Thursday, September 23, 2010

9.8a Neurology of Sensation and Perception

Updated 16 May 2019

Sensation and Perception is what we as humans experience when we see and perceive, hear and listen, taste and distinguish, and feel and react to external stimuli, which can be in the outer environment of our everyday lives or one of the inner sensations from the tissues of our bodies.
   First an example of sensation and perception: A reader should quickly glance at the below Figure and make an immediate judgment what she or he thinks it shows. 

Of course what one sees is the picture: That is sensation. But what is it? A Vase? Or 2 profiles of facing faces? Or both? That is perception. And note the use of the verb-pairs: see and perceive to indicate the difference between sensation and perception; similarly hear and listen, taste and distinguish, sniff an odor and smell a thing (perfume, etc.), feel and react appropriately to the feel. (Withdraw from pain)
   Sensation precedes perception and comes, bottom up to one from the environment through one's sensory nerves. Perception is an effect of the central nervous system, mostly the cerebral cortex sensory areas. It comes to us top-down, from the highest levels in our minds.
   First, sensory nerves and sensation. Keep in mind that sensory means all the outside and inside body stimuli based on physical change, like temperature, pressure, tearing tissue, crushing, electromagnetic waves such as light, vibrations in air, etc. And on and in the skin are many varied specialized sensory receptor bodies made up of cells specialized to sense a change. In the case of sensory systems like vision or hearing, these have developed special organs - the eyes, the ears. On the skin (and also in internal tissues) there are a variety of micro sensory receptor bodies. They are various but each sensation from the environment - temperature change, damaging tear of tissue, pinprick damage, vibration, light-touch, etc. - has a specific sensory end-body - a specialized cell. One type of end-body, for example in its response to potentially damaging external heat, is set to discharge an electric current when it contacts a temperature above 42 degrees Celsius (c.108 Fahrenheit; you can note that 42 to 45 degrees C is the maximum temperature for a hot bath).
   The electric current that discharges from a skin cell in response to a sensory stimulus is called a generator potential because it generates the current from the stimulus. It is analog, i.e., smooth increase or decrease. Once it reaches a certain high level, it discharges an electric signal that becomes an action potential, AP; a digital signal, moving up in the sensory nerve fiber toward the central nervous system. This AP moves, signaling, for example, excess skin-touch heat. The sensory fibers come together as part of a main sensory nerve and go into the spinal cord at the skin-touch body level. The ganglia are groups of similar sensory neuron cells that have grown out the sensory fibers to the skin and that receive the first signal of, in this example, the sensation of excess heat. Right and left pairs of these sensory ganglia are located at each of the 31 pairs of spinal cord segments, taking care of each horizontal level of the body from the toes to the neck; and in the head, at cranial nerve sensory ganglia. The sensory ganglia are located in the spinal cord dorsally (Toward the body's back) in a thickened bulb just outside the spinal cord, so they are called dorsal root ganglia. 
   Below you see, a sensory neuron from a dorsal root ganglion. The neuron as you may see below sends out a single fiber that makes a T division into 2 horizontal fibers, the T horizontal fiber (on your left in the Figure) transmitting a stimulus from the periphery and the other horizontal fiber (On your right in the Figure) transmitting the signal into the spinal cord on its way up to thalamus and cerebral cortex. This avoids the need for a connecting synapse to pass the signal; it gives a more direct feeling and also is faster transmitting, an important factor for survival in the face of noxious stimuli.

Labeled Lines and Topography of Sensory Neurons  Each sensation has its own neural wiring system. So the nerve endings for excess heat to the skin transmit to nerve fibers that run to dorsal spinal root neurons that are solely used to transmit the feeling of a specific temperature excess heat; and this separation continues up the spinal cord into the thalamus. Each system of sensation - temperature, touch, sharp pain, tearing pain, vibration, etc - has its own parallel transmission all through much of the nervous system. Furthermore, the feeling of a too hot object touching your left hand index finger (as one example) can be got just as well by putting an electric current into the nerve fiber or into its spinal cord transmitting neuron, to stimulate a neuron in the thalamus that is part of this system. This explains that our feeling of a sensation is not intrinsic in the cause of the sensation (i.e., the hot object touching the skin) but rather is a construct most likely of one's thalamus. This goes for every type of sensation except smell, which is uniquely not routed through the Thalamus..
   Second, not only the sensation but its location, so called topography is separated out in the a levels of the Central Nervous System. So each type of sensation at every CNS level is constructed into a map of its skin locations that locates its point of stimulus.
   This gives an idea of what is called labeled lines in the CNS - the parallel running of particular types of neural impulses.
The Thalamus and Cerebral Cortex in Perception  What we perceive from a sensation is based on the sensation getting connected, starting with the thalamus and then into the cerebral cortex, with our previous experiences as part of memories of facts and feelings. In this way we connect up what we see or feel, with what we perceive in the image or the feeling. If, for example, you run a business that produces glass vases for flowers or drinks, you might first of all perceive the sensation of the image at the start of this chapter as a vase. But if you were a portrait painter, you might first perceive it as 2 facing faces.
  The Special Senses are Vision, Hearing, Balance and Smell. Much about them is in the chapter on Cranial Nerves.

Each sensory modality is mediated by a distinct neural system with multiple components that contribute to perception. Sensory pathways include neurons that link the receptors at the periphery with the spinal cord, brain stem, thalamus, and cerebral cortex. The perception of a touch on the hand begins when cutaneous mechanoreceptors cause a population of sensory peripheral neurons to discharge action potentials that are transmitted forward in the neuronal afferent axon fibers, thus setting up a propagated response in the dorsal column spinal cord nuclei and then in the thalamus. From the thalamus, sensory information streams to several areas of the cerebral cortex, each of which analyzes particular aspects of the original stimulus and produces all the aspects that give us perception of what we see, hear, feel, etc. This cortical representation is closely correlated with our conscious perception. For example, an illusion of sensation in the hand, albeit a slightly blunted one, can be elicited by electrical stimulation of the cortical area that represents the hand.

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