Download Nervous Systems: Cells and Functions

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The Mammalian Nervous
System: Structure and Higher
Functions
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Nervous Systems: Cells and Functions
• Neurons are specialized cells of the nervous
system that receive, encode, and transmit
information.
• Neurons with their support cells (glial cells) make
up nervous systems.
• Information is received by sensory cells and
converted into electrical signals that are
transmitted and processed by neurons.
• To cause behavioral or physiological responses, a
nervous system communicates these signals to
effectors, such as muscles and glands.
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Nervous Systems: Cells and Functions
• The neuron’s plasma membrane generates
electrical signals called nerve impulses (or
action potentials) and conducts the signals from
one location on a cell to the most distant reaches
of that cell.
• Neurotransmission is the electrochemical
process by which nerve cells communicate.
• Most neurons have four regions: a cell body,
dendrites, an axon, and axon terminals.
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Nervous Systems: Cells and Functions
• The cell body contains the nucleus and most of
the cell’s organelles.
• Many projections sprout from the cell body; most
of them are dendrites, which bring information
from other neurons or sensory cells to the cell
body.
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Nervous Systems: Cells and Functions
• The axon usually carries information away from
the cell body.
• Axons conduct information to target cells, which
can be other neurons, muscle cells, or gland cells.
• At its end, the axon divides into many fine nerve
endings. At the tip of each nerve ending is a
swelling called the axon terminal.
• The axon terminal is positioned very close to the
target cell.
• At the axon, terminal nerve impulses cause the
release of neurotransmitters (chemical
messengers) into the synapse.
Figure 44.2 Neurons (Part 1)
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Nervous Systems: Cells and Functions
• The synapse is a specialized region in which nerve cells
make contact with one another.
• It is here that information, in chemical form, is transferred
between neurons.
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Nervous Systems: Cells and Functions
• Variation between different types of neurons is
considerable.
• Length of the axon differs in different cell types.
Some axons can be very long.
• Gilal cells supply neurons with nutrients. Some
consume foreign particles, and some maintain
ionic balance around neurons.
Figure 44.2 Neurons (Part 2)
Neurons with bushy dendrites
collect information from many
other cells.
Neurons with fewer
dendrites process
fewer imputs.
Figure 44.2 Neurons (Part 3)
Some neurons branch
over a broad area.
Some
neurons
provide
local links
to a small
number of
cells.
Some communicate
long distances via
long axons.
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Nervous Systems: Cells and Functions
• It is important to remember that nervous systems
depend on neurons working together.
• The simplest neural network consists of three
cells: a sensory neuron connected to a motor
neuron connected to a muscle cell.
• Most neuronal networks are more complex. The
human brain has an estimate 1011 neurons and
1014 synapses.
• The neurons and synapses in the human brain
are divided into thousands of distinct but
interacting networks that function in parallel.
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Neurons: Generating and
Conducting Nerve Impulses
• The difference in voltage across the plasma
membrane of a neuron is called its membrane
potential.
• In an unstimulated neuron, the voltage difference
is called a resting potential.
• An action potential is the sudden and rapid
reversal in voltage across a portion of the plasma
membrane. For 1 to 2 milliseconds, the inside of
the cell becomes more positive than the outside.
• Nerve impulses are action potentials that travel
along axons.
Figure 44.13 Synaptic Transmission Begins with the Arrival of a Nerve Impulse
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The Nervous System:
Structure, Function, and Information Flow
• The brain and spinal cord together constitute the
central nervous system (CNS).
• Information is transmitted from sensory cells to
the CNS and from the CNS to effectors via
neurons, which extend or reside outside of the
brain and spinal cord.
• In the CNS, sensory information is assimilated,
responses formulated, and motor responses sent
out.
• In addition to controlling physiological activities,
the brain encodes and stores information in the
form of memory, and generates ideas.
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The Nervous System:
Structure, Function, and Information Flow
• Information is brought to and from the CNS by
means of the enormous network of nerves that
make up the peripheral nervous system.
• The peripheral nervous system reaches every
tissue of the body; it connects to the CNS via
spinal and cranial nerves.
• The PNS consists of pairs of nerve extensions
from the brain, the spinal cord, and the autonomic
nervous system.
• Some axons in a nerve may be carrying
information from the CNS while other axons in the
same nerve may be carrying information to the
CNS.
Figure 44.1 Nervous Systems Vary in Size and Complexity (Part 2)
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The Nervous System:
Structure, Function, and Information Flow
• The nervous system is a complicated information
processing system.
• The afferent portion of the peripheral nervous
system carries information to the CNS.
• We are consciously aware of vision, hearing, pain,
and limb position, which are types of information
that move through afferent pathways.
• We are unaware of other afferent information such
as blood pressure, deep body temperature, and
blood oxygen supply.
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The Nervous System:
Structure, Function, and Information Flow
• The efferent portion of the peripheral nervous
system carries information from the CNS to the
muscles and glands of the body.
• Efferent pathways can be divided into a voluntary
division (conscious movements) and an
involuntary, or autonomic, division
(physiological functions).
• In addition to receiving neural information, the
CNS receives chemical information from
hormones circulating in the blood or from
neurohormones released by neurons into the
extracellular fluids of the brain.
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The Nervous System:
Structure, Function, and Information Flow
• The Autonomic Nervous System is the portion
of the peripheral nervous system that regulates
the activity of the human body’s internal organs.
• It is in direct connection with the cardiac muscle,
smooth muscles, and glands.
• It controls motor activity such as speeding up and
slowing down heartbeat and dilating and
constricting the pupils.
• It also works on the conscious level by relaying
sensory information, such as the stuffed feeling
after eating a big meal, to the CNS.
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Figure 46.1 Organization of the Nervous System
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The Nervous System:
Structure, Function, and Information Flow
• Early in vertebrate development, a hollow tube of
neural tissue forms.
• At the anterior end, the neural tube forms three
swellings that become the basic divisions of the
brain: the hindbrain, midbrain, and forebrain.
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Figure 46.2 Development of the Human Nervous System (Part 1)
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Figure 46.2 Development of the Human Nervous System (Part 3)
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The Nervous System:
Structure, Function, and Information Flow
• Different brain structures develop from the three
embryonic regions.
• The medulla, pons, and cerebellum are derived
from the hindbrain.
• The medulla and pons contain distinct groups of
neurons that are involved in the control of physical
functions such as breathing and circulation.
• All information traveling from the spinal cord must
pass through the pons and medulla.
• The cerebellum refines motor commands to the
joints and muscles.
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The Nervous System:
Structure, Function, and Information Flow
• The part of the brain from which the spinal cord
extends is called the brain stem.
• The brain stem controls the functioning of vital
body organs.
• It is the site of the reticular activating system,
which regulates the brain’s level of awareness.
• The brain stem filters sensory information that
stimulates the reticular activating system, which
then stimulates activity and alterness in the brain.
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The Nervous System:
Structure, Function, and Information Flow
• The core of the forebrain and consists of the
thalamus and the hypothalamus.
• Cerebrum consists of two cerebral hemispheres,
left and right.
• In humans, the cerebrum is the largest part of the
brain and plays major roles in sensory perception,
learning, memory, and conscious behavior.
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Functional Subsystems of the Nervous System
• The spinal cord conducts information between
the brain and organs of the body, integrates
information coming from the peripheral nervous
system, and issues motor commands.
• In the nervous system, gray matter is tissue rich
in neuronal cell bodies, and white matter
contains axons.
• Spinal nerves leave the spinal cord at regular
intervals; each one has two roots, one connecting
it to the dorsal horn of the gray matter and the
other connecting with the ventral horn.
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Figure 46.3 The Spinal Cord Processes Information
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Functional Subsystems of the Nervous System
• The cerebral cortex is the largest part of the
central nervous system.
• It is divided into two hemispheres and four paired
lobes, which are separated in terms of function
but which share information.
• It is the highest level at which sensory information
is perceived and identified and motor responses
initiated.
• Higher functions such as thought, speech, and
music appreciation take place here.
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Figure 46.5 The Human Cerebrum (Part 1)
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Functional Subsystems of the Nervous System
• Proprioception is the sense of position and
movement of the body.
• Sense receptors located in muscles, tendons, and
joints, relay information concerning the position of
body parts, muscular coordination, and the
degree of stretch in muscles and tendons.