neurons are organized into nervous...
TRANSCRIPT
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N E U R O N S A R E O R G A N I Z E D I N T O
N E R V O U S S Y S T E M S3 4 . 5
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INTRODUCTION
• The cnidarians have nerve nets, the most simple type of nervous system.
The sea anemone has a nerve net that serves simple behaviours such as contraction and relaxation
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EVOLUTION
• Evolution of nervous systems followed two major trends:
– Centralization—integrating neurons became clustered together in centralized organs (e.g., brain and spinal cord).
– Cephalization—major integrating areas became concentrated toward the anterior end of the animal (head).
• The anterior end meets the environment first; sensory organs are also concentrated there.
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CENTRAL NERVOUS SYSTEM
• Central nervous system (CNS)—brain and spinal cord
• Composed mostly of integrating neurons and glial cells; it must interact with sensors and effectors.
• Effectors are cells or tissues that perform actions, that “carry out orders,” such as muscle cells.
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PERIPHERAL NERVOUS SYSTEM• Peripheral nervous system (PNS)—neurons outside the CNS
• They bring sensory information from sense organs to the CNS and carry orders from the CNS to effectors.
• Nerves are bundles of axons in the PNS.
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TYPES OF NEURONS
• Interneurons—neurons confined to the CNS
• Sensory neurons—sensory receptor cells or neurons that carry signals from sensory cells to the CNS (afferent neurons)
• Efferent neurons—convey signals from the CNS to muscles or other effectors
• Motor neurons—carry signals to skeletal muscles
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AUTONOMIC NERVOUS SYSTEM
• Autonomic nervous system (ANS)—controls effectors other than skeletal muscles (autonomic effectors)
• Controls smooth muscle in organs, exocrine glands, some endocrine glands, acid secreting cells in the stomach, and other effectors.
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3 DIVISIONS
• Vertebrate ANS has three divisions:
• Enteric division—nerve cells internal to the gut wall
• Sympathetic division prepares the body for emergencies—“fight or flight”
• Parasympathetic division slows the heart, lowers blood pressure and increases digestion
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AUTONOMIC NERVOUS SYSTEM
• Autonomic efferent pathways begin with preganglionic neurons with cell bodies in the CNS.• Axons of preganglionic neurons synapse on a second neuron outside the CNS in a collection
of nerve cell bodies called a ganglion.
• The second neuron is postganglionic—its axon leaves the ganglion and synapses in the target organs.
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AUTONOMIC NERVOUS SYSTEM• Parasympathetic preganglionic neurons exit the CNS from the brain
and sacral region of the spinal cord.– The ganglia are near the target organs.
• Sympathetic preganglionic neurons exit the CNS at the thoracic and lumbar regions of the spinal cord.
– Most of the ganglia lie next to the spinal cord.
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AUTONOMIC NERVOUS SYSTEM
• Sympathetic postganglionic neurons use norepinephrine as the neurotransmitter.
• Parasympathetic postganglionic neurons use acetylcholine.
• In organs that receive both inputs, the target cells usually repond in opposite ways.
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AUTONOMIC NERVOUS SYSTEM• The sympathetic and
parasympathetic divisions often work in opposition; acting together, they can adjust effector functions up or down as needed.
• Example: Sympathetic stimulation of the pace maker causes the heart rate to increase, and parasympathetic stimulation causes it to decrease.
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AUTONOMIC NERVOUS SYSTEM
• The fight-or-flight response is an effect of the sympathetic division.
• When activated, it increases the heart rate, force of contraction, and cardiac output; it dilates lung passageways and increases release of glucose from the liver.
• At the same time, it reduces less urgent activities such as digestion.
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SPINAL REFLEXES• Many neurons that control
skeletal muscles enter or leave the CNS in spinal nerves.
• Spinal nerves have both sensory and motor neurons.
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CONCEPT 34.5 NEURONS ARE ORGANIZED INTO NERVOUS SYSTEMS
• Spinal reflex—afferent information converts to efferent activity without going through the brain
• The knee-jerk reflex:
• Stretch receptors in the patellar tendon send action potentials to the spinal cord.
• The sensory neuron synapses with a motor neuron, sending an action potential to the leg muscle.
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1. A hammer tap stretches the tendon in the knee, stretching receptors in the knee
2. Stretch receptors fire action potentials.
3. The sensory neuron synapses with a motor neuron in the spinal cord
4. The motor neuron conducts action potentials to the quadriceps, causing contraction
5. Simultaneously a spinal interneuron inhibits firing in the motor neuron for the antagonistic muscle.
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VERTEBRATE EVOLUTION OF THE FOREBRAIN
• Vertebrate brains have three main regions: forebrain, midbrain, and hindbrain.
• The brain and spinal cord must pass through the medulla oblongata, the most posterior part of the hindbrain.
– This area has changed little over the course of vertebrate evolution.
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VERTEBRATE EVOLUTION OF THE FOREBRAIN• In contrast, the cerebral
hemispheres have undergone dramatic changes.
– They are important in carrying out high-order sensory, motor, and integrative functions.
• The evolution of enhanced functionality in mammals and birds has gone hand in hand with large increases in the numbers of neurons and larger brain size.
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VERTEBRATE EVOLUTION OF THE FOREBRAIN• However, it is important to note that some animals with small brains exhibit stunning
behavioral capacities.
• In humans, all available evidence indicates that individual intelligence is not correlated with individual brain size.
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SPECIFICITY IN MAMMALIAN CEREBRAL HEMISPHERES • Cerebral cortex—outer-most layer of the
cerebral hemispheres, with many cell bodies
– It is folded into convolutions, which increases its size.
• The left side of the body is served mostly by the right side of the brain, and vice versa.
• In each cerebral hemisphere, specific regions are specialized to carry out specific sensory and motor functions.
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SPECIFICITY IN MAMMALIAN CEREBRAL HEMISPHERES
• Combined sensory and motor functions often occur in localized brain areas.
• Imaging methods such as PET (Positron) allow us to visualize areas where neurons exhibit increased electrical activity correlated with specific activities, such as language functions.
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SPECIFICITY IN MAMMALIAN CEREBRAL HEMISPHERES
• Functional magnetic resonance imaging (fMRI) is another technique to pinpoint brain activity.
• Example: In a person experiencing fear, increased activity is seen in the amygdala in the forebrain.
• Even memories of frightening situations can activate the amygdala.
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SPECIFICITY IN MAMMALIAN CEREBRAL HEMISPHERES • Parts of the brain that
serve various anatomical regions of the body are physically related to each other in ways that mirror the rest of the body.
• Example: Map of the somatosensory (“body sensing”) part of the cerebral cortex
– The size of each body part in the drawing reflects the amount of cortical area devoted to the part.