Please enable JavaScript.

Coggle requires JavaScript to display documents.

Chapter 12: Fundamentals of the Nervous System & Nervous Tissue -…

- - - - Parallel Processing
        
        Neurons are joined in parallel
        
        Info from a single neuron is sent along 2 or more parallel pathways
        
        Occurs when a single sensory stimulus results in many perceptions
        
        Occurs almost instantaneously
        
        Allows brain to evaluate stimuli with incredible speed & enables info to be integrated along numerous pathways
      - Serial Processing
        
        Neurons that synapse 1:1 in a sequence
        
        Joined in series
        
        Neurons linked in series pass their signal along a single pathway from one neuron to the next
        
        Like links in a chain
        
        Ex: reflex arc
- - - - Gray Matter
        
        Gray colored zone that surrounds the hollow central cavity of the CNS
        
        In the spinal cord, the gray matter is a butterfly-shaped region
        
        Dorsal half contains cell bodies of interneurons
        
        Ventral half contains cell bodies of motor neurons
        
        Is the site where neuron cell bodies are clustered
        
        Mix of neuron cell bodies, dendrites, short, nonmyelinated neurons & neuroglia
        
        Synapses occur
        
        Cortex
        
        Throughout CNS, white matter is external to gray matter
        
        In 2 regions of the brain (cerebrum & cerebellum), there's an additional layer of superficial gray matter
        
        Called the cortex
        
        Neuronal Integration
        
        Structural link between the PNS & CNS occurs in the gray matter of the CNS
        
        Reflex Arcs
        
        Simplest example of neuronal integration
        
        Simple chains of neurons that cause of simplest, reflexive behaviors
        
        Reflect the basic structural plan of the nervous system
        
        Account for reflexes
        
        Reflexes
        
        Rapid, automatic motor responses to stimuli
        
        Unlearned, unpremeditated & involuntary
        
        Reflexes are either somatic reflexes or visceral reflexes
        
        Somatic reflex results in the contraction of skeletal muscles
        
        Ex: jerking away your hand after it accidentally touches a hot stove
        
        Visceral reflex activate smooth muscle, cardiac muscle or glands
        
        Ex: Vomiting in response to some food that irritates your stomach
        
        5 Essential Components of Every Reflex Arc
        
        Each component activates the next
        
        1: Receptor
        
        Is the site where the stimulus acts
        
        Receptors are located at the terminal end of the peripheral process of a sensory neuron
        
        2: Sensory Neuron
        
        Transmits the afferent impulses to the CNS
        
        3: Integration Center
        
        Consists of 1 or more synapses in the gray matter of the CNS
        
        In the simplest reflexes, the integration center is a single synapse between a sensory & motor neuron
        
        In more complex reflexes, the integration center can involve multiple synapses that send signals through long chains of interneurons to other portions of the CNS
        
        4: Motor Neuron
        
        Conducts efferent impulses from the integration center to an effector
        
        5: Effector
        
        Muscle/gland cell that responds to the efferent impulses by contracting/secreting
        
        Monosynaptic Reflex
        
        Simplest & fastest of all reflexes
        
        No interneuron between the sensory & motor neuron
        
        Only 1 synapse in this reflex arc
        
        Ex: knee-jerk reflex
        
        Polysynaptic Reflex
        
        More common type of reflex
        
        1 or more interneurons are part of the pathway
        
        Interneurons signal the motor neuron to contract the muscle involved
        
        Ex: withdrawal reflexes
      - White Matter
        
        External to gray matter
        
        Contains no neuron cell bodies
        
        Contains millions of axons & neuroglia
        
        White color comes from the myelin sheets around many of the axons
        
        Tracts
        
        Bundles of axons that are traveling to similar destinations
        
        Within white matter
    - - Neuronal Circuits
        
        Interneurons of the CNS may be interconnected in multiply ways, called neuronal circuits
        
        Form networks of interneurons that are interposed between each sensory & motor neuron
        
        Ex: painful stimulus
        
        Immediate response is spinal reflex (moving hand away from heat)
        
        Sensory info passed along to brain
        
        Pain is felt after reflexive withdrawal
        
        Diverging Circuit
        
        One presynaptic neuron synapses with several other neurons (divergence)
        
        Converging Circuit
        
        Many neurons synapse on a single postsynaptic neuron (convergence)
        
        Reverberating Circuit
        
        Circuit that receives feedback via a collateral axon from a neuron in the circuit
  - - - Nerves
        
        Cablelike organ in the PNS
        
        Each nerve consists of many long axons (nerve fibers) arranged in parallel bundles & enclosed by successive wrappings of connective tissue
        
        Almost all nerves contain both myelinated & nonmyelinated sensory & motor axons
        
        Nerve Structure
        
        Within a nerve, each axon is surrounded by Schwann cells
        
        Layer of loose connective tissue called endoneurium covers the Schwann cells
        
        Groups of axons are bound into bundles called nerve fascicles
        
        Wrapped by connective tissue called perineurium
        
        The whole nerve is surrounded by a tough fibrous sheath called the epineurium
    - - Sensory/Afferent Division
        Carrying toward
        
        Signals are picked up by sensory receptors throughout the body & carried by nerve fibers of the PNS into the CNS
        
        Visceral Sensory
        
        Sensory innervation of the viscera
        
        General: stretch, pain, temperature, chemical changes & irritation in viscera; nausea & hunger
        
        Special: taste & smell
        
        General Visceral Senses
        
        General means widespread
        
        Stretch, pain & temperature
        
        Can be felt widely in the digestive & urinary tracts, reproductive organs & other viscera
        
        Hunger & nausea sensations
        
        Special Visceral Senses
        
        Special means localized
        
        Taste & smell
        
        Also referred to as chemical senses
        
        Sensory receptors are localized to the tongue & nasal cavity, respectively
        
        Somatic Sensory
        
        Sensory innervation of the skin, body wall & limbs
        
        Receptors are spread widely throughout the surface of the body
        
        General Somatic Senses
        
        General means widespread
        
        Touch, pain, pressure, vibration, temperature & proprioception from the skin, body wall & limbs
        
        Proprioception
        Sensing one's own body
        
        Another type of general somatic sensation
        
        Detects the amount of stretch in muscles, tendons & joint capsules
        
        Informs you of the position & movement of your body in space
        
        Giving you a "body sense"
        
        Special Somatic Senses
        
        Special means localized
        
        Receptors are confined to relatively small areas of the body
        
        Most special senses are confined to the head
        
        Hearing, equilibrium & vision
      - Motor/Efferent Division
        Carrying away
        
        Signals are carried away from the CNS by fibers of the PNS to the effectors (muscles & glands)
        
        Causing these organs to contract/secrete
        
        Visceral Motor
        
        Involuntary motor innervation of smooth muscle, cardiac muscle & glands
        
        Also arrestor pili muscles, smooth muscle in vessels & sweat glands
        
        Autonomic Nervous System
        
        General visceral motor neurons make up the autonomic nervous system (ANS)
        
        Controls the function of the visceral organs
        
        Also called the involuntary nervous system
        
        Parasympathetic Division
        Rest & Digest
        
        Conserves energy & promotes digestion
        
        Sympathetic Division
        Fight or Flight
        
        Gets the body ready for activity
        
        Somatic Motor
        Voluntary nervous system
        
        Stimulates contraction of the skeletal muscles
        
        Voluntary control over skeletal muscle contraction
- - - - Conduction
        
        Highly specialized cells that conduct electrical signals from 1 part of the body to another
        
        Signals are transmitted along the plasma membrane (neurilemma) in the form of nerve impulses/action potentials
      - Extreme Longevity
        
        Neurons can live & function for a lifetime (100+ years)
      - Don't Divide
        
        Neurons don't divide
        
        Exception: neural stem cells have been identified in certain areas of the CNS
      - High Metabolic Rate
        
        Require continuous & abundant supplies of oxygen & glucose
        
        Neurons can't survive for more than a few minutes without oxygen
    - - Cell Body
        Soma
        
        Consist of a single nucleus surrounded by cytoplasm
        
        Focal point for the outgrowth of the neuron processes during embryonic development
        
        Plasma membrane of the cell body typically acts as a receptive surface that receives signals from other neurons
        
        Most cell bodies are located in the CNS
        
        Clusters of cell bodies (ganglia) lie along the nerves in the PNS
        
        Contains usual organelles & other structures
        
        Chromatophlic Bodies (Nissl Bodies)
        
        Clusters of rough ER & free ribosomes
        
        Function to renew membranes of the neuron
      - Neuron Processes
        
        Armlike processes extends from the cell bodies of all neurons
        
        2 types of processes, which differ in structure & functional properties
        
        Types of Processes
        
        Dendrites
        
        Most neurons have numerous dendrites
        
        Processes that branch from the cell body like the limbs on a tree
        
        Function as receptive sites
        
        Provide an enlarged surface area for receiving signals from other neurons
        
        Conduct electrical signals toward the cell body
        
        Axon
        
        Neuron has only 1 axon
        
        Arises from a cone-shaped region of the cell body (axon hillock)
        
        Thin processes of uniform diameter throughout their length
        
        Impulse generators & conductors
        
        Transmit nerve impulses away from the cell body
        
        Components
        
        Lack organelles involved in protein synthesis
        
        Receive protein components from cell body
        
        Neurofilaments, actin microfilaments & microtubules provide structural strength
        
        Also aid in the transport of substances to & from cell body
        
        Axonal transport
        
        Length/Size
        
        Axon of some neurons is short, but can be very long in others
        
        Long axons are called nerve fibers
        
        Axons w/ large diameters conduct impulses faster
        
        Branches
        
        Axons can occasionally branch out
        
        Branches are called axon collaterals
        
        Extend from the axon at ~right angles along the length
        
        Axons branch at its end, called terminal arborization
        
        Terminal branches end in knobs called terminal boutons or axon terminals
        
        Terminal Boutons
        
        Nerve impulse is typically generated where the axon extends from axon hillock to the terminal boutons
        
        Nerve impulse reaches the terminal boutons
        
        Causes the release of neurotransmitter chemicals into extracellular space
        
        Mitochondria are abundant in the terminal bouton
        
        Secretion of neurotransmitters requires a lot of energy
        
        Neurotransmitters
        
        Excite/inhibit the neurons/target organ in contact with the axon bouton
      - Synpases
        
        Sites where neurons communicate
        
        Most synapses transmit info through chemical messengers
        
        Some neurons in the CNS transmit signals electrically through gap junctions
        
        Determine the direction of information flow through the nervous system
        
        At the synapse, the plasma membranes of the 2 neurons are separated by a synaptic cleft
        
        Presynaptic Neuron
        
        Neuron that conducts signals toward a synapse
        
        Synaptic Vesicles
        
        Terminal bouton on the presynaptic side contains synaptic vesicles
        
        Membarne-bound sacs filled with neurotransmitters
        
        Postsynaptic Neuron
        
        Neuron that transmits signals away from the synapse
        
        2 Main Types of Synapses
        
        Axodendritic Synapses
        
        Synapse occurs between the terminal boutons of 1 neuron and the dendrites of another neuron
        
        Most synapses are like this
        
        Axosomatic Synapses
        
        Synapse occurs between axons & neuron cell bodies
        
        How Does a Synapse Function?
        
        Impulse travels along the axon of the presynaptic neuron
        
        Impulse stimulates the synaptic vesicles to fuse with the presynaptic membrane
        
        The fused area ruptures, causing the vesicles to release their neurotransmitter molecules
        
        Neurotransmitters diffuse across the synaptic cleft & bind to the postsynaptic membrane
        
        This binding changes the membrane charge on the postsynaptic neuron
        
        Influences the membrane's ability to generate a nerve impulse
    - - Structural Classification of Neurons
        
        Neurons are grouped structurally according to the number of processes that extend from the cell body
        
        Unipolar
        
        Have a short, single process that emerges from the cell body & divides like an inverted T into 2 long branches
        
        Most start out as bipolar neurons whose 2 processes fuse together near the cell body during development
        
        More properly called pseudounipolar neurons
        
        Found in sensory ganglia in the PNS, where they function as sensory neurons
        
        Don't have dendrites
        
        Central Process
        
        The branch that runs centrally into the CNS
        
        Is an axon
        
        Peripheral Process
        
        The branch that runs peripherally to the receptors
        
        Called an axon, but is ambiguous
        
        Conducts impulses (like an axon), but toward the cell body (like a dendrite)
        
        Receptive endings at the terminal end of the peripheral axon receives sensory stimuli
        
        Transmit signals through the peripheral axon toward the cell body
        
        Multipolar
        
        Have more than 2 processes
        
        Most have numerous dendrites & a single axon
        
        Some only have dendrites
        99%+ of neurons in the body are multipolar
        
        Major neuron type in the CNS
        
        Bipolar
        
        Have 2 processes that extend from opposite sides of the cell body
        
        Very rare neurons that occur in some of the special sensory organs
        
        Inner ear, olfactory epithelium of the nose, retina of the eye
        
        Mostly serve as sensory neurons
      - Functional Classification of Neurons
        
        Neurons are grouped functionally according to the direction the nerve impulse travels relative to the CNS
        
        Sensory Neurons
        Afferent Neurons
        
        Make up the sensory division of the PNS
        
        Transmit impulses toward the CNS from sensory receptors in the PNS
        
        Most sensory neurons are pseudounipolar
        
        Their cell bodies are in ganglia outside the CNS
        
        The peripheral process extends from a sensory receptor
        
        Central process terminates in the CNS
        
        These 2 processes function as 1, carrying impulses directly from peripheral receptors to the CNS
        
        Some sensory neurons are bipolar
        
        These are restricted to some of the special sense organs
        
        Motor Neurons
        Efferent Neurons
        
        Make up the motor division of the PNS
        
        Carry impulses away from the CNS to effector organs (muscles & glands)
        
        Most motor neurons are multipolar
        
        Cell bodies are in the CNS
        
        Except for some neurons of the ANS
        
        Motor neurons form junctions with effector cells
        
        Stimulating muscles to contract or glands to secrete
        
        Interneurons
        
        Lie between motor & sensory neurons
        
        Multipolar neurons that are confined to the CNS
        
        Interneurons link together into chains to form complex neuronal pathways
  - - - Astrocytes
        
        Most abundant glial cells of the CNS
        
        Part of the blood brain barrier
        
        Have many radiating processes with bulbous ends
        
        Some bulbs cling to neurons or capillaries
        
        Known Functions
        
        Regulating neurotransmitter levels
        
        Increase the rate of neurotransmitter uptake in regions of high neuronal activity
        
        Signaling increased blood flow through capillaries in active regions of the brain
        
        Controlling the ionic environment around neurons
        
        Other Functions
        
        Help synapses form in developing neural tissue
        
        Produce molecules necessary for neural growth
        
        Propagate calcium signals that may be involved with memory
      - Microglial Cells
        
        Smallest & least abundant neuroglia of the CNS
        
        Have elongated cell bodies & cell processes with many pointed projections
        
        Phagocytes
        
        Macrophages of the CNS
        
        Migrate to & engulf invading organisms and injured/dead neurons
        
        Do not originate in nervous tissue
        
        Derived from blood cells (monocytes)
      - Ependymal Cells
        
        CNS originates in the embryo as a hollow neural tube & retains a central cavity throughout life
        
        Form a simple epithelium that lines the central cavity of the spinal cord & brain
        
        Provide a fairly permeable layer between the cerebrospinal fluid that fills the cavity & the tissue fluid that bathes the cells of the CNS
        
        Bear cilia that help circulate the cerebrospinal fluid
      - Oligodendrocytes
        
        Have fewer branches than astrocytes
        
        Produce insulating coverings called myelin sheaths around the axons in the CNS
        
        Myelin Sheaths
        
        Segmented structures composed of myelin lipoprotein & surround the thicker axons of the body
        
        Each segment consists of the plasma membrane of a glial cell rolled in concentric layers around the axon
        
        Function
        
        Form an insulating layer that prevents leakage of electrical current from axon
        
        Increase the speed of impulse conduction along the axon
        
        Makes impulse propagation more energy-efficient
        
        Myelin Sheaths in the CNS
        
        Formed by oligodendrocytes
        
        Oligodendrocytes have multiple processes that coil around several different axons
        
        Myelin sheath gaps are also present, but are more widely spaced out than in the PNS
        
        Thick axons are myelinated
        
        Thin axons are nonmyelinated
        
        Covered by the many long processes of glial cells abundant in the CNS (ex: astrocytes)
        
        Myelin Sheaths in the PNS
        
        Formed by Schwann cells
        
        Develop during fetal period & in the first year of postnatal life
        
        Schwann cells wrap in concentric layers around the axon
        
        Cover axon in a tightly packed coil of membranes
        
        Thick axons are myelinated
        
        Thin axons are nonmyelinated
        
        Conduct impulses more slowly
        
        Found in portions of the ANS & in some sensory fibers
        
        Nodes of Ranvier
        
        Myelin sheath gaps
        
        Gaps along the axon
        
        Speed up nerve transmission
    - - Satellite Cells
        
        Surround neuron cell bodies within ganglia
      - Schwann Cells
        
        Surround all axons in the PNS
        
        Form myelin sheaths around many of these axons