Chapter 6A The Peripheral Nervous System: Afferent Division

1. Chapter 6A The Peripheral Nervous System: Afferent Division

2. Outline • Pathways, perceptions, sensations • Receptor Physiology • Receptors have differential sensitivities to various stimuli. • A stimulus alters the receptor’s permeability, leading to a graded receptor potential. • Receptor potentials may initiate action potentials in the afferent neuron. • Receptors may adapt slowly or rapidly to sustained stimulation. • Each somatosensory pathway is “labeled” according to modality and location. • Acuity is influenced by receptive field size and lateral inhibition. • PAIN • Stimulation of nociceptors elicits the perception of pain plus motivational and emotional responses. • The brain has a built-in analgesic system.

3. Cortex • Higher processing • (Alzheimers)http://scienceblogs.com/neurophilosophy/2007/11/alois_alzheimers_first_case.php • Basal nuclei • Control of movement, inhibitory, negative • http://www.vin.com/proceedings/Proceedings.plx?CID=TUFTSBG2007&PID=18599&Category=3036&O=Generic • Thalamus • Relay and processing of sensory information • Awareness, a positive screening center for information • Hypothalamus • Hormone secretion, regulation of the internal environment • Cerebellum • Important in balance and in planning and executing voluntary movement • http://neuro.psychiatryonline.org/cgi/content/full/16/3/367 • Brain Stem • Relay station (posture and equilibrium), cranial nerves, control centers, reticular integration, sleep control

4. Peripheral Nervous System • Consists of nerve fibers that carry information between the CNS and other parts of the body • Afferent division • Sends information from internal and external environment to CNS • Visceral afferent • Incoming pathway for information from internal viscera (organs in body cavities) • Sensory afferent • Somatic (body sense) sensation • Sensation arising from body surface and proprioception • Special senses • Vision, hearing, taste, smell

5. Perception • Conscious interpretation of external world derived from sensory input • Why sensory input does not give true reality perception • Cerebral cortex further manipulates the data • Sensation vs. perception

6. What Do You Perceive?

7. Receptors • Structures at peripheral endings of afferent neurons • Detect stimuli (change detectable by the body) • Convert forms of energy into electrical signals (action potentials) • Process is called transduction

8. Types of Receptors • Photoreceptors • Responsive to visible wavelengths of light • Mechanoreceptors • Sensitive to mechanical energy • Thermoreceptors • Sensitive to heat and cold • Osmoreceptors • Detect changes in concentration of solutes in body fluids and resultant changes in osmotic activity • Chemoreceptors • Sensitive to specific chemicals • Include receptors for smell and taste and receptors that detect O2 and CO2 concentrations in blood and chemical content of digestive tract • Nociceptors • Pain receptors that are sensitive to tissue damage or distortion of tissue

9. Epidermis Ruffini ending Dermis Free nerve ending Meissner’s corpuscle Pacinian corpuscle Hair receptor

10. Golgi tendon organ Type II sensory neuron Spinal cord Intrafusal muscle fibers Nuclear bag fiber Type lA sensory neuron Nuclear chain fiber Nuclei of muscle fibers Motor end plate Alpha motor neuron Extrafusal muscle fibers Gamma motor neuron

11. Uses For Perceived Information • Afferent input is essential for control of efferent output • Processing of sensory input by reticular activating system in brain stem is critical for cortical arousal and consciousness • Central processing of sensory information gives rise to our perceptions of the world around us • Selected information delivered to CNS may be stored for further reference • Sensory stimuli can have profound impact on our emotions

12. Receptors • May be • Specialized ending of an afferent neuron • Separate cell closely associated with peripheral ending of a neuron • Stimulus alters receptor’s permeability which leads to graded receptor potential • Usually causes nonselective opening of all small ion channels • This change in membrane permeability can lead to the influx of sodium ions. This produces receptor (generator) potentials. • The magnitude of the receptor potential represents the intensity of the stimulus. • A receptor potential of sufficient magnitude can produce an action potential. This action potential is propagated along an afferent fiber to the CNS.

13. Conversion of Receptor and Generator Potentials into Action Potentials Receptor Potential Generator Potential

14. Receptors • May adapt slowly or rapidly to sustained stimulation • Types of receptors according to their speed of adaptation • Tonic receptors • Do not adapt at all or adapt slowly • Muscle stretch receptors, joint proprioceptors • Phasic receptors • Rapidly adapting receptors • Tactile receptors in skin

15. Fig. 6-5, p. 185

16. Somatosensory Pathways • Pathways conveying conscious somatic sensation • Consists of chains of neurons, or labeled lines, synaptically interconnected in particular sequence to accomplish processing of sensory information • First-order sensory neuron • Afferent neuron with its peripheral receptor that first detects stimulus • Second-order sensory neuron • Either in spinal cord or medulla • Synapses with third-order neuron • Third-order sensory neuron • Located in thalamus

18. Table 6-1, p. 186

19. Fig. 5-11, p. 145

20. Acuity • Refers to discriminative ability • Influenced by receptive field size and lateral inhibition

21. Lateral inhibition Fig. 6-7, p. 187

22. Pain • Primarily a protective mechanism meant to bring a conscious awareness that tissue damage is occurring or is about to occur • Storage of painful experiences in memory helps us avoid potentially harmful events in future • Sensation of pain is accompanied by motivated behavioral responses and emotional reactions • Subjective perception can be influenced by other past or present experiences

23. Cortex • Higher processing • Basal nuclei • Control of movement, inhibitory, negative • Thalamus • Relay and processing of sensory information • Awareness, a positive screening center for information • Hypothalamus • Hormone secretion, regulation of the internal environment • Cerebellum • Important in balance and in planning and executing voluntary movement • Brain Stem • Relay station (posture and equilibrium), cranial nerves, control centers, reticular integration, sleep control

24. Pain • Presence of prostaglandins (lower nociceptors threshold for activation) greatly enhances receptor response to noxious stimuli • Nociceptors do not adapt to sustained or repetitive stimulation • Three categories of nociceptors • Mechanical nociceptors • Respond to mechanical damage such as cutting, crushing, or pinching • Thermal nociceptors • Respond to temperature extremes • Polymodal nociceptors • Respond equally to all kinds of damaging stimuli

25. Characteristics of Pain

26. Pain • Two best known pain neurotransmitters • Substance P • Activates ascending pathways that transmit nociceptive signals to higher levels for further processing • Glutamate • Major excitatory neurotransmitter • Brain has built in analgesic system • Suppresses transmission in pain pathways as they enter spinal cord • Depends on presence of opiate receptors • Endogenous opiates – endorphins, enkephalins, dynorphin

27. Somatosensory cortex (Location of pain) (Perception of pain) Higher brain Thalamus (Behavioral and emotional responses to pain) Hypothalamus limbic system Reticular formation Brain stem ( Alertness) Noxious stimulus Spinal cord Afferent pain fiber Substance P Nociceptor Fig. 6-8a, p. 189

28. No perception of pain To thalamus Periagueductal gray matter Reticular formation Opiate receptor Noxious stimulus Endogenous opiate Transmission of pain impulses to brain blocked Afferent pain fiber Substance P Nociceptor Fig. 6-8b, p. 189