ANSWER THE ESSAYS AND THE BULLET (-) QUESTIONS

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APLECTURE17OVERVIEWOFTHENERVOUSSYSTEM.ppt

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Overview of Nervous System

Internal Coordination Maintenance

endocrine system -

-chemical messengers (hormones)

-secreted into to the blood

-slow communication

nervous system -

-electrical and chemical signals

-transmit messages from cell to cell

-fast communication

Steps of nerve message transmission

Sense organs (afferent)

-receive external stimuli

-transmit messages to spinal cord and brain

Brain and spinal cord

-processes this information

-determine appropriate response

-issue commands to effectors (muscles & glands)

Effectors (muscle & gland cells) carry out response

Nervous System
Anatomical Subdivisions

central nervous system (CNS)
brain and spinal cord

-enclosed by cranium and vertebral column

peripheral nervous system (PNS)
all other nerves & ganglia of nervous system

-nerve – nerve fibers bundles wrapped in fibrous connective tissue

-ganglion – knot-like swelling in nerve

- composed of many neuron cell bodies

Subdivisions of Nervous System

Brain

Nerves

Ganglia

Peripheral nervous

system (PNS)

Central nervous

system (CNS)

Spinal

cord

Function- transmit sensory signals from various receptors to the CNS

Somatic sensory

-from receptors in skin, muscles, bones,& joints

visceral sensory division

-from receptors in viscera of thoracic & abdominal cavities (heart, lungs, & stomach)

Sensory Divisions of PNS

Motor Divisions of PNS

transmits signals from the CNS to responding gland & muscle cells (effectors)

Visceral motor division (autonomic NS)

Transmits signals to glands, cardiac & smooth muscle

Involuntary, visceral reflexes

Divisions

-Sympathetic- stimulating (e.g. increase heart rate)

-Parasympathetic- calming (e.g. slows heart rate)

Somatic motor division

transmits signals to skeletal muscles

yields muscular contraction & somatic reflexes

Brain

Central nervous system

Peripheral nervous system

Spinal

cord

Sensory

Motor

Visceral

sensory

Somatic

sensory

Visceral

motor

Somatic

motor

Sympathetic

Parasympathetic

Universal Properties of Neurons

excitability (irritability)
respond to environmental stimuli

conductivity
neurons produce electrical signals conducted to other neurons at distant locations

secretion
chemical neurotransmitter secreted
crosses the gap and stimulates the next cell

Functional Types of Neurons

sensory (afferent) neurons
detect stimuli
transmit information to CNS

interneurons (association) neurons
solely in CNS, 90% neurons
process, store, retrieve information, & determine response to stimuli
interconnect sensory pathways & motor pathways of CNS

motor (efferent) neuron
transmit signals to muscles and gland cells (the effectors)
Motor- signal to muscles
Efferent- conduct signals away from the CNS

Functional Types of Neurons

Peripheral nervous system

Central nervous system

Sensory (afferent)

neurons conduct

signals from

receptors to the CNS.

Motor (efferent)

neurons conduct

signals from the CNS

to effectors such as

muscles and glands.

Interneurons

(association

neurons) are

confined to

the CNS.

Neuron Structure

soma (cell body)– neuron control center

Uninucleate w/ large nucleolus

cytoskeleton
Dense mesh microtubules & neurofibrils (actin filaments)
compartmentalizes rough ER into dark staining Nissl bodies

no centrioles – no further cell division

inclusions – glycogen granules, lipid droplets, melanin, and lipofuscin

Lipofuscin- brown pigment
produced by lysosomes from organelle degradation
lipofuscin accumulates with age
wear-and-tear granules
most abundant in old neurons

Soma

dendrites – many branches from soma

resemble bare branches of a tree in winter

primary site for receiving signals from other neurons

more dendrites = more information received

provide precise pathway for reception & processing of neural information

Figure 12.4a

Dendrites

Soma

Nucleus

Nucleolus

Axon

Node of Ranvier

Internodes

Synaptic knobs

Axon hillock

Initial segment

Myelin sheath

Schwann cell

Axon collateral

(a)

Trigger zone:

Direction of

signal transmission

Terminal

arborization

Neuron Structure

axon (nerve fiber)-

originates from axon hillock

cylindrical, unbranched except at distal end

specialized for rapid conduction of nerve signals to points remote to the soma

axoplasm – cytoplasm of axon

axolemma – plasma membrane of axon

one axon per neuron

Schwann cells and myelin sheath enclose axon

Axon distal end- extensive branches
synaptic knob– swelling forming a junction (synapse) with the next cell
contains synaptic vesicles full of neurotransmitter

Figure 12.4a

Dendrites

Soma

Nucleus

Nucleolus

Axon

Node of Ranvier

Internodes

Synaptic knobs

Axon hillock

Initial segment

Myelin sheath

Schwann cell

Axon collateral

(a)

Trigger zone:

Direction of

signal transmission

Terminal

arborization

Neuron Structure

Variation in Neuron Structure

multipolar neuron
one axon and multiple dendrites
most common (brain & spinal cord)

bipolar neuron
one axon and one dendrite
olfactory cells, retina, inner ear

unipolar neuron
single process leading away from the soma
sensory from skin & organs to spinal cord

anaxonic neuron
many dendrites but no axon
help in visual processes

Dendrites

Axon

Dendrites

Axon

Unipolar neuron

Multipolar neurons

Bipolar neurons

Anaxonic neuron

Axonal Transport- 2-way passage of proteins, organelles along axon

Function- repair axolemma,

gated ion channel proteins,

enzymes or neurotransmitters

Types-
anterograde– down the axon away from soma
retrograde– up the axon toward the soma

Microtubules guide materials along axon
motor proteins transport materials along microtubules
kinesin – anterograde transport
dynein – retrograde transport

Neuroglial Cells

~trillion neurons in the nervous system

neuroglia outnumber the neurons by as much as 50 to 1

neuroglia or glial cells
support and protect the neurons
bind neurons together & form framework for nervous tissue
in fetus, guide migrating neurons to their destination

Six Types of Neuroglial Cells

oligodendrocytes
form myelin sheaths in CNS
wrap around neuron forming an insulating layer & speeds up signal conduction

ependymal cells
lines internal cavities of the brain
cuboidal epithelium with cilia on apical surface
secretes and circulates cerebrospinal fluid (CSF)

microglia
migrating macrophages (white blood cells, monocytes)
Phagocytize cellular debris

astrocytes
most abundant glial cell in CNS
cover entire brain surface and most nonsynaptic regions of the neurons in the gray matter of the CNS
diverse functions
form a supportive framework of nervous tissue
extensions contact capillaries that stimulate them to form a tight seal called the blood-brain barrier
convert blood glucose to lactate for neuronal energy
secrete nerve growth factors
communicate w/ neurons & influence synaptic signaling
absorbing excess neurotransmitters and ions

Six Types of Neuroglial Cells

Schwann cells
envelope nerve fibers only in PNS
wind repeatedly around a nerve fiber
produces a myelin sheath similar to the ones produced by oligodendrocytes in CNS
assist in the regeneration of damaged fibers

satellite cells
surround the neurosomas in ganglia of only the PNS
provide electrical insulation around the soma
regulate the chemical environment of the neurons

Neuroglial Cells of CNS

Ependymal cell

Cerebrospinal fluid

Neurons

Astrocyte

Perivascular feet

Microglia

Oligodendrocyte

Capillary

Myelinated axon

Myelin (cut)

Myelin- insulating layer (sheath) around a nerve fiber

formed by
Oligodendrocytes- CNS

Schwann cells- PNS

consists of the plasma membrane of glial cells

many cells are needed to cover one nerve fiber

Myelin

myelin sheath is segmented
nodes of Ranvier – gap between segments

internodes – myelin covered segments between gaps

initial segment – nerve fiber section between the axon

hillock & first glial cell

trigger zone – the axon hillock & the initial segment
play an important role in initiating a nerve signal

Myelin Sheath in PNS

Myelin sheath

Axolemma

Axoplasm

Neurilemma

(c)

Schwann cell

nucleus

-thick outermost coil of

myelin sheath

Endoneurium-external to neurilemma is basal lamina &

a thin layer of fibrous connective tissue

Myelination in PNS

Axon

(a)

Myelin sheath

Schwann cell

Nucleus

Basal lamina

Neurilemma

Endoneurium

Myelination in CNS

(b)

Oligodendrocyte

Nerve fiber

Myelin

No wrapping around neuron

No neurolemma

No endoneurium

Myelin Sheath Degenerative Disorders

multiple sclerosis
oligodendrocytes and myelin sheaths in the CNS deteriorate
myelin replaced by hardened scar tissue
nerve conduction disrupted (double vision, tremors, numbness)
onset between 20 and 40 and fatal from 25 to 30 years after diagnosis
cause may be autoimmune triggered by virus

Tay-Sachs disease - a hereditary disorder Eastern European Jewish
accumulation of ganglioside (GM2) disrupts conduction of nerve signals
normally decomposed by lysosomal enzyme
enzyme missing in individuals homozygous for Tay-Sachs allele
blindness, loss of coordination, and dementia
fatal before age 4

Unmyelinated Axons of PNS

Schwann cells hold w/ small nerve

fibers in grooves on its surface

membrane folds once around each

fiber overlapping along edges

mesaxon – neurilemma wrapping of

unmyelinated nerve fibers

(c)

Myelin sheath

Neurilemma

3µm

Myelinated

axon

Schwann

cell cytoplasm

Basal

lamina

Unmyelinated

axon

Conduction Speed of Nerve Fibers

speed determined by
diameter of fiber
presence or absence of myelin

signal conduction occurs along the surface of a fiber

conduction speed
small, unmyelinated fibers - 0.5 - 2.0 m/sec
small, myelinated fibers - 3 - 15.0 m/sec
large, myelinated fibers - up to 120 m/sec
slow signals- stomach and dilate pupil
fast signals- skeletal muscles & sensory signals

Regeneration of Peripheral Nerves

regeneration of a damaged peripheral nerve fiber can occur if:
its soma is intact
at least some neurilemma remains

regeneration tube – formed by Schwann cells, basal lamina, and the neurilemma near the injury

NO regeneration of damaged nerve fibers in the CNS

Nerve Growth Factor

nerve growth factor (NGF) –
a protein secreted by a gland, muscle, & glial cells
binds to receptors on the axon terminals of the neurons.
functions-
prevents apoptosis (programmed cell death) in growing neurons
enables growing neurons to make contact with their target cells

isolated by Rita Levi-Montalcini in 1950s, won Nobel prize

use of growth factors is now a vibrant field of research