs - detect movement of objects across the skin
5. Ruffini's end-organs - 



Nerve ending of Ruffini. (After A. Ruffini.)
Corpuscles of Ruffini.—Ruffini described a special variety of nerve-ending in the subcutaneous tissue of the human finger (Fig. 937); they are principally situated at the junction of the corium with the subcutaneous tissue. They are oval in shape, and consist of strong connective-tissue sheaths, inside which the nerve fibers divide into numerous branches, which show varicosities and end in small free knobs.

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6. Pacinian Corpuscles



Pacinian corpuscle, with its system of capsules and central cavity. a. Arterial twig, ending in capillaries, which form loops in some of the intercapsular spaces, and one penetrates to the central capsule. b. The fibrous tissue of the stalk. n. Nerve tube advancing to the central capsule, there losing its white matter, and stretching along the axis to the opposite end, where it ends by a tuberculated enlargement.
3. Somatosensory Pathways in the Central Nervous System
1. The main pathways for the transmission of somatosensory signals are the dorsal column - medial lemniscal system and the anterolateral system
2. Enter through the Dorsal Roots
3. One of two pathways
1. Thermoreceptors and Nociceptors - anteriolateral system
2. Mechanoreceptors - dorsal column-medial lemniscal (DC-ML)
4. Transmission in the Dorsal Column-Medial Lemniscal System
1. The anatomy of the dorsal column-medial lemniscal system is characterised by high degree of somatotopic (spacial) organisation as follows:
1. Primary Sensory Neurons
1. Discriminative touch, vibration, and proprioception - sensation via myelinated fibers - enter the spinal root canal.
2. Many simply pass into the dorsal column area and ascend without synapsing until they reach the dorsal column nuclei in the caudal Medulla. 
3. Collateral branches of these axons synapse locally with interneurons - to mediate reflexes stretch reflex
4. The fibers from the lower extremities (Sacral and Lumbar) synapse in the nucleus gracilus medially, whereas those from the upper extremity (thoracic and Cervical) terminate in the nucleus cuneatus laterally. So from Medial to Lateral, The Lumbar, sacral, Thoracic, and Cervical Axons group by location and function. 
2. Dorsal Column Nuclei
1. Axons from the cuneate and gracile form the medial leminiscus, which cross the midline in the caudal medulla as the sensory decussation. 
2. This fiber bundle continues rostrally to the to the thalamus, where the axons terminate in the ventrobasal complex, mainly in the ventroposterior lateral  nucleus (VPL). Axons of the VPL then enter the posterior limb of the internal capsule and project to the primary somatosensory cortex (SI) in the post central gyrus.
3. The Medial Lemniscal Pathway
1. Fibers from the lower extremity pass upward through the medial portion of the dorsal column, terminate in the gracile nucleus, and form the ventral and lateral portion of the medial lamniscus. They eventually terminate laterally in the VPL; neurons here project to the most medial part of the SI, on the medial wall of the hemisphere.
2. Information for upper extremity travels in the lateral part of the dorsal column, terminated in the cuniate nucleus, and enters the dorsal and medial portion of the medial lemniscus. These fibers synapse in the medial part of the VPL,  and finally they reach the arm territory of the SI in the hemisphere contralateral to the body surface, where the signal actually originate.
3. Throughout the system there is a point-to-point relationship between the origin in the peripheries and the termination in the SI.
4. Somatosensory Signals from the face
1. Tactile somatosensory signals from the face travel in the trigeminal nerve and enter the brain stem at the midpontine levels, where the primary sensory fibers terminate in the principle trigeminal sensory nucleus. 
2. From here the axons cross the midline and course rostrally, adjacent to the medial lemniscus, and eventually terminate medially in a portion of the ventrobasal complex, the Ventral Posteromedial (VPM) nucleus. 
3. This system of fibers is comparable to the DC-ML system and conveys similar types of somatosensory information from the face.
5. Somatosensory Areas of the Cerebral Cortex
1. The postcentral gyrus comprises of the primary somatosensory cortex, which corresponds to Brodmann's areas 3,1 and 2. 
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3. A second somatosensory area (SII) is much smaller than SI and is located just posterior to the face region of SI bordering on the lateral fissure. 
4. Within SI the segregation of the body is maintained such that face region is ventrally located nearest to to the lateral fissure, the upper extremity continues medially and dorsally from the face region and extends from the convexity of the hemisphere, and the lower extremity projects onto the medial surface of the hemisphere.
5. In fact, there is a complete but separate body representation in area 3,1, and 2. 
6. Within each of these body representations, there is an unequal volume of cortex devoted to each body part. 
7. These body surfaces with a high density of sensory receptors are represented by a larger areas in the cortex than those with relatively low density of receptors.
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6. Functional Anatomy of the Primary Somatosensory Cortex
1. Contains 6 different horizontally arranged cellular layers numbered I to VI beginning with layer one at the cortical surface
1. The most characteristic is layer IV 
1. receives projections from VPL and VPM of the ventrobasal thalamus. 
2. From here it spreads dorsally to layers I to III and ventrally to layers V and VI
2. Vertically organised columns of neurons that extend through all 6 layers. These are functionally determined columns that vary in width from 0.3 to 0.5 millimeters and contain 100,000 neurons each. 
3. The most anterior part of area 3 of the SI, the vertical columns process muscle afferents, where the posterior part of area 3 they process cutaneous input.
4. Area 2, the vertical columns process pressure and proprioception.
5. The functions of the primary and association somatosensory areas can be inferred from studies of patients with lesions in these areas as follows:
1. primary somatosensory cortex
1. inability to precisely localise cutaneous stimuli on the body surface; some crude localizing ability may be retained
2. Inability to judge degrees of pressure, or weight of objects touching the skin; and
3. the inability to identify objects by touch to texture (astereognosis)
2. association cortex for somatic sensation - 
1. Lesions that involve Brodmann's areas 5 and 7 will damage the association cortex for somatic sensation. 
2. inability to recognise objects that have a relatively complex shape or texture when palpated with the contralateral hand; and
3. loss of awareness of the contralateral side of the body - hemineglect. This symptom is most acute with lesions in the non-dominant parietal lobe; and
4. when feeling an object, patients will only explore the side that is ipsilateral to their lesion and ignore the contralateral side (amorphosynthesis). 
2. Characteristics of Signal Transmission and Processing in the Dorsal Column-Medial Lemniscal System
1. receptive field
1. The receptive field of the SI cortical neuron is determined by the combination of the primary sensory nerons, dorsal column nuclear neurons, and thalamic neurons that provide afferent projections to that SI neuron.
2. Two-point discrimination is used to test the DC-ML system
1. Used to determine the patients ability to distinguish two simultaneously applied cutaneous stimuli as two separate points - two point discrimination  
2. Two-point discrimination varies:
1. Finger tips and lips - can distinguish 1mm to 2 mm as separate points
2. back - 30 to 70 mm
3. This function depends on the central processing elements in the DC-ML pathway to recognise that the two excitatory signals generated peripherally are separate and non-overlapping.
3. Lateral inhibition is a mechanism that is used throughout the nervous system to "sharpen" signal transmission
1. Inhibition of input from the peripheral portion of a specific receptive field to better define the boundaries of the excited zone. In the DC-LM system, lateral inhibition occurs at the level of the dosal column nuclei and in the thalamic nuclei.  
4. The DC-ML system is particularly effective in sensing rapidly changing and repetitive stimuli, and this is the basis of vibratory sensation.
1. This capability resides in the rapidly adapting pacinian corpuscules, which are able to detect vibrations upto 700 Hz, and in the Meissener's corpuscules, 200Hz
5. The awareness of body position or body movement is called proprioceptive sensation
1. Kinesthetic Sense or dynamic proprioception
2. Combines tactile, muscle, and joint capsule receptors used to produce proprioception. 
3. Fingers rely on tactile receptors and the limbs on muscle spindles
3. Transmission of Poorly Localised Tactile Input: Anterolateral System 
1. Signals travelling on small myelinated fibers and unmyelinated C fibers can arise from tactile receptors (free nerve endings in the skin). This information is transmitted with pain and temperature signals in the anterolateral portion of the spinal cord white matter. 
2. The anterolateral system extends to the ventrobasal thalamus as well as to the intralaminar and posterior thelamic nuclei. 
3. Diffuseness, lacks the point-to-point discrimination of the DC-ML, account for the less effective localising ability. 

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TRANSMISSION IN THE DORSAL