chapter 17 nervous tissue
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Chapter 17 Nervous Tissue. Controls and integrates all body activities within limits that maintain life Three basic functions sensing changes with sensory receptors fullness of stomach or sun on your face interpreting and remembering those changes reacting to those changes with effectors - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 17Nervous Tissue
• Controls and integrates all body activities within limits that maintain life
• Three basic functions– sensing changes with sensory receptors
• fullness of stomach or sun on your face
– interpreting and remembering those changes– reacting to those changes with effectors
• muscular contractions
• glandular secretions
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Major Structures of the Nervous System
• Brain, cranial nerves, spinal cord, spinal nerves, ganglia, enteric plexuses and sensory receptors
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Organization of the Nervous System
• CNS is brain and spinal cord• PNS is everything else
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Nervous System Divisions
• Central nervous system (CNS) – consists of the brain and spinal cord
• Peripheral nervous system (PNS)– consists of cranial and spinal nerves that contain both
sensory and motor fibers– connects CNS to muscles, glands & all sensory
receptors
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Subdivisions of the PNS• Somatic (voluntary) nervous system (SNS)
– neurons from cutaneous and special sensory receptors to the CNS
– motor neurons to skeletal muscle tissue
• Autonomic (involuntary) nervous systems– sensory neurons from visceral organs to CNS– motor neurons to smooth & cardiac muscle and glands
• sympathetic division (speeds up heart rate)
• parasympathetic division (slow down heart rate)
• Enteric nervous system (ENS)– involuntary sensory & motor neurons control GI tract– neurons function independently of ANS & CNS
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Neurons• Functional unit of nervous system• Have capacity to produce action potentials
– electrical excitability• Cell body
– single nucleus with prominent nucleolus
– Nissl bodies (chromatophilic substance)
• rough ER & free ribosomes for protein synthesis
– neurofilaments give cell shape and support
– microtubules move material inside cell– lipofuscin pigment clumps (harmless
aging)• Cell processes = dendrites & axons
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Nucleus with Nucleolus
Parts of a Neuron
Axons or Dendrites
Cell body
Neuroglial cells
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Dendrites
• Conducts impulses towards the cell body
• Typically short, highly branched & unmyelinated
• Surfaces specialized for contact with other neurons
• Contains neurofibrils & Nissl bodies
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Axons• Conduct impulses away from
cell body• Long, thin cylindrical
process of cell • Arises at axon hillock • Impulses arise from initial
segment (trigger zone)• Side branches (collaterals)
end in fine processes called axon terminals
• Swollen tips called synaptic end bulbs contain vesicles filled with neurotransmitters Synaptic boutons
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Axonal Transport
• Cell body is location for most protein synthesis– neurotransmitters & repair proteins
• Axonal transport system moves substances – slow axonal flow
• movement in one direction only -- away from cell body
• movement at 1-5 mm per day
– fast axonal flow• moves organelles & materials along surface of microtubules
• at 200-400 mm per day
• transports in either direction
• for use or for recycling in cell body
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Axonal Transport & Disease
• Fast axonal transport route by which toxins or pathogens reach neuron cell bodies– tetanus (Clostridium tetani bacteria)– disrupts motor neurons causing painful muscle spasms
• Bacteria enter the body through a laceration or puncture injury– more serious if wound is in head or neck because of
shorter transit time
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Functional Classification of Neurons
• Sensory (afferent) neurons– transport sensory information from skin,
muscles, joints, sense organs & viscera to CNS
• Motor (efferent) neurons– send motor nerve impulses to muscles & glands
• Interneurons (association) neurons– connect sensory to motor neurons– 90% of neurons in the body
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Structural Classification of Neurons
• Based on number of processes found on cell body– multipolar = several dendrites & one axon
• most common cell type
– bipolar neurons = one main dendrite & one axon• found in retina, inner ear & olfactory
– unipolar neurons = one process only(develops from a bipolar)• are always sensory neurons
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Association or Interneurons• Named for histologist that first described them or
their appearance
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• Half of the volume of the CNS
• Smaller cells than neurons
• 50X more numerous
• Cells can divide – rapid mitosis in tumor formation (gliomas)
• 4 cell types in CNS– astrocytes, oligodendrocytes, microglia & ependymal
• 2 cell types in PNS– schwann and satellite cells
Neuroglial Cells
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Neuroglia of the central nervous system (CNS).
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Astrocytes
• Star-shaped cells• Form blood-brain
barrier by covering blood capillaries
• Metabolize neurotransmitters
• Regulate K+ balance• Provide structural
support
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Oligodendrocytes
• Most common glial cell type
• Each forms myelin sheath around more than one axons in CNS
• Analogous to Schwann cells of PNS
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Microglia
• Small cells found near blood vessels• Phagocytic role -- clear away dead cells• Derived from cells that also gave rise to
macrophages & monocytes
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Ependymal cells
• Form epithelial membrane lining cerebral cavities & central canal
• Produce cerebrospinal fluid (CSF)
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Satellite Cells
• Flat cells surrounding neuronal cell bodies in peripheral ganglia
• Support neurons in the PNS ganglia
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Schwann Cell
• Cells encircling PNS axons• Each cell produces part of the myelin sheath
surrounding an axon in the PNS
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Axon Coverings in PNS
• All axons surrounded by a lipid & protein covering (myelin sheath) produced by Schwann cells
• Neurilemma is cytoplasm & nucleusof Schwann cell
– gaps called nodes of Ranvier• Myelinated fibers appear white
– jelly-roll like wrappings made of lipoprotein = myelin
– acts as electrical insulator– speeds conduction of nerve
impulses• Unmyelinated fibers
– slow, small diameter fibers– only surrounded by neurilemma
but no myelin sheath wrapping
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Myelination in PNS
• Schwann cells myelinate (wrap around) axons in the PNS during fetal development
• Schwann cell cytoplasm & nucleus forms outermost layer of neurolemma with inner portion being the myelin sheath
• Tube guides growing axons that are repairing themselves
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Myelination in the CNS
• Oligodendrocytes myelinate axons in the CNS
• Broad, flat cell processes wrap about CNS axons, but the cell bodies do not surround the axons
• No neurilemma is formed
• Little regrowth after injury is possible due to the lack of a distinct tube or neurilemma
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Gray and White Matter
• White matter = myelinated processes (white in color)• Gray matter = nerve cell bodies, dendrites, axon terminals,
bundles of unmyelinated axons and neuroglia (gray color)– In the spinal cord = gray matter forms an H-shaped inner core
surrounded by white matter
– In the brain = a thin outer shell of gray matter covers the surface & is found in clusters called nuclei inside the CNS
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Neuronal Circuits• Neurons in the CNS are organized into neuronal
networks• A neuronal network may contain thousands or
even millions of neurons.• Neuronal circuits are involved in many important
activities– breathing
– short-term memory
– waking up
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Neuronal Circuits
• Diverging -- single cell stimulates many others
• Converging -- one cell stimulated by many others
• Reverberating -- impulses from later cells repeatedly stimulate early cells in the circuit (short-term memory)
• Parallel-after-discharge -- single cell stimulates a group of cells that all stimulate a common postsynaptic cell (math problems)
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Regeneration & Repair
• Plasticity maintained throughout life– sprouting of new dendrites– synthesis of new proteins– changes in synaptic contacts with other neurons
• Limited ability for regeneration (repair)– PNS can repair damaged dendrites or axons– CNS no repairs are possible
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Neurogenesis in the CNS• Formation of new neurons from stem cells was not
thought to occur in humans– 1992 a growth factor was found that stimulates adult mice
brain cells to multiply– 1998 new neurons found to form within adult human
hippocampus (area important for learning)
• Factors preventing neurogenesis in CNS– inhibition by neuroglial cells, absence of growth
stimulating factors, lack of neurolemmas, and rapid formation of scar tissue
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• Axons & dendrites may be repaired if– neuron cell body remains intact– schwann cells remain active and form a tube – scar tissue does not form too rapidly
• Chromatolysis– 24-48 hours after injury, Nissl bodies break up into
fine granular masses
Repair within the PNS
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• By 3-5 days, – wallerian degeneration occurs
(breakdown of axon & myelin sheath distal to injury)
– retrograde degeneration occurs back one node
• Within several months, regeneration occurs– neurolemma on each side of
injury repairs tube (schwann cell mitosis)
– axonal buds grow down the tube to reconnect (1.5 mm per day)
Repair within the PNS
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Multiple Sclerosis (MS)• Autoimmune disorder causing destruction of
myelin sheaths in CNS– sheaths becomes scars or plaques– 1/2 million people in the United States– appears between ages 20 and 40 – females twice as often as males
• Symptoms include muscular weakness, abnormal sensations or double vision
• Remissions & relapses result in progressive, cumulative loss of function
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• The second most common neurological disorder – affects 1% of population
• Characterized by short, recurrent attacks initiated by electrical discharges in the brain– lights, noise, or smells may be sensed– skeletal muscles may contract involuntarily– loss of consciousness
• Epilepsy has many causes, including;– brain damage at birth, metabolic disturbances,
infections, toxins, vascular disturbances, head injuries, and tumors
Epilepsy
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Neuronal Structure & Function