Physiology of Tactile Sensation

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1 Physiology of Tactile Sensation Objectives: 1. Describe the general structural features of tactile sensory receptors how are first order nerve fibers specialized to receive tactile stimuli? 2. Understand how tactile stimuli are transformed into electrical signals 3. Describe the generator potential and production of action potentials 4. Describe how stimulus features are encoded in action potential activity. What is the role of action potential frequency and duration; rapidly and slowly adapting receptors? 5. Understand the concept of receptive fields, their significance, and how do they change from 1 to 2 to 3 to cortical neurons in the tactile sensory pathway. 6. Understand the meaning of cortical columns and their significance. Think about it! 1. Overview 1. Tactile stimuli on the skin are transformed into the language of electrical activity 2. Electrical signals follow the 3-neuron pathway to the cortex 3. Electrical signals are modified along the pathway so that they represent features inherent in the stimulus (eg heavy vs light, sharp vs dull, brief vs continuous). Mechanisms include: a. Encoding how a physical stimulus is transformed into the electrical activity of sensory receptors (1 neurons) b. Processing how neurons in the pathway reconstruct the shape of objects that touch the skin. This involves integration of activity by synaptic connections at relays (nuclei) in pathway divergence, convergence, inhibition. 2. First Order Neuron (tactile sensory receptor) A. What are the physical properties of sensory receptors that respond to touch Terminals of DRG fibers in the skin are specialized to respond to touch (deformation) Most terminals (ie skin receptors) have encapsulated endings influence how mechanical energy is transformed into electrical activity. TYPES of receptors: Pacinian corpuscle, Meissner s corpuscle, Merkel disks, Ruffini ending, hair follicle, naked nerve endings, (muscle spindle, golgi tendon organ) Pacinian Corpuscle 2mm x 1mm 1

2 LOCATIONS: skin, subcutaneous tissue, viscera, muscle, cardiovascular sys, bones and joints glabrous (non-hairy) skin contains the greatest variety and density of receptors and is therefore more sensitive compared to hairy skin. Receptor Modality Depth Density Adapt Receptive Field Resolution Meissner s corpuscle motion across skin Superficial 150/ cm 2 RA -Small- Fine <3mm Merkel disks edges, form, texture Superficial SA Finger tip Pacinian corpuscle vibration Deep 15/ cm 2 RA -Large- Coarse>10mm Ruffini ending Skin stretch Deep SA Entire finger/hand hair follicle Hair bending RA Coarse Each of these receptors provides a basic quality of tactile sensation. When the skin is touched, their collective activity encodes all the elements inherent in that stimulus. Multiple types of tactile receptors are necessary for encoding the different features of a stimulus. Other characteristics such as location and branching in the skin and adaptation determine how a receptor responds to a particular stimulus (eg small vs large deformation) B. How do receptors transform touch into electrical activity i) Mechanosensitive ion channels in the nerve terminal open when the nerve membrane is physically deformed. This causes depolarization. The depolarization is called the generator potential. It is a graded potential. The depolarization spreads along the nerve terminal. If the depolarization reaches threshold, it produces action potentials at the first Node of Ranvier where Na + channels are clustered. stimulus 2

3 ii) Sensory receptors encode 4 important features of stimuli in their action potential activity a) MODALITY what activates the receptor Law of Specific Energies: A sensory receptor can be activated only by a specific type of energy/stimulus determined by the receptor s structure and physiology. Tactile receptors respond only to mechanical deformation. The retina responds only to light. Modality information is preserved in the pathway to cortex as axons carry these signals in parallel and from relay to relay. This is the important principle of labeled lines in sensory systems, i.e. specific axons carry specific modalities of sensory information. Different types of tactile receptors respond to pressure, stretch (skin/muscle), vibration, joint position/movement based upon differences in their structure. b) LOCATION on body each receptor has a specific location on the body surface where it responds to stimuli. This is called its receptive field. It is the location on skin where a stimulus affects a neuron s activity. It indicates where cells are analyzing stimuli. RF/Body location is preserved along the route to the cortex by the somatotopic map. RFs of different receptors overlap on skin. The size of the receptive field is determined by the capsule properties (size, skin depth) and branching pattern of the nerve terminal. Superficial receptors (Meissner s, Merkel) have the smallest RFs (2-10 mm for one DRG fiber) and provide the finest spatial information. The deepest receptors (Pacini, Ruffini) have the largest RFs (70-100mm in size) and provide coarse spatial information. Pacinian Corp are ultra sensitive (3nm displacement), but their deep location in skin makes their RF the size of the hand, which easily detects vibrations transmitted to it. Each nerve cell in the tactile pathway has a RF. Spatial Resolution of skin depends on size of RF and receptor density. Spatial resolution is measured by two point discrimination. It refers to our ability to detect fine tactile stimuli on body. Two point discrimination requires a perceptual comparison of tactile information, so it depends on cortical function. Spatial resolution of skin varies over the body. 3

4 c) INTENSITY of stimulus encoded by action potential frequency. Increasing the stimulus intensity causes a larger generator potential and a higher frequency of action potentials. d) DURATION of stimulus the length of time a stimulus touches the skin is encoded by the duration of electrical activity. Receptors differ in their ability to respond temporally to stimuli duration. Most receptors decrease their activity during a sustained stimulus (adaptation). Slowly adapting (SA) receptors respond during the entire duration of contact with skin, providing static information. Rapidly adapting (RA) receptors respond only briefly to sustained contact with skin and at onset and cessation of contact, providing dynamic (on/off) information. Adaptation is determined by the physical/mechanical properties of the capsule that surrounds the nerve terminal. 4

5 3. Second and third order neurons What happens to activity along the pathway? Relay nuclei (Nucleus Gracilis/Cuneatus and VPL) process tactile information: synaptic connections integrate the activity from presynaptic neurons through divergence, convergence and inhibition. Nucleus Gracilis and Cuneatus Divergence increases the number of active neurons Incoming presynaptic 1 axons make connections onto many postsynaptic 2 neurons, increasing the number of neurons transmitting the activity to cortex. Activity in one 1 axon can affect many 2 and 3 neurons. Thus, a tactile stimulus is represented by a population of active neurons, which ensures that the cortex is sufficiently excited by a stimulus. Convergence decreases neurons, decreases spatial resolution 1 axons with neighboring RFs converge onto individual 2 neurons. Thus, the RFs of 2 neurons are larger than RFs of 1 neurons. This reduces the number of 2 neurons necessary to cover areas that aren t as important for tactile information (eg, the back, leg) but it lowers spatial resolution of those areas. For more important areas like the finger tip, there is a higher density of receptors and LESS convergence to preserve spatial resolution. Divergence and convergence increase excitatory activity in the pathway, ensuring that a stimulus on the skin causes a significant increase in activity at the cortex. Inhibition improves resolution Used in areas where spatial detail is important, eg fingers. Overlapping RFs normally would decrease spatial resolution, reducing our ability to discriminate fine tactile detail. To prevent this, relay nuclei contain inhibitory interneurons, which improve the detection of tactile details. Within relay nuclei, 1 axons inhibit the activity of 2 neurons of neighboring RFs. This limits the excitation to a narrower array of axons, ie those most closely associated with the simulated RF. It increases the contrast between neighboring RFs (ie when one is stimulated as opposed to the other) and, therefore, improves 2-point discrimination. In a more general sense, most sensory systems employ this mechanism of lateral inhibition inhibition of neighbors to improve contrast (and therefore detectability) between stimuli. - RF 1-2 inhib Divergence, convergence and inhibition occur onto VPL neurons too. These effects make RFs of neurons at each relay nucleus larger and more complex as synaptic interactions progressively modify how higher order neurons integrate information. However, as activity travels along parallel 3-neuron pathways to the brain, the location and modality information from receptors is preserved by their axonal routes and not intermixed. The properties of divergence, convergence, and inhibition are general features of many sensory systems that enable them to reconstruct stimulus features faithfully. 5

6 4. Cortical neurons What happens when information gets to the cortex? Cortical neurons in the Postcentral gyrus continue to process tactile information in similar ways to previous levels. Along with neighboring cortex, this ultimately results in perception. Receptive fields become more complex as convergence onto cortical neurons combines 1 RFs into more complex shapes. This allows the cortex to analyze important features of the stimulus such as edges, which aids in identifying object shape (round, rectangular, etc), object orientation, and object motion. From Principles of Neural Science, Kandel, Schwartz, The amount of cortex devoted to a region of the body is related to receptor density in that region. This accounts for the larger amounts of cortex devoted to the face, hand, and foot. 6

7 How does the cortex organize all the tactile information that arrives there? Axons from VPL penetrate the cortex and travel vertically to terminate on neurons in layer 4. Axons and dendrites from these neurons spread vertically to make synaptic connections with neurons in other layers. Thus, around each 3 axon is a cylindrical population of neurons that process the activity from that axon. This cylindrical population is called a cortical column. Each column actually contains a small group of 3 axons. All neurons in a column respond pia 0.5mm Modified from Concise Text of Neuroscience, Kingsley, 2 Ed., 2000, Lippincott. to the same type of stimulus in the same region of the body (RF). This type of processing indicates that the cortex analyzes information in a modular manner based on sensory modality and skin location. The pattern of layered columns is repeated in other functional regions of the cortex and it is a fundamental feature by which the brain processes information for normal brain function. Thus, developmental problems that disrupt cortical layering have profound effects on brain connectivity and function leading to seizures, mental retardation, and cognitive deficits. Plasticity Defined broadly as a change in the organization or function of the brain. This can occur over a wide scale from changes in the properties of single nerve cells to recovery of function that occurs when part of the brain is damaged. The somatotopic map of the postcentral gyrus can be modified by experience (use it or lose it principle). Increased use of part of the body can expand the cortical area for that part. Likewise, lack of use, as would occur with trauma, results in decreased cortical representation of that part. Changes in peripheral sensory inputs from regions of the body have been used in experiments to explore cortical plasticity. Initial experiments showed that if a finger was amputated, the corresponding area on the postcentral gyrus had no activity at first. After 7

8 several weeks, the silent area responded to adjacent parts of the intact hand, ie the remaining functional areas spread out to use the cortical area devoid of input (imagine a patient with an amputation). Subsequent experiments showed that similar results were obtained simply by using a local anesthetic to eliminate activity in a nerve temporarily. The somatotopic map changed reversibly while the anesthetic was applied and then after it was removed. These results were caused by rather dramatic changes in sensory information. However, further experiments showed that everyday-types of activity can lead to cortical reorganization. Animals trained to use their fingers in tasks that required lots of practice expanded the cortical somatosensory areas associated with those fingers while the areas for non-practicing fingers decreased. Thus, our brains continuously change structurally and functionally to enable us to accomplish our tasks more effectively. Experience/use is the key to change so that brain function can be shaped by what we do rather than what genetic information can specify. This capability also allows the brain to compensate for injury by reorganizing and remapping damaged regions to new functional areas. The Amazing Property of Perception Perception is the process by which we recognize, organize, and make sense of sensations we receive from stimuli. It occurs at the level of the cortex after 1) encoding of the stimulus, 2) progression/modification of that activity along the pathway, and 3) cortical analysis first at the specific location where a sensory pathway terminates and then at neighboring cortical areas. However, encoding is NOT sufficient for perception. Perception ALSO requires attention to the stimulus, which is limited especially when multiple stimuli are received simultaneously. An engrossing conversation on a cell phone readily leads to lack of perception of objects in the visual world, which is one of the current dangers while driving. Is stimulation necessary for perception? NO! A recent study showed that when subjects imagined visual objects in the complete absence of input, visual cortical areas were activated and the activity was specific to the types of objects the subjects imagined. Similarly, people with schizophrenia and people with seizure disorders have visual, auditory and other sensory experiences (hallucinations) in the absence of external stimuli. These experiences are generated by activity that begins in the cortex. Thus, a more complete understanding of perception involves the idea that what we perceive and what we think is real is actually a combination of stimulusevoked activity and our own internal representations of information about the world. 8

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