neural control mechanism -2 Flashcards
neurons
The basic unit of the nervous system is the individual
nerve cell, or neuron .
Neurons operate by generating electrical signals that
move from one part of the cell to another part of the same
cell or to neighboring cells.
nervous system is divided into
(1) the central nervous system (CNS) , composed of the
brain and spinal cord; and
(2) the peripheral nervous system (PNS) ,consisting of the
nerves that connect the brain and spinal cord with the body’s
muscles, glands, sense organs, and other tissues
neurtransmitters
The electrical signal causes the release of chemical
messengers—neurotransmitters —to communicate with
other cells.
cell body
- Cell body (Soma): contains the nucleus and ribosomes and
thus has the genetic information and machinery necessary for
protein synthesis.
dendrites
- Dendrites: series of highly branched outgrowths of the cell
body
Branching dendrites increase a cell’s surface area—some
neurons may have as many as 400,000 dendrites
Knoblike outgrowths called dendritic spines increase the
surface area
The presence of protein synthesis machinery allows dendritic
spines to remodel their shape in response to variation in
synaptic activity (like learning and memory)
axon
- Axon(nerve fiber): a long process that extends from the cell body and carries outgoing signals
to its target cells
Range in length from a few microns to over a meter
Region of the axon that arises from the cell body (initial segment or axon hillock ) also
termed as “trigger zone”
The axon may have branches, called collaterals . The greater the degree of branching of the
axon and axon collaterals, the greater the cell’s sphere of influence
Each branch ends in an axon terminal , which is responsible for releasing neurotransmitters
from the axon
Alternatively, some neurons release their chemical messengers from a series of bulging areas
along the axon known as varicosities .
The axons of many neurons are covered by sheaths of myelin, usually consists of 20 to 200
layers of highly modified plasma membrane
In the brain and spinal cord, these myelin-forming cells are the oligodendrocytes . Each
oligodendrocyte may branch to form myelin on as many as 40 axons
schwann cells
In the PNS, cells called Schwann cells form individual myelin sheaths surrounding 1- to
1.5-mm-long segments at regular intervals along some axons
node of ranvier
The spaces between adjacent sections of myelin where the axon’s plasma membrane is
exposed to extracellular fluid are called the nodes of Ranvier
maintence of structure and function of cell axon
Various organelles and other materials must move as far as 1 meter between the cell
body and the axon terminals. This movement, termed axonal transport , depends
on a scaffolding of microtubule “rails” running the length of the axon and
specialized types of motor proteins known as kinesins and dyneins ( Figure 6.3 ).
At one end, these double-headed motor proteins bind to their cellular cargo, and the
other end uses energy derived from the hydrolysis of ATP to “walk” along the
microtubules.
kinesin
Kinesin transport mainly occurs from the cell body toward the axon terminals (
anterograde ) and is important in moving nutrient molecules, enzymes,
mitochondria, neurotransmitter-filled vesicles, and other organelles.
dyein
Dynein
movement is in the other direction ( retrograde ), carrying recycled membrane
vesicles, growth factors, and other chemical signals that can affect the neuron’s
morphology, biochemistry, and connectivity. Retrograde transport is also the route
by which some harmful agents invade the CNS, including tetanus toxin and the
herpes simplex, rabies, and polio viruses.
neurons can be divided into
affert neurns
effernt neurons
interneurons
afferent neuron
Afferent nuerons: convey information from the tissues and organs
of the body toward the CNS
At their peripheral ends have sensory receptors , which respond to
various physical or chemical changes in their environment by
generating electrical signals in the neuron
Structurally different, two branched axon, one is peripheral process,
begins where the dendritic branches converge from the receptor
endings. The other branch, central process, enters the CNS to form
junctions with other neurons
Cell body and the long axon are outside the CNS
efferent neurons
convey information away from the CNS to
effector cells like muscle, gland, or other cell types
Conventional neuronal structure (refer to figure) Their cell bodies and
dendrites are within the CNS
interneuron
Interneurons: connect neurons within the CNS
Lie entirely within the CNS
They account for over 99% of all neurons and have a wide range
of physiological properties, shapes, and functions.
The number of interneurons interposed between specific afferent
and efferent neurons varies according to the complexity of the
action they control.
Example: The knee-jerk reflex elicited by tapping below the
kneecap activates thigh muscles without interneurons. In
contrast, to hear a song or smell a certain perfume that evokes
memories of someone you know, millions of interneurons may
be involved
synapse
Synapse: The anatomically specialized junction between two
neurons where one neuron alters the electrical and chemical
activity of another.
Most synapses occur between an axon terminal of one neuron
and a dendrite or the cell body of a second neuron
A neuron that conducts a signal toward a synapse is called a
presynaptic neuron , whereas a neuron conducting signals
away from a synapse is a postsynaptic neuron
glial cells
Neurons account for only about half of the cells in the
human CNS. The remainder are glial cells ( glia, “glue”).
Glial cells surround the soma, axon, and dendrites of
neurons and provide them with physical and metabolic
support
Glial cells retain the capacity to divide throughout life.
Consequently, many CNS tumors actually originate from
glial cells
types of glial cells
oligodendrocyte
astrocyte
microglia
epndymal cells
schwann cells
oligodendrocyte
which forms the myelin sheath of CNS axons
astrocyte
helps regulate the composition of the
extracellular fluid in the CNS by removing potassium ions
and neurotransmitters around synapses
formation of tight junctions between the cells that make up the
walls of capillaries in CNS, which forms Blood brain barrier
Astrocytes also sustain the neurons metabolically—for example,
by providing glucose and removing ammonia.
In developing embryos, astrocytes guide neurons as they migrate
to their ultimate destination, and they stimulate neuronal growth
by secreting growth factors.
microglia
The microglia: a third type of glial cell, are specialized,
macrophage-like cells (Chapter 18) that perform immune functions
in the CNS, and may also contribute to synapse remodeling and
plasticity.
eondymal cells
ependymal cells line the fluid-filled cavities within the brain and
spinal cord and regulate the production and flow of cerebrospinal
fluid
schwann cells
Schwann cells, the glial cells of the PNS, have most of the
properties of the CNS glia.
neural growth and regenerations
Neuronal cells or glia develops from stem cells in embryo
Each neuronal daughter cell differentiates, migrates to its
final location, and sends out processes that will become its
axon and dendrites.
A specialized enlargement, the growth cone , forms the
tip of each extending axon and is involved in finding the
correct route and final target for the process