bio l 105—lab 8b-nerve histology-----1 the nervous system 11/14 divisions central nervous system...

BIO L 105—lab 8B-Nerve Histology-----1

The Nervous System 11/14

Divisions•Central nervous system (CNS)

– Brain and spinal cord (dorsal body cavity)– Integration and command center

• Interprets sensory input and dictates motor output

•Peripheral nervous system (PNS)– Portion of nervous system outside CNS;

brings information from body– Consists mainly of nerves that extend from

brain and spinal cord• Spinal nerves to and from spinal cord• Cranial nerves to and from brain

Lab 8B-BIO 105

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Peripheral Nervous System (PNS)

• Two functional divisions– Sensory (afferent) division

• Somatic sensory fibers—convey impulses from skin, skeletal muscles, and joints to CNS

• Visceral sensory fibers—convey impulses from visceral organs to CNS

– Motor (efferent) division • Transmits impulses from CNS to effector organs

– Muscles and glands

• Two divisions– Somatic or Voluntary nervous system– Autonomic nervous system

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Motor Division of PNS: Somatic Nervous System

• Voluntary nervous system– Conscious control of skeletal muscles

• Conducts impulses from CNS to skeletal muscle

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Motor Division of PNS: Autonomic Nervous System

• Involuntary nervous system– Regulates smooth muscle, cardiac muscle,

and glands– Visceral motor nerve fibers

• Two functional subdivisions which work in opposition to each other

• Sympathetic• Parasympathetic

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Histology of Nervous Tissue

• Two principal cell types– Neurons (nerve cells)—excitable cells that

transmit electrical signals– Neuroglia or Glia – small cells that surround,

wrap, and generally support neurons

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• Neuroglial cells:– Provide a supportive scaffolding for neurons– Insulate neurons– Guide young neurons so they can make

proper connections– Promote health and growth

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Neuroglia

• Astrocytes (CNS)

• Microglial cells (CNS)

• Ependymal cells (CNS)

• Oligodendrocytes (CNS)

• Satellite cells (PNS)

• Schwann cells (PNS)

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Astrocytes•Most abundant, highly branched cell•Cling to neurons, synaptic endings, and covers capillaries•Functions include

– Support neurons and help maintain position – Play role in exchanges between capillaries and

neurons; help with nutrient supply – Control chemical environment around neurons

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Microglial Cells

•Small cells with processes that monitor neurons

•Migrate toward injured neurons

•Can transform to phagocytize microorganisms and neuronal debris

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Ependymal Cells

• May be ciliated– Cilia help CerebroSpinalFluid circulate

throughout CNS• Line central cavities of brain and spinal column

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Myelin Sheath• Whitish, protein-lipoid segmented sheath around

most long or large-diameter axons– Myelinated fibers

•Function of myelin– Protects and electrically insulates axon– Increases speed of nerve impulse transmission•Nonmyelinated fibers conduct impulses more slowly

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Oligodendrocytes

•Branched cells that wrap CNS nerve fibers and form insulating myelin sheaths•Can wrap up to 60 axons at once•Have Nodes of Ranvier (gaps in myelin)•No neurilemma

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Satellite Cells and Schwann Cells: in PNS

•Schwann cells – Surround all peripheral nerve fibers and form

myelin sheaths– Wrap around axon in jelly roll fashion– One cell forms one segment of myelin sheath

• Similar function as oligodendrocytes

– Vital to regeneration of damaged peripheral nerve fibers

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• Neurilemma– Outermost layer of Schwann cell containing

nucleus and most of cytoplasm– Essential for axon healing and repair

• Nodes of Ranvier – Myelin sheath gaps between adjacent

Schwann cells

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Neurons– Structural units of nervous system--form gray

matter of CNS

– Highly specialized cells that conduct impulses

– Extreme longevity

– High metabolic rate—requires continuous supply of oxygen and glucose

– All have cell body, axon and one or more dendrites

•Plasma membrane functions in:– Electrical signaling

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Neuron Cell Body (Perikaryon or Soma)•Contains nucleus and nucleolus•Major biosynthetic center

– Synthesizes proteins, membranes, and other chemicals

– Rough ER is very active; also called nissl bodies • Synthesizes neurotransmitters

•Plasma membrane receives information from other neurons

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Dendrites– short, tapering, diffusely branched processes – Increase surface area of cell so can receive

messages

•Receptive (input) region of neuron•Convey incoming electrical signals toward cell body as graded potentials (short distance signals) not action potentials

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The Axon •Generates and transmits nerve impulses along axolemma (neuron cell membrane) to axon terminal

– Neurotransmitters released into extracellular space• Either excite or inhibit neurons with which axons are in close

contact

•Carries on many conversations with different neurons at same time

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Structural Classification of Neurons•Grouped by number of processes•Three types

– Multipolar – 3 or more processes• 1 axon, other processes dendrites• Most common; major neuron in CNS

– Bipolar – 2 processes• Rare, e.g., Retina and olfactory mucosa

– Unipolar – 1 short process– One process –sensory or afferent receptor – Other process – motor or efferent branch

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Functional Classification of Neurons

• Grouped by the direction in which impulse travels in relation to CNS

• Sensory or afferent– Transmit impulses from sensory receptors toward CNS– Almost all are Unipolar

• Motor or efferent– Carry impulses away from CNS to effectors– Multipolar

• Interneurons (association neurons)– Lie between motor and sensory neurons– Shuttle signals through CNS pathways; most are entirely within

CNS

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Structure of a Nerve•Bundle of myelinated and non-myelinated axons enclosed by connective tissue•Cell bodies lie in CNS or in ganglia near CNS•Connective tissue coverings include

– Endoneurium—loose connective tissue that encloses axons and their myelin sheaths

– Perineurium—coarse connective tissue that bundles fibers into fascicles

– Epineurium—tough fibrous sheath around a nerve

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LABWORK

1.Explain organization of nervous system.

2. Describe neuroglia and process of myelin formation.

3. Describe and identify parts of a neuron and the different classifications of neurons on models and microscope slides.

4. Describe and identify structure of a nerve (microscope slides and models).