Pathways and Tracts Suzanne DAnna 1 Pathway l

Pathways and Tracts Suzanne D'Anna 1

Pathway l route followed by a nerve impulse as it travels through the nervous system l a reflex arc is simplest of these pathways Suzanne D'Anna 2

Pathways l 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 l motor - descending l Suzanne D'Anna 4

Sensory Pathways (ascending) l start at sensory receptors and end in cerebral cortex of brain l 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 l extend up to medulla on same side of body l axon terminals form synapses with second-order neurons l Suzanne D'Anna 6

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

Third-Order Neurons (associated neurons) l 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 l consist of upper and lower motor neurons l Two basic pathways: - direct or pyramidal - indirect or extrapyramidal l Suzanne D'Anna 10

Direct Motor Pathways (pyramidal) carry impulses from cerebral cortex directly to lower motor neurons l simplest pathway consists of two neurons; upper and lower motor neurons l Suzanne D'Anna 11

Upper Motor neurons (pyramidal) in cortex l fibers pass through bulges called pyramids on medulla oblongata - hence the name l conduct impulses from motor cortex to motor nuclei of the cerebral nerves or to the ventral gray columns of spinal cord l 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 l responses are semivoluntary and automatic l Suzanne D'Anna 13

Direct Pathway Impulses l 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 l carry lower motor neurons through other parts of brain l Pathways involve: - motor cortex, basal ganglia, thalamus, cerebellum, reticular formation, nuclei in the brain stem l Suzanne D'Anna 15

Tracts bundles of nerve fibers (axons) in CNS l 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 l Suzanne D'Anna 16

Tracts l (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 l located outside central nervous system in the peripheral nervous system l Suzanne D'Anna 19

Four Basic Types of Ganglia posterior root ganglia l sympathetic trunk ganglia l prevertebral ganglia l terminal ganglia l Suzanne D'Anna 20

Posterior Root Ganglia (dorsal root ganglia) contain cell bodies of sensory nerves l located near spinal cord in posterior (dorsal) roots of spinal nerves l Suzanne D'Anna 21

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

Prevertebral Ganglia (sympathetic division) located anterior to vertebral column l close to the abdominal arteries l contain cell bodies of postganglionic sympathetic neurons l also called collateral ganglia l Suzanne D'Anna 23

Terminal Ganglia (parasympathetic division) located near or inside internal organs (visceral effectors) l consist of clusters of cell bodies of postganglionic parasympathetic neurons l also known as intramural ganglia l Suzanne D'Anna 24

Shingles or Herpes Zoster caused by same virus that causes chicken pox (Herpes varicella-zoster) l chicken pox virus may survive in dormant state in dorsal root ganglia l stress or advancing age may cause the virus to become active l 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 l Symptoms: - painful raised red lesions - follow course of nerve on skin external to it - no specific treatment l Suzanne D'Anna 26

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

Spinal Nerve l 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) l 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 l merge to form single mixed spinal nerve l pass through intervetebral foramen (IVF) l after passing IVF spinal nerve divides into two main branches - large ventral (anterior) ramus - smaller dorsal (posterior) ramus l 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 ganglion spinal nerve anterior root Suzanne D'Anna sympathetic trunk ganglion vertebrae 32

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

Examples of Nuclei Suzanne D'Anna 34

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

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

Ventricles four cavities within brain - two lateral ventricles - third ventricle - fourth ventricle l each ventricle contains capillary network called choroid plexus which forms cerebrospinal fluid from blood plasma l Suzanne D'Anna 37

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

Brain Stem nuclei for most of the cranial nerves are located in brain stem l other nuclei located in brain stem control breathing, the force and rate of heart contractions, and blood vessel diameter l Suzanne D'Anna 39

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

Nuclei 3 rd. 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 l occurs in gray matter l Change can be: - external - outside the body - internal - inside the body l Suzanne D'Anna 42

Homeostasis l internal environment of body is maintained at a relatively constant level l blood pressure, plasma glucose, p. H, and body temperature are examples of body conditions that must be consistently maintained Suzanne D'Anna 43

Homeostatic Mechanism l sequence of events that maintains a consistent internal environment l homeostatic mechanisms are called negative feedback control systems Suzanne D'Anna 44

Negative Feedback l principle dictating most control systems l 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 l autonomic reflexes: - involve the contraction of smooth muscles, cardiac muscles, and glands l Suzanne D'Anna 46

Reflex Arc basic structural and functional unit of nervous system l begins with a receptor at end of a sensory nerve fiber l Suzanne D'Anna 47

Sequence of Events in Response to Stimulus receptor l sensory pathway l integrating center l motor pathway l effector l Suzanne D'Anna 48

Receptor specialized sensory nerve ending l detects environmental change (stimulus) l responds by initiating a nerve impulse in a sensory neuron l Suzanne D'Anna 49

Sensory Pathway (afferent) l carries nerve impulse from receptor to central nervous system Suzanne D'Anna 50

Integrating Center receives signals l determines responses l sends out instructions via motor neurons to effector cells l Suzanne D'Anna 51

Motor Pathway (efferent) l carries instructions from CNS to effector (muscles or glands) Suzanne D'Anna 52

Effector muscle or gland that contracts, or secretes l response is called a reflex l usually counteracts original stimulus i. e. , negative feedback effect l Suzanne D'Anna 53