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Fundamentals of the NS and Nervous Tissue Chapter 12 Part II Basic Divisions of the Nervous System Figure 12.2 Figure 12.3 Nervous Tissue Cells are densely packed and tightly intertwined Two main cell types 1) neurons, the excitable nerve cells that transit electrical signals

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nervous tissue
Nervous Tissue
  • Cells are densely packed and tightly intertwined
  • Two main cell types

1) neurons, the excitable nerve cells that transit electrical signals

2) neuroglia, nonexcitable supporting cells that surround and wrap the neurons

  • Both developed from the embryonic neural tube and neural crest
the neuron
The Neuron
  • The human body contains billions of neurons, or nerve cells
  • Basic structural unit of the NS

- specialized cells conduct signals from one part of the body to another

- signals are transmitted along the plasma membrane as nerve impulses, or action potentials

neuron special characteristics
Neuron - Special Characteristics

1. Longevity - can live and function for a lifetime

2. Do not divide – fetal neurons lose their ability to undergo mitosis; neural stem cells are an exception

3. High metabolic rate – require abundant oxygen and glucose. Neurons cannot survive for more than a few minutes without oxygen

  • Neurons typically are large, complex cells and all have a cell body with one or more processes
the cell body
The Cell Body
  • Also called a soma, all have a single nucleus surrounded by cytoplasm (perikaryon)

- size of cell body varies from 5 – 140 um

- contain usual organelles

- plus chromatophilic (Nissl) bodies, clusters of RER and free ribosomes that stain darkly and continually renew the membranes of the cell

  • Neurofibrils – bundles of intermediate filaments (neurofilaments)

- form a network between chromatophilic bodies

the cell body8
The Cell Body
  • Most neuronal cell bodies are located within the CNS

- protected by bones of the skull and vertebral columns

  • Ganglia (sing. ganglion) – clusters of cell bodies

- lie along the nerves in the PNS

neuron processes
Neuron Processes
  • Armlike processes extend from the cell bodies of all neurons – dendrites and axons
  • Dendrites

- extensively branch from the cell body

- transmit electrical signals toward the cell body

- chromatophilic bodies – only extend into the basal part of dendrites and to the base of the axon hillock

- function as receptive sites for receiving signals from other neurons

axons
Axons
  • Neuron has only one

- initial segment called the axon hillock

  • Impulse generator and conductor
  • Transmits impulses away from the cell body
  • Chromatophilic bodies are absent

- lack GA, ribosomes, all organelles involved in protein synthesis

  • No protein synthesis occurs in the axon
axons12
Axons
  • Neurofilaments, actin microfilaments, and microtubules

- provide strength along length of axon

- aid in the transport of substances to and from the cell body (axonal transport)

  • Branches along the length are infrequent

- axon collaterals

  • Multiple branches at the end or terminus

- terminal branches (telodendria) end in knots called axon terminals (aka end bulbs or boutons)

nerve impulse
Nerve Impulse
  • Generated at the initial segment of the axon
  • Conducted along the axon
  • Releases neurotransmitters at axon terminals

- neurotransmitters excite or inhibit the neuron or target organ

  • Axon diameter varies

- larger diameter faster the impulse according to the basic law of physics: resistance to the passage of an electrical current decreases as the diameter of any ‘cable’ increases

  • Neuron receives and sends signals
synapses
Synapses
  • Synapse (‘union’) - site at which neurons communicate
  • Signals pass across synapse in one direction
  • Presynaptic (information-sending) neuron – conducts the signal toward a synapse
  • Postsynaptic (information-receiving) neuron – transmits electrical signals away from a synapse
  • Most CNS neurons function as both presynaptic and postsynaptic

- getting signals from some neurons and dispatching it to others

types of synapses
Types of Synapses

Two main types:

  • Axodendritic – between axon terminals of one neuron and dendrites of another

- most common type of synapse

  • Axosomatic – between axons and neuronal cell bodies
  • Axoaxonic, dendrodendritic, and dendrosomatic are uncommon types of synapses
synapse
Synapse

Structurally synapses are elaborate cell junctions

  • Axodendritic synapses – representative type
  • Synaptic vesicles on the presynaptic side

- membrane-bound sacs containing neurotransmitters

- abundant mitochondria in axon terminals

  • Synaptic cleft

- separates the PM of the two neurons

  • Presynaptic and postsynaptic densities

– regions of dense material of opposing cell membranes

- presynaptic region a honeycombed grid

- postsynaptic region a perforate plate

the signals carried by neurons
The Signals Carried by Neurons
  • PMs of neurons conduct electrical signals
  • Resting neuron – membrane is polarized

- inner, cytoplasmic side is negatively charged

- high [K+] intercellular and high [Na+] extracellular

  • Stimulation of the neuron  depolarization

- permeability of PM changes allowing Na+ ions to rush in

action potential
Action Potential
  • Strong stimulus applied to the axon triggers

- nerve impulse/action potential

  • Membrane at the axon’s initial segment is depolarized

- positive charge inside the axon

- negative charge outside the axon

  • Impulse travels the length of the axon

- impulse stimulates the synaptic vesicle to fuse with the presynaptic membrane, fused area ruptures

- NT diffuses across the synaptic cleft binds to the postsynaptic membrane at the postsynaptic density

- binding changes the postsynaptic membrane charge

excitatory synapses
Excitatory Synapses
  • Neurotransmitters alter the permeability of the postsynaptic membrane to certain ions

- influx of positive ions depolarizes the postsynaptic membrane

- drives the postsynaptic neuron toward impulse generation

inhibitory synapses
Inhibitory Synapses
  • Increases membrane polarization

- the external surface of the postsynaptic membrane becomes more positive

- reduces the ability of the postsynaptic neuron to generate an action potential

  • 1000s of exitatory and inhibitory synapses act on every neuron
structural classification of neurons
Structural Classification of Neurons

According to the number of processes they contain

  • Multipolar – possess more than two processes

- numerous dendrites and one axon

- 99% of neurons including motor and most interneurons

  • Bipolar – possess two processes

- rare neurons found in some special sensory organs (inner ear, olfactory epithelium, retina)

  • Unipolar (pseudounipolar) – possess one short single process

- most start as bipolar neurons, 2 processes fuse together

- make up the typical sensory neurons

neurons classifed by function
Neurons Classifed by Function

According to the direction the nerve impulse travels: sensory neurons, motor neurons, interneurons

  • Sensory or afferent (arriving) neurons make up the sensory division of the PNS

- transmit impulses toward the CNS

- short, single process divides into the central process that runs centrally into the CNS

- the peripheral process extends peripherally to the receptors

motor neurons
Motor Neurons
  • Or efferent (exiting) neurons make up the motor division of the PNS

- carry impulses away from the CNS to effector organs

- most are multipolar

- cell bodies are within the CNS (except for some in the ANS)

- form junctions with effector cells, stimulating muscle contraction or glandular secretion

interneurons
Interneurons
  • Or association neurons lie between motor and sensory neurons

- confined entirely to the CNS

- link into chains that form complex neuronal pathways

- make up 99.98% of all neurons

- almost all are multipolar but have a great diversity in size and branching patterns of their processes

supporting cells
Supporting Cells
  • Six types – 4 in the CNS and 2 in the PNS
  • Provide supportive functions for neurons
  • Cover nonsynaptic regions of the neurons
neuroglial in the cns
Neuroglial in the CNS
  • Or glial cells include the: astrocyte, ependymal cells, microglia, and oligodendrocytes

- have branching processes and a central cell body

- outnumber neurons 10 to 1

- make up half the mass of the brain

- can divide throughout life

astrocytes
Astrocytes
  • The most abundant glial cell type

- sense when neurons release glutamate

- extract blood sugar from capillaries for energy

- take up and release ions in order to control environment around neurons

- involved in synapse formation in developing neural tissue

- produce molecules necessary for neuronal growth (brain-derived trophic factor, BDTF)

- propagate calcium signals involved with memory

slide38

Figure 12.12b

  • Microglia – smallest, least abundant glial cell
    • Macrophages of the CNS
    • Engulf invading microorganisms and dead neurons
    • Derive from blood cells called monocytes
ependymal cells and oligodendrocytes
Ependymal Cells and Oligodendrocytes
  • Ependymal cells

- line the central cavity of the spinal cord and brain

- bear cilia – help circulate the CSF

  • Oligodendrocytes – have few branches

- wrap their cell processes around axons in the CNS

- produce myelin sheaths

neuroglia in the pns
Neuroglia in the PNS
  • Satellite cells –surround neuron cell bodies within ganglia
  • Schwann cells (neurolemmocytes) – surround PNS axons

- form myelin sheath around axons of the PNS

Figure 12.13

myelin sheaths
Myelin Sheaths
  • Segmented structures composed of the lipoprotein myelin
  • Surround thicker axons
  • Form an insulating layer
    • Prevent leakage of electrical current
  • Increase the speed of impulse conduction along the axon

- makes impulse propagation more energy-efficient

myelin sheaths in the pns
Myelin Sheaths in the PNS
  • Formed by Schwann cells (neurolemmacytes)
  • Develop during fetal period and in the first year of postnatal life
  • Schwann cells (neurolemmacytes) wrap in concentric layers around the axon
    • cover the axon in a tightly packed coil of membranes
  • Neurilemma – nucleus and most of the cytoplasm of the neurolemmacytes end up just external to the myelin layers
myelin sheaths in the pns49
Myelin Sheaths in the PNS
  • Nodes of Ranvier – gaps along axon
  • Thick axons are myelinated

- nerve impulses jump from the membrane of one node to the next

  • Thin axons are unmyelinated

- conduct impulses more slowly

myelin sheaths in the cns
Myelin Sheaths in the CNS

Figure 12.15b

  • Oligodendrocytes form myelin sheaths in the CNS - have multiple processes, coil around several different axons

- Nodes of Ranier present but more widely spaced

gray matter in the cns
Gray Matter in the CNS
  • Gray-colored zone that surrounds the hollow central cavities of the CNS
  • Forms H-shaped region in the spinal cord

- dorsal half contains cell bodies of interneurons

- ventral half contains cell bodies of motor neurons

  • Primarily composed of neuronal cell bodies

- also contains dendrites, unmyelinated axons

- surrounds white matter of CNS in cerebral cortex and cerebellum

white matter in the cns
White Matter in the CNS
  • Lies external to the gray matter of the CNS
  • Composed of myelinated axons
    • consists of axons passing between specific regions of the CNS
  • Tracts are bundles of axons traveling to similar destinations
nerves
Nerves
  • Cablelike organs in the PNS
    • consists of numerous axons (nerve fibers)
    • Arranged in parallel bundles enclosed in CT
  • Almost all nerves contain both myelinated and unmyelinated sensory and motor fibers
    • each axon is surrounded by Schwann cells

Note: terms neuron, nerve fiber, and nerve

  • A neuron is a nerve cell
  • A nerve fiber is a long axon
  • A nerve is a collection of nerve fibers in the PNS
nerves54
Nerves
  • Endoneurium – layer of delicate connective tissue surrounding the axon
  • Perineurium – connective tissue wrapping surrounding a nerve fascicle
    • Nerve fascicles – groups of axons bound into bundles
  • Epineurium – whole nerve is surrounded by tough fibrous sheath
integration between the pns and cns
Integration Between the PNS and CNS
  • The CNS and PNS are functionally interrelated
  • Nerves of the PNS - information pathways to and from body periphery
  • Afferent PNS fibers respond to sensory stimuli
  • Efferent PNS fibers transmit motor stimuli from CNS to muscles and glands
integration between the pns and cns57
Integration Between the PNS and CNS

The CNS is composed of interneurons that

  • Process and receive sensory information
  • Direct information to specific CNS regions
  • Initiate appropriate motor responses
  • Transport information from one area of the CNS to another
reflex arcs
Reflex Arcs
  • Simple chains of neurons that cause our simplest, reflexive behaviors

- reflects the basic structural plan of the NS

  • Responsible for reflexes - rapid, autonomic motor responses

- unlearned, unpremeditated, and involuntary

  • Can be somatic reflexes or visceral reflexes

- jerking away your hand after touching something hot

- vomiting in response to food that irritates your stomach

five essential components to the reflex arc
Five Essential Components to the Reflex Arc

1. Receptor – site where stimulus acts

2. Sensory neuron – transmits afferent impulses to the CNS

3. Integration center – consists of one or more synapses in the CNS

4. Motor neuron – conducts efferent impulses from integration center to an effector

5. Effector – muscle or gland cell that responds to efferent impulses by contracting or secreting

3 neuron reflex arc
3-neuron Reflex Arc

a pin stimulates a receptor 

sends an impulse to a sensory

neuron  activates an integration

center  signals a motor neuron

Figure 12.17

types of reflexes
Types of Reflexes
  • Monosynaptic reflex- simplest of all reflexes

- fastest of all reflexes with just one synapse (2 neurons)

- example: knee-jerk reflex

  • Polysynaptic reflex – far more common

- one or more interneurons between the sensory and motor neurons

- most contain a single interneuron (total of 3 neurons)

- withdrawal reflexes

types of reflex arcs
Types of Reflex Arcs

Figure 12.18a, b

simplified design of the nervous system
Simplified Design of the Nervous System
  • Three-neuron reflex arcs – exemplify fundamental design of the entire NS
  • NS more than just a series of reflex arcs

- complexity arises from the organization of interneurons into neuronal circuits that process information

- circuits form chains of interneurons interposed between each sensory and motor neuron

simplified design of the nervous system64
Simplified Design of the Nervous System
  • Sensory neurons – located dorsally

- cell bodies outside the CNS in sensory ganglia

- central processes enter dorsal aspect of the spinal cord

  • Motor neurons – located ventrally

- axons exit the ventral aspect of the spinal cord

  • Interneurons – located centrally

- neurons are confined to the CNS

- synapse with sensory neurons

- long chains of interneurons lie between sensory and motor neurons

neuronal circuits
Neuronal Circuits
  • Diverging circuit – one presynaptic neuron synapses with several other neurons (divergence)
  • Converging circuit – many neurons synapse on a single postsynaptic neuron (convergence)
  • Reverberating circuit – circuit that receives feedback via a collateral axon from a neuron in the circuit
input processing
Input Processing
  • Serial processing - neurons pass a signal to a specific destination along a single pathway from one to another
  • Parallel processing - input is delivered along many pathways

- a single sensory stimulus results in multiple perceptions

disorders of the ns
Disorders of the NS
  • Multiple sclerosis – autoimmune disease and a common cause of neural disability
    • Immune system attacks the myelin around axons in the CNS
    • Varies widely in intensity among those affected
    • More women than men are affected
    • When men are affected disease develops quicker and is more devastating
    • cause is incompletely understood
nervous tissue throughout life
Nervous Tissue Throughout Life
  • Nervous system develops from dorsal ectoderm
    • invaginates to form the neural tube and neural crest
    • neural tube walls begin as neuroepithelial cells
    • these cells divide and become neuroblasts
neuronal regeneration
Neuronal Regeneration
  • Neural injuries may cause permanent dysfunction
  • If axons alone are destroyed, cells bodies often survive and the axons may regenerate

- if a nerve is severed in the PNS macrophages invade and destroy the axon distal to the injury

- axon filaments grow peripherally from the injured site

- partial recovery is sometimes possible

neuronal regeneration74
Neuronal Regeneration
  • CNS – neuroglia never form bands to guide re-growing axons and may hinder axon growth with growth-inhibiting chemicals
  • No effective regeneration after injury to the spinal cord and brain