Nervous System III PDF

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NERVOUS SYSTEM, Pt. 3 lecture - Professor Tonya Penkrot...

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Nervous System III: Spinal Cord & PNS 12.10 Spinal Cord Gross Anatomy and Protection  Spinal cord is enclosed in vertebral column o Begins at the foramen magnum o Ends at L1 or L2 vertebra  Functions o Provides two-way communication to and from brain and body o Major reflex center: reflexes are initiated and completed at spinal cord  Protected by bone, meninges, and CSF  Spinal dura mater is one layer thick o Does not attach to vertebrae  Epidural space o Cushion of fat and network of veins in space between vertebrae and spinal dura mater  CSF fills subarachnoid space between arachnoid and pia maters  Dural and arachnoid membranes extend to sacrum, beyond end of cord at L1 or L2 o Site of lumbar puncture or tap  

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Spinal cord terminates in cone-shaped structure called conus medullaris Filum terminale extends to coccyx o Fibrous extension of conus covered with pia mater o Anchors spinal cord Denticulate ligaments o Extensions of pia mater that secure cord to dura mater o Cervical and lumbar enlargements: areas where nerves servicing upper and lower limbs arise from spinal cord Spinal nerves o Part of PNS o Attach to spinal cord by 31 paired roots Cervical and lumbosacral enlargements o Nerves serving upper and lower limbs emerge here Cauda equina o Collection of nerve roots at inferior end of vertebral canal

Spinal Cord Cross-sectional Anatomy 

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Two lengthwise grooves that run length of cord partially divide it into right and left halves o Ventral (anterior) media fissure o Dorsal (posterior) media sulcus Gray matter is located in core, white matter outside Central canal runs length of cord o Filled with CSF Gray matter and spinal roots o Cross section of cord resembles butterfly or letter "H" o Three areas of gray matter are found on each side of center and are mirror images:  Dorsal horns: interneurons that receive somatic and visceral sensory input  Ventral horns: some interneurons; somatic motor neurons  Lateral horns (only in thoracic and superior lumbar regions): sympathetic neurons o Gray commissure: bridge of gray matter that connects masses of gray matter on either side  Encloses central canal

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Ventral roots: bundle of motor neuron axons that exit the spinal cord Dorsal roots: sensory input to cord Dorsal root (spinal) ganglia: cell bodies of sensory neurons Spinal nerves: formed by fusion of dorsal and ventral roots Gray matter divided into four groups based on if somatic or visceral innervation:  Somatic sensory (SS), visceral sensory (VS), visceral (autonomic) motor (VN), and somatic motor (SM)

White matter o Myelinated and non-myelinated nerve fibers allow communication between parts of spinal cord, and spinal cord and brain o Run in three directions  Ascending: up to higher centers (sensory inputs)  Descending: from brain to cord or lower cord levels (motor outputs)  Transverse: from one side to other (commissural fibers) o White matter is divided into three white columns (funiculi) on each side  Dorsal (posterior)  Lateral  Ventral (anterior) o Each spinal tract is composed of axons with similar destinations and functions

Spinal Cord Trauma and Disorders 

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Spinal cord trauma o Localized injury to spinal cord or its roots leads to functional losses  Paresthesias: caused by damage to dorsal roots or sensory tracts  Leads to sensory function loss  Paralysis: caused by damage to ventral roots or ventral horn cells  Leads to motor function loss  Two types of paralysis: flaccid or spastic o Flaccid paralysis: severe damage to ventral root or ventral horn cells  Impulses do not reach muscles; there is no voluntary or involuntary control of muscles  Muscles atrophy o Spastic paralysis: damage to upper motor neurons of primary motor cortex  Spinal neurons remain intact; muscles are stimulated by reflex activity  No voluntary control of muscles  Muscles often shorten permanently o Transection (cross sectioning) of spinal cord at any level results in total motor and sensory loss in regions inferior to cut  Paraplegia: transection between T1 and L1  Quadriplegia: transection in cervical region o Spinal shock: transient period of functional loss caudal to lesion Poliomyelitis o Destruction of ventral horn motor neurons by poliovirus o Muscles atrophy o Death may occur from paralysis of respiratory muscles or cardiac arrest o Survivors often develop postpolio syndrome many years later from neuron loss Amyotrophic lateral sclerosis (ALS) o Also called Lou Gehrig's disease o Destruction of ventral horn motor neurons and fibers of pyramidal tract o Symptoms: loss of ability to speak, swallow, and breathe o Death typically occurs within 5 years  Stephen Hawking an extreme exception o Caused by environmental factors and genetic mutations involving RNA processing

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Involves glutamate excitotoxicity Drug riluzole interferes

12.11 Neuronal Pathways  

Major spinal tracts are part of multi-neuron pathways Four key points about spinal tracts and pathways: o Decussation: most pathways cross from one side of CNS to other at some point o Relay: consist of chain of two or three neurons o Somatotopy: precise spatial relationship in CNS correspond to spatial relationship in body o Symmetry: pathways are paired symmetrically (right and left)

Ascending Pathways 

Conduct sensory pathways upward through a chain of three neurons: o First-order neuron  Conducts impulses from cutaneous receptors and proprioceptors  Branches diffusely as it enters spinal cord or medulla  Synapses with second-order neuron o Second-order neuron  Interneuron  Cell body in dorsal horn of spinal cord or medullary nuclei  Axons extend to thalamus or cerebellum o Third-order neuron  Also an interneuron  Cell bodies in thalamus  Axons extends to somatosensory cortex  No third-order neurons in cerebellum

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Somatosensory signals travel along three main pathways on each side of spinal cord: o Two pathways transmit somatosensory information to sensory cortex via thalamus  Dorsal column-medial lemniscal pathways  Spinothalamic pathways  Provide for discriminatory touch and conscious proprioception o Third pathway, spinocerebellar tracts, terminate in the cerebellum o Dorsal column-medial lemniscal pathways  Transmit input to somatosensory cortex for discriminative touch and vibrations  Composed of paired fasciculus cuneatus and fasciculus gracilis in psinal cord and medial lemniscus in brain (medulla to thalamus) o Spinothalamic pathways  Lateral and ventral spinothalamic tracts  Transmit pain, temperature, coarse touch, and pressure impulses within lateral spinothalamic tract o Spinocerebellar tracts  Ventral and dorsal tracts  Convey information about muscle or tendon stretch to cerebellum  Used to coordinate muscle activity

Descending Pathways and Tracts  

Deliver efferent impulses form brain to spinal cord Two groups: o Direct pathways: pyramidal tracts  Impulses from pyramidal neurons in precentral gyri pass through pyramidal (lateral and ventral corticospinal) tracts

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Descend directly without synapsing until axon reaches end of tract in spinal cord In spinal cord, axons synapse with interneurons (lateral tract) or ventral horn motor neurons (ventral tract)  Direct pathway regulates fast and fine (skilled) movements o Indirect pathways: all others  Also referred to as multi-neuronal pathways  Complex and multi-synaptic  Includes brain stem motor nuclei and all motor pathways except pyramidal pathways  These pathways regulate:  Axial muscles, maintaining balance and posture  Muscles controlling coarse limb movements  Head, neck, and eye movements that follow objects in visual field Motor pathways involve two neurons: o Upper motor neurons  Pyramidal cells in primary motor cortex o Lower motor neurons  Ventral horn motor neurons  Innervate skeletal muscles

Developmental Aspects of Central Nervous System 

Starting in a 3-week old embryo: 1. Ectoderm thickens, forming neural plate  Invaginates, forming neural groove flanked by neural folds 2. Neural crest forms from migrating neural fold cells 3. Neural groove deepens, edges fuse forming neural tube

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Neural tube, formed by week 4, differentiates into CNS o Brain forms rostrally o Spinal cord forms caudally By week 6, both sides of spinal cord bear a dorsal alar plate and a ventral basal plate o Alar plate becomes interneurons o Basal plate becomes motor neurons o Neural crest cells form dorsal root ganglia

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Hypothalamus is one of last areas of CNS to develop o Premature infants have poor body temperature regulation Visual cortex develops slowly over first 11 weeks Neuromuscular coordination progresses in superior-to-inferior and proximal-to-distal directions along with myelination

Clinical -- Homeostatic Imbalance 12.2 

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Cerebral palsy: neuromuscular disability involving poorly controlled or paralyzed voluntary muscles o Due to brain damage, possibly form lack of oxygen during birth o Spasticity, speech difficulties, motor impairments can be seen o Some patients have seizures, are intellectually impaired, and/or are deaf o Visual impairment is common Anencephaly: cerebrum and parts of brain stem never develop o Neural fold fails to fuse o Child is vegetative o Death occurs soon after birth Spina bifida: incomplete formation of vertebral arches o Spina bifida occulata: least serious, involves only one or few missing vertebrae  Causes no neural problems

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Spina bifida cystica: more severe and most common Saclike cyst protrudes dorsally from spine Cyst may contain CSF (meningocele) or portions of spinal cord and nerve roots (myelomeningocele)  Larger cysts cause neurological impairment  Usually also see hydrocephalus Most cases caused by lack of B vitamin folic acid  US incidence has dropped with mandatory supplementation  

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The Peripheral Nervous System  

PNS provides links from and to world outside our body Consists of all neural structures outside brain and spinal cord that can be broken down into four parts: 1. Sensory Receptors 2. Transmission Lines: Nerves and Their Structure and Repair 3. Motor Endings and Motor Activity 4. Reflex Activity

Part 1 -- Sensory Receptors and Sensation 13.1 Sensory Receptors   

Sensory receptors: specialized to respond to changes in environment (stimuli) o Activation results in graded potentials that trigger nerve impulses Awareness of stimulus (sensation) and interpretation of meaning of stimulus (perception) occur in brain Three ways to classify receptors: by type of stimulus, body location, and structural complexity

Classification by Stimulus Type     

Mechanoreceptors: respond to touch, pressure, vibration, and stretch Thermoreceptors: sensitive to changes in temperature Photoreceptors: respond to light energy (example: retina) Chemoreceptors: respond to chemicals (examples: smell, taste, changes in blood chemistry) Nociceptors: sensitive to pain-causing stimuli (examples: extreme heat or cold, excessive pressure, inflammatory chemicals)

Classification by Location 

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Exteroceptors o Respond to stimuli arising outside body o Receptors in skin for touch, pressure, pain, and temperature o Most special sense organs Interoceptors (visceroceptors) o Respond to stimuli arising in internal viscera and blood vessels o Sensitive to chemical changes, tissue stretch, and temperature changes o Sometimes cause discomfort but usually person is unaware of their workings Proprioceptors o Respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles o Inform brain of one's movements

Classification by Receptor Structure 

Majority of sensory receptors belong to one of two categories: o Simple receptors of the general senses  Modified dendritic endings of sensory neurons

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Are found throughout body and monitor most types of general sensory information Receptors for special senses  Vision, hearing, equilibrium, smell, and taste  All are housed in complex sense organs  Covered in Chapter 15

Simple receptors of the general senses o General senses include tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle sense  No "one-receptor-one-function" relationship  Receptors can respond to multiple stimuli o Receptors have either:  Nonencapsulated (free) nerve endings  Abundant in epithelia and connective tissues  Most are nonmyelinated, small-diameter group C fibers; distal terminals have knoblike swellings  Respond mostly to temperature, pain, or light touch  Thermoceptors  Cold receptors are activated by temps from 10 to 40*C  Located in superficial dermis  Heat receptors are activated from 32 to 48*C located in deeper dermis  Outside those temperature ranges, nociceptors are activated and interpreted as pain  Nociceptors: pain receptors triggered by extreme temperature changes, pinch, or release of chemicals from damaged tissue  Vanilloid receptor: protein in nerve membrane is main player  Acts as ion channel that is opened by heat, low pH, chemicals (examples: capsaicin in red peppers)  Itch receptors in dermis: can be triggered by chemicals such as histamine  Tactile (Merkel) discs: function as light touch receptors  Located in deeper layers of epidermis  Hair follicle receptors: free nerve endings that wrap around hair follicles  Act as light touch receptors that detect bending of hairs  Example: allows you to feel a mosquito landing on your skin  Encapsulated nerve endings  Almost all are mechanoreceptors whose terminal endings are encased in connective tissue capsule  Vary greatly in shape and include:  Tactile (Meissner's) corpuscles: small receptors involved in discriminative touch  Found just below skin, mostly in sensitive and hairless areas (fingertips)  Lamellar (Pacinian) corpuscles: large receptors respond to deep pressure and vibration with first applied (then turn off)  Located in deep dermis  Bulbous corpuscles (Ruffini endings): respond to deep and continuous pressure  Located in dermis  Muscle spindles: spindle-shaped proprioceptors that respond to muscle stretch  Tendon organ: proprioceptors located in tends that detect stretch  Joint kinesthetic receptors: proprioceptors that monitor joint position and motion

13.2 Sensory Processing

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Survival depends upon: o Sensation: the awareness of changes in the internal and external environment o Perception: the conscious interpretation of those stimuli

General Organization of the Somatosensory System    

Somatosensory system: part of sensory system serving body wall and limbs Receives inputs from: o Exteroceptors, proprioceptors, and interoceptors Input is relayed toward head, but processed along the way Levels of neural integration in sensory systems: o Receptor level: sensory receptors o Circuit level: processing in ascending pathways o Perceptual level: processing in cortical sensory areas

Perception of Pain    

Warns of actual or impeding tissue damage so protective action can be taken Stimuli include extreme pressure and temperature, histamine, K+, ATP, acids, and bradykinin Impulses travel on fibers that release neurotransmitters glutamate and substance P Some pain impulses are blocked by inhibitory endogenous opioids (example: endorphins)

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Pain tolerance o All perceive pain at same stimulus intensity o Pain tolerance varies o "Sensitive to pain" means low pain tolerance, not low pain threshold o Genes help determine pain tolerance as well as response to pain medications  Research in use of genetics to determine best pain treatment is ongoing

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Visceral and referred pain o Visceral pain results from stimulation of visceral organ receptors  Felt as vague aching, gnawing, burning  Activated by tissue stretching, ischemia, chemicals, muscle pains o Referred pain: pain from one body region perceived as coming form different region  Visceral and somatic pain fibers travel along same nerves, so brain assumes stimulus comes from common (somatic) region  Example: left arm pain during heart attack

Clinical -- Homeostatic Imbalance 13.1 

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Long-lasting or intense pain, such as limb amputation, can lead to hyperalgesia (pain amplification), chronic pain, and phantom limb pain o NMDA receptors are activated by long-lasting or intense pain  Allow spinal cord to "learn" hyperalgesia  Early pain management critical to prevent Phantom limb pain: pain felt in limb that has been amputated o Now use epidural anesthesia during surgery to reduce phantom pain

Part 2 -- Transmission Lines: Nerves and Their Structure and Repair 13.3 Nerves and Associated Ganglia Structure and Classification

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Nerve: cordlike organ of PNS Bundle of myelinated and non-myelinated peripheral axons enclosed by connective tissue Two types of nerves: spinal or cranial, depending on where they originate Connective tissue coverings include: o Endoneurium: loose connective tissue that encloses axons and their myelinated sheaths (Schwann cells) o Perineurium: coarse connective tissue that bundles fibers into fascicles o Epineurium: tough fibrous sheath around all fascicles to form the nerve

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Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers Nerves are classified according to the direction they transmit impulses o Mixed nerves: contain both sensory and motor fibers  Impulses travel both to and from CNS o Sensory (afferent) nerves: impulses only toward CNS o Motor (efferent) nerves: impulses only away from CNS

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Pure sensory (afferent) or pure motor (efferent) nerves are rare; most nerves are mixed Types of fibers in mixed nerves: o Somatic afferent (sensory from muscle to brain) o Somatic efferent (motor from brain to muscle) o Visceral afferent (sensory from organs to brain) o Visceral efferent (motor from brain to organs)

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Ganglia: contain neuron cell bodies associated with nerves in PNS o Ganglia associated with afferent nerve fibers contain cell bodies of sensory neurons  Dorsal root ganglia (sensory, somatic) o Ganglia associated with efferent nerve fibers contain autonomic motor neurons  Autonomic ganglia (motor, visceral)

Regeneration of Nerve Fibers 

Mature neurons are amitotic, but if the soma (cell body) of the damaged nerve is intact, the peripheral axon may regenerate in PNS; does not occur in CNS

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CNS axons o Most CNS fibers never regenerate o CNS oligodendrocytes bear growth-inhibiting proteins that prevent CNS fiber regeneration o Astrocytes at injury site form scar tissue o Treatment: neutralizing growth inhibitors, blocking receptors for inhibitory proteins, destroying scar tissue components PNS axons o PNS axons can regenerate in damage is not severe 1. Axon fragments and myelin sheaths distal to injury degenerate (Wallerian degeneration); degeneration spreads down axon 2. Macrophages clean dead axon debris; Schwann cells are stimulated to divide 3. Axon filaments grow through regeneration tube 4. Axon regenerates, and new myelin sheath forms

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Shingles 

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Chickenpox: common disease of early childhood o Caused by varicella-zoster virus o Produces itchy rash that clears up without complications Virus remains for life in the posterior root ganglion

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o Kept in check by the immune system Shingles (herpes zoster): l...