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Nervous Tissue

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Nervous Tissue Consists of two main cell types:

- neurons - sensory control, and regulation

- neuroglia - support, protection, and homeostasis

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Parts of a Neuron

cell body dendrites axon

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Cell Body

contains nucleus, surrounded by granular cytoplasm

contains organelles (i.e., lysosomes, mitochondria, Golgi complex, etc.)

Nissl bodies (chromatophilic substance)

- arrangement of rough endoplasmic reticulum (site of protein synthesis)

neurofibril

- provides support and shape for cell

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Two Kinds of Nerve Fibers dendrites

- neurons usually contain many axons

- neurons contain only one

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Dendrites dendro = tree

highly branched processes not myelinated extend out from cell body receive stimuli carry a nerve impulse toward the cell

body

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Axon

long, thin, cylindrical projection may or may not be myelinated contains mitochondria and neurofibrils but no

rough ER; no protein synthesis cytoplasm - axoplasm surrounded by axolemma

(lemma = sheath or husk) carries a nerve impulse away to other cells

i.e., (nerves, muscles, glands)

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Parts of an Axon long nerve cell fibers are covered with

myelin

- fatty substance

- protects and insulates fibers

- increases transmission rate of

- nerve impulses

- forms myelin sheath

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Parts of an Axon (cont.)

neurolemma sheath surrounds myelin Schwann cells are surrounded by

neurolemma

- Schwann cells are also called neurolemmocytes

narrow gaps in myelin sheath between adjacent Schwann cells are called nodes of Ranvier

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Parts of an Axon (cont.)

axon hillock

- cone-shaped elevation where axon joins cell body

initial segment

- first portion of axon trigger zone

- junction of the hillock and initial segment where nerve impulses arise

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Parts of an Axon (cont.)

axon collateral

- side branch along length of axon axon terminals

- fine branched end portions of an axon and its collaterals

- when impulse reaches axon terminal, neurotransmitter is released

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Parts of an Axon (cont.)

neurons are close to each other but separated by synaptic gap

- this junction is called a synapse

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Parts of an Axon (cont.)

synaptic end-bulbs

- bulb-shaped structures

- tips of axons synaptic vesicles

- membrane-enclosed sacs within synaptic end-bulbs

- store neurotransmitters (different neurons have different neurotransmitters)

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Neuroglia

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Neuroglia constitute one-half of volume of CNS outnumber neurons 5-50 times can multiply and divide, unlike neurons Functions:

- nerve glue

- supports

- insulates

- protects

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Types of Neuroglial Cells astrocytes oliogodendrocytes microglia ependymal cells Schwann cells satellite cells

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Astrocytes (astro = star)

star-shaped many processes participate in metabolism of neurotransmitters maintain potassium (K+) balance for

generation of nerve impulses participate in brain development aid in formation of blood-brain barrier link between neurons and blood vessels

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Oliogodendrocytes (oligo = few, dendro = tree)

few processes smaller than astrocytes form supporting network by twining

around neurons produce a lipid and protein wrapping

called a myelin sheath

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Microglia(micro = small, glia = glue)

small protect CNS from disease phagocytic

- engulf microbes and clear away debris

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Ependymal Cells (ependyma = upper garment)

shape ranges from squamous to columnar

may be ciliated line brain ventricles and central canal of

spinal cord

ventricles are spaces that produce and circulate cerebrospinal fluid

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Schwann Cells (Neurolemmocytes)

found in peripheral nervous system form myelin sheath support, protect, and nourish neurons plasma membrane of neurolemmocytes

forms myelin sheath around axon in PNS myelin sheath insulates speed of impulse

transmission

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Satellite Cells found in peripheral nervous system support neurons in ganglia (clusters of

neuron cell bodies) of PNS

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Neuron Classification structural classification

- based on number of processes extending from cell body

functional classification

- based on direction in which a neuron transmits a nerve impulse

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Structural Classification unipolar (pseudounipolar)

- have a single process extending from cell body

- always are sensory neurons

- originate in embryo as bipolar

- during development, axon and dendrite fuse together into a single process

- single process divides into two branches a short distance from cell body

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Structural Classification bipolar

- have one dendrite and one axon

- found in retina, internal ear, and olfactory area of brain

multipolar

- several dendrites and one axon

- most neurons in brain and spinal cord are of this type

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Functional Classification based on the direction in which a

neuron transmits a nerve impulse Two types of neurons in PNS:

- afferent (toward CNS)

- efferent (away from CNS)

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Afferent carry impulses (sensory information)

toward CNS also known as sensory neurons

Efferent carry impulses away from CNS affect activity of muscles or glands also known as motor neurons

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Sensory = Afferent

Motor = Efferent

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Afferent or Sensory Neurons Four categories:

- general somatic

- special somatic

- general visceral

- special visceral

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Afferent or Sensory Neurons (cont.)

general somatic

- carry impulses from skin, skeletal muscles, and joints

(pain, temperature, touch, vibration, and pressure)

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Afferent or Sensory Neurons (cont.)

special somatic

- carry or relay impulses from retina and internal ear

(vision, hearing, and balance)

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Afferent or Sensory Neurons (cont.)

general visceral

- carry impulses from internal organs and receptors of body fluids

(distension of internal organs and chemical conditions in body)

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Afferent or Sensory Neurons (cont.)

special visceral

- carry impulses from tongue and olfactory mucosa

(taste and smell)

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Efferent or Motor Neurons general somatic

- carry impulses to most skeletal muscles general visceral

- carry impulses to smooth muscle, cardiac muscle, and glands

special visceral

- carry impulses to skeletal muscles that control facial expression

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Associated Neurons located in CNS carry impulses from one neuron to

another inside brain and spinal cord establish interrelationship with other

neurons forming neuronal circuits