Information Flow and the Neuron
Animals sense and respond to the environment. The ability to perceive such danger, for example of an approaching tornado, and take evasive action is dependent on the nervous system. However to understand how our nervous system works, you should have to start with how it’s neuron functions and its structure.
I.
Cells
of the Nervous System
A. The neuron is the basic
unit of communication in the nervous system.
1.
Three
Classes
a) Sensory Neurons are
receptors for any specific stimulus to spinal cord and brain.
b) Interneurons integrate input and output
signals in the brain and spinal cord.
c) Motor Neuron relay
information away from the motor brain and spinal cord to the muscle or glands
(effectors)
B. Neuroglia – variety of specialized cells that
protect, structurally support, and functionally assist the neuron
II.
Neuron
– The Communication Specialists
A. Functional
Zones of a Neuron
1.
The
numerous, usually short extension that receives information (input zones) are dendrites.
2.
The
usually single, often long extension (conducting zones) that transmits impulses
to other cells at its branched endings (output zones) are axons.’
3.
Signals
arise in trigger zones
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B. A Neuron at
Rest, Then Moved to Action
1.
Neuron
maintains a voltage difference across plasma membrane if not bothered.
a) The
inside, more negatively charged than the outside
b) The resting membrane
potential
2.
(Action Potential)
when neuron receives signals, an abrupt, temporary reversal in the voltage
difference, the inside becomes more positive.
3.
Stimulation
of a neuron disturbs the distribution of electric charge across its plasma
membrane.
C. Restoring and
Maintaining Readiness
1.
Three
factors resulting membrane potential
a) The
concentration of K+, Na+, and other charged molecules are
not the same on the two sides of the plasma membrane.
b) Channel
proteins spanning the membrane actively influence the diffusion of specific
types of ions.
c) Transport
proteins spanning the membrane actively pump Na+ and K+
ions.
2.
There
are more K+ ions inside and more Na+ ions outside the resting
neuron membrane.
a) Na+
ions have a tendency to leak out through the channel proteins.
b) Most of
the Na+ channels are gated and is close most of the time, keeping
the outside high.
c) Still small amount of sodium do leak in and must be pumped out (K+ pumped in) by the sodium-potassium pump.
III.
Action
Potentials
A. Approaching
Threshold
1.
Graded – the signals at the input
zones vary in magnitude depending on the intensity and duration of the
stimulus.
2.
Local – the signal does not usually
spread beyond the input zone
a) If
stimulation is strong enough, an adjacent trigger zone may respond.
3.
When
minimum threshold
level is reached by a stimulus, gated channels open and Na+
rushes in.
B.
All
or Nothing Spike
1.
Action
potential = all or nothing events
2.
Sodium
gates closes, potassium gates open at the membrane site if charged reversal.
3.
Sodium
potassium membrane pumps are operational for restoring the resting potential.
C. Propagation of
Action Potentials
1.
Actions
potential = self-propagating, don’t diminish in magnitude
2. In a period after disturbance, membrane remain insensitive to stimulation; gate is shut, potassium gates open.
IV.
Node
to Node Hopping along Sheathed Axons
A. Axons are
wrapped in a myelin sheath, which consists of the plasma membrane of neuroglial
cell called schwann
cells.
B. Action
potentials move quickly along myelinated axons for they jump from node to node.
C. With Multiple
Sclerosis, myelin sheaths in the spinal cord’s nerve tract deteriorate.
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 neurotransmitter substance is released.
1.
Presynaptic cell – neuron that releases the
neurotransmitter molecules into the cleft.
a) Gated
proteins open to allow calcium ions enter the neuron.
b) Calcium
causes synaptic vesicles to fuse with the plasma membrane, releasing
neurotransmitters into the cleft.
2.
The
neurotransmitter binds to receptors on the membrane of the postsynaptic cell.
a) Have
excitatory effects – drive cell’s membrane to the threshold of an action
potential.
b) Have inhibitory effects – drive the membrane
away from threshold.
3.
Acethylcholine
(Ach) – has both excitatory and inhibitory effects
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VI.
Synaptic
Integration
A. Smorgasbord of
Signals
1.
Serotin
2.
Norepinephrine
3.
Dopamine
4.
GABA
5.
Neuromodulators are substances that enhance or
reduce the effects of a neurotransmitter on target neurons.
B. Synaptic
Integration
1.
Excitatory
and Inhibitory signals compete at the input zone
a) EPSP – summation of signals
that brings the membrane closer to threshold (depolarizating effect)
b) IPSP – drives the membrane
away from the threshold by hyperpolarizing effect.
C. Removing
Neurotransmitter from the Synaptic Cleft
1.
Some
molecules diffuse out; others are actively pumped back into the preynaptic
cells by the membrane transport proteins; acetylcholinesterase degrades many.
VII.
Paths
of Information Flow
A. Blocks and
Cables of Neurons
1.
Neuron
circuits or pathways determine the direction a signal will travel.
2.
In
the brain, neurons are organized into regional blocks that receive, integrate,
and then send out signals.
B. Signals
between brain or spinal cord and regions travel by nerves.
1.
Axons
of sensory neurons, motor neurons, or both are bundled together in a nerve.
2.
Nerve
pathways, or “tracts”, bundles within the brain and spinal cord
C. Reflex Arcs
1.
Reflexes are simple, stereotyped
movements made in response to sensory stimuli.
In stretch
reflex, receptors of sensory neurons (muscle spindles) transmit impulses to the spinal
cord where direct synapses with motor neurons occur.
In the withdrawal reflex, interneurons in the spinal cord can activate or suppress motor neurons as necessary for a coordinated response.
