vander lecture

1. Sensory System John Paul L. Oliveros, MD, DPPS

2. General Principles Awarenesss of our external and internal world is brought about by neural mechanisms that process afferent information Stimulus energy  receptor potentials (graded potentials)  action potentials (Nerve fibers) Sensory system Part of the nervous system that consists of sensory receptors Neural pathways Processing areas of the brain Sensory information Information processed by a sensory system May or may not lead to conscious awareness of the stimulus Sensation Sensory information that reaches consciousness Perception A persron’s understanding of the sensation’s meaning

3. Receptors Sensory Receptors Initiates neural activity at the border betwee the nervoussystem and the outside world Change stimulus energy (pressure, temperature, light, soundwaves, etc) Can either be: Specialized ending s of afferent neurons Separate cellthat affect the ends of afferent neurons Stimulus Energy that impinges upon and activates a sensory receptor Stimulus transduction The process by which stimulus is transformed into an electrical response Adequate stimulus The type of energy to which a receptor responds in normal functioning Receptors respond best to only a very narrow range of stimulus energy (lowest threshold)

4. Receptor Potential Transduction process in all sensory receptors involve the opening and closing of ion channels that receive information about the outside world Receptor potential A change in the membrane potential on a specialized receptor membrane It is a Graded potential separate receptors: graded potential causes release of neurotransmitter Receptors on afferent neuons: A local current must flow to a part of an axon that can produce an action potential 1st node of Ranvier Part of myelinated afferent neurons capable of producing action potentials

5. Receptor Potential Graded potential magnitude determines action potential frequency but not action potential magnitude Factors controlling receptor potential magnitude Stimulus strength Rate of change of stimulus strength Temporal summation of successive receptor potentials Adaptation Decrease in receptor sensitivity Results in decrease in frequency of action potentials in an afferent neuron despite maintenance of the stimulus at a constant strength

6. Neural pathways in sensory system Sensory pathway A bundle of parallel 3-neuron chains Sensory units A single afferent neuron with all its receptor endings Receptive field Portion of the body that when stimulated leads to activity in a particular afferent neuron

7. Ascending pathways Central processes Part of afferent neurons that enter the brain or spinal cord and synapse with interneurons 2nd order neurons Interneurons that synapse with afferent neurons Synapse with 3rd, 4th, etc interneurons until the cerebral cortex is reached

8. Ascending pathways Specific ascending pathways Ascending pathways in the brain and spinal cord that carry information about single types of stimuli Somatosensory cortex Lies in the parietal lobe of the brain behind the junction of the parietal and frontal lobes Where information from somatic recepotrs are transmitted Information from skin, skeletal muscles, tendon and joints Visual cortex At the occipital lobe Where spefic pathways from the eyes transmit Auditory cortex Where specific pathways from the ears transmit Loacted at the temporal lobe

9. Ascending pathways Nonspecific ascending pathways Activated by sensory units of several different types Signal general information Polymodal neurons 2nd order neurons that respond to inputs from several afferent neurons, each activated by a different stimulus

10. Association Cortex and Perceptual Processing Cortical Association Areas Areas of the brain outside the primary cortical sensory areas but are adjacent to them Elaborates perception information from the primary sensory cortical areas Regions closests to the primary sensory cortical areas process information in fairly simple ways and serves basic sensory function Regions farther from the primary sensory cortical areas process information in more complicated ways Arousal Attention Memory Language Emotional and motivational significance (frontal lobe/ limbic system)

12. 1. there is heirarchical processing of afferent information along individual pathways

13. 2. information is processed by parallel pathways, each of which handles a limited aspect of neural signals generated by the sensory transducers

15. Primary sensory coding Stimulus intensity Distinguishing intensity Frequency of action potentials Inc. Stimulus strengthinc. Receptor potential  inc. Action potential frequency single receptor Other receptors of the same neuron Recruitment Calling in of receptors on additional afferent neurons

16. Primary sensory coding Stimulus location Factors: Main factor: Site of the stimulated receptor amount of convergence of neuronal input in ascending pathways: inversely related to acuity/precision Size of the receptive fieldcovered by a receptor Overlap of nearby receptive fields

17. Primary sensory coding

18. Primary sensory coding Lateral inhibition More important in localization than the different sensitivites of receptors throughout the receptor field Information from afferent neurons whose receptors are at the edge of the stimulus is inhibitted compared to information from the stimulus’ center Increases contrast between relevant and irrelevant information May occur at any levels of the pathway but mostly on the early stages

19. Primary sensory coding Stimulus duration Receptors differ in the way they respond to a constantly maintained stimulus adaptation Rapidly adapting receptors: Important in signaling rapid change On response On-off response Slowly adapting receptors: Maintain response at or near the initial level of firing regardless of the stimulus duration For prolonged events (posture)

20. Central control of afferent information Reticular formation and cortex: main control

21. End