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Chapter Two
Nerve Cells and Nerve Impulses
WHAT, THEN, IS THE BRAIN?
The brain, perhaps the most complex system.
What is the functional organization of the nervous system?
1. Reticular doctrine: Is the brain a reticular structure, a syncithium? NO
2. Neuron doctrine: The Neuron is the functional unit of the nervous system
Camilo Golgi invented Golgi stain in the 1870's (19th century)
Santiago Ramon Y Cajal
The neuron is a very specialized cell. It consists of:
Body or soma, dendrites, axon, axon hillock, axon terminal, synaptic button
Myelin is a fatty sheath that covers the axon.
Node of Ranvier: gaps in the myelin
Node of Ranvier
Figure 2.5 Vertebrate motor neuron
Cells of the Nervous System
Neurons and Glia
Membrane: separates the inside of the cell from the outside
Nucleus: contains the chromosomes
Mitochondrion: useful for metabolic activities
Ribosomes: sites for synthesizing new protein molecules
Endoplasmic reticulum: network of thin tubes that transport
synthesized proteins to other locations
Ion channels
Figure 2.3 The membrane of a neuron
protein channels permit certain ions to cross through the
membrane
Figure 2.2 An electron micrograph of parts of a neuron
The nucleus, membrane, and other structures are characteristic of most
animal cells. The plasma membrane is the border of the neuron.
The Structure of a Neuron
Dendrites: branching fibers that get narrower as they
extend from the cell body toward the periphery;
information receiver
Dendritic spines: short outgrowths that increase the surface
area available for synapses
Cell body :contains the nucleus and other structures found
in most cells
Axon: thin fiber of constant diameter, in most cases longer
then the dendrites; information-sender
Myelin: sheath-insulating material covering the axons;
speed up communication in the neuron
Presynaptic terminal: the point on the axon that releases
chemicals
The brain has about 100 billion neurons and about 100 trillion
connections between them
Neurons can be classified according to:
1. Number of processes
Unipolar
Bipolar
Miltipolar
2. Function:
Afferent (sensory)
Efferent (motor)
Interneuron.
Figure 2.6 A vertebrate sensory neuron
Figure 2.8 Cell structures and axons
It all depends on the point of view. An axon from A to B is an efferent axon from A
and an afferent axon to B.
Not all living systems have these three kinds of functional neurons.
One-stage system (sensory-motor neuron): sea anemones (hydras)
Two-stage system (sensory and motor neurons): jellyfish.
Three-stage system (sensory-interneuron-motor): from mollusks (e.g., mussels) on.
GLIA: In addition to neurons, the brain is made of glial cells.
Glial cells are about 10 times more numerous than neurons
Functions of glia:
1. Structural support
2. Nutritive functions and general housekeeping functions
3. Help in forming the blood-brain barrier
4. guidance for neuron migration during development
5. Producing the insultain myelin for faster nervous conduction.
Oligodendrocites in brain (central nervous system)
Schwann cells in nerves (peripheral nervous system).
Macroglia: three kinds
Oligodendrocites--produce myelin in Central NS
Schawann cells-- produce myelin in Peripheral NS
Astrocytes--participate in nutrition and blood-brain barrier
Figure 2.11 (a) Shapes of some glia cells.
Oligodendrocytes produce myelin sheaths in the CNS. Each oligodendrocyte
forms such segments for 30 to 50 axons.
Schwann produce myelin in the PNS.
Astrocytes pass chemicals back and forth between neurons and blood and
among various neurons in an area.
Microglia proliferate in areas of brain damage and remove toxic materials.
The Blood-Brain Barrier
Why we need a blood-brain barrier?
To keep out harmful substances such as viruses, bacteria, and
harmful chemicals. (Neurons cannot divide).
How the blood-brain barrier works?
Endothelial cells are tightly joined to one another, and many
molecules, including some drugs to fight cancer or
Parkinson , cannot pass into the brain.
What can pass the blood-brain barrier ?
Passive Transport: requires no energy to pass
Small uncharged molecules-oxygen and carbon dioxide
Molecules that can dissolve in the fats of the capillary walls
Active Transport: requires energy to pass
Glucose, amino acids, vitamins and hormones
Figure 2.13 The blood-brain barrier
Most large molecules and electrically charged molecules cannot cross from the
blood to the brain. A few small uncharged molecules such as O2 and CO2 can
cross; so can certain fat-soluble molecules. Active transport systems pump
glucose and certain amino acids across the membrane.
Nourishment of Vertebrate Neurons
Glucose-primary energy source for the brain
Oxygen-needed to metabolize glucose
Thiamine-necessary for the use of glucose
The Nerve Impulse
The Resting Potential of the Neuron
Resting potential: results from a difference in distribution of
various ions between the inside and outside of the cell
(-70mV inside compared with outside the cell)
Measurement of the Resting Membrane Potential
Microelectrodes
Why a Resting Potential?
Prepares neuron to respond rapidly to a stimulus
Figure 2.14 Methods for recording activity of a neuron
Diagram of the apparatus and a sample recording.
Neurons are the functional units of the nervous system.
What is the property that allows them to interact with each other?
Neurons are capable of signaling
Neurons communicate by sending electrical signals
called Action Potentials
Action Potentials are produced by the movement of ions in
and out of the neuron, through the cell membrane.
Ions are charged particles: Positive charges: cations
Negative charges: anions
The Nerve Impulse
What are the forces that move the ions across the cell membrane?
Ions move along gradients of potential energy. What is potential energy?
In the neuron, ions are moved by two forces (potential energy):
Concentration Gradients: difference in distribution for various ions
between the inside and outside of the membrane
Electrical Gradient: the difference in positive and negative
charges
across the membrane
The cell membrane is a lipid bilayer which does not allow
the passage of ions
However, the membrane has protein channels that allow
the passage of ions
Protein channels are very selective
1. Concentration gradient
Due to Concentration gradient between inside and oustside
the membrane, K+, Na+, A-, Cl- ions tend to go:
K+: OUT
A- : OUT (large ions, proteins, RNA, DNA, etc, cannot leave)
Na+: IN
Cl- : IN
2. Electrical gradient
Model of neuron: what happens if K+ channels open?
Movement of K+ along a CONCENTRATION gradient creates
an ELECTRICAL gradient.
RESTING POTENTIAL:
-70 mV (inside negative with respect to outside).
Figure 2.16 The sodium and potassium gradients for a resting membrane
.
Animation
What happens to Na+?
CONCENTRATION & ELECTRICAL GRADIENTS PUSH NA+ IN !!
What happens if Na+ channels open? AN ACTION POTENTIAL !
Momentary reversal of potential: positive inside, negative outside
Outside
Na+ cannels closed
Na+ channels open
++++++++++
-----------
membrane____________________________________________________
Inside
-----------
++++++++++++
Resting Potential
(-70 mV inside)
Action potential
(+50 mV inside)
The Action Potential
Important Definitions
Hyperpolarization: increasing the negative charge inside the
neuron
Depolarization: decreasing the negative charge inside the
neuron
Threshold of excitation: Level above which a stimulation
produces a sudden depolarization of the membrane
Action Potential: rapid depolarization and slight reversal of
the usual polarization
Molecular Basis of the Action Potential
Sodium channels open once threshold is reached causing an
influx of sodium: depolarization to +50 mv
Potassium channels open as the action potential approaches
its peak allowing potassium to flow out of the cell:
hyperpolarization to -70mv.
Fig 2.17 Sodium ions cross during the peak of the action potential
Potassium ions cross later in the opposite direction, returning the
membrane resting potential
Why ion channels open or close?
they are GATED by several stimuli:
-electrical stimuli: differences in voltage: voltage gated.
-chemical stimuli: chemical transmitters, (in synapses).
-mechanical: for instance, the tap in the knee that produces
the knee jerk reflex.
The Action Potential
The All-or-None Law
The size of an action potential (120 mv) and its speed are
independent of the intensity of the stimulus that initiated it.
Similar to firing a gun: when trigger reaches threshold, the bullet
is fired with the same speed no matter how strongly the trigger
is pulled.
The Action Potential
The Refractory Period: after an action potential, the
neuron resists the production of further action
potentials
Two Refractory Periods
1. Absolute Refractory Period (1-2 msec)
The sodium gates are firmly closed
The membrane cannot produce an action potential,
regardless of the stimulation.
-Limits the maximum firing frequency: 1000/sec
-Action potential cannot reverse direction
2. Relative Refractory Period
A stronger than normal stimulus can result in an action
potential.
CHARACTERISTICS OF THE ACTION POTENTIAL
-Na+ and K+ channels in axon are voltage gated.
-Action Potential are triggered by positive change in membrane potential.
-Threshold potential: 10 mV (from -70 mV to -60 mV)
-Size of action potential: 120 mV: from -70 mV to + 50 mV (all or nothing)
-Action potentials are triggered in the axon hillock.
No action potentials in soma or dendrites (but new data suggest otherwise)
The first ionic event in the generation of an action potential
is the opening of Na+ channels
Duration of Action Potential: about 1 msec
The action potential ends because
-The gate for Na+ closes,
-The gates for K+ opens: outflow of K+, accumulates + charges outside,
bringing the potential inside back to -70 mV.
-Inflow of Cl- attracted by the + charges inside
(gates for Cl- are always open).
Propagation of the Action Potential
Axon Hillock-where the action potential begins
Terminal Buttons-the end point for the action potential
The action potential flows toward the terminal and does not reverse
directions because the area where the action potential just came
from are still in refractory period
Propagation of Action Potential
Passive membrane properties
The propagation of action potential is mediated by voltage-gated channels.
A potential at one place triggers the neighboring place (domino effect)
Homology with the burning of a flame down a wick. Heat-gated channel.
A flame, like the action potential, cannot go back.
Speed of conduction
The Myelin Sheath and Saltatory Conduction
Saltatory conduction. By isolating a segment of the axon, myelin forces
the action potential to jump from one node of Ranvier to the next.
Na+ channels accumulate in the nodes of Ranvier
In large myelinated axons, the conduction can be as much as 100 m/sec,
or 220 miles per hour.
The propagation speed is slower in small, unmyelinated fibers.
Myelosclerosis, multiple sclerosis: slow down or stop conduction
Figure 2.20 Saltatory conduction in a myelinated axon
An action potential at the node triggers flow of current to the next
node, where the membrane regenerates the action potential.
Effect of action potentials on the concentration of ions
inside the cell is Very small.
There is a Na+-K+ pump that kicks Na+ out and brings K+ in to
maintain the concentrations at a stable value.
This pump requires metabolic energy (ATP).
After blockade of the Na+-K+ pump (with DNP, dinitrophenol),
there can be thousands of action potentials.
Mechanisms of action of local and general anesthetics & venoms:
Local anesthetics (Novocain, xylocaine) attach to Na+ channels,
preventing Na+ inflow
General anesthetics (ether, chloroform) Open K+ channels: clamp potential
Scorpion Venom: Keeps Na+ channels open and K+ channels closed
Tetrodotoxin (TTX, from puffer fish) blocks Na+ channels
Cyanide blocks ATP-dependent Na+-K+ pump