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Pathways and Tracts Suzanne D'Anna 1 Pathway route followed by a nerve impulse as it travels through the nervous system a reflex arc is simplest of these pathways Suzanne D'Anna 2 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 Suzanne D'Anna 3 Pathways (cont.) sensory - ascending motor - descending Suzanne D'Anna 4 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 Suzanne D'Anna 5 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 Suzanne D'Anna 6 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 secondorder neurons synapse with third-order neurons Suzanne D'Anna 7 Third-Order Neurons (associated neurons) carry impulses from thalamus to cerebral cortex (where conscious sensation is produced) Suzanne D'Anna 8 Pathway Suzanne D'Anna 9 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 Suzanne D'Anna 10 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 Suzanne D'Anna 11 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 Suzanne D'Anna 12 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 Suzanne D'Anna 13 Direct Pathway Impulses Channeled into three tracts: - lateral corticospinal tracts - anterior corticospinal tracts - corticobulbar tracts direct pathways result in precise voluntary movements Suzanne D'Anna 14 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 Suzanne D'Anna 15 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 Suzanne D'Anna 16 Tracts (cont.) Tracts are named according to: - location in spinal cord - origin - termination Suzanne D'Anna 17 Ganglia and Nuclei Suzanne D'Anna 18 Ganglia (ganglion - singular) group of neuron cell bodies located outside central nervous system in the peripheral nervous system Suzanne D'Anna 19 Four Basic Types of Ganglia posterior root ganglia sympathetic trunk ganglia prevertebral ganglia terminal ganglia Suzanne D'Anna 20 Posterior Root Ganglia (dorsal root ganglia) contain cell bodies of sensory nerves located near spinal cord in posterior (dorsal) roots of spinal nerves Suzanne D'Anna 21 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 Suzanne D'Anna 22 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 Suzanne D'Anna 23 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 Suzanne D'Anna 24 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 Suzanne D'Anna 25 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 Suzanne D'Anna 26 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 Suzanne D'Anna 27 Spinal Nerve Attached to spinal cord by: - dorsal (posterior) root is composed of sensory fibers - ventral (anterior) root is composed of motor fibers Suzanne D'Anna 28 Dorsal Root (posterior) exhibits small enlargement called dorsal root ganglion which contains cell bodies of sensory neurons Suzanne D'Anna 29 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 Suzanne D'Anna 30 Spinal Cord Showing Roots and Ganglia spinal cord posterior root ganglion (dorsal) sympathetic trunk ganglion Suzanne D'Anna 31 Spinal Cord Showing Roots and Ganglia spinal cord posterior root posterior root ganglion spinal nerve anterior root Suzanne D'Anna sympathetic trunk ganglion vertebrae 32 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 Suzanne D'Anna 33 Examples of Nuclei Suzanne D'Anna 34 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 Suzanne D'Anna 35 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 Suzanne D'Anna 36 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 Suzanne D'Anna 37 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 Suzanne D'Anna 38 Brain Stem nuclei for most of the cranial nerves are located in brain stem other nuclei located in brain stem control breathing, the force and rate of heart contractions, and blood vessel diameter Suzanne D'Anna 39 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 Suzanne D'Anna 40 Nuclei 3rd. Ventricle head of caudate nucleus 2 divisions of lenticular nucleus tail of caudate nucleus posterior lateral nucleus of thalamus choroid plexus medial pulvinar nucleus of thalamus Suzanne D'Anna 41 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 Suzanne D'Anna 42 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 Suzanne D'Anna 43 Homeostatic Mechanism sequence of events that maintains a consistent internal environment homeostatic mechanisms are called negative feedback control systems Suzanne D'Anna 44 Negative Feedback principle dictating most control systems response in which a stimulus counteracts, reverses or reduces original stimulus (back to original value) Suzanne D'Anna 45 Types of Reflexes somatic reflexes: - involve contraction of of skeletal muscles autonomic reflexes: - involve the contraction of smooth muscles, cardiac muscles, and glands Suzanne D'Anna 46 Reflex Arc basic structural and functional unit of nervous system begins with a receptor at end of a sensory nerve fiber Suzanne D'Anna 47 Sequence of Events in Response to Stimulus receptor sensory pathway integrating center motor pathway effector Suzanne D'Anna 48 Receptor specialized sensory nerve ending detects environmental change (stimulus) responds by initiating a nerve impulse in a sensory neuron Suzanne D'Anna 49 Sensory Pathway (afferent) carries nerve impulse from receptor to central nervous system Suzanne D'Anna 50 Integrating Center receives signals determines responses sends out instructions via motor neurons to effector cells Suzanne D'Anna 51 Motor Pathway (efferent) carries instructions from CNS to effector (muscles or glands) Suzanne D'Anna 52 Effector muscle or gland that contracts, or secretes response is called a reflex usually counteracts original stimulus i.e., negative feedback effect Suzanne D'Anna 53