The Nervous System:
Neurons and Synapses
Homeostasis is maintained by an appropriate balance between sympathetic and parasympathetic
activity.
Nerve- Any bundle of nerve fibers running to various organs and tissues of the body
Neuron- A cell specialized
to conduct and generate electrical impulses and to carry information from one part of the brain to another.
Neuron is a single nerve cell; a nerve is a bundle of many nerve fibers (axons and dendrites of neurons) Neurons
are responsible for integrating information received from the internal and external environment; they are specialized for
excitability and conductivity and consist of a cell body, dendrites and an axon. There are three types of neurons
in the body. We have sensory neurons, interneurons, and motor neurons. Neurons are a major class of cells in the nervous system
and the functional unit of the nervous system (but they are outnumbered by glial cells).
Neurons are
sometimes called nerve cells, though this term is technically imprecise, as many neurons do not form nerves. In vertebrates,
neurons are found in the brain, the spinal cord and in the nerves and ganglia of the peripheral nervous system. Their main
role is to process and transmit information.
Neurons have
excitable membranes, which allow them to generate and propagate electrical impulses. Sensory neuron takes nerve impulses or
messages right from the sensory receptor and delivers it to the central nervous system. A sensory receptor is a structure
that can find any kind of change in it's surroundings or environment.
Each neuron can carry a signal in
only one direction (from the cell body down the axon). They are long lived cells, having been in your body since before birth
or in the few years after birth.
Neurons of the peripheral nervous system can be very long cells, since the cell body must
reside in or near the central nervous system. Motor neurons of the PNS have short dendrites and their cell body resides in
the spinal cord. They have a long axon that goes to the muscle or gland they innervate. Sensory neurons of the PNS have long
dendrites going from the region they receive sensory input from to the basal ganglia next to the spinal cord (where their
cell bodies are located). Sensory neurons have a short axon going into the spinal cord and synapsing with other neurons.
Check
out the Mouse Party and other cool demonstrations about how drugs work!
Types of Neurons
Structure of a neuron
Neurons have three different parts to them. They all have an axon, a cell body
and dendrites. The axon is the part of the neuron that conducts nerve impulses. Axons can get to be quite long. When an axon
is present in nerves, it is called a nerve fiber. A cell body has a nucleus and it also has other organelles. The dendrites
are the short pieces that come off of the cell body that receive the signals from sensory receptors and other neurons.
Synapse:
Chemical synapses are specialized junctions through which the cells of the nervous system signal
to each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses allow the neurons of the central
nervous system to form interconnected neural circuits.
Transmissions across the majority of synapses in the nervous system
is one-way and occurs through the release of chemical neurotransmitters from the presynaptic axon endings called
terminal boutons.
Myelin Sheath
Schwann cells contain a lipid substance called myelin
in their plasma membranes. When schwann cells wrap around axons, a myelin sheath forms. There are gaps that have no myelin
sheath around them; these gaps are called nodes of Ranvier. Myelin sheathes make excellent insulators. Axons that are longer
have a myelin sheath, while shorter axons do not. The disease multiple sclerosis is an autoimmune disease where the body attacks
the myelin sheath of the central nervous system.
Neuroglia- Support cells, also
known as glial cells. They are more abundant than neurons. They do not transport messages.
Astrocytes, Oligodendrocytes, and Schwann cells
-Oligodendrocytes have extensions that form the myelin sheath around
axons in the Central Nervous System. Nerve impulses travel faster along myelinated axons than unmyelinated axons (saltatorial
conduction is the term to describe jumping from one node to another).
-Schwann cells perform the same function as oligodendrocytes, but are found
in the peripheral nervous system. Instead of having extensions that wrap around axons, the entire cell wraps itself around
and around.
-Astrocytes
are most common in the brain. They have lots of small feet that go to capillaries and neurons.This is the cell in
which all things must go through to get to the brain. They must perform this duty because the capillaries have tight junctions
in which messages cannot be passed. This is what is called the blood brain barrier. Many drugs cannot enter the astrocytes.
They must instead use a precursor drug. Astrocytes are found in the CNS.
Electrical Activity of Axons
The permeability of the
axon membrane of sodium to potassium is regulated by gated channels.
-Resting membrane potential is -70 mV (voltage). This
is sightly permeable to potassium and impermeable to sodium.
-An excess positive charge on the outside of the cell and
the excess of the negative charge inside the cell collect close to the plasma membrane These exceeds charges are only a very
small fraction of the total number of ions in and outside the cell.
-
A more in depth look at Electrical Activity of Axons
The Nerve Impulse
When a nerve is stimulated the resting potential changes. Examples of such stimuli are pressure,
electricity. chemicals., etc. Different neurons are sensitive to different stimuli(although most can register pain). The stimulus
causes sodium ion channels to open. The rapid change in polarity that moves along the nerve fiber is called the "ACTION POTENTIAL."
This moving change in polarity has several stages: Depolarization The upswing is caused when positively charged sodium ions(Na+)
suddenly rush through open sodium gates into a nerve cell.The membrane potential of the stimulated cell undergoes a localized
change from-65 millivolts to 0 in a limited area. As additional sodium rushes in, the membrane potential actually reverses
its polarity so that the outside of the membrane is negative relative to the inside. During this change of polarity the membrane
actually develops a positive value for a moment(+40 millivolts). The change in voltage stimulates the opening of additional
sodium channels (they are voltage-gated). This is an example of a positive feedback loop.
Repolarization (The down-swing)
is caused by the closing of sodium ion channels and the opening of potassium ion channels. Release of positively charged potassium
ions (K+) from the nerve cell when potassium gates open. Again, these are opened in response to the positive voltage--they
are voltage gated. This expulsion acts to restore the localized negative membrane potential of the cell (about -65 or -70
mV is typical for nerves).
Refractory phase is a short period of time after the depolarization stage. Shortly after the
sodium gates open they close and go into an inactive conformation. The sodium gates cannot be opened again until the membrane
is repolarized to its normal resting potential. The sodium-potassium pump returns sodium ions to the outside and potassuim
ions to the inside. During the refractory phase this particular area of the nerve cell membrane cannot be depolarized. This
refractory area explains why action potentials can only move forward from the point of stimulation. Increased permeability
of the sodium channel occurs when there is a deficit of calcium ions. when there is a deficit of calcium ions (Ca+2) in the
interstitial fluid the sodium channels are activated (opened) by very little increase of the membrane potential above the
normal resting level. The nerve fiber can therefore fire off action potentials spontaneously, resulting in tetany. Could be
caused by the lack of hormone from parathyroid glands. Could be caused by hyperventilation, which leads to a higher pH, which
causes calcium to bind and become unavailable. Speed of conduction. This area of depolarization/repolarization/recovery moves
along a nerve fiber like a very fast wave. In nonmyelinated fibers, conduction is hundreds of times faster because the action
potential only occurs at the nodes of Ranvier by jumping from node to node. This is called "saltatory" conduction. Damage
to the myelin sheath by the disease can cause severe impairment of nerve cell function. Some poisons and drugs interfere with
nerve impulses by blocking sodium channels in nerves. See discussion on drug at the end of this outline.
If you don't
understand this a great illustrated site to look into... http://outreach.mcb.harvard.edu/animations/actionpotential.swf
All or none law
Muscle cells and neurons function on an all-or-none principle. Once the stimulus exceeds a
certain threshold, the contraction or nerve impulse in that single cell will happen, and it will happen all the way. It's
like when you play dominoes and you line them up and you tilt the first one only so far and not have it fall over, but once
you pass the point of no return, they are going down, and all the others will fall down in a quick motion. This principle
is fully dependent on those gated ion channels opening, letting sodium ions flood into the cell.
Acetylcholine as a neurotransmitter
Acetylcholine(ACh) is used as an excitatory neurotransmitter by some neurons in the
CNS and by somatic motor neurons at the neuromuscular junction. At autonomic nerve endings, ACh may be either excitatory or
inhibitory, depending on the organ involved.
The stimulatory effects on ACh on skeletal muscle cells is produced by
the binding of ACh to nicotinic ACh receptors. Effects of ACh on other cells occur when ACh binds to muscarinic ACh receptors.
Chemically regulated Channels
The binding of a neurotransmitter to its receptor protein can cause the opening of ion channels through two different
mechanisms. These two mechanisms can be illustrated by the actions of ACh on the nicotonic and muscarinic subtypes of the
ACh receptors.
Ligand-Gated Channels
A neurotransmitter molecule is the ligand that binds to its specific receptor
protein. Part of the protein has extracellular sites that bind to the neurotransmitter ligands, while part of the protein
spans the plasma membrane and has a central ion channel.
The nicotinic ACh receptor can serve as an example of ligand-gated
channels. Two of its five-polypeptide subunits contain ACh binding sites, and the channel opens when both sites bind to ACh.
The opening of this channel permits the simultaneous diffusion of Na+ into and K+ out of the postsynaptic cell. The effects
of the inward flow of Na+ predominate, however, because of its steeper electrochemical gradient. This produces the depolarization
of an excitatory postsynaptic potential (EPSP)
Although the inward diffusion of Na+ predominates in an EPSP, the simultaneous
outward diffusion of K+ prevents the depolarization from overshooting 0mV.
G-Protein-Coupled Channels
The muscarinic ACh receptors are formed from only a single subunit which can bind to
one ACh molecule. Unlike the nicotinic receptors, these receptors do not contain ion channels. Binding of ACh (the ligand)
to the muscarinic receptor causes it to activate a complex of proteins in the cell membrane known as G-Proteins. They are
named because their activity is influenced by guanosine nucleotides.
There are 3 G-protein subunits
1-Alpha
2-Beta
3Gamma
In
response to the binding of ACh to its receptor, the alpha subunit dissociates from the other two subunits, which stick together
to form a beta-gamma complex. On the specific case, either the alpha subunit or the beta-gamma complex then diffuses through
the membrane until it binds to an ion channel, causing the channel to open or close.
The binding of ACh to its muscarinic
receptors indirectly affects permeability of K+ channels. This can produce hyperpolarization in some organs and depolarization
in other organs.
Scientists have learned that it is the beta-gamma complex that binds to the K+ channels in the heart
muscle cells and causes these channels to open. Which leads to the diffusion of K+ out of the postsynaptic cell. The cell
the becomes hyperpolarized production an inhibitory postsynaptic potential (IPSP). Such an effect is produced in the heart
for example, when autonomic nerve fibers synapse with pacemaker cells and slow the rate of beat.
In a smooth muscle
cells of the stomach the binding of ACh to its muscarinic receptors causes a different type of G-protein alpha subunit to
dissociate and bind to gated K+ channels. The binding of the G-protein subunit to the gated K+ channels causes them to close
rather than open. The diffusion of K+ which occurs at an outgoing rate in the resting cell is reduced to below resting levels.
The resting membrane potiential is maintained by a balance between cations flowing into the cell and cations following out
a reduction in the outward flow of K+ produces a depolarization. This depolarization produced in these smooth muscle cells
results in contraction of the stomach
Acetylcholinesterase (AChE)
The inactivation of ACh is achieved by means of an enzyme called acetylcholinesterase or AChE which
is present on the postsynaptic membrane or immediately outside the membrane with its active site facing synaptic cleft.
Acetylcholine in the PNS
Somatic motor neurons form synapses with skeletal muscle cells (muscle fibers). At these synapes,
or neuromuscular junctions, the postsynaptic membrane of the muscle fiber is known as a motor end plate. The EPSPs produced
by ACh in the skeletal muscle fibers are often called end-plate potentials. Voltage-regulated channels produce action potentials
in the muscle fiber, and these are reduced by other voltage-regulated channels along the muscle plasma membrane
If
in any stage in the process of neuromuscular transmission is blocked, muscle weakness sometimes leading to paralysis and death
may result
Autonomic motor neurons innervated cardiac muscle, smooth muscles in blood vessels and
visceral organs, and glands. There are two classifications of autonomic nerves, sympathetic and parasympathetic. Most of the
parasympathetic axons that innervate the effector organs use ACh as their neurotransmitter.
