UNIT 4: Homeostasis
Chapter 11: The Nervous System
pg. 514
11.2: Nerve Signals
pg. 522 - 529
Through research by many scientists the difference between neural and
electrical transmission was determined. Nerve impulses travel much slower
then electrical current, the cause is the cytosol of the cell body creates a
resistance to the current. Although electrical current is faster, it also
decreases over distance, where the nerve impulse intensity does not diminish
over distance, the same from start to end. Nerves use internal cellular energy
to generate current, where electrical current comes from an external energy
source.
In the earlier 1900’s Julius Bernstein proposed the idea that nerve impulses
are the result of ions moving through the nerve cell membrane. Evidence
was then determined by K. S. Cole and H. J. Curtis while studying a squid.
When the nerve became excited (stimulated) the electrical potential across
the membrane rose rapidly from -70 mV (at rest) to +40 mV.
Neural Communication via Synapses
Synapse – is a functional connection between neurons or between neurons
and effectors.
Chemical synapse – is a synapse in which a neurotransmitter moves from a
pre-synaptic cell to a postsynaptic cell through the synaptic cleft.
Neurotransmitter – is a chemical that is released from vesicles into synapses
to facilitate nerve signal transmission.
Synaptic cleft – is the tiny gap between pre-synaptic and post synaptic cells
in a chemical synapse, across which the neurotransmitter diffuses.
Electrical synapse – is a synapse in which the pre-synaptic cell makes direct
contact with the postsynaptic cell, allowing current to flow via gap junctions
between the cells
The synapse is the junction where one neuron interacts with another neuron
or an effector neuron. As impulse travels from dendrite, cell body, to the
axon, therefore the impulse reaches the end of one nerve cell (axon terminal
of the pre-synaptic cell) and must be passed on to the adjacent nerve cell
(dendrite of the post-synaptic cell). Depending on the kind of neuron the
signal across the synaptic cleft can be either chemical or electrical.
The more common form is the chemical synapse is the chemical messenger,
called a neurotransmitter. The axon terminal at the synapse will release the
neurotransmitter entering the synaptic cleft (25 nm wide) and then stimulate
the post-synaptic cell.
The electrical synapse is different, the pre and post- synaptic membranes are
in direct contact allowing the current to flow directly from one cell to the
next. Ions which create this current, move directly between the two cell
membranes, providing an unbroken transmission of the signal.
Figure 2: a) In a chemical synapse, the neurotransmitter diffuses across the synaptic cleft
and binds to a receptor in the plasma membrane of the postsynaptic cell. B) Electrical
signals transfer directly across gap junctions in an electrical synapse.
Conduction of Electrical Signals by Neurons
Membrane potential – is the electrical potential of a membrane, which is
caused by an imbalance of charges on either side of the membrane.
Ion channel – is a protein embedded in the plasma membrane that allows
ions to pass through it.
Cells maintain a positive and negative charge across their plasma membrane.
The outside is positive and the inside is negative, and this charge separation
can produce a voltage, or electrical potential difference, or membrane
potential. The potential difference is caused by the uneven distribution of
Na+ and K+ ions on the inside and to the outside of the plasma membrane.
This caused by the selectively permeability of the plasma membrane and the
ion channels.
In most cells, the plasma membrane potential remains stable. In nerve,
muscle respond to chemical, electrical, mechanical, and certain other stimuli
can cause a change in the membrane potential rapidly. The cells become
excited, and the ion channels and plasma membrane become more
permeable to the movement of ions across the membrane.
Resting Membrane
Resting potential – is the voltage difference across a nerve cell membrane of
an unstimulated neuron; usually negative.
The ion channels actively pump Na+ and K+ ions across the plasma
membrane. To do this energy is required; ATP hydrolysis is used to pump
three Na+ in out of the cell for every two K+ pumped in. Since there are now
more [Na+] ions out side then [K+] ions in side, this concentration difference
creates a net positive charge outside the cell.
The typical neuron not conducting an impulse has a steady negative plasma
membrane potential of -70 mV, and is known as the resting potential. A
membrane is this stated is said to be polarized.
Figure 3: The distribution of ions inside and outside an axon produces the resting
potential, -70 mV. Note that the Na+ and K+ ion channels are closed when the membrane
is at the resting potential.
The Na+/K+ pumps are responsible for maintaining a difference between Na+
and K+ inside and outside the cell. The concentrations of anions in side the
cell remain unchanged, creating a negative charge inside the cell and a
positive charge outside the cell.
Action Potential
Action potential – is the voltage difference across a nerve cell membrane
when the nerve is excited.
Threshold potential – is the potential of which an action potential is
generated by a neuron.
Refractory period – is the period of time during which the threshold
required for the generation of an action potential is much higher than normal.
When a nerve cell receives an impulse, there is sudden and temporary
change in the membrane potential. This is called an action potential, where
a stimulus causes positive charges from the outside the membrane to flow
inward, causing the interior of the cell (cytosol) to become less negative.
The action potential can be broken down into six phases.
Phase 1: The initial stimulus causes the plasma membrane to be more
permeable and incoming ions raise the membrane potential (polarized) to a
less negative value and is now depolarized. The membrane potential will
eventually reach the threshold potential (-50 to -55 mV). This causes the ion
channels to open.
Phase 2: The ion channels continue to open passing more Na+ ions into the
cell. In less then a milli second the action potential increases sharply, the
inside of the cell and is now positively charged.
Phase 3: The action potential reaches its peak, +30 mV or higher, the sodium
pumps close and become inactive. Potassium channels open and allow K+
ions to leave.
Phase 4: As K+ ions leave the cell the membrane potential decreases rapidly
and the membrane becomes re-polarized.
Phase 5: The potassium channels close slowly as the membrane potential
changes from positive to negative. The membrane eventually reaches its
resting potential.
Phase 6: At this time the resting potential is stabilized and the membrane is
ready for the next action potential.
Figure 4: Changes in
membrane potential
during the six phases of
the action potential.
The time required for the membrane to go from resting potential to action
potential and back again is approximately 5 ms. An action potential can only
be achieved if the stimulus is strong enough to cause depolarization to reach
the threshold potential (all or nothing principle).
Once the threshold potential has been reached the impulse will travel from
the dendrite to the axon without any further stimulus, this is called the
propagation of the action potential.
The refractory period begins to occur once the action potential has reached
its peak. The refractory period lasts until the resting potential of the
membrane has been stabilized. This process ensures that the impulse will
only travel in one direction. Some proteins channels are still open and are
not prepared to pump ions again, they must reset themselves. Only down
stream channels can open, causing the impulse to move along the axon to the
axon terminals.
Figure 5: The action potential proceeds along the axon with a domino effect. Each rapid
change in potential triggers a change in potential in the adjacent region, causing the
action potential to move along the axon in a wave or depolarization.
Conduction across Chemical Synapses
Most neurons communicate by means of neurotransmitters. The action
potentials are transmitted directly across electrical synapses, but they cannot
jump across the synaptic cleft in a chemical synapse. The process creates a
slight time delay, because of diffusion and binding of neurotransmitters to
the post-synaptic cell.
Neurotransmitters are stored in pre-synaptic vesicles (axon terminal) in the
cytosol. Ca+ pumps work continuously to pump Ca+ into the synaptic cleft
maintaining a Ca+ ion imbalance. Once the action potential arrives the
calcium channels open allowing the Ca+ to move from the cleft into the axon
terminal cytosol. This increase in Ca+ in the cytosol causes the synaptic
vesicles to move to the membrane and release the neurotransmitter
molecules into the synaptic cleft.
The neurotransmitter molecules diffuse across the synaptic cleft to the postsynaptic cell (dendrite). The neurotransmitter stimulates the ion channels to
open causing an action potential to occur.
Figure 6: A chemical synapse, facilitated by a neurotransmitter.
Neurotransmitters
Acetylcholine is the best known neurotransmitter, it triggers muscle
contractions, stimulates hormone secretions and is involved in wakefulness,
attentiveness, memory, speech, learning, anger, aggression, and sexuality.
Alzheimer’s disease maybe caused by the degeneration of neurons in the
brain and the neuron’s inability to release acetylcholine.
11.3: The Central Nervous System
pg. 530 - 536
11.4: The Peripheral Nervous System
pg. 537 - 541
11.5: The Senses
pg. 542 - 548
11.6: The Body and Stress
pg. 549 - 553
Chapter 11: Summary
pg. 558
Chapter 11: Self-Quiz
pg. 559
Chapter 11: Review
pg. 560 – 565
Unit 4: Self – Quiz
pg. 568 – 569
Unit 4: Review
pg. 570 - 577