Transcript Nerve Cells and Nerve Impulses

Chapter Two
Nerve Cells and Nerve Impulses
Cells of the Nervous System
Neurons and Glia
The Structures of an Animal Cell
Membrane-a structure that separates the inside of the cell
from the outside
Nucleus-the structure that contains the chromosomes
Mitochondrion-structure where the cell performs metabolic
activities
Ribosomes-sites at which the cell synthesizes new protein
molecules
Endoplasmic reticulum-a network of thin tubes that
transport newly synthesized proteins to other locations
Figure 2.3 The membrane of a neuron
Embedded in the membrane are protein channels that permit
certainions to cross through the membrane at a controlled rate.
Figure 2.2 An electron micrograph of parts of a neuron
from the cerebellum of a mouse
The nucleus, membrane, and other structures are characteristic of most
animal cells. The plasma membrane is the border of the neuron.
Magnification approximately 3 23,000.
Cells of the Nervous System
Neurons and Glia
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
Figure 2.5 The components of a vertebrate motor neuron
The cell body of a motor neuron is located in the spinal cord. The various
parts are not drawn to scale; in particular, a real axon is much longer
in proportion to the size of the soma.
Cells of the Nervous System
Neurons and Glia
Terms associated with Neurons
Motor neuron-receives excitation from other neurons and
conducts impulses from its soma in the spinal cord to
muscle of gland cells
Sensory neuron-specialized at one end to be highly
sensitive to a particular type of stimulation
Local neuron-small neuron with no axon or a very short
one
Efferent axon-carries information away from the structure
Afferent axon-brings information into a structure
Intrinsic/interneuron-the cell’s dendrites and axon’s are
entirely contained within a single structure
Figure 2.6 A vertebrate sensory neuron
Note that the soma is located in a stalk off the main trunk of the axon.
(As in Figure 2.5, the various structures are not drawn to scale.)
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, just as a train from Washington to New York is exiting
Washington and approaching New York.
Cells of the Nervous System
Neurons and Glia
Glia-supportive cells in the nervous system
Types
Astrocytes-star-shaped glia that wrap around the
presynaptic terminals of several axons
Radial Glia-a type of astrocyte that guides the
migration of neurons and the growth of their
axons and dendrites during embryonic
development
Oligodendrocytes-located in the CNS and provide
myelin sheaths for axons
Schwann Cells-located in the PNS and provide myelin
sheaths for axons
Figure 2.11 (a) Shapes of some glia cells.
Oligodendrocytes produce myelin sheaths that insulate certain vertebrate axons
in the central nervous system; Schwann cells have a similar function in the
periphery. The oligodendrocyte is shown here forming a segment of myelin
sheath for two axons; in fact, each oligodendrocyte forms such segments for 30
to 50 axons. 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. Radial glia (not shown here)
guide the migration of neurons during embryological development.
Glia have other functions as well.
The Blood-Brain Barrier
Why we need a blood-brain barrier
To keep out harmful substances such as viruses, bacteria, and
harmful chemicals
How the blood-brain barrier works
Tight Gap Junctions
What can pass the blood-brain barrier
Passive Transport-require no energy to pass
Small uncharged molecules-oxygen and carbon dioxide
Molecules that can dissolve in the fats of the capillary walls
Active Transport-require 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)
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
(a) Diagram of the apparatus and a sample recording. (b) A microelectrode and
stained neurons magnified hundreds of times by a light microscope. (Fritz Goro)
The Nerve Impulse
The Forces Behind the Resting Potential
Selective Permeability-the membrane allows some molecules
to pass more freely than others
The Forces
Sodium-Potassium Pump-actively transports three sodium
ions out of the cell while simultaneously drawing two
potassium ions into the cell
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
Figure 2.16 The sodium and potassium gradients for a resting membrane
Sodium ions are more concentrated outside the neuron; potassium ions are more
concentrated inside. However, because the body has far more sodium than
potassium, the total number of positive charges is greater outside the cell than
inside. Protein and chloride ions (not shown) bear negative charges inside the cell.
At rest, very few sodium ions cross the membrane except by the sodium-potassium
pump. Potassium tends to flow into the cell because of an electrical gradient but
tends to flow out because of the concentration gradient.
Animation
The Action Potential
Important Definitions
Hyperpolarization-increasing the negative charge inside the
neuron
Depolarization-decreasing the negative charge inside the
neuron
Threshold of excitation-Any stimulation beyond a certain level
producing a sudden, massive 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
Potassium channels open as the action potential approaches
its peak allowing potassium to flow out of the cell
Cell overshoots resting membrane potential
Figure 2.17 The movement of sodium and potassium ions
during an action potential
Note that sodium ions cross during the peak of the action potential and that
potassium ions cross later in the opposite direction, returning the membrane to
its original polarization.
The Action Potential
The All-or-None Law
The size, amplitude, and velocity of an action potential are
independent of the intensity of the stimulus that initiated it.
The Action Potential
The Refractory Potential
Defined-During this time the cell resists the production of further
action potentials
Two Refractory Periods
Absolute Refractory Periods
The sodium gates are firmly closed
The membrane cannot produce an action potential,
regardless of the stimulation.
Relative Refractory Periods
The sodium gates are reverting to their usual state, but
the potassium gates remain open.
A stronger than normal stimulus can result in an action
potential.
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
The Myelin Sheath and Saltatory Conduction
Myelin Sheaths increase the speed of neural transmission
Nodes of Ranvier-Short area’s of the axon that are unmyelinated
Saltatory Conduction-jumping action of actions potentials from
node of Ranvier to node of Ranvier
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.
Signaling Without Action Potentials
Depolarizations and hyperpolarizations of dendrites and cell bodies
Small Local neurons-produce graded potentials (membrane
potentials that vary in magnitude and do not follow the all-ornone law)