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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
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

  • 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
nervous system terms
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
structure of a nerve
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

nervous systems across animal groups
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
the vertebrate central nervous system
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

cranial and spinal nerves
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
gray and white matter
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

the cns is isolated
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

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

Figure 8.9

  • 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)
  • 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
  • Involved in processing and integrating sensory information, and in coordinating behavior
  • Main regions
    • Cerebrum
    • Thalamus
    • Epithalamus
    • Hypothalamus
  • 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
  • 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
cortical lobes
Cortical Lobes
  • Based on the names of the overlying bones or function

Figure 8.14

cortical topology
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

  • Located at the base of the forebrain
  • Maintains homeostasis
  • Interacts with the autonomic nervous system
  • Regulates secretion of pituitary hormones
limbic system
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

  • 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
  • Located above the thalamus
  • Contains
    • Habenular nuclei – communicates with the tegmentum of the midbrain
    • Pineal complex – Establishes circadian rhythms and secretes melatonin
autonomic pathways
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
maintaining homeostasis
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
dual innervation
Dual Innervation

Figure 8.17

similarities in autonomic pathways
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

differences in autonomic pathways
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
differences in autonomic pathways cont
Differences in Autonomic Pathways, Cont.
  • Type of neurotransmitter released at the effector

Figure 8.19

only sympathetic innervation
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

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

Figure 8.22

somatic motor pathways
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
somatic pathway characteristics
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
reflex arcs38
Reflex Arcs
  • Least complex integrated responses
  • Can involve as few as two neurons (monosynaptic) or more than two (polysynaptic)

Figures 8.23 & 8.25

reflex arcs cont
Reflex Arcs, Cont.
  • Can be arranged in two ways
    • Convergence – allows spatial summation
    • Divergence – can amplify signals

Figure 8.24

learning and memory
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
invertebrate learning and memory
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

invertebrate learning and memory cont
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
memory in mammals
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