PSB2000 Exam 2 Lecture Summary C Robison Instructor Lecture 18 The Senses We have five main senses plus a number of additional senses such as vestibular balance and interoceptive internal sensations The Senses All senses must be transformed from a physical event into a nerve signal by a process called transduction There are several common themes that are seen across all of the senses Each sense has one or more types of specialized receptors For instance touch receptors for touch and photoreceptors for light Signals from the external environment such as images or sounds must be turned into action potentials that can be encoded by the brain This is called sensory transduction In order to perceive a sensation it must be relayed from the sense organ e g the eye the ear etc to the CNS o For the most part the sensory signal has to make it to the cortex to be consciously o Each sense takes a different path to the cortex All except smell are relayed through the perceived thalamus before going to cortex After sensory signals are sent to the various sensory cortices the cortex projects to other cortex areas called association areas where more complex processing is done Sensory reflexes such as your pupils dilating to bright light operate below the conscious level The senses also undergo sensitization and habituation at an unconscious level o Habituation when a stimulus is repeated the response to that stimulus decreases Example if somebody keeps snapping their fingers next to your ear your startle response will decrease o Sensitization but when you pair a repeated stimulus with a noxious unpleasant event the response to that stimulus increases Example after snapping their fingers the person flicks your ear o You can test sensitization and habituation in Aplysia sea slugs by measuring their gill withdrawal reflex The senses are under cortical control For instance the brain can alter how noises are perceived Page 1 of 16 PSB2000 Exam 2 Lecture Summary C Robison Instructor Lecture 19 Somatosensation Somatosensation Somatosensation is comprised of various sensations of touch such as pressure movement and vibration It also includes nociceptive sensations of pain heat and cold Somatosensation means the senses of the body Proprioception is part of somatosensation it is also interoceptive Somatosensory signals enter the brain through the spinal cord or through the cranial nerves Pain and touch take slightly different pathways Each sensory neuron occupies a receptive field Areas with more sensitive touch reception have smaller receptive fields that are closer together Thus the fingertips have small closely packed receptive fields many touch receptors while the forearm only has a few big ones o The density of the receptive fields can be measured by determining the two point threshold of the body part Each spinal nerve provides sensory innervation to a long strip of skin This pattern is called the dermatome and represents which spinal nerve provides somatosensation to a given area of skin How Somatosensation Works The touch receptors in the skin are called mecanoreceptors because they detect mechanical force There are also mechanoreceptors in the joints and muscles that detect stretch or tension The combined actions of various receptors lead to the overall sense of touch Stretching displacing or vibrating the skin activates different types of mechanoreceptors When activated the mechanoreceptor depolarizes and sends a signal to the spinal cord o Different receptors have different adaptation rates and different receptive field sizes o Fast adapting fibers detect vibration or applied pressure on the skin o Slow adapting fibers detect sustained pressure or stretch o Touch receptors have thick axons with lots of myelination Pain signals are called nociception Pain is not perceived until it gets to the cortex o Pain receptors are called free nerve endings o They are multipotent This means multiple stimuli can activate them For instance some pain receptors respond to both tissue inflammation and heat o Pain receptors have thin axons with little myelination This means their signals are slow Repeated or sustained pain can result in increased sensitivity to pain hyperalgesia This is why it hurts to peel a band aid off after you ve been hurt Hot and cold sensations are thermoception These use the same pathway as pain They are not always nociceptive since some heat and cold signals don t cause pain Proprioception is your ability to detect how your body is positioned o Muscle spindles receptors inside your muscles measure muscle stretch o Golgi tendon organs GTOs measure muscle tension and determine limb position The Touch and Pain Pathways The touch and pain pathways both have two synapses between the skin and the cerebral cortex Page 2 of 16 PSB2000 Exam 2 Lecture Summary C Robison Instructor They also both decussate cross over to the other side of the brain However these two pathways cross over at different locations The touch pathway o Touch signals enter the spinal cord at the dorsal root o The touch fibers then ascend up to the brainstem They synapse at the medulla o The touch fibers then decussate cross over at the medulla o The fibers project up to the thalamus where it synapses again This is where the touch and pain pathways converge The pain pathway o Pain enters the spinal cord at the dorsal root o The pain fiber decussates crosses over in the spinal cord and synapses o The fiber projects up to the thalamus where it synapses again This is where the touch and pain pathways converge From the thalamus both pathways project up to the somatosensory cortex This is just behind the central sulcus which separates the frontal and parietal lobes As the touch and pain fibers go up to the cortex they converge The receptive fields of each fiber become larger This lets the brain do more complex calculations Pain and Temperature There are two types of pain discriminative and affective o Discriminative pain is processed in the somatosensory cortex It tells you where the pain is and how bad it is This sensation isn t too different from touch o Affective pain is processed in the emotional centers the cingulate cortex amygdala and hypothalamus This is the part of pain that makes it feel bad There is a gate theory of pain According to this theory pain must pass through several gates on the way to the cortex Brain signals or analgesic pain killer drugs can shut the gate o Blocking the chemical signals caused by the
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