Post on 19-Oct-2020
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Fundamentals of the Nervous System and Nervous Tissue: Part A
Func?ons of the Nervous System
1. Sensory input – Informa?on gathered by sensory receptors about
internal and external changes
2. Integra?on – Interpreta?on of sensory input
3. Motor output – Ac?va?on of effector organs (muscles and
glands) produces a response
Figure 11.1
Sensory input
Motor output
Integration
Divisions of the Nervous System
• Central nervous system (CNS) – Brain and spinal cord – Integra?on and command center
• Peripheral nervous system (PNS) – Paired spinal and cranial nerves carry messages to and from the CNS
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Peripheral Nervous System (PNS)
• Two func?onal divisions 1. Sensory (afferent) division
• Soma?c afferent fibers—convey impulses from skin, skeletal muscles, and joints
• Visceral afferent fibers—convey impulses from visceral organs
2. Motor (efferent) division • Transmits impulses from the CNS to effector organs
Motor Division of PNS
1. Soma?c (voluntary) nervous system – Conscious control of skeletal muscles
Motor Division of PNS
2. Autonomic (involuntary) nervous system (ANS)
– Visceral motor nerve fibers – Regulates smooth muscle, cardiac muscle, and
glands – Two func?onal subdivisions
• Sympathe?c • Parasympathe?c
Figure 11.2
Central nervous system (CNS) Brain and spinal cord Integrative and control centers
Peripheral nervous system (PNS) Cranial nerves and spinal nerves Communication lines between the CNS and the rest of the body
Parasympathetic division
Conserves energy Promotes house- keeping functions during rest
Motor (efferent) division Motor nerve fibers Conducts impulses from the CNS to effectors (muscles and glands)
Sensory (afferent) division Somatic and visceral sensory nerve fibers Conducts impulses from receptors to the CNS
Somatic nervous system
Somatic motor (voluntary) Conducts impulses from the CNS to skeletal muscles
Sympathetic division Mobilizes body systems during activity
Autonomic nervous system (ANS)
Visceral motor (involuntary) Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands
Structure Function Sensory (afferent) division of PNS Motor (efferent) division of PNS
Somatic sensory fiber
Visceral sensory fiber
Motor fiber of somatic nervous system
Skin
Stomach Skeletal muscle
Heart
Bladder Parasympathetic motor fiber of ANS
Sympathetic motor fiber of ANS
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Histology of Nervous Tissue
• Two principal cell types 1. ______________________—excitable cells that
transmit electrical signals
Histology of Nervous Tissue
2. ______________________ (glial cells)—suppor?ng cells: • Astrocytes (CNS) • Microglia (CNS) • Ependymal cells (CNS) • Oligodendrocytes (CNS) • Satellite cells (PNS) • Schwann cells (PNS)
______________________
• Most abundant, versa?le, and highly branched glial cells
• Cling to neurons, synap?c endings, and capillaries
• ______________________ and brace neurons
Astrocytes
• Help determine capillary permeability • Guide migra?on of young neurons • Control the chemical environment • Par?cipate in informa?on processing in the brain
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Figure 11.3a
(a) Astrocytes are the most abundant CNS neuroglia.
Capillary
Neuron
Astrocyte
______________________
• Small, ovoid cells with thorny processes • Migrate toward injured neurons • Phagocy?ze microorganisms and neuronal debris
Figure 11.3b
(b) Microglial cells are defensive cells in the CNS.
Neuron
Microglial cell
______________________ Cells
• Range in shape from squamous to columnar • May be ______________________ • Line the central cavi?es of the brain and spinal column – Separate the CNS inters??al fluid from the cerebrospinal fluid in the cavi?es
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Figure 11.3c
Brain or spinal cord tissue
Ependymal cells
Fluid-filled cavity
(c) Ependymal cells line cerebrospinal fluid-filled cavities.
______________________
• Branched cells • Processes wrap CNS nerve fibers, forming insula?ng myelin sheaths
Figure 11.3d
(d) Oligodendrocytes have processes that form myelin sheaths around CNS nerve fibers.
Nerve fibers
Myelin sheath
Process of oligodendrocyte
______________________ Cells and ______________________ Cells
• Satellite cells – Surround neuron cell bodies in the PNS
• Schwann cells (neurolemmocytes) – Surround peripheral nerve fibers and form myelin sheaths
– Vital to regenera?on of damaged peripheral nerve fibers
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Figure 11.3e
(e) Satellite cells and Schwann cells (which form myelin) surround neurons in the PNS.
Schwann cells (forming myelin sheath)
Cell body of neuron Satellite cells
Nerve fiber
______________________ (Nerve Cells)
• Special characteris?cs: – Long-‐lived (→ 100 years or more) – Amito?c—with few excep?ons – High metabolic rate—depends on con?nuous supply of oxygen and glucose
– Plasma membrane func?ons in: • Electrical signaling • Cell-‐to-‐cell interac?ons during development
Cell Body (Perikaryon or Soma)
• Biosynthe?c ______________________ of a neuron
• Spherical nucleus with nucleolus • Well-‐developed ______________________ • Rough ER called Nissl bodies (chromatophilic substance)
Cell Body (Perikaryon or Soma)
• Network of neurofibrils (neurofilaments) • _________ _____________—cone-‐shaped area from which axon arises
• Clusters of cell bodies are called nuclei in the CNS, ganglia in the PNS
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Figure 11.4b
Dendrites (receptive regions)
Cell body (biosynthetic center and receptive region)
Nucleolus
Nucleus Nissl bodies
Axon (impulse generating and conducting region)
Axon hillock Neurilemma
Terminal branches
Node of Ranvier Impulse direction
Schwann cell (one inter- node)
Axon terminals (secretory region)
(b)
Processes
• ______________________ and axons • Bundles of processes are called
– Tracts in the CNS – Nerves in the PNS
Dendrites
• Short, tapering, and diffusely branched • ______________________ (input) region of a neuron
• Convey electrical signals toward the cell body as graded poten?als
The Axon • One axon per cell arising from the axon hillock • Long axons (nerve fibers)
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The Axon
• Numerous ______________________ branches (telodendria)
• Knoblike axon terminals (synap?c knobs or boutons) – ______________________ region of neuron – Release neurotransmifers to excite or inhibit other cells
Axons: Func?on
• ______________________ region of a neuron • Generates and transmits nerve impulses (ac?on poten?als) away from the cell body
Axons: Func?on
• Molecules and organelles are moved along axons by motor molecules in two direc?ons: – ______________________—toward axonal terminal
• Examples: mitochondria, membrane components, enzymes
– ______________________—toward the cell body • Examples: organelles to be degraded, signal molecules, viruses, and bacterial toxins
Figure 11.4b
Dendrites (receptive regions)
Cell body (biosynthetic center and receptive region)
Nucleolus
Nucleus Nissl bodies
Axon (impulse generating and conducting region)
Axon hillock Neurilemma
Terminal branches
Node of Ranvier Impulse direction
Schwann cell (one inter- node)
Axon terminals (secretory region)
(b)
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Myelin Sheath
• Segmented protein-‐lipoid sheath around most long or large-‐diameter axons
• It func?ons to: – Protect and electrically ______________________ the axon
– Increase ______________________ of nerve impulse transmission
Myelin Sheaths in the PNS
• ______________________ cells wraps many ?mes around the axon – Myelin sheath—concentric layers of Schwann cell membrane
• Neurilemma—peripheral bulge of Schwann cell cytoplasm
Myelin Sheaths in the PNS
• ______________________ of Ranvier – Myelin sheath gaps between adjacent Schwann cells
– Sites where axon collaterals can emerge
Figure 11.5a
(a) Myelination of a nerve fiber (axon)
Schwann cell cytoplasm Axon
Neurilemma Myelin sheath
Schwann cell nucleus
Schwann cell plasma membrane
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A Schwann cell envelopes an axon.
The Schwann cell then rotates around the axon, wrapping its plasma membrane loosely around it in successive layers.
The Schwann cell cytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath.
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Unmyelinated Axons
• Thin nerve fibers are ______________________
• One Schwann cell may incompletely enclose 15 or more unmyelinated axons
Myelin Sheaths in the CNS
• Formed by processes of ______________________, not the whole cells
• Nodes of Ranvier are present • No neurilemma • Thinnest fibers are ______________________
Figure 11.3d
(d) Oligodendrocytes have processes that form myelin sheaths around CNS nerve fibers.
Nerve fibers
Myelin sheath
Process of oligodendrocyte
White Mafer and Gray Mafer
• ______________________ mafer – Dense collec?ons of myelinated fibers
• ______________________ mafer – Mostly neuron cell bodies and unmyelinated fibers
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Structural Classifica?on of Neurons
• Three types: 1. ______________________—1 axon and several
dendrites • Most abundant • Motor neurons and interneurons
2. ______________________—1 axon and 1 dendrite • Rare, e.g., re?nal neurons
Structural Classifica?on of Neurons
3. ______________________ (pseudounipolar)—single, short process that has two branches:
• Peripheral process—more distal branch, oien associated with a sensory receptor
• Central process—branch entering the CNS
Table 11.1 (1 of 3) Table 11.1 (2 of 3)
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Func?onal Classifica?on of Neurons
• Three types: 1. ______________________ (afferent)
• Transmit impulses from sensory receptors toward the CNS
2. ______________________ (efferent) • Carry impulses from the CNS to effectors
Func?onal Classifica?on of Neurons
3. ______________________ (associa?on neurons) • Shufle signals through CNS pathways; most are
en?rely within the CNS
Table 11.1 (3 of 3)