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UWL BIO 312 - Nervous system IV: sensory reception

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Lecture 11Lecture Outline:I Sensory Physiologya. Terminologyb. Transduction of stimulus energyc. Somatosensory cortexII Spinal corda. Structureb. functionCurrent Lecture: Sensory Physiology:- General senses: conscious awareness of touch, pressure, vibration, proprioception, temperature, and pain- Special senses: vision, hearing, balance, smell, taste- We are constantly exposed to various forms of stimulus energy originating from within and outside of the bodyo Electromagnetico Mechanicalo Thermalo Chemicalo Nervous system can extract meaning only from action potentials- Sensory receptors: convert stimulus energy into APs and transmit via afferent neurons to CNS for processingo Stimulus energy converted into electrical signal by transductiono Many types of sensory receptors throughout body, each are most responsive to a particular type of stimulus (most receptors can be activated by several types of energy if the intensity is great enough)o Stimulation of a given receptor (no matter how it was activated) gives rise to only one sensation.o Doctrine of specific nerve energies: for every type of sensation there is a special type of sensory receptor whose activation always gives rise to that sensationo Modality: quality or type of sensation produced; some conscious, some are noto Each type of sensory receptor has its own unique adequate stimuluso Sensory receptors are either specialized endings of afferent neurons or separate cells that signal the afferent neurono Classifications: Mechanoreceptors: activated by mechanical force, touch, audition, vestibular Photoreceptors: activated by light, used for vision Chemoreceptors: activated by the changes in concentration of a chemical in a solution, olfaction, taste, arterial PO2, pH of CSF Thermoreceptors: activated by heat, temperature Nociceptors: activated by pain and extreme temperatures Exteroceptors: stimulated by energy from outside of the body Enteroceptors: stimulated by energy from inside the body, blood pressure Bio 312 1st Edition Proprioceptors: awareness of limb and body position- Transduction of stimulus energy:o Receptor potential: a graded/local potential, amplituted related to stimulus intensityo Stimulus energy causes the opening of a cation channel in sensory receptor membrane, causing depolarizationo If depolarization reaches threshold potential an action potential is formed and sent down neurono More action potentials that are fired determines the intensity of the stimulus received by the NSo Adaptation: a reduction in response (the number of action potentials) to a constant stimulus; helps prevent sensory overload Receptors vary in their speed of adaptation Rapidly adapting such as touch receptors are important in situations where CNS needs info about rate of change in stimulus intensity called phasic Slowly adapting such as proprioreceptors, baroreceptors, and pain receptors are important in situations where CNS needs constant info about the stimulus called tonic- General Senses: (somatosensory system) touch, pressure, proprioreception, vibration, temperature, paino Afferent info much reach somatosensory cortex to discriminate quality, intensity and location of stimuluso Somatosensory info has to pass through a series of neurons (usually 3) to get form receptor to somatosensory cortex First order neuron: first neuron in sensory pathway, unipolar Second order neuron: synapses with first order neuron and third order neuron Third order neuron: synapses with second order neuron, travels to correct portion of braino Spinal cord  brain stem  thalamus  somatosensory cortexo Somatosensory cortex: outermost layer of the brain Somatotopic representation: each part of the body is mapped out/represented by a specific region on the somatosensory cortex Law of projection: all somatosensory information from a given area goes only to its designated area on the somatosensory cortex Amount of cortex devoted to any body part is not proportional to the size of that body part Decussation: crossing over of sensory information to be processed by the brain on the opposite side of that sensory informationo Receptive fields: specific area that when stimulated will activate a specific somatic sensory neuron More sensitive that area of skin, the smaller the receptive field- How does Somatosensory info reach somatosensory cortex:o Somatosensory signals from most of body first enter spinal cord through spinal nerveso General senses afferents from face, mouth and head use cranial nerveso All 31 pairs of spinal nerves are mixed nerves (some of the axons are those of 1st order afferents, others are axons of efferent neurons)o Each spinal nerve is designated to a specific dermatomes or specific areas of skin A dermatome receives its densest innervation from its corresponding spinal nerve (some leaves at nerves adjacent to that area) If T10 spinal nerve is severed, T10 dermatome would lose most of its sensory ability, but not allo Spinal nerves, split into dorsal and ventral root immediately before it attaches to the spinal cord Dorsal root: sensory information (afferent)- Dorsal root ganglion: collection of nerve cells outside of CNS  Ventral root: motor information (efferent)- Structure and function of Spinal cord: extends from foramen magnum to L1 vertebrao Very well protected, provides 2-way communication pathway to and from the brain (ascending and descending tracts)o Cauda equina: portion where spinal cord splits into several large nerves, extend down through foramina of sacrumo Thickness varies, differing amounts of information coming in at different areas requires varying amounts of nerves designated to that informationo Intervertebral foramina: hole where spinal nerves enter bony vertebral canal Herniated disc puts pressure on nerve as they enter intervertebral foraminao Protection of spinal cord consists of meninges, cerebrospinal fluid, and adipose tissueo Epidural block: local anesthetic being injected into meninges of spinal cord blocking pain signals Performed in the lumbar segments to avoid hitting spinal cordo Internal structure: in cross section, spinal cord is divided into outer white matter and inner gray matter. Gray matter divided into dorsal and ventral horns- Dorsal horn: cell bodies of interneurons some of which are 2nd order neurons- Lateral horn: cell bodies of autonomic pre-ganglionic efferent neurons-


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