Download Structure of the Nervous System

Structure of the Nervous
System
Organization of the Nervous System
•Nervous system can be classified in
terms of information flow: Afferent
neurons (sensory neurons) send signals
into the central nervous system (CNS)
for processing. The processed signal is
sent out along efferent neurons to
activate the required cellular response in
effector cells.
•The afferent and efferent neurons form
the peripheral nervous system (PNS).
•The PNS can be divided into the
somatic motor division and the
autonomic division. The autonomic
divisions can be further divided into
sympathetic and parasympathetic
divisions.
•The enteric nervous system is viewed as the 3rd division of the nervous
system.
Structure of a Neuron
• The nervous system is composed
of primarily neurons and glia.
•Neuron is the functional unit of
the nervous system.
•Information flow is from the cell
body to the axon terminals via
axons.
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Cell Bodies & Dendrites
•Cell Body: Makes up approx. 1/10 of the cell
volume. It contains the nucleus where genes are
transcribed.
•It maintains the well being of the cell and protein
synthesis takes place in the cell. Proteins are
packaged into vesicles and transported out to other
regions along microtubules.
•A number of important organelles are found
there: rough endoplasmic reticulum (ER),
mitochondria, Golgi complex.
•Dendrites: are highly branched structures
which receive incoming signals from other
neurons. They increase the surface area of the
neuron, allowing multiple inputs from several
neurons.
•They contain rough ER, mitochondria and
other organelles.
Pathologies of Spine Distribution And Shapes
Fiala et al (2002)
Axons
•Axons propagate electrical
signals along their length
to their terminals.
•Axons contain
mitochondria (source of
ATP) and microtubules (act
like railroad tracks,
enabling transport of
organelles and molecules
(packaged into secretory
vesicles) to and from axon
terminals).
•Slow axonal transport can move material at a rate of 0.2- 5 mm/day.
Carries material not consumed rapidly (eg. enzymes and cytoskeletal
proteins).
• Fast axonal transport can move material at a rate of 400 mm/day. Moves
material consumed rapidly, such as, synaptic vesicles. Ships back old
membrane components for recycling to the cell body.
Synapses
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Small Synaptic Boutons in
the Mammalian Brain
Mouse Neuromuscular
Junction
Parasympathetic
Varicosities formed by
Preganglionic Axons
Innervating Cell Bodies of
Ganglion Cells
•At synapses, the neuron which delivers the signal is called a
presynaptic cell, and the cell which receives the signal is termed a
postsynaptic (or target) cell.
•The electrical signal transmitted along the axon is translated into a
chemical signal before its relayed across the synapse onto the target
cell. In the target cell, the signal is again electrical.
Three Functional Types of Neurons
Motor Neurons
Sensory Neurons
CNS Interneurons
Neural Circuits
Definition: A functional group of neurons which process a special
kind of information.
•Neurons link together to form neural circuits which perform
special tasks. Many of these are reflexes.
•Signaling within these circuits gives rise to higher cognitive
functions, such as thinking.
•Since circuits are needed for even the most basic function, it has
been suggested that the functional unit of the nervous system is a
group of neurons, rather than an individual neuron.
•How do these circuits link together to create behaviour? How do
they adapt to change?
Glial Cells
•Glial cells out-number
neurons by 10-50:1.
•Play a supportive role in the
nervous system. They
communicate with neurons
and amongst each other.
•Myelin (concentric layer of
phospholipid insulating sheath wrapped
around portions of a neuron) is formed
from oligodendrocytes in the CNS and
Schwann cells in the PNS.
•In the CNS myelin wraps around
several axons, whereas in the PNS,
around a single axon. Nodes of Ranvier
are exposed regions of axons.
Brain
•Weighs about
1400 g and
contains 1012
neurons.
•Each neuron
receives about 200,
000 synapses.
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•Different regions of the brain subserve different functions. When a single
function is carried out by more than one region of the brain, its termed
parallel processing.
•Network of neurons can alter their connections based on past experience,
thus displaying plasticity.
Neurons Are Grouped in Nuclei and Tracts
in the Cerebral Cortex
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•Cell bodies form layers in parts of the brain or cluster into groups
called nuclei.
•Grey matter consists of cell bodies and white matter, of axons.
Cerebral Cortex
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•Within this layer, our higher brain functions arise (eg. Thought).
•Contain 3 functional specializations: (a) sensory areas which direct
perception; (b) motor areas which direct movement; (c) associated
areas which integrate information and direct voluntary areas.
Einstein’s Brain
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•His brain was the same as that of
most others, except his lacked a
small wrinkle (the parietal
operculum) which most have.
•The lack of this region may have
provided a compensatory increase
in other regions of the brain - the
inferior parietal lobes; a region
associated with visual imagery and
mathematical thinking.
•Einstein’s own words about his
thinking process,”…words do not
seem to play any role” but there is
“associative play” of “more or less
clear images” of the “visual and
muscular type”.
Witelson et al (1999)
Cerebral Lateralization or Left Brain-Right
Brain Dominanance
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Split-Brain Experiments
Blood-Brain Barrier Protects the Brain From
Harmful Substances
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•Protects the brain from harmful substances entering the interstatial
fluid.
•The endothelial cells which form the walls of the capillaries are
connected together by tight junctions, not leaky junctions and
pores.
•Paracrines secreted by astrocytes induce the formation of tight
junctions.
•Some areas are not protected, such as the vomiting centre of the
medulla.
Spinal Cord
•Contains major
pathways which shuttle
information between the
brain and the periphery
of the body.
•Contain interneurons
which do not extend out
of the CNS. They
modify information
passing through them.
•Severed spinal cord
leads to paralysis.
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•Cell bodies of sensory neurons are in the dorsal root ganglia. They
form synapses in the dorsal horn.
•Cell bodies of somatic motor neurons are found in the ventral horns.
CNS is Protected by the Skull and Vertebral
Column
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Meninges Stops Neural Tissue From Bruising
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Autonomic Nervous System
Symapthetic & Parasympathetic Divisions
•In fight-or-flight situations, digestion is of low priority. Heart and
skeletal muscle prepare for high level of activity.
•The maintenance of homeostasis in the body involves balancing
sympathetic and parasympathetic activity.
References
1.
Tortora, G.J. & Grabowski, S.R (2003). Principles of
Anatomy & Physiology.New Jersey: John Wiley & Sons.
Ch.12: pp.385-395; Ch.13: pp.419-425, Ch.14: relevant
sections.
2.
Silverthorn, D.U (1998). Human Physiology: An
Integrated Approach. New Jersey: Prentice Hall. Ch.8:
pp.208-224; Ch.9: pp.235-244, pp.248-250; Ch.11:
pp.307-310.
3.
Witelson, S.F. et al., (1999). The exceptional brain of
Albert Einstein. Lancet 353: pp.2149-2153.
Einstein’s Brain Removed Within 7hrs of
His Death in 1955
T&G(13.
5 & 13.6)
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•Cell bodies of sensory neurons are in the dorsal root ganglia. They
form synapses in the dorsal horn.
•Cell bodies of somatic motor neurons are found in the ventral horns.