Information Flow and
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
of the Nervous System
A. The neuron is the basic
unit of communication in the nervous system.
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
B. Neuroglia – variety of specialized cells that
protect, structurally support, and functionally assist the neuron
– The Communication Specialists
Zones of a Neuron
numerous, usually short extension that receives information (input zones) are dendrites.
usually single, often long extension (conducting zones) that transmits impulses
to other cells at its branched endings (output zones) are axons.’
arise in trigger zones
B. A Neuron at
Rest, Then Moved to Action
maintains a voltage difference across plasma membrane if not bothered.
inside, more negatively charged than the outside
b) The resting membrane
when neuron receives signals, an abrupt, temporary reversal in the voltage
difference, the inside becomes more positive.
of a neuron disturbs the distribution of electric charge across its plasma
C. Restoring and
factors resulting membrane potential
concentration of K+, Na+, and other charged molecules are
not the same on the two sides of the plasma membrane.
proteins spanning the membrane actively influence the diffusion of specific
types of ions.
proteins spanning the membrane actively pump Na+ and K+
are more K+ ions inside and more Na+ ions outside the resting
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.
small amount of sodium do leak in and must be pumped out (K+ pumped
in) by the sodium-potassium
Graded – the signals at the input
zones vary in magnitude depending on the intensity and duration of the
Local – the signal does not usually
spread beyond the input zone
stimulation is strong enough, an adjacent trigger zone may respond.
level is reached by a stimulus, gated channels open and Na+
or Nothing Spike
potential = all or nothing events
gates closes, potassium gates open at the membrane site if charged reversal.
potassium membrane pumps are operational for restoring the resting potential.
C. Propagation of
potential = self-propagating, don’t diminish in magnitude
a period after disturbance, membrane remain insensitive to stimulation; gate is
shut, potassium gates open.
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
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.
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.
Presynaptic cell – neuron that releases the
neurotransmitter molecules into the cleft.
proteins open to allow calcium ions enter the neuron.
causes synaptic vesicles to fuse with the plasma membrane, releasing
neurotransmitters into the cleft.
neurotransmitter binds to receptors on the membrane of the postsynaptic cell.
excitatory effects – drive cell’s membrane to the threshold of an action
b) Have inhibitory effects – drive the membrane
away from threshold.
(Ach) – has both excitatory and inhibitory effects
A. Smorgasbord of
Neuromodulators are substances that enhance or
reduce the effects of a neurotransmitter on target neurons.
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.
Neurotransmitter from the Synaptic Cleft
molecules diffuse out; others are actively pumped back into the preynaptic
cells by the membrane transport proteins; acetylcholinesterase degrades many.
of Information Flow
A. Blocks and
Cables of Neurons
circuits or pathways determine the direction a signal will travel.
the brain, neurons are organized into regional blocks that receive, integrate,
and then send out signals.
between brain or spinal cord and regions travel by nerves.
of sensory neurons, motor neurons, or both are bundled together in a nerve.
pathways, or “tracts”, bundles within the brain and spinal cord
C. Reflex Arcs
Reflexes are simple, stereotyped
movements made in response to sensory stimuli.
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.