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Pathways and Tracts

Pathways and Tracts. Pathway. route followed by a nerve impulse as it travels through the nervous system a reflex arc is simplest of these pathways. Pathways. Include: - nerve fibers in nerves; bundles called fascicles

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Pathways and Tracts

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  1. Pathways and Tracts

  2. Pathway • route followed by a nerve impulse as it travels through the nervous system • a reflex arc is simplestof these pathways

  3. Pathways • Include: - nerve fibers in nerves; bundles called fascicles - nerve fibers inside the brain and spinal cord; bundles called tracts - bundles of nerve fibers linking the two halves of the brain called commissures

  4. Pathways (cont.) • sensory - ascending • motor - descending

  5. Sensory Pathways(ascending) • start at sensory receptors and end in cerebral cortex of brain • Consist of a series of three neurons: - first-order neurons - second-order neurons - third-order neurons

  6. First-Order Neurons • sensory neurons that convey impulses from sensory receptors to CNS • extend up to medulla on same side of body • axon terminals form synapses with second-order neurons

  7. Second-Order Neurons (associated neurons) • carry sensory impulses to the thalamus (integrating center) • axon of the second-order neuron crosses to the opposite side of the medulla to thalamus • in thalamus, axon terminals of second-order neurons synapse with third-order neurons

  8. Third-Order Neurons (associated neurons) • carry impulses from thalamus to cerebral cortex (where conscious sensation is produced)

  9. Pathway

  10. Motor Pathways(descending) • start in brain and terminate at muscles or glands • consist of upper and lower motor neurons • Two basic pathways: - direct or pyramidal -indirect or extrapyramidal

  11. Direct Motor Pathways(pyramidal) • carry impulses from cerebral cortex directly to lower motor neurons • simplest pathway consists of two neurons; upper and lower motor neurons

  12. Upper Motor neurons (pyramidal) • in cortex • fibers pass through bulges called pyramids on medulla oblongata - hence the name • conduct impulses from motor cortex to motor nuclei of the cerebral nerves or to the ventral gray columns of spinal cord

  13. Lower Motor Neurons (pyramidal) • peripheral neurons whose cell bodies lie in the ventral gray column of spinal cord and terminate in skeletal muscles • responses are semivoluntary and automatic

  14. Direct Pathway Impulses • Channeled into three tracts: - lateral corticospinal tracts - anterior corticospinal tracts - corticobulbar tracts direct pathways result in precise voluntary movements

  15. Indirect Motor Pathways(extrapyramidal) • impulses follow complex polysynaptic circuits • carry lower motor neurons through other parts of brain • Pathways involve: - motor cortex, basal ganglia, thalamus, cerebellum, reticular formation, nuclei in the brain stem

  16. Tracts • bundles of nerve fibers (axons) in CNS • Types of tracts: -sensory or ascending contain nerve fibers that carry impulses up the spinal cord to the brain - motor or descending contain nerve fibers that carry impulses down SC

  17. Tracts (cont.) • Tracts are named according to: - location in spinal cord - origin - termination

  18. Ganglia and Nuclei

  19. Ganglia (ganglion - singular) • group of neuron cell bodies • located outside central nervous system in the peripheral nervous system

  20. Four Basic Types of Ganglia • posterior root ganglia • sympathetic trunk ganglia • prevertebral ganglia • terminal ganglia

  21. Posterior Root Ganglia(dorsal root ganglia) • contain cell bodies of sensory nerves • located near spinal cord in posterior (dorsal) roots of spinal nerves

  22. Sympathetic Trunk Ganglia (sympathetic division) • form a chain of ganglia on each side of the vertebral column • extend from neck to coccyx • contain cell bodies of postganglionic sympathetic neurons • also known as paravertebral ganglia and sympathetic chain ganglia • can be dissected out separately like a string of pearls

  23. Prevertebral Ganglia(sympathetic division) • located anterior to vertebral column • close to the abdominal arteries • contain cell bodies of postganglionic sympathetic neurons • also called collateral ganglia

  24. Terminal Ganglia(parasympathetic division) • located near or inside internal organs (visceral effectors) • consist of clusters of cell bodies of postganglionic parasympathetic neurons • also known as intramural ganglia

  25. Shingles or Herpes Zoster • caused by same virus that causes chicken pox (Herpes varicella-zoster) • chicken pox virus may survive in dormant state in dorsal root ganglia • stress or advancing age may cause the virus to become active

  26. Shingles or Herpes Zoster (cont.) • virus is present in sensory trunk but damage caused by the virus is seen in the skin over the affected nerve • Symptoms: - painful raised red lesions - follow course of nerve on skin external to it - no specific treatment

  27. Shingles or Herpes Zoster (cont.) • if 7th. cranial nerve is affected, Bell’s palsy (facial paralysis) results • if optic nerve is affected blindness will occur

  28. Spinal Nerve • Attached to spinal cord by: - dorsal (posterior) root is composed of sensory fibers - ventral (anterior) root is composed of motor fibers

  29. Dorsal Root (posterior) • exhibits small enlargement called dorsal root ganglionwhichcontains cell bodies of sensory neurons

  30. Dorsal and Ventral Roots • pass laterally from spinal cord • merge to form single mixed spinal nerve • pass through intervetebral foramen (IVF) • after passing IVF spinal nerve divides into two main branches - large ventral (anterior) ramus - smaller dorsal (posterior) ramus

  31. Spinal Cord Showing Roots and Ganglia spinal cord posterior root ganglion (dorsal) sympathetic trunk ganglion

  32. Spinal Cord Showing Roots and Ganglia spinal cord posterior root ganglion posterior root spinal nerve sympathetic trunk ganglion vertebrae anterior root

  33. Nuclei • located in brain or spinal cord • cluster of neuron cell bodies • CNS nuclei are isolated regions of gray matter • located within white matter of brain and spinal cord • neurons in a given nucleus perform specific functions

  34. Examples of Nuclei

  35. Basal Ganglia (cerebral nuclei or basal nuclei) • several groups of nuclei • located within white matter of cerebral hemispheres • they integrate semi-voluntary automatic movements like walking, swimming, and laughing

  36. Thalamus • consists of a pair of oval masses on each side of 3rd ventricle in diencephalon • mostly gray matter • made up of many nuclei • Functions include: - language, memory, emotion, integration and relay of sensory impulses to the cerebral cortex

  37. Ventricles • four cavities within brain - two lateral ventricles - third ventricle - fourth ventricle • each ventricle contains capillary network called choroid plexus which forms cerebrospinal fluid from blood plasma

  38. Hypothalamus • in region of diencephalon • located below two halves of thalamus • consists of a variety of nuclei and nuclear areas - most important control area for internal environment • Functions: - thirst, hunger, hormone production, and fear and rage reactions

  39. Brain Stem • nuclei for most of the cranial nerves are located in brain stem • other nuclei located in brain stem controlbreathing, the force and rate of heart contractions, and blood vessel diameter

  40. Cerebellum • cerebellar nuclei are regions of gray matter located deep within cerebellum • Concerned with: - balance, proprioception, (self-awareness), and the planning and coordination of complex muscular activities

  41. Nuclei 3rd. Ventricle 2 divisions of lenticular nucleus head of caudate nucleus posterior lateral nucleus of thalamus tail of caudate nucleus medial pulvinar nucleus of thalamus choroid plexus

  42. Reflex (stimulus) • fast, predictable, automatic, unconscious response to change in the environment that helps to maintain homeostasis • occurs in gray matter • Change can be: - external - outside the body - internal - inside the body

  43. Homeostasis • internal environment of body is maintained at a relatively constant level • blood pressure, plasma glucose, pH, and body temperature are examples of body conditions that must be consistently maintained

  44. Homeostatic Mechanism • sequence of events that maintains a consistent internal environment • homeostatic mechanisms are called negative feedback control systems

  45. Negative Feedback • principle dictating most control systems • response in which a stimulus counteracts, reverses or reduces original stimulus (back to original value)

  46. Types of Reflexes • somatic reflexes: - involve contraction of of skeletal muscles • autonomic reflexes: - involve the contraction of smooth muscles, cardiac muscles, and glands

  47. Reflex Arc • basic structural and functional unit of nervous system • begins with a receptor at end of a sensory nerve fiber

  48. Sequence of Events in Response to Stimulus • receptor • sensory pathway • integrating center • motor pathway • effector

  49. Receptor • specialized sensory nerve ending • detects environmental change (stimulus) • responds by initiating a nerve impulse in a sensory neuron

  50. Sensory Pathway(afferent) • carries nerve impulse from receptor to central nervous system

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