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Chapter 12

Nervous System

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Outline

• Nervous Tissue• The Nerve Impulse

– Action Potential• Central Nervous System

– The Spinal Cord– The Brain

• Peripheral Nervous System– Nerves and Ganglia

• Drug Abuse• Degenerative Nervous System Diseases

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Nervous Tissue

• Nervous Tissue contains two types of cells.– Neurons transmit nerve impulses between

parts of the nervous system.– Neuroglia support and nourish neurons.

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Neuron Structure

• Neurons are classified according to function.– Sensory neurons take impulses from a

sensory receptor to the CNS.– Interneurons receive input from sensory

neurons, and other neurons, and then communicate with motor neurons.

– Motor neurons take nerve impulse away from the CNS to an effector that carries out responses to environmental change.

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Neuron Structure

• Neurons contain three basic parts.– Cell body contains nucleus and other

organelles.– Dendrites receive signals from sensory

receptors or other neurons.– Axon conducts nerve impulses.

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Types of Neurons

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Myelin Sheath

• Some axons are covered by a protective myelin sheath.– Formed by Schwann cells containing

myelin in plasma membranes.– Nodes of Ranvier are gaps on the axon

with no myelin sheath.

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Myelin Sheath

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The Nerve Impulse

• The nervous system uses the nerve impulse to convey information.– Resting potential is the voltage level when

an axon is not conducting an impulse.• Sodium-potassium pump causes greater

concentration of Na+ outside the axon, and greater concentration of K+ inside the axon.– Unequal ion distribution causes inside of

axon to be negative relative to the outside.

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Action Potential

• An action potential is a rapid change in polarity across an axomembrane as the nerve impulse occurs.– All-or-none once threshold is reached.

Sodium gates open, allowing Na+ to move inside the axon.

Potassium gates open, allowing K+ to move outside the axon.

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Transmission across a Synapse

• Transmission across a synaptic cleft is carried out by neurotransmitters stored in synaptic vesicles.– Depending on the neurotransmitter and

the receptor, the response of the postsynaptic neuron can be towards excitation or inhibition.

Integration is the summing of signals received by a postsynaptic neuron.

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Integration

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The Central Nervous System

• The central nervous system (CNS) is made up of the spinal cord and the brain.– Both are wrapped in protective

membranes, meninges, with spaces between meninges filled with cerebrospinal fluid.

• CNS is composed of two types of nervous tissue.– Gray matter – Short, nonmyelinated fibers.– White matter - Myelinated axons.

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The Spinal Cord

• The spinal cord extends from the base of the brain through the foramen magnum into the vertebral canal.

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Functions of the Spinal Cord

• The spinal cord provides a means of communication between the brain and the peripheral nerves that leave the cord, and is a center for reflex actions.

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The Brain

• The Cerebrum.– The cerebrum, telencephalon, is the

largest portion of the human brain.Communicates with, and coordinates

activities of, other parts of the brain.Divided into left and right cerebral

hemispheres. Divided by longitudinal fissure.

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The Human Brain

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The Brain

• Gray matter of cerebrum consists of cerebral cortex and basal nuclei.– Cerebral cortex is a highly convoluted

outer layer that covers the cerebral hemispheres.

– Basal nuclei are masses located deep within white matter.

• White matter consists of long myelinated axons organized into tracts.

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Lobes of Cerebral Hemisphere

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The Brain

• Diencephalon is made up of hypothalamus and thalamus, and circles the third ventricle.

• Cerebellum is separated from the brain stem by the fourth ventricle.

• The brain stem contains the midbrain, pons, and medulla oblongata.– Reticular formation is a complex network

of nuclei and fibers extending the length of the brain stem.

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Reticular Activating System

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The Limbic System

• Limbic system is a complex network of tracts and nuclei.– Blends primitive emotions and higher

mental functions into a united whole.– Composed of hippocampus and

amygdala.

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Limbic System

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Higher Mental Functions

• Memory is the ability to hold a thought in mind or to recall events from the past.– Short-term versus long-term.– Skill memory.

• Learning takes place when we retain and utilize past memories.

• Language and speech depends on motor centers in the occipital and temporal lobes.

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Language and Speech

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Language and Speech

• Left and right brain have different functions.– Left Hemisphere:

Verbal, Logical, Analytical, Rational.– Right Hemisphere:

Nonverbal, Intuitive, Creative.

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Peripheral Nervous System

• Composed of nerves and ganglia.

Cranial nerves take impulses to and from the brain.

Spinal nerves take impulses to and from the spinal cord.

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Cranial and Spinal Nerves

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Somatic System

• Somatic system includes nerves that take sensory information from external sensory receptors to the CNS and motor commands away from the CNS to the skeletal muscles.– Reflexes are automatic responses to a

stimulus.

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Reflex Arc

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Autonomic System

• Autonomic system regulates the activity of cardiac and smooth muscles and glands.– Sympathetic division brings about fight or

flight responses.– Parasympathetic division brings about

relaxed responses.

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Drug Abuse

• Drugs that affect the nervous system have two general effects.– Impact limbic system.– Promote or decrease action of a particular

neurotransmitter.• Drug abuse is apparent when a person

takes a drug at a dose level and under circumstances that increase the potential for a harmful effect.

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Drug Actions at a Synapse

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Drug Abuse

• Alcohol.• Nicotine.

– Causes neurons to release dopamine.Excess dopamine has reinforcing effect

that leads to dependence.• Cocaine.

– Prevents synaptic uptake of dopamine.Continued use causes body to produce

less dopamine.

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Drug Abuse

• Heroin.– Binds to receptors meant for endorphins.

Continued use causes body to produce fewer endorphins.

• Marijuana.– Binds to receptor for anandamide.

Brain impairment?

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Homeostasis

• Governance of internal organs and the regulation of blood and tissue fluid usually takes place below the level of consciousness.– Heart Rate.– Breathing Rate.

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Degenerative Nervous System Diseases

• Alzheimer disease.– Presence of abnormal neurons.

Plaques.Neurofibrillary tangles.

• Parkinson disease.– Overactive basal nuclei due to the

degeneration of dopamine-releasing neurons in the brain.

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Review

• Nervous Tissue• The Nerve Impulse

– Action Potential• Central Nervous System

– The Spinal Cord– The Brain

• Peripheral Nervous System– Nerves and Ganglia

• Drug Abuse• Degenerative Nervous System Diseases

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