Exam 2 Study Guide Somatic Chemical Senses Sensation Perception Test Objectives 1 Stimuli Somatosensation is not one sense but many What types of matter or energy are sensed on the body surface skin Mechanical Energy o Pressure Mg this is touch o Vibration Hz this is low high frequency textures What types of matter or energy are sensed beneath the body surface muscles Thermal Energy o Temperature cold warm hot Chemical o Injury Pain tendons proprioception Stretch skeletal position Tension mass or force What does this say about Somatoperception Body Surface Proprioception Haptic Perception Test Objectives 2 Receptors and Anatomy From Skin to Brain Skin sensations must go to the brain to be processed In some cases the information has to travel up to two meters Information proceeds to the brain via two major pathways the evolutionarily older spinothalamic pathway which is slower and transmits information from thermoreceptors and nociceptors The dorsal column medial lemniscal DCML pathway includes wider diameter axons and fewer synapses and it therefore quicker to get to the brain How are the many stimulus dimensions of somatosensation communicated to cortex Lemniscal Pathway are these receptors All mechanoreception is transmitted via this pathway What Tactile pressure Hair Follicle Receptor Detect Tactile shape texture Meissner Merkel Ruffini Pacinian ID Proprioception stretch Muscle Spindle ID Proprioception tension Golgi Tendon Organ ID Spinothalamic Pathway pathway What are these receptors All thermal and injury information is transmitted by this This pathway is gated in the brainstem Tissue Damage pain Nociceptor Detect Temperature hot cold Thermal Receptor ID The brainstem not the telencephalon has the ability to directly suppress transmission of information through the Spinothalamic Pathway Test Objectives 3 Lemniscal Pathway Receptors How do we characterize mechanoreceptors Stimulation Type Pressure Detect or ID shape vs Texture ID All mechanoreceptors have stretch sensitive Na channels Mechanical force on the receptor opens these channels Not so different from inner hair cells SA II FA II Muscle Spindle responds to muscle stretch Red is motor Muscle fiber Blue is sensory Golgi Tendon Organ Tension Receptor M u s c l e B o n e Provides information about mass of held object or object that you are trying to move Size of Receptive Field The size and density of mechanoreceptor receptive fields varies widely across the skin surface Stimulation anywhere within this large receptive field goes to one cortical column Stimulation within this tiny receptive field goes to one cortical column Mechanoreceptors with large receptive fields tend to show Many One connectivity which lowers thresholds for detection Those with small receptive fields tend to show One One connectivity which produces better identification Many O ne pattern Less discrimin ation Lower threshol d Good for detectio n One On e pattern Better discrimi nation Higher threshol d Good for Identific ation Larger lower thresholds smaller higher thresholds Absolute threshold the smallest amount of skin displacement stimulus that elicits the perception of touch Point Localization the ability to localize touch sensations on a stimulated region of skin o Varies with the region of the body stimulated more sensitive areas areas with higher density of mechanoreceptors are better able to localize touch sensations o Stimulation of regions with more fine muscular control hands and mouth result in more accurate point localization o Two point threshold The smallest separation of two separate but adjacent points of stimulation on the skin that produces two distinct impressions of touch Rate of Adaptation Slow Detect or ID shape vs Fast ID texture Meissner Pacinian Rapidly Adapting RA Tuned for low or high frequency textures Location relative to Skin Surface and Body Region ID of high below surface vs low near surface frequency textures Again tension between detection and identification Haptic Perception to act is to sense Haptic Perception We rarely experience the tactile richness of the world passively We use our motor systems to move our skin across and to grasp physical objects in order to identify them Our unitary perceptions of these objects are a synthesis of sensory information from all of the receptors listed below as well as information from our muscle spindles e g the position of your hand as you grasp an object and golgi tendon organs e g how heavy is the object Test Objectives 4 Organization of the Lemniscal Pathway What is the anatomical relationship between somatosensory and motor information in the spinal cord Sensory Neurons D o r s a l S e n s o r y Thalamus V e n t r a l M o t o r Hypothalamus What are the anatomical relationships between somatosensory and motor information in cortex Motor frontal The Lemniscal Pathway 3 neurons from skin to cortex arranged in somatotopy A somatosensory pathway will typically have three long neurons primary secondary and tertiary or first second and third Lemniscal system Muscles 1 The first neuron has its cell body in the dorsal root ganglion of the spinal nerve 2 The second neuron has its cell body either in the spinal cord or in the brainstem This neuron s ascending axons will cross decussate to the opposite side at the hindbrain The axons of these neurons terminate in the thalamus 3 The third neuron has its cell body in the VPN of the thalamus and ends in the postcentral gyrus of the parietal lobe Fibers from the body enter the spinal cord at the dorsal horn and travel up the spinal cord Their first synapse is at the cuneate and gracile nuclei which are just ventral to the brain When the information arrives in the medulla at the medial lemniscus the information crosses to the contralateral side of the body and then is sent to the ventral posterior nucleus of the thalamus From the thalamus somatosensory information is sent to the somatosensory area 1 S1 in the cortex located in the parietal lobe Lemniscal Somatotopy How does variation in the in receptive field size and density relate to distortions of somatosensory maps in cortex What do cortical homunculi tell us about an animal s somatosensory experience of the world Touch sensations are spatially represented in area S1 in the same way they are on the skin This is called cortical somatotopy and is illustrated by the homunculus The homunculus is a drawing depicting the amount of cortical space a given area of the body has The
View Full Document
Unlocking...