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Overview of the Nervous System. One of the body’s homeostatic control systems Contains sensors, integrating centers, and output pathways More interneurons in a pathways  greater ability to integrate information. Figure 8.1. Cnidarians.

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Overview of the nervous system l.jpg
Overview of the Nervous System

  • One of the body’s homeostatic control systems

  • Contains sensors, integrating centers, and output pathways

  • More interneurons in a pathways  greater ability to integrate information

Figure 8.1


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Cnidarians

  • Most nervous systems are organized into three functional divisions

  • Cnidarians are an exception

  • Their nervous system is an interconnected web or nerve net

  • Neurons are not specialized into different divisions

  • Neurons are functionally bipolar and impulses radiate out from the stimulus

  • Can still perform complex behaviors


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Nervous System Terms

  • Bilaterally symmetrical – anterior and posterior end and a right and left side

  • Cephalization - sense organs are concentrated at the anterior end

  • Brain – a complex integrating center made up of clusters of ganglia

  • Ganglia– groupings of neuronal cell bodies

  • Nuclei– groupings or neuronal cell bodies within the brain

  • Tracts – groupings of axons within the brain

  • Nerves – axons of afferent and efferent neurons


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Structure of a Nerve

  • Parallel bundles of myelinated and unmyelinated axons enclosed in several layers of connective tissue

    • Endoneurium

    • Perineurium

    • Epineurium

  • Fasicles – bundle of axons

  • Mixed nerves – contain both afferent and efferent neurons

Figure 8.3


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Nervous Systems Across Animal Groups

  • Cephalization occurs in most animals and becomes more apparent in more complex nervous systems

  • Cnidarians and Echinoderms are exceptions

  • Organisms with more complex nervous systems have more neurons


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The Vertebrate Central Nervous System

  • Among the most highly cephalized animals

  • Unique in having a hollow dorsal nerve cord

  • Portion of nervous system is encased within cartilage or bone

  • Central nervous system (CNS) – brain and spinal cord

  • Peripheral nervous system (PNS) – rest of the nervous system

Figure 8.5a


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Cranial and Spinal Nerves

  • Cranial nerves

    • Exit directly from the braincase

    • 13 pairs (labeled with roman numerals)

    • Some are afferent and some are efferent

  • Spinal nerves

    • Emerge from the spinal cord

    • Named based on the region of the spine where they originate


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Gray and White Matter

  • Brain and spinal cord contain two types of tissue

    • Gray matter – neuronal cell bodies

    • White matter – bundles of axons and their myelin sheaths

  • Spinal chord white matter is on the surface and gray matter is inside (opposite for cerebral cortex)

Figure 8.5b


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The CNS is Isolated

  • Meninges– layers of connective tissue that surround the brain and spinal cord

  • Number of layers vary across taxa (fish have one, mammals have three)

  • Cerebral spinal fluid (CSF) fills the space within the meninges and acts as a shock absorber

  • Blood-brain barrier – tight junctions in brain capillaries prevent material from leaking out of the bloodstream and into the CNS

Figure 8.6


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The Vertebrate Brain

  • The brain is an extension of the spinal cord

  • It is hollow inside and central cavities called ventricles contains CSF

  • Three main regions

    • Rhombencephalon (hindbrain)

      • Reflexes and involuntary behaviors

    • Mesencephalon (midbrain)

      • Coordination of sensory information

      • Relay center in mammals

    • Prosencephalon (forebrain)

      • Integration of olfactory information with other senses

      • Regulation of body temperature, reproduction, eating, emotion

      • Learning and memory in mammals


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Brain Size

  • Most groups have the same major brain structures, although these structures vary in relative size

Figure 8.9



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Hindbrain

  • Three regions

  • Pons – located above the medulla

    • Pathway between the medulla, the cerebellum, and the forebrain

    • Controls alertness and initiates sleep and dreaming

  • Cerebellum – two hemispheres at the back of the brain

    • Responsible for motor coordination

    • Contains half of the neurons in the brain

  • Medulla oblongata – located at the top of the spinal cord

    • Regulates breathing, heart rate, diameter of blood vessels, and blood pressure

    • Contain pathways between the spinal cord and the brain

    • Many cross over (e.g., left to right)


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Midbrain

  • Primary center for coordinating and initiating behavioral responses in fish and amphibians

  • Size and function reduced in mammals

    • Primarily serves as a relay center

  • Sometimes grouped with the pons and medulla and termed the brainstem


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Forebrain

  • Involved in processing and integrating sensory information, and in coordinating behavior

  • Main regions

    • Cerebrum

    • Thalamus

    • Epithalamus

    • Hypothalamus


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Cerebrum

  • Outer layer is the cortex

  • Divided into two cerebral hemispheres

    • Left side controls the right side of the body

    • Right side controls the left side of the body

  • Connected by the corpus callosum


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Cortex

  • Integrates and interprets sensory information and initiates voluntary movements

  • Has taken over many of the midbrain functions in lower vertebrates

  • Six layers

  • Isocortex (outer layer) is necessary for cognition and higher brain functions

    • More folded in more advanced mammals

    • Gyri – folds

    • Sulci – grooves


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Cortical Lobes

  • Based on the names of the overlying bones or function

Figure 8.14


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Cortical Topology

  • Each part of the cortex corresponds to the specific part of the body that it governs

  • The areas devoted to various parts of the body are disproportionate

Figure 8.5


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Hypothalamus

  • Located at the base of the forebrain

  • Maintains homeostasis

  • Interacts with the autonomic nervous system

  • Regulates secretion of pituitary hormones


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Limbic System

  • A network of connected structures that lie between the cortex and the rest of the brain

  • Influences emotions, motivation, and memory

  • Sometimes called the “emotional brain”

  • Includes the hypothalamus and other parts

    • Amygdala – aggression and fear responses

    • Hippocampus– converts short-term memory to long-term memory

    • Olfactory bulbs – sense of smell

Figure 8.11


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Thalamus

  • Large grouping of gray matter above the hypothalamus

  • Part of the reticular formation

  • Receives input from the limbic system and all senses except olfaction

  • Relays information to the cortex

  • Acts as a filter


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Epithalamus

  • Located above the thalamus

  • Contains

    • Habenular nuclei – communicates with the tegmentum of the midbrain

    • Pineal complex – Establishes circadian rhythms and secretes melatonin



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Autonomic Pathways

  • Involved in homeostasis

  • “Involuntary nervous system”

  • Systems

  • Sympathetic

    • Most active during periods of stress or physical activity

    • “Fight-or-flight” system

  • Parasympathetic

    • Most active during periods of rest

    • “Resting and digesting” system

  • Enteric

    • Independent of other two systems

    • Affects digestion by innervating the GI tract, pancreas, and gall bladder


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Maintaining Homeostasis

  • Balancing of the sympathetic and parasympathetic systems

  • Three features of maintaining homeostasis

    • Dual innervation – most internal organs receive input from both systems

    • Antagonistic action – one system stimulates while the other inhibits

    • Basal tone – Even under resting conditions autonomic neurons produce APs


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Dual Innervation

Figure 8.17



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Similarities in Autonomic Pathways

  • Pathways contain two neurons in series

    • Preganglionic– may synapse with many postganglionic neurons and intrinsic neurons

    • Postganglionic – release neurotransmitter at the effector from varicosities

  • These neurons synapse with each other in the autonomic ganglia

Figure 8.18


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Differences in Autonomic Pathways

  • Differences between the sympathetic (S) and parasympathetic (PS) branches

    • Preganglionic cell body location

      • S: thoracic and lumbar regions of the spinal cord

      • PS: hindbrain and sacral region of the spinal cord

    • Ganglia location

      • S: chain that runs close to the spinal cord

      • PS: close to the effector

    • Number of postganglionic neurons that synapse with a single preganglionic neuron

      • S: 10 or more

      • P: three or less


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Differences in Autonomic Pathways, Cont.

  • Type of neurotransmitter released at the effector

Figure 8.19


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Only Sympathetic Innervation

  • Some effectors receive only sympathetic innervation

    • Adrenal medulla – modified postganglionic neuron

    • Sweat glands

    • Arrector pili muscles in the skin

    • Kidneys

    • Most blood vessels

Figure 8.20




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Reflex Arcs

  • Most autonomic changes occur via simple neural circuits that do not involve conscious centers of the brain

Figure 8.22


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Somatic Motor Pathways

  • Control skeletal muscle

  • Usually under conscious control

  • The “Voluntary nervous system”

  • Some pathways are not under conscious control, e.g., knee-jerk reflex


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Somatic Pathway Characteristics

  • Control only one type of effector, skeletal muscle

  • Cell bodies are located in the CNS

  • Monosynaptic, therefore very long

  • Axons split into a cluster of axon terminals at the neuromuscular junction

  • Synaptic cleft between the motor neuron and the muscle is very narrow

  • Release the neurotransmitter acetylcholine

  • Effect on the muscle is always excitatory


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Reflex Arcs

  • Least complex integrated responses

  • Can involve as few as two neurons (monosynaptic) or more than two (polysynaptic)

Figures 8.23 & 8.25


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Reflex Arcs, Cont.

  • Can be arranged in two ways

    • Convergence – allows spatial summation

    • Divergence – can amplify signals

Figure 8.24


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Learning and Memory

  • Most animals can form memories and learn due to the plasticity of the nervous system

  • Learning – process of acquiring new information

  • Memory – retention and retrieval of information

  • Plasticity – ability to change both synaptic connections and functional properties of neurons in response to stimuli


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Invertebrate Learning and Memory

  • Well studied in the sea slug, Aplysia (20,000 neurons)

  • Habituation – decline in response to a stimulus due to repeated exposure

    • Allows animal to ignore unimportant stimuli and focus on novel stimuli

    • Occurs because of changes in the presynaptic axon terminal

      • Inactivation of Ca2+ channels   neurotransmitter release

Figure 8.29


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Invertebrate Learning and Memory, Cont.

  • Sensitization – increase in the response to a gentle stimulus after exposure to a strong stimulus

    • Occurs because of changes in the presynaptic axon terminal

      •  Ca2+ entry   neurotransmitter release

      • Involves a secondary circuit

        • Releases serotonin  binds to G-protein-coupled receptors  cascade of reactions  inactivation of K+ channels   AP duration   Ca2+ influx   neurotransmitter release


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Serotonin Effects

Figure 8.31


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Memory in Mammals

  • The hippocampus is involved in long-term memory, but the memories are stored elsewhere

  • Long-term potentiation – repetitive stimulation of hippocampal tissue leads to an increase in the response of the postsynaptic neuron


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