Chapter 34 Information Flow and The Neuron

I. Cells of the Nervous System
A. The neuron, or nerve cell, is the basic unit of communication in all nervous systems.
B. Three classes of neurons work together:
1 Sensory neurons are receptors for specific sensory stimuli.
2 Interneurons in the brain and spinal cord integrate input and output signals.
3 Motor neurons send information from integrator to muscle or gland cells (effectors).
C. Neuroglial cells physically support and protect the neurons in various ways.

II. Functional Zones of the Neuron
A. A neuron has three basic parts:
1 The cell body contains the nucleus and metabolic machinery for protein synthesis.
2 Dendrites are numerous, usually short extensions that receive stimuli (input zones).
3 An axon is usually a single, rather long extension that transmits impulses to other cells (output zones).
B. There are variations of these features depending on the particular neuron described.


III. Neural Messages
A. Membrane Excitability
1 A neuron at rest maintains a steady voltage difference across its plasma membrane.
a. The inside is more negatively charged than the outside.
b. This is called the resting membrane potential.
2 When a neuron receives signals, an abrupt, temporary reversal—the inside becomes more
positive—in the polarity is generated (an action potential).
3 Any membrane that can produce action potentials is said to show membrane excitability.
B. Neurons “At Rest”
1 The resting membrane potential is the result of three factors:
a. The concentrations of potassium, sodium ions, and other charged molecules are not the same on
the two sides of the plasma membrane.
b. Channel proteins spanning the membrane influence the diffusion of specific types of ions.
c. Transport proteins spanning the membrane actively pump sodium and potassium ions.
2 There are more potassium ions inside and more sodium ions outside the resting neuron membrane. a. Potassium ions have a tendency to leak out by facilitated diffusion through channel proteins.
b. Most of the sodium channels are “gated” and remain closed most of the time, keeping the
concentration outside high.
c. However, small amounts of sodium do leak in and must be pumped out (and potassium pumped
in) by the sodium-potassium pump.
d. The differences in ion concentrations produce a resting membrane potential of about 70 millivolts.
C. Local Disturbances in Membrane Potential
1 “Graded” signals vary in magnitude depending on the intensity and duration of the stimulus.
2 “Local” means the signal does not usually spread beyond the input zone; however, if the stimulation
is strong enough, an adjacent trigger zone may respond.

IV. A Closer Look at Action Potentials
A. Mechanism of Excitation
1 Measurements of voltage difference across a membrane before, during, and after action potentials
reveal three states:
a. A neuron at rest is polarized—inside is more negative than outside.
b. During an action potential, the membrane is depolarized—inside becomes more positive than outside.
c. After a potential, the membrane is repolarized—resting conditions are restored.
2 When a stimulus reaches a certain minimum—a threshold—gated channels open and sodium rushes in.
a. In an accelerating way, more and more gates open (example of positive feedback).
b. Action potentials are all-or-nothing events.
B. Duration of Action Potentials
1 When depolarization in one region is ended, the sodium gates close and potassium gates open.
2 The sodium-potassium membrane pumps also become operational to fully restore the resting potential.
C. Propagation of Action Potentials
1 Refractory Period
a. The action potential is self-propagating and moves away from the stimulation site to adjacent
regions of the membrane undiminished.
b. A brief refractory period follows at each depolarization site—sodium gates shut, potassium gates
open—during which the membrane is insensitive to stimulation.
2 Sheathed Axons
a. Many axons are covered by a myelin sheath derived in part from Schwann cells.
b. Each section of the sheath is separated from adjacent ones by a node where the axon membrane
(plentiful in gated sodium channels) is exposed.
c. The action potentials jump from node to node (saltatory conduction), which is fast and efficient.

V. Chemical Synapses
A. A chemical synapse is a junction between a neuron and an adjacent cell, separated by a synaptic cleft
into which a transmitter substance is released.
1 The neuron that releases the transmitter into the cleft is called the presynaptic cell.
2 The cell whose behavior is affected by the substance is the postsynaptic cell.
B. When an action potential reaches the axon endings of a neuron, transmitter substances are released. 1 First, gated protein channels open to allow calcium ions to enter the neuron.
2 Calcium causes the vesicles to fuse with the membrane and release the transmitter substance into
the cleft.
3 The transmitter binds to receptors on the membrane of the postsynaptic cell.
C. Effects of Transmitter Substances
1 Neurotransmitters may have excitatory effects if they drive a cell’s membrane to the threshold of an
action potential.
a. Acetylcholine is the transmitter at neuromuscular junctions.
b. Serotonin acts on brain cells to govern sleeping, sensory perception, temperature regulation, and
emotional states.
c. Norepinephrine apparently affects brain regions concerned with emotions, dreaming, and awaking.
2 Neurotransmitters may have inhibitory effects if they help drive the membrane away from threshold;
examples include dopamine and GABA.
D. Neuromodulators
1 These substances enhance or reduce membrane responses in target neurons.
2 Endorphins and enkephalins inhibit perceptions of pain.
E. Synaptic Integration
1 Excitatory and inhibitory signals compete at the input zone.
2 An excitatory postsynaptic potential (EPSP) is a summation of signals that brings the membrane
closer to threshold (depolarizing effect).
3 An inhibitory postsynaptic potential (IPSP) drives the membrane away from threshold by a
hyperpolarizing effect.


VI. Paths of Information Flow
A. Neuron circuits or pathways will determine the direction a signal will travel.
1 The brain has many “local” circuits.
2 Signals between brain or spinal cord and body regions travel by nerves.
a. Axons of sensory neurons, motor neurons, or both, are bundled together in a nerve.
b. Within the brain and spinal cord, such bundles are called nerve pathways, or “tracts.”
B. Reflexes are simple, stereotyped movements made in response to sensory stimuli.
1 In a reflex arc such as the stretch reflex, receptors of sensory neurons transmit impulses to the spinal
cord where direct synapses with motor neurons occur.
2 In the withdrawal reflex, interneurons in the spinal cord can activate or suppress motor neurons as
necessary for a coordinated response.