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3.E.2 Nervous System
Animals have nervous systems that
detect external and internal signals,
transmit and integrate information, and
produce responses.
The Nervous System can
be divided into the
Central Nervous System
(CNS) and the Peripheral
Nervous System (PNS).
The CNS includes the
brain and spinal cord.
The PNS consists of all
nerves outside of the
brain and spinal cord.
A typical neuron has a cell body, axon,
dendrites and an axon terminal.
The direction of the neural impulse is
from the dendrites (input) to the axon
terminal (output).
Dendrites receive and integrate signals
from other neurons and conduct the
signal towards the cell body.
Axons conduct signals away from the cell
body. Axons can be as long as many
meters in large vertebrates. A “nerve” is a
bundle of axons.
Many axons have a myelin sheath that acts as
an electrical insulator. Myelinated axons appear
white: the “white matter” of the brain.
Unmyelinated axons appear gray: the “gray
matter” of the brain.
Glial cells called Schwann cells form the
myelin sheath in the PNS. Swann cells are
separated by gaps of unsheathed axon
called Nodes of Ranvier.
Two types of glial cells, oligodendrocytes and
astrocytes, form the myelin sheath in the CNS.
Astrocytes form the "blood-brain barrier”, separates
brain cells from the blood and protects the brain from
many common bacterial infections.
The structure of the neuron allows for
the detection, generation, transmission
and integration of signal information.
Neurons are excitable; their membranes
can depolarize and produce an action
potential, or electrical impulse.
Action potentials propagate impulses
along neurons.
Membranes of neurons are polarized by
the establishment of electrical potentials
across the membranes.
At rest, the membrane potential (Vm) of a
neural membrane is about -70 mV. This is due
to an unbalanced electric charge distribution.
Sodium is maintained outside the cell,
potassium inside the cell. The cell membrane is
impermeable to anions.
This voltage difference across the
membrane produces a kind of
electromotive force.
The cell membrane has protein channels, called
leak channels that allow Na+ or K+ to diffuse
down their concentration gradients.
The sodium-potassium pump, powered by ATP,
counters the effect of the leak channels,
working to maintain the resting membrane
potential at -70 mV.
An action potential is a rapid change in
membrane potential in response to a
stimulus.
An action potential:
1. A stimulus from a sensory cell or another
neuron causes the target cell to depolarize
toward the threshold potential.
2. If the stimulus exceeds threshold (-55 mV),
voltage-gated sodium channels open.
3. When the voltage-gated sodium channels
open, sodium floods into the cell. The
membrane depolarizes (the inside of the cell
becomes more positive).
4. At the peak action potential, the voltage-gated
sodium channels close and potassium channels
open, allowing K+ to diffuse out of the cell down its
electrochemical gradient. The membrane
repolarizes (the inside of the cell becomes more
negative.)
4. The membrane hyperpolarizes (becomes more
negative than resting potential). During this
period of recovery, called the refractory period,
the nerve cell cannot be stimulated again. The
sodium-potassium pump returns the membrane
potential to -70 mV.
Diagram showing the opening and
closing of voltage-gated ion channels:
The action potential propagates as a wave down the
axon. An action potential at one place on the axon
depolarizes the adjacent sections of its membrane. If
sufficiently strong, this depolarization provokes a
similar action potential at the neighboring membrane
patches.
Transmission of information between
neurons occurs across synapses.
Synapse
Chemical messengers called
neurotransmitters transmit the neural
message across synapses.
Neurotransmitter
Function
Acetylcholine
Neuromuscular junctions
Epinephrine (adrenalin)
Stimulates sympathetic nervous system
Norepinephrine
Stimulates sympathetic nervous system
Dopamine
CNS neurotransmitter involved with reward –
motivated behavior
Glutamate
The most common neurotransmitter in the
brain
Serotonin
CNS neurotransmitter involved with mood,
memory and learning
GABA
Inhibitory neurotransmitter of the CNS
The response produced by transmission of
a signal can be stimulatory or inhibitory.
Different regions of the vertebrate brain
have different functions.
The human brain is divided into left and
right cerebral hemispheres.
The forebrain consists of the cerebrum, thalamus,
and hypothalamus. The brainstem is composed of
both the midbrain and the hindbrain.
The cerebellum plays an important role
in motor control.
The cerebrum is a major part of the brain,
controlling emotions, hearing, vision,
personality and much more. It controls all
voluntary actions.
The hypothalamus produces neurohormones,
which are hormones produced and released
by neuroendocrine cells into the blood.
Example Neurohormones
Oxytocin
Dopamine
Epinephrine
Norepinephrine
Vasopressin
Learning Objectives:
LO 3.43 The student is able to construct an explanation, based on
scientific theories and models, about how nervous systems detect
external and internal signals, transmit and integrate information,
and produce responses. [See SP 6.2, 7.1]
LO 3.44 The student is able to describe how nervous systems detect
external and internal signals. [See SP 1.2]
LO 3.45 The student is able to describe how nervous systems transmit
information. [See SP 1.2]
LO 3.46 The student is able to describe how the vertebrate brain
integrates information to produce a response. [See SP 1.2]
LO 3.47 The student is able to create a visual representation of
complex nervous systems to describe/explain how these systems
detect external and internal signals, transmit and integrate
information, and produce responses. [See SP 1.1]
LO 3.48 The student is able to create a visual representation to
describe how nervous systems detect external and internal signals.
[See SP 1.1]
LO 3.49 The student is able to create a visual representation to
describe how nervous systems transmit information. [See SP 1.1]
LO 3.50 The student is able to create a visual representation to
describe how the vertebrate brain integrates information to
produce a response. [See SP 1.1]