The Somatosensory System: Transduction and Pathways

2+3 Subject Physiology Done by Correction … Doctor The Somatosensory System The somatosensory system is a complex system of sensory neurons and neural pathways that responds to changes at the surface or inside the body. Recall: the nervous tissue is made up of grey matter (cell bodies) and white matter (axons). Types of sensations 1- General sensations: somatic. 2- Special sensations: taste, smell, vision, etc. Sensation receptors - Sensory receptors are activated by different stimuli, transforming it into signals which are transmitted along axons (afferent neurons) to the spinal cord while being processed. Signals end up in the brain for further processing. - The receptors only identify the signal, but the response depends more on the processing throughout the pathway. Thus, one receptor can give 2 different sensations depending on the pathway of processing. 1|Page - Types of sensory receptors: 12345- Mechanoreceptors: detect deformation. Thermoreceptors: detect changes in temperature. Nociceptors: detect damage (pain receptors). Electromagnetic: detect light. Chemoreceptors: detect taste and smell. - Somatic sensations are divided mainly into: 1- Mechanoreceptive: stimulated by mechanical displacement. a- Tactile: • Touch • Pressure • Vibration • Tickle and itch b- Position or proprioceptive: • Static position • Rate of change 2- Thermoreceptive: detect heat and cold. 3- Nociceptive: detect pain and are activated by any factor that damages tissue by an exaggerated amount of energy. Excitation of sensory receptors - Receptor potential: it is a type of graded potential produced by activation of a sensory receptor and is proportionally graded according to the stimulus intensity. When the receptor potential rises above the threshold, action potentials appear. The greater the intensity of the stimulus, the greater the receptor potential, and the greater the amount of generated action potentials is.  As seen in this figure, when the membrane receptor potential increased above the threshold, action potential was generated. The frequency of action potentials increased in proportion with the increased intensity of the stimulus. 2|Page Extra: Somatosensory pathway - Information is received through receptors inside or at the surface of the body. This information, about pain, temperature, touch, position, and vibration, are processed throughout the somatosensory pathway.  Signals are transmitted by axons (white matter) and processed in cell bodies (ganglia / nucleus). - Sensory pathways consist of a chain of neurons, from the receptor reaching the brain.  The number of neurons making up a pathway solely depends on the complexity of the signal; i.e. as the complexity of the signal increases, the number of neurons in a pathway increase. Types of sensory pathways The main three pathways are: 1- Linear: allows one neuron to relay information to the following one in a linear manner. This pathway causes the signals to be more precise. 2- Convergence: allows a neuron to receive input from many neurons. In this pathway, the summation of inputs facilitates detecting low sensations, however, it decreases preciseness and discrimination of signals. 3- Divergence: allows one neuron to communicate with many other neurons. Sensation destination 1- Cortex ‘mainly’: perception and consciousness. 2- Unconscious involuntary reactions, e.g. unconsciously, we react to the smell, sight and even thought of food with increased secretion of saliva. 3- Subcortex / spinal cord: reflexes. 3|Page Cortical Representation - A cortical map is a representation of the human body within the brain, based on the nerves dedicated to processing motor functions, or sensory functions, for different parts of the body. - Nerve fibers conducting somatosensory information from all over the body, terminate in an area of the parietal lobe in the cerebral cortex, which is known as the primary somatosensory area (physiologically) or the postcentral gyrus (anatomically). Note: gyrus is the term used for the prominent raise or outward fold in the brain. On the other hand, the sulcus is the depression or the shallow groove surrounding the gyrus. - Brodmann, a German anatomist, divided the areas of the cerebral cortex based on histological features and numbered them. Brodmann areas 1, 2 and 3 comprise the primary somatosensory cortex of the human brain. 4|Page The relationship between cortical representation and receptive fields: The receptive field is a portion of sensory space that can elicit neuronal responses when stimulated. The greater the density of sensory receptors on a body part, the smaller the required receptive field is. This relationship allows better discrimination in sensory inputs. In the cortex, the smaller the receptive area, the larger its represented cortical area is and vice versa.  For example: the hand has a high density of receptors, thus its receptive area is small while its cortical area representation is large as seen in the picture above. ---------------------------------------------------------------------------------------- 5|Page Sensation Modalities - Stimulation of a receptor usually produces one sensation; receptors are modalityspecific. An exception is the free nerve ending sensory receptors, in which they can sense more than one sensory modality. - There are 2 types of sensation modalities: 1- Fast → Sensations are transmitted through the ‘Posterior column-medial lemniscus’. It detects: a- Proprioception: the ability to know where and how your body’s orientation. b- Fine touch and pressure (two-point discrimination). c- Vibration 2- Slow → Sensations are transmitted through the ‘Anterolateral system’ (also known as the spinothalamic pathway). It detects: a- Temperature b- Crude touch and pressure (itching) c- Pain Note: Fine touch (or discriminative touch) allows a subject to sense and localize touch. It permits ‘Two-point discrimination’, which is the ability to distinguish that two nearby objects touching the skin are truly two distinct points, not one. Crude touch (or non-discriminative touch) allows the subject to sense that something has touched them, without being able to localize where they were touched. - Labeled line theory: it suggests that each sensory modality is processed throughout a fixed, direct-line communication system from the skin to the brain.  In conclusion: Fast modalities are transmitted by the Posterior column-medial lemniscus pathway. Whereas slow modalities are transmitted by the Anterolateral pathway. 6|Page Now, we will discuss major ascending (from the body to the brain) sensory pathway in details: 1- Posterior column-medial lemniscus (PCML) This pathway conveys sensations of fine touch, two-point discrimination, vibration, and proprioception. It transmits information from the body to the primary somatosensory cortex in the postcentral gyrus of the parietal lobe of the brain. 1- First-order neurons ‘blue’: The first-order neurons are pseudounipolar sensory neurons that pass through the dorsal root ganglia and send their afferent fibers through the two posterior columns / tracts of white matter: the gracile fasciculus (lower body) and the cuneate fasciculus (upper body). 3 Note: The gracile fasciculus is present throughout the length of the spinal cord and contains afferents from the lower trunk (below the T6). The cuneate fasciculus is present laterally to the gracile fasciculus only in the upper thoracic and cervical spinal cord segments (above T6) and contains afferents from the upper trunk. 2 2- Second-order neurons ‘red’: The first-order axons synapse with secondorder neurons in the medulla. First-order axons of the gracile tract and the cuneate tract synapse with the posterior nucleigracile nucleus and the cuneate nucleusin the medulla respectively (these nuclei contain the cell bodies of the second-order neurons). 1 The axons of second-order neurons that leave the gracile and cuneate nuclei of the medulla are called ‘internal arcuate fibers’. Upon crossing (decussation) from one side of the medulla to the other, they ascend as ‘medial lemniscus’, i.e. the medial lemniscus is a large ascending bundle of secondary-order axons that decussated in the medulla. 7|Page Note: processing in the PCML does not occur along the spinal cord, it occurs starting from the medulla where synapse between first and second-order neurons happens. 3- Third-order neurons ‘green’: The second-order neurons bundle of axons (medial lemniscus) synapse with the thirdorder neurons in the thalamus (ventral posterolateral nucleus) where further processing occurs. The third-order neurons fibers ascend to the ‘primary somatosensory area’. Note: the thalamus is responsible for processing most of the somatosensory signals.  Summary of the PCML pathway: The pathway receives information from sensory receptors throughout the body, and carries this in nerve tracts in the white matter of the dorsal columns of the spinal cord to the medulla, where it is continued as the medial lemniscus, on to the thalamus and from there to the primary somatosensory area. PCML Function As mentioned, PCML conveys sensations of two-point discrimination, vibration, and proprioception. Thus, it is important for: 1- Stereognosis: the ability to perceive and recognize the form of an object in the absence of visual and auditory information. 2- Graphesthesia: the ability to recognize writings on the skin by the sensation of touch. 3- Movement and weight recognition. PCML lesion associated symptoms ‘lesions in the posterior columns’ \ Dysfunction of the PCML pathways causes loss of two-point discrimination, vibration, and proprioception sensations. This causes: 1- Astereognosis. 2- Agraphesthesia. 3- Abarognosis: loss of the ability to detect the weight of an object held in the hand or to tell the difference in weight between two objects, i.e. loss of the ability to sense weight. 4- Sensory Ataxia: it is a form of ataxia (loss of coordination) caused not by cerebellar dysfunction but by loss of sensory input into the control of movement. Note: Ataxia is the lack of voluntary coordination of muscle movements that can cause gait abnormality, speech changes, and abnormalities in eye movements. 8|Page 2- Anterolateral system (Spinothalamic tract) This pathway conveys sensations of pain, temperature, and crude touch. The spinothalamic tract, like the dorsal column medial lemniscus pathway, uses three neurons to convey sensory information from the periphery to conscious level at the cerebral cortex: 1- First-order pseudounipolar neurons located within the dorsal root ganglion extend from the skin to the posterior gray horn of the spinal cord. 2- First-order neurons synapse with the nuclei of the second-order neurons in the grey matter of the spinal cord. 3- The axons of the secondary neurons cross over (decussate) to the other side in the spinal cord. The axons travel up the length of the spinal cord through the spinothalamic tract (which is present in the ventral column) into the medulla. Note: the PCML pathway fibers ascend in the spinal cord through the posterior column (gracile and cuneate tracts), whereas in the anterolateral system, they ascend through the anterior column (spinothalamic tract). 4- Traveling up the medulla, the tract moves dorsally. The neurons ultimately synapse with third-order neurons in the thalamus. From there, signals go to the primary somatosensory cortex.  Notice: the second-order neurons in both pathways, PCML and the anterolateral, are where crossing-over (decussation) and processing happen. a- PCML pathway: decussation of 2nd order neurons occur in the lower medulla. b- Anterolateral pathway: decussation of 2nd order neurons occurs in the spinal cord. 9|Page Somatotopic Arrangement - Somatotopy is the point-for-point correspondence of an area of the body to a specific point on the central nervous system. 1- In the spinal cord: In the PCML pathway, as we ascend in the spinal cord, fibers are added laterally. Thus, as seen in the picture, the lower extremities are medially represented then fibers are added laterally as we ascend to the upper extremities.  Medial to lateral: Legs → Trunk → Arms → Neck As for the Anterolateral pathway, it is the opposite; fibers are added medially. Somatosensory cortex 2- Subcortex: ‘this is for the PCML pathway only, the picture to the right represents the PCML pathway, check slide 34 for the anterolateral system’ Position of ML At first, the medial lemniscus in the lower part of the brainstem is oriented in which the fibers from the lower body ‘red’ are anterior, whereas the upper body ‘blue’ is posterior. Then, the medial lemniscus rotates where the fibers of the lower body become lateral, whereas the upper body becomes medial. Finally, in the upper part of the brain stem, the medial lemniscus further rotates. The lower body fibers end up being posterolateral and the upper body remains medial. ML ML Key: ML: medial lemniscus L: lower body ‘leg’ T: trunk A: upper body ‘arm’ Note: in the anterolateral system the fibers ascend directly with the upper body being medially, whereas the lower body is lateral; similar to the spinal cord. 3- In the cortex: ‘in both pathways’ Somatosensory information that arrives at the thalamus is brought to the primary somatosensory cortex. In the cortex, similar to the spinal cord, the information is somatotopically organized with the upper extremities (e.g. face) also being most lateral, whereas the lower extremities (e.g. legs) are most medial. 10 | P a g e  Being able to visualize the somatotropic organization of the body is important clinically to distinguish what would an injury to the CNS cause. For example, if there is a medial injury in the medial lemniscus of the upper brain stem, then the loss of sensations will affect the upper body. Clinical Aspects 1- Injury in the spinal cord: - Recall that, decussation of fibers in the PCML pathways occurs in the medulla, whereas decussation in the anterolateral pathway occurs almost at the level of sensation in the spinal cord. Therefore ↓ :  Injury to the posterior and anterior columns (in the spinal cord) on one side results in: a- Loss of PCML sensations (proprioception, discriminative touch, and vibration) below the level of the lesion on the same side. b- Loss of Anterolateral sensations (pain, temperature, and crude touch) below the level of the lesion on the opposite side. - Recall that, in the posterior columns, the gracile fasciculus is present throughout the length of the spinal cord and contains afferents from the lower trunk (below the T6), while the cuneate fasciculus is present laterally to the gracile fasciculus only in the upper thoracic and cervical spinal cord segments (above T6) and contains afferents from the upper trunk.  Therefore, when the injury of the spinal cord is at the cervical level, consequently, both the upper and lower trunks PCML sensations will be affected. Whereas if the injury is at the lumbar level, only the lower trunk will be affected sparing the upper trunk. Example 1: Injury of a lumbar segment on the right side: ➔ affect lower trunk only: loss of PCML sensations on the right side and loss of anterolateral sensations on the left side. Example 2: Injury to the upper cervical segment on the right side: ➔ affect lower and upper trunk: loss of PCML sensations on the right side and loss of anterolateral sensations on the left side. 11 | P a g e 2- Injury in the brainstem (medulla) or higher: - If a lesion occurs in the brainstem (medulla) or higher, the patient presents with loss of both, the anterolateral and PCML pathways, sensations on the opposite side; since the decussation in both pathways already occurred. 12 | P a g e