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35-2 The Nervous System
Photo Credit: AP/Wide World Photos.
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35-2 The Nervous System
What are the functions of the nervous system?
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35-2 The Nervous System
The nervous system controls and coordinates functions throughout the body and responds to internal and external stimuli.
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Neurons
Neurons
The messages carried by the nervous system are electrical signals called impulses.
The cells that transmit these impulses are called neurons.
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Neurons
Neurons are classified according to the direction in which an impulse travels.
Sensory neurons carry impulses from the sense organs to the spinal cord and brain.
Motor neurons carry impulses from the brain and spinal cord to muscles and glands.
Interneurons connect sensory and motor neurons and carry impulses between them.
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Neurons
Structures of a Neuron
Nucleus
Dendrites
Axon terminals
Cell body
Myelin sheath
The nervous system controls and coordinates functions throughout the body. The basic units of the nervous system are neurons.
Axon
Nodes
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Neurons
The largest part of a typical neuron is the cell body.
It contains the nucleus and much of the cytoplasm.
Cell body
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Neurons
Dendrites extend from the cell body and carry impulses from the environment toward the cell body.
Dendrites
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Neurons
The axon is the long fiber that carries impulses away from the cell body.
Axon terminals
Axon
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Neurons
The axon ends in axon terminals.
Axon terminals
Axon
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Neurons
The axon is sometimes surrounded by an insulating membrane called the myelin sheath.
There are gaps in the myelin sheath, called nodes, where the membrane is exposed.
Impulses jump from one node to the next.
Myelin sheath
Nodes
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The Nerve Impulse
The Resting Neuron (aka Resting Potental) 
When resting, the outside of the neuron has a net positive charge.
The inside of the neuron has a net negative charge.
Also, the inside of the cell has more potassium (K+) ions and few sodium (Na+) ions. Because…
The sodium-potassium pump in the nerve cell membrane pumps potassium (K+) ions into the cell and (Na+) ions out of the cell by active transport.
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The Nerve Impulse
Sodium-Potassium Pump
The sodium-potassium pump in the neuron cell membrane uses the energy of ATP to pump sodium out of the cell and, at the same time, to pump potassium in. This ongoing process maintains resting potential.
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The Nerve Impulse
How is a nerve impulse transmitted?
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The Nerve Impulse
The Moving Impulse
An impulse begins when a neuron is stimulated by another neuron or by the environment.
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The Nerve Impulse
During activity, the resting stage is reversed and an impulse moves along the axon. This is called an action potential.
An impulse begins when a neuron is stimulated by another neuron. At the leading edge of an action potential, gates in the sodium channels open, allowing Na+ ions to flow into the cell. This flow of ions causes the action potential to move.
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The Nerve Impulse
During an action potential, the potassium pumps open, allowing K+ ions to flow out of the neuron.
At the trailing edge of an action potential, gates in the potassium channels open, allowing positive ions to flow out and restoring the resting potential of the neuron.
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The Nerve Impulse
The impulse continues to move along the axon untill it reaches the axon terminal.
At the trailing edge of an action potential, gates in the potassium channels open, allowing positive ions to flow out and restoring the resting potential of the neuron.
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The Nerve Impulse
Threshold 
A stimulus MUST be strong enough to cause a neuron to send an impulse.
The minimum level of a stimulus is called the threshold.
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The Nerve Impulse
Just Read (Do Not Write)
A stimulus that is stronger than the threshold produces an impulse.
A stimulus that is weaker than the threshold produces no impulse.
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The Synapse
The Synapse
At the end of the neuron, the impulse reaches an axon terminal. Usually the neuron makes contact with another cell at this site.
The neuron may pass the impulse along to the second cell.
The spot where an impulse is transferred to another cell is called a synapse.
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The Synapse
A Synapse
When an impulse reaches the end of the axon of one neuron, neurotransmitters are released into the synaptic cleft. The neurotransmitters bind to receptors on the membrane of an adjacent dendrite.
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The Synapse
Neurotransmitters are chemicals used by a neuron to transmit an impulse across a synapse to another cell.
Vesicle
Neurotransmitter
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The Synapse
Neurotransmitters attach to membrane receptors on the next cell.
A new impulse begins on the next neuron cell.
Receptor
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The Synapse
Moments after binding to receptors, neurotransmitters are released from the cell surface.
The neurotransmitters may then be broken down by enzymes, or taken up and recycled by the axon terminal.
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26. 35-3 Divisions of the Nervous System
Photo Credit: AP/Wide World Photos.
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The Nervous System
The human nervous system has two major divisions:
central nervous system (CNS)
peripheral nervous system (PNS)
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28. The Central Nervous System
The CNS relays messages, processes information, and analyzes information.
The CNS consists of the brain and the spinal cord.
The brain and spinal cord are wrapped in three layers of connective tissue known as meninges.
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29. The Central Nervous System
Between the meninges and the CNS tissue is a space filled with cerebrospinal fluid.
Cerebrospinal fluid acts as a shock absorber that protects the central nervous system.
Cerebrospinal fluid also permits exchange of nutrients and waste products between blood and nervous tissue.
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The Brain
Parts of The Human Brain
Cerebrum
Thalamus
Pineal gland
Hypothalamus
The brain helps to relay messages, process information, and analyze information. The brain consists of the cerebrum, cerebellum, and brain stem.
Pituitary gland
Cerebellum
Pons
Brain stem
Medulla oblongata
Spinal cord
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The Brain
The Brain
The brain is the place to which impulses flow and from which impulses originate.
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The Brain
The Cerebrum 
Is the largest region of the brain.
It controls the voluntary (conscious) activities.
Used for intelligence, learning, and judgment.
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The Brain
A deep groove divides the cerebrum into hemispheres, which are connected by a band of tissue called the corpus callosum.
Each hemisphere is divided into regions called lobes.
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The Brain
Lobes of the Cerebrum
This view of the cerebrum shows the four different lobes of the brain. Different functions of the body are controlled by different lobes of the brain.
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The Brain
Each half of the cerebrum deals with the opposite side of the body:
The left half of the cerebrum controls the right side of the body.
The right half of the cerebrum controls the left side of the body.
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The Brain
The outer layer of the cerebrum is called the cerebral cortex and consists of gray matter.
The inner layer of the cerebrum consists of white matter, which is made up of bundles of axons with myelin sheaths.
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The Brain
Cerebellum
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The Brain
The Cerebellum 
Is second largest region of the brain.
It coordinates and balances the actions of the muscles so that the body can move gracefully and efficiently.
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The Brain
Pons
Brain stem
Medulla oblongata
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The Brain
The Brain Stem
The brain stem connects the brain and spinal cord.
It has two regions: the pons and the medulla oblongata. Each region regulates information flow between the brain and the rest of the body.
Blood pressure, heart rate, breathing, and swallowing are controlled in the brain stem.
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The Brain
Thalamus
Hypothalamus
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The Brain
The Thalamus and Hypothalamus 
The thalamus receives messages from all sensory receptors throughout the body and relays the information to the proper region of the cerebrum for further processing.
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The Brain
The hypothalamus controls recognition and analysis of hunger, thirst, fatigue, anger, and body temperature.
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The Spinal Cord
The Spinal Cord
The spinal cord links the brain and the rest of the body.
Information, including some kinds of reflexes, are processed directly in the spinal cord.
A reflex is a quick, automatic response to a stimulus.
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45. The Peripheral Nervous System
The PHS is all of the nerves and associated cells that are not part of the brain and the spinal cord.
The PHS includes cranial nerves, spinal nerves, and ganglia.
Ganglia are collections of nerve cell bodies.
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46. The Peripheral Nervous System
Two divisions of the peripheral nervous system.
The sensory division transmits impulses from sense organs to the central nervous system.
The motor division transmits impulses from the central nervous system to the muscles or glands.
The motor division is divided into the somatic nervous system and the autonomic nervous system.
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47. The Peripheral Nervous System
The Somatic Nervous System
The SNS regulates conscious control activities such as the movement of skeletal muscles.
Some somatic nerves are involved with reflexes.
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48. The Peripheral Nervous System
A reflex arc includes a sensory receptor, sensory neuron, motor neuron, and effector that are involved in a quick response to a stimulus.
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49. The Peripheral Nervous System
Sensory neuron
Reflex Arc
Motor neuron
Interneuron
Spinal cord
Effector (responding muscle)
The peripheral nervous system transmits impulses from sense organs to the central nervous system and back to muscles or glands. When you step on a tack, sensory receptors stimulate a sensory neuron, which relays the signal to an interneuron within the spinal cord. The signal is then sent to a motor neuron, which in turn stimulates a muscle in your leg to lift your leg.
Sensory receptors
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50. END OF SECTION

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35-2
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35-2
Neurons that carry impulses from the brain and spinal cord to the muscles are
interneurons.
sensory neurons.
resting neurons.
motor neurons.
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35-2
The part of the neuron that carries impulses toward the cell body is the
axon.
myelin sheath.
dendrite.
nodes.
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35-2
The minimum level of a stimulus that is required to activate a neuron is called its
action potential.
resting potential.
threshold.
synapse.
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35-2
Chemicals that are used by a neuron to transmit impulses are called
neurotransmitters.
synapses.
axons.
inhibitors.
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35-2
An action potential begins when
sodium ions flow into the neuron.
potassium ions flow into the neuron.
sodium and potassium ions flow into the neuron.
sodium and potassium ions flow out of the neuron.
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