Chapter 15 PDF

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PROFESSOR WILLIAMS-HOGARTH - chapter 15 detailed notes...

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Chapter 15 Serial processing • Information from one pool of neurons to another pool • Somatic sensory neurons • In the serial processing, each of cell bodies represent a collection of neurons in the same place o This nervous tissue is called gray matter. Gray matter can come in various forms --- dorsal root ganglion, horns, nuclei (basal nuclei and lateral geniculate), and cortex (cerebrum). These all send an axon out of structure o Collection of axons are called white matter. White matter structures can be roots, nerves, tracts (fascicule), peduncles, etc. o all axons send info to another pool of neurons, continuously flows down the line.

o o tracts: bundles of CNS axons that share a common origin and destination o columns: several tracts that form an anatomically distinct mass. o LOOK AT ANATOMICAL HEAD ATTACHMENT*** Receptors • Classifying sensory receptors o Exteroceptors: provide information about the external environment o Proprioceptors: a type of exteroceptors that report the positions of skeletal muscles and joints o Interoceptors: monitor visceral organs and functions of internal environment ▪ Growth, sexual reproduction etc Exteroceptors • Somatic sensory receptors that monitor skeletal muscles, joints and skin surface • Provide information about general sensations and position sense • General sensations o Pain- nociceptors o Temperature – thermoreceptors o Touch, pressure physical distortion – tactile receptors and mechanoreceptors ▪ Root hair plexuses, free nerve endings and Merkel discs/

Proprioceptors • Monitor the position of joints and muscles • Proprioceptors (muscle spindles and Golgi tendon organs). Interoceptors • Monitor visceral organs and functions – visceral sensations • Mechanoreceptors: contain mechanically gated ion channels whose gates open or close in response distortions of the plasma membrane o Baroreceptors – detect pressure changes in the walls of blood vessels and in portions of digestive, respiratory and urinary tracts o Chemoreceptors: monitor pH, carbon dioxide, and oxygen levels in arterial blood.

Sensory Receptors • Specialized cells that monitor specific conditions • In the body or external environment • When stimulated, a receptor passes information to the CNS • In the form of action potentials along the axon of a sensory neuron Sensory Receptors • Specialized cells that monitor specific conditions • In the body or external environment • When stimulated, a receptor passes information to the CNS • In the form of action potentials along the axon of a sensory neuron Sensory Pathways

Deliver somatic and visceral sensory information to their final destinations inside the CNS using: • Nerves • Nuclei • Tracts 2 ways for stimulus to activate a sensory neuron Dendrite itself is receptor: free nerve endings (drinking somtheing cold) Once stimulate dendrite of sensory neuron brings info to CNS (PICTURE ON PHONE OF OTHER TYPE) Difference between two is that one is the receptor cell whereas the other one has a separate receptor cell but still do same thing Activity of translating sensory info into an action potential is TRANSDUCTION (conversion of sensa7on into an ac7on poten7al). •

Unipolar neuron • Plasmalemma vs axolemma o Concentration of voltage gated channels o Voltage gated channels found in axon • The dendrite becomes the axon. • These neurons that monitor the general sensations: pain touch temp SENSORY PATHWAYS Pathways are serial processing pathways The first neuron in the pathway is known as the first order neuron. • First order neuron o Sensory neuron delivers sensations to the CNS (can be unipolar neuron). o Cell body of a first order general sensory neuron is located in dorsal root ganglion or cranial nerve ganglion. • Second Order neuron o Axon of the sensory neuron synapses on an interneuron in the CNS o May be located in spinal cord or brain stem (depends on pathway and what determines pathway is the sensation). • Third order neuron o Always located in same place; thalamus. ▪ Purpose of thalamus is to bring sensory info to consciousness. o If the sensation is to reach our awareness, the second order neuron synapses • Somatic Sensory Pathways o Carry sensory information from the skin and musculature of the body wall, head, neck, and limbs o Three major somatic sensory pathways o Each pathway has 2 spinal tracts associated ▪ The spinothalamic pathway (1st neuron: spinal cord to thalamus to the cortex--- afferent) (conscious) ▪ The posterior column pathway (conscious)

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The spinocerebellar pathway (only reaches cerebellum, and does not process conscious level --- afferent). (unconscious).

Difference between pathway and a tract A pathway is the entire series of activity that brings it to destination. Think of gps one destination to next. Part of pathway that travels in a spinal cord is the tract. Spinothalamic pathway • Pain, temp, crude touching pressure, • Divided into two tracts o Lateral spinothalamic tract o Anterior spinothalamic tract • Posterior column pathway o Fine touch, conscious proprioception o Fasciculus is the same as tract o Divided into two tracts ▪ Fasciculus gracilis ▪ Fasciculus cuneatus • Spinocerebellar pathway o Two tracts ▪ Posterior spinocerebellar ▪ Anterior spinocerebellar tract The psinothalamic Pathway • Provides conscious sensations of poorly localized crude touch, pressure, pain and temp • Crude touch and fine touch is being able to differentiate where and what is touching you. • Anterior spinothalamic Tract o First order neurons o Bring info (crude touch) from skin to spinal nerve, through dorsal root ganglion. o Synapses on second order neuron within posterior gray horn (sensory neurons are here). o Axon from dorsal horn neuron crosses over to opposite side of spinal cord (crossing from one side to another side of spinal cord is decussa7on. o The axon of second order neuorn travels through white matter through spinal cord which forms the anterior spinothalamic tract. o Synapses in ventral nucleus group of the thalamus o After the sensations have been sorted and processed, they are relayed to primary sensory cortex. Lateral spinothalamic tract • Pain and temperature • Same thing as other one except where the second order decussates it travels along the lateral spinothalamic tract not the anterior one.

Posterior Column Pathway • Called this because axons form posterior white column • Carries sensations of fine touch (can define what you are touching), pressure, vibration and proprioception (knowing what your doing in space, closing eyes know if your right leg is flexed or extended). • Tracts o Left and right fasciculus gracilis o Left and right fasciculus cuneatus • Axon goes into spinal cord through dorsal root ganglion and does not synapse until brain stem so it travels up spinal cord which forms the fasciculus gracilis and fasciculus cuneatus. Faciculus gracilis carries sensations from lower part of body. Cuneatus is upper body. • Second order neuron gets synapsesd in nucelues gracilis and nucles cuneatus. Decussates over and travels to ventral nuclei in thalamus to the primary sensory cortex. • Conscious pathways have same info red, white black, consciousness, always in dorsal root ganglion, 3rd destination and consciousness. Somatopic organization – organized accoding to your body. Post central gyrus has very specific parts where certain sensations of body come from. Sensation rom tongue, hand foot, etc. Some places have a lot of representation of neurons whereas others don’t Ex: more neurons for fine touch on face then on back. So face is bigger on diagram. Sensory homunculus • Functional map of primary sensory cortex • Distortions occur because area of sensory cortex devoted to particular body region is not proportional Spinocerebellar Pathway • Unconscious • Receives proprioceptive information about position of skeletal muscles, tendons, joints. • Only two neurons in this one. No third order neuron because never reaches consciousness does not go to thalamus. • First order neuron in dorsal root ganglion. Synapses on seconf order neuron on the dorsal horn. Second neuron decussates and forms a spinal tract and goes to the cerebellum. • Two tracts- one decussates one doesn’t o Anterior spinocerebellar tract does (goes to superior peduncle) o The posterior spinocerebellar tract does not decussate. (goes to inferior peduncle). • Goes to cerebellum to adjust upright posture.

Somatic Motor Pathways • Always involve at least two motor neurons

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o Upper motor neuron o Lower motor neuron Upper Motor Neuron o Cell body lies in a CNS processing center o Synapses on the lower motor neuron o Innervates a single motor unit in a skeletal muscle ▪ Activity in upper motor neuron may facilitate or inhibit lower motor neuron Lower Motor Neuron o Cell body lies in a nucleus of the brain stem or spinal cord o Triggers a contraction in innervated muscle ▪ Only axon of lower motor neuron extends outside CNS ▪ Destruction of or damage to lower motor neuron eliminates voluntary and reflex control over innervated motor unit Conscious and Subconscious Motor Commands o Control skeletal muscles by traveling over three integrated motor pathways ▪ Cor:cospinal pathway ▪ Medial pathway ▪ Lateral pathway

None of the motor pathways are located in the posterior white column because posterior column pathway is a sensory pathway and occupies that whole area. The corticospinal pathway • Sometimes called the pyramidal system • Provides voluntary control over skeletal muscles o System begins at pyramidal cells of primary motor cortex o Axons of these upper motor neurons descend into brain stem and spinal cord to synapse on lower motor neurons that control skeletal muscles. • Contains three pairs of descending tracts o 1. Corticobulbar tracts o 2. Lateral corticospinal tracts o 3. Anterior corticospinal tracts Corticobulbar tract • Provide conscious control over skeletal muscles that move the eye, jaw, face, and some muscles of the neck and pharynx • Innervate motor centers of medial and lateral pathways Corticospinal tracts • As they descend, lateral corticospinal tracts are visible along the ventral surface of medulla oblongata as a pair of thick bands, the pyramids • At spinal segment it targets, an axon in anterior corticospinal tract crosses over to opp side of spinal cord in anterior white commissure before synapsing on lower motor neurons in anterior gray horns

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Motor homunculus

o Primary motor cortex corresponds point by point with specific regions of body o Cortical areas have been mapped out in diagrammatic form o Homunculus provides indication of degree of fine motor control available ▪ Hands, face, tongue, which are capable of varied and complex movements, appear very large, while trunk is relatively small ▪ These proportions are similar to the sensory homunculus The medial and lateral pathways • Several centers in cerebrum, dienceohalon, and brain stem may issue somatic motor commands as result of processing performed at subconscious level • These nuclei and tracts are grouped by their primary functions o Components of medial pathway help control gross movements of trunk and proximal limb muscles o Components of lateralpathway help control distal limb muscles that perform more orecise movements The Medial Pathway • Primarily concerned with control of muscle tone and gross movements of neck, trunk, and proximal limb muscles • Upper motor neurons of medial pathway are located in: o Vestibular nuclei o Superior and inferior colliculi o Reticular formation • Vestibular nuclei o Receive info over the vestibulocochlear nerve from receptors in inner ear that monitor position and movement of the head ▪ Primary goal is to maintain posture and balance ▪ Descending fibers of spinal cord constitute vestibulospinal tracts • Superior and inferior colliculi o Are located in the roof of the mesencephalon, or the tectum o Colliculi receive visual and auditory sensations o Axons of upper motor neurons in colliculi descend in tectospinal tracts o These axons cross to opp side, before descending to synapse on lower motor neurons in brain stem or spinal cord • Reticular formation o Loosely organized network of neurons that extends throughtout brain stem o Axons of upper motor neurons in reticular formation descend into reticulospinal tracts without crossing to opp side The Lateral pathway • Primarily concerned with control of muscle tone and more precise movements of distal parts of limbs • Axons of upper motor neurons in red nuclei cross to opposite side of brain and descend into spinal cord in rubrospinal tracts

• The Basal Nuclei and cerebellum • Responsible for coordination and feedback control over muscle contractions o Whether contractions are consciously or subconsciously directed o Provide background patterns of movement involved in voluntary motor activities ▪ Some axons extend to the premotor cortex, the motor association area that directs activities of the primary motor cortex • Alters the pattern of instructions carried by the corticospinal tracts • Other axons alter the excitatory or inhibitory output of the reticulospinal tracts The cerebellum • Monitors o Proprioceptive sensation o Visual info from eyes o Vestibular sensations from inner ear as movements are udnerway • Levels of Processing and Motor Control o All sensory and motor pathways involve a series of synapses, one after the other o General pattern ▪ Spinal and cranial reflexes provide rapid, involuntary, preprogrammed responses that preserve homeostasis over short term o Cranial and spinal reflexes ▪ Control the most basic motor activities o Integrative centers in the brain ▪ Perform more elaborate processing ▪ As we move from medulla oblongata to cerebral cortex, motor patterns become increasingly complex and variable o Primary motor cortex ▪ Most complex and variable motor activities are directed by primary motor cortex of cerebral hemispheres o Neurons of the primary motor cortex

▪ Innervate motor neurons in the brain and spinal cord responsible for stimulating skeletal muscles o Higher centers in the brain ▪ Can suppress or facilitate reflex responses o Reflexes ▪ Can complement or increase the complexity of voluntary movements o...