Download The Nervous System

The Nervous System
Chapter 25
Conduction of Impulses
• An impulse is a response to a stimulus that causes an
action by the body. The impulse is like an electric
signal that triggers the nervous system to react.
• The nervous system receives information from
internal and external stimuli and responds to that
info.
• While bacteria, protists, and plants are capable of
nervous response, only animals have true nervous
systems.
Nervous Response
• There are 4 criteria that must be met for a nervous
response to occur.
– 1) there must be a way to detect a stimulus. In most cases,
this is done by sensory receptors located all over the body.
These receptors might be individual nerve cells or nerve
cells that form part of a sense organ, like the eye or nose.
These cells turn the sensory input into electrical impulses.
• 2) impulses must be transmitted to other parts of the
body. They are carried along a network of neurons,
which are cells designated for carrying nerve
impulses.
• 3) impulses have to be interpreted and analyzed to
determine the proper response. This is done in the
brain or spinal cord.
• 4) an appropriate response must be carried
out by an effector, usually a muscle or gland.
This would be seen when you pull your hand
away from a hot stove, or when you get a rush
of adrenaline from your adrenal glands.
Nerve Tissue
• There are 4 specific types of cells that form the
tissues of the nervous system.
– 1) sensory neurons transmit incoming impulses from
receptors in sense organs (eyes, ears, skin, nose) to the
brain or spinal cord, where they are interpreted.
– 2) motor neurons act once the sensory neuron sends its
message, which is analyzed in the brain. They transmit the
outgoing response to the effectors.
• 3) interneurons carry impulses from sensory
to motor neurons, forming a sort of bridge
between them.
• 4) nerves are many neurons grouped together,
like an electrical cord made of many wires.
Anatomy of a Neuron
• Part of a neuron looks like all other animal
cells, and is called the soma (or body remember your prefixes?).
• Coming off one side of the soma is the axon.
• It is long and narrow and has branches at the
tail that form the end brush.
Neuron Structure
• The axon transmits impulses away (axon-away) from
the soma and is covered in a material called the
myelin sheath.
• The myelin sheath helps to protect it from being
exposed, and also helps impulses travel faster down
the axon.
• It also provides nourishment to the axon and helps in
axon regeneration.
• From the other side of the soma come
dendrites.
• Dendrite means tree or branches, so you can
imagine what these extensions look like.
• The dendrites conduct impulses toward the
cell body.
• The impulse is picked up by the dendrites,
where it moves toward the soma, down the
axon, out the end brush, and to the dendrites
of another neuron until it reaches the control
center (or an effector on the way back).
*Test Question Alert*
• Identify the following parts of a neuron:
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Axon
Dendrite
Soma
Axon terminal
Myelin sheath
Nucleus
Transmitting Impulses from Neuron
to Neuron
• Virtually all nerve impulses must travel through many
neurons before reaching their destinations: the
brain, spinal cord, or effector.
• However, the neurons don’t make a solid chain
through the whole body. Instead, there are small
fluid-filled spaces between the dendrites of one
neuron and the axons of another. These spaces are
called synapses.
Synapses
• The impulse travels across the synapses with the
help of neurotransmitters.
• These are chemicals found in vesicles located on the
end brush of the axon. The vesicles fuse with the
axon membrane, causing the chemical to be
released, leaving the cell membranes and combining
with receptor molecules found in nearby dendrites.
Neurotransmitters
Nervous Systems
• Vertebrates, including humans, have 2 major nervous
systems. The CNS consists of a brain and nerve cords.
• These structures act as control centers, receiving
messages from sensory neurons and sending
responses via motor neurons.
• The PNS consists of the sensory and motor neurons
mentioned before.
CNS and PNS
• In humans, which are bilaterally symmetrical
animals, the nerves come in pairs - one for
each side of the body.
• There are 12 pairs of cranial nerves attached
to the brain and 31 pairs of spinal nerves.
*Test Question Alert*
• Match the structure with the nervous system
it belongs to (CNS, PNS).
– Brain
– Spinal nerves
– Spinal cord
– Cranial nerves
The CNS of Humans
• Vertebrate brains are made up of 3 sections:
the forebrain, midbrain, and hindbrain.
• These 3 sections are further divided and
named for their function.
Sections of the Brain
• In humans, the forebrain
is made up of the
cerebrum, which has two
hemispheres.
• Each hemisphere contains
4 lobes: frontal, temporal,
parietal, and occipital.
*Test Question Alert*
• Identify the lobes of
the brain by the color
they are on the
diagram.
• The surface of the cerebrum is the cerebral
cortex, made of neuron somas that are not
covered in myelin sheath. They are referred to
as gray matter.
• The interior of the cerebrum is made of
myelinated neurons and called white matter.
Gray and White Matter
• Also part of the forebrain, and located under the
cerebrum, are the thalamus and hypothalamus.
• The thalamus sorts sensory information.
• The hypothalamus controls hunger, body
temperature, aggression, and metabolism. It also
helps in endocrine (hormone) control.
The Forebrain
• The midbrain functions as a station for
message exchange between fore- and
hindbrain.
• The hindbrain consists of the cerebellum,
pons, and medulla oblongata.
The Midbrain and Hindbrain
• The cerebellum is the motor area of the brain,
coordinating impulses sent from the cerebrum.
• The medulla oblongata controls many responses of
internal organs, like breathing rate, heart rate, and
gland secretion.
• The medulla oblongata and pons make up the brain
stem
• The pons links the brain to the spinal cord
and, along with the medulla, helps control
respiration.
*Test Question Alert*
• Match the function listed below with the section
of the brain responsible for carrying it out.
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Controlling respiration
Gland secretion
Coordinating impulses from the cerebrum
Controls heart rate
Controls hunger
Controls aggression
Sorts sensory information
Controls body temperature
Controls metabolism
Controls breathing rate
*Test Question Alert*
• Locate the following parts
of the brain on the
diagram:
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Cerebrum
Thalamus
Hypothalamus
Pons
Medulla
Cerebellum
*Test Question Alert*
• Match the part of the brain with the section it
belongs to (mid-, hind-, or forebrain).
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Cerebrum
Thalamus
Hypothalamus
Pons
Medulla
Cerebellum
• The spinal cord has a fluid-filled central canal. The
cord itself is protected by bony vertebrae.
• The exterior cord is white matter, while the interior
surface is gray matter.
• The spinal cord connects the brain with the PNS. It
consists of the 62 spinal nerves, which contain both
sensory and motor neurons.
The ANS of Humans
• The ANS controls reflexes in the body that you
don’t typically regulate, such as heartbeat and
digestion.
• It is made up of 2 groups of nerves that work
like a checks-and-balances.
• The first group is the sympathetic nerves.
• The sympathetic nerves speed up heart and
breathing rate during emergencies or
strenuous exercise, causing adrenaline and
other hormones to be released, giving you a
burst of energy.
• The parasympathetic nerves slow those processes
down. It causes blood vessels to dilate (or expand),
decreasing blood pressure.
• Sympathetic nerves cause your pupils to dilate in low
light, while your parasympathetic nerves relax your
pupils, making them smaller in high light levels.
• Sympathetic deals with stress/emergencies.
Parasympathetic does the opposite.
*Test Question Alert*
• Match the action with the system (sympathetic or
parasympathetic) responsible for carrying it out.
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Dilation of blood vessels
Dilation of pupils
Speeding up heart rate
Adrenaline release
Relaxation of pupils
Stress/emergency situations
Increasing breathing rate