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Chapter 14

PS262 Chapter 14 Summary

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Elizabeth Olds

PS262 Chapter 14 Summary – Cutaneous Senses Somatosensory system includes (1) the cutaneous senses which are responsible for perceptions such as touch and pain that are usually caused by stimulation of the skin; (2) proprioception – the ability to sense the position of the body and limbs and (3) kinesthesis - the ability to sense the movement of the body and limbs Overview of the Cutaneous System The Skin  Comel called the skin the monumental façade of the human body (it is the heaviest organ in the human body)  Skin maintains the integrity of what’s inside and protects us from what’s outside but it also provides us with information about the various stimuli that contact it  Our outer layer of skin is called epidermis; below that we find the dermis then finally the mechanoreceptors, receptors tat respond to the mechanical stimulation such as pressure, stretching and vibration o Two mechanoreceptors: merkel receptor and meissner corpuscle o The merkel receptor fires continuously as long as the stimulus is on; the Meissner corpuscle fires only when the stimulus is first applied and when it is removed o The other two mechanoreceptors are: Ruffini cylinder and Pacinian corpuscle (deeper skin) o The Ruffini cylinder responds continuously to stimulation and the Pacinian corpuscle responds when the stimulus is applied and removed  Nerve fibers from receptors in the skin travel in bundles called peripheral nerves that enter the spinal cord through the dorsal root  Two major pathways: medial leminscal pathway (large fibers that carry signals related to sensing the positions of the limbs) and spinothalamic pathway (consists of small fibers that transmit signals related to temperature and pain o Ex// case of 17 yr old Ian – he lost his ability to feel touch and sense the position of his limbs (lemniscal), he was still able to sense pain and temperature (spinothalamic) Maps of the Body on the Cortex  Somatosensory receiving area = S1; secondary somatosensory cortex = S2  An important characteristic of the somatosensory cortex is that it is organized into maps that correspond to locations on the body  Penfield found that stimulating the ventral part of S1 (lower parietal lobe) caused sensations on the lips and face, stimulating higher on S1 caused sensations in the hands and fingers and stimulating the dorsal S1 caused sensations in the legs and feet The Plasticity of Cortical Body Maps  The cortical representation of a particular function can become larger if that function is used often  Some experiments showed that increasing stimulation of a specific area of the skin causes an expansion of the cortical area receiving signals from that area of skin Perceiving Details  The properties of the receptors are one of the things that determines what we experience when the skin is stimulated  Merkel receptor is sensitive to details o The spacing between Merkel receptors is the same on all fingertips o This means that while receptor spacing is part of the answer, the cortex also plays a role in determining tactile acuity Cortical Mechanisms for Tactile Acuity  There is a parallel between the representation of the body in the brain and the acuity at different locations on the body  A way to demonstrate the connection between cortical mechanisms and acuity is to determine the receptive fields of neurons in different parts of the cortical homunculus  REMEMBER: the receptive field for a neuron in the visual system is the area on the retina that when stimulated influences the firing rate of a neuron o The receptive field for a neuron in the cutaneous system is the area on the skin that, when stimulated, influences the firing rate of the neuron  We can see that cortical neurons representing parts of the body with better acuity (fingers) have smaller receptive fields o This means that two points that are close together on the fingers might fall on receptive fields that don’t overlap o So would cause neurons that are separated in the cortex to fire  Having small receptive fields of neurons receiving signals from the fingers translates into more separation on the cortex which enhances the ability to feel two close-together points on the skin as two separate points Perceiving Vibration  The mechanoreceptor that is responsible for sensing vibration is the Pacinian corpuscle (PC)  The presence of PC determines which pressure stimuli actually reach the fiber o The PC consists of a series of layers with fluid between each layer, it transmits rapidly applied pressure like vibration to the nerve fiber but does not transmit continuous pressure  The corpuscle causes the fiber to receive rapid changes in pressure, but not to receive continuous pressure  The properties of the corpuscle cause the fiber to respond poorly to continuous stimulation but to respond well to changes in the stimulation that occur at the beginning and end of a pressure stimulus or when stimulation is changing rapidly, as occurs in vibration Perceiving Texture  Spatial cues are caused by relatively large surface elements, such as bumps and grooves, that can be felt both when the skin moves across the surface elements and when it is pressed onto the elements o Ex// Braille dots  Temporal cues occur when the skin moves across a textured surface like fine sandpaper o This cue provides information in the form of vibrations that occur as the result of
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