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A&P > PPT Notes Chapter 11 > Flashcards

PPT Notes Chapter 11 Flashcards

1
Q

Functions of the Nervous System

A
  1. Sensory input
  2. Information gathered by sensory receptors about internal and external changes
  3. Integration
  4. Interpretation of sensory input
  5. Motor output
  6. Activation of effector organs (muscles and glands) produces a response
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2
Q

Divisions of the Nervous System

A

Central nervous system (CNS)

  • Brain and spinal cord Integration and command center

Peripheral nervous system (PNS)

Paired spinal and cranial nerves carry messages to and from the CNS

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3
Q

Peripheral Nervous System

A

Two functional divisions:

  1. Sensory (afferent) division
  • Somatic afferent fibers—convey impulses from skin, skeletal muscles, and joints
  • Visceral afferent fibers—convey impulses from visceral organs
  1. Motor (efferent) division
    * Transmits impulses from the CNS to effector organs
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4
Q

Motor Division of PNS

A

Somatic (voluntary) nervous system

  • Conscious control of skeletal muscles
  • Examples: walking, talking, playing piano, etc.

Autonomic (involuntary) nervous system (ANS)

Visceral motor nerve fibers

Regulates smooth muscle, cardiac muscle, and glands

Examples: digestion, heart beat, sweating, etc.

Two functional subdivisions

Sympathetic

Parasympathetic

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5
Q

Histology of Nervous Tissue

A

Two principal cell types

Neurons—excitable cells that transmit electrical signals

Neuroglia (glial cells)—supporting cells:

  • Astrocytes (CNS)
  • Microglia (CNS)
  • Ependymal cells (CNS)
  • Oligodendrocytes (CNS)
  • Satellite cells (PNS)
  • Schwann cells (PNS)
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6
Q

Astrocytes

A
  • Most abundant, versatile, and highly branched glial cells
  • Cling to neurons, synaptic endings, and capillaries
  • Support and brace neurons
  • Help determine capillary permeability
  • Guide migration of young neurons
  • Control the chemical environment
  • Participate in information processing in the brain
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7
Q

Microglia

A
  • Small, ovoid cells with thorny processes
  • Migrate toward injured neurons
  • Phagocytize microorganisms and neuronal debris
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8
Q

Ependymal Cells

A
  • Range in shape from squamous to columnar
  • May be ciliated
    • Line the central cavities of the brain and spinal column
    • Separate the CNS interstitial fluid from the cerebrospinal fluid in the cavities
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9
Q

Oligodendrocytes

A
  • Branched cells
  • Processes wrap CNS nerve fibers, forming insulating myelin sheaths
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10
Q

Satellite Cells and Schwann Cells

A
  • Satellite cells
  • Surround neuron cell bodies in the PNS
  • Schwann cells (neurolemmocytes)
  • Surround peripheral nerve fibers and form myelin sheaths
  • Vital to regeneration of damaged peripheral nerve fibers
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11
Q

Neurons (Nerve Cells)

A

Special characteristics:

  • Long-lived ( 100 years or more)
  • Amitotic—with few exceptions
  • High metabolic rate—depends on continuous supply of oxygen and glucose
  • Plasma membrane functions in:

Electrical signaling

Cell-to-cell interactions during development

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12
Q

Cell Body (Perikaryon or Soma)

A
  • Biosynthetic center of a neuron
  • Spherical nucleus with nucleolus
  • Well-developed Golgi apparatus
  • Rough ER called Nissl bodies (chromatophilic substance)
  • Network of neurofibrils (neurofilaments)
  • Axon hillock—cone-shaped area from which axon arises
  • Clusters of cell bodies are called nuclei in the CNS, ganglia in the PNS
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13
Q

Processes

A
  1. Dendrites and axons
  2. Bundles of processes are called
  3. Tracts in the CNS
  4. Nerves in the PNS
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14
Q

Dendrites

A
  • Short, tapering, and diffusely branched
  • Receptive (input) region of a neuron
  • Convey electrical signals toward the cell body as graded potentials
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15
Q

The Axon

A
  • One axon per cell arising from the axon hillock
  • Long axons (nerve fibers)
  • Occasional branches (axon collaterals)
  • Numerous terminal branches (telodendria)
  • Knoblike axon terminals (synaptic knobs or boutons)
  • Secretory region of neuron
  • Release neurotransmitters to excite or inhibit other cells
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16
Q

Axons: Function

A
  • Conducting region of a neuron
  • Generates and transmits nerve impulses (action potentials) away from the cell body
  • Molecules and organelles are moved along axons by motor molecules in two directions:
  • Anterograde—toward axonal terminal
  • Examples: mitochondria, membrane components, enzymes
  • Retrograde—toward the cell body
  • Examples: organelles to be degraded, signal molecules, viruses, and bacterial toxins
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17
Q

Myelin Sheath

A
  • Segmented protein-lipoid sheath around most long or large-diameter axons
  • It functions to:
  • Protect and electrically insulate the axon
  • Increase speed of nerve impulse transmission
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18
Q

Myelin Sheath in PNS

A
  • Schwann cells wraps many times around the axon
  • Myelin sheath—concentric layers of Schwann cell membrane
  • Neurilemma—peripheral bulge of Schwann cell cytoplasm
  • Nodes of Ranvier
  • Myelin sheath gaps between adjacent Schwann cells
  • Sites where axon collaterals can emerge
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19
Q

Unmyelinated axons

A
  • Thin nerve fibers are unmyelinated
  • One Schwann cell may incompletely enclose 15 or more unmyelinated axons
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20
Q

Myelins Sheaths in the CNS

A
  • Formed by processes of oligodendrocytes, not the whole cells
  • Nodes of Ranvier are present
  • No neurilemma
  • Thinnest fibers are unmyelinated
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21
Q

White Matter and Gray Matter

A
  • White matter
  • Dense collections of myelinated fibers
  • Gray matter
  • Mostly neuron cell bodies and unmyelinated fibers
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22
Q

Structural Classification of Neurons

A

Three types:

Multipolar—1 axon and several dendrites

  • Most abundant
  • Motor neurons and interneurons

Bipolar—1 axon and 1 dendrite

  • Rare, e.g., retinal neurons

Unipolar (pseudounipolar)—single, short process that has two branches:

Peripheral process—more distal branch, often associated with a sensory receptor

Central process—branch entering the CNS

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23
Q

Functional Classification of Neurons

A

Three types:

  • Sensory (afferent)
  • Transmit impulses from sensory receptors toward the CNS
  • Motor (efferent)
  • Carry impulses from the CNS to effectors
  • Interneurons (association neurons)
  • Shuttle signals through CNS pathways; most are entirely within the CNS
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24
Q

Neurophysiology

A
  • Neurons are highly irritable
  • Action potentials, or nerve impulses, are:
    • Electrical impulses carried along the length of axons
    • Always the same regardless of stimulus
    • The underlying functional feature of the nervous system
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25
Q

Electricity Definitions

A

Voltage (V) – measure of potential energy generated by separated charge

Potential difference – voltage measured between two points

Current (I) – the flow of electrical charge between two points

Resistance (R) – hindrance to charge flow

Insulator – substance with high electrical resistance

Conductor – substance with low electrical resistance

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26
Q

Electrical current and the body

A
  • Reflects the flow of ions rather than electrons
  • There is a potential on either side of membranes when:
    • The number of ions is different across the membrane
    • The membrane provides a resistance to ion flow
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27
Q

Role of Ion Channels

A

4 Types of plasma membrane ion channels:

  1. Passive, or leakage, channels – always open
  2. Chemically gated channels – open with binding of a specific neurotransmitter
  3. Voltage-gated channels – open and close in response to membrane potential
  4. Mechanically gated channels – open and close in response to physical deformation of receptors
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28
Q

Operation of a Gated Channel

A
  • Example: Na+-K+ gated channel
  • Closed when a neurotransmitter is not bound to the extracellular receptor
  • Na+ cannot enter the cell and K+ cannot exit the cell
  • Open when a neurotransmitter is attached to the receptor
  • Na+ enters the cell and K+ exits the cell
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29
Q

Operation of a voltage-gated channel

A

Example: Na+ channel

  • Closed when the intracellular environment is negative
  • Na+ cannot enter the cell
  • Open when the intracellular environment is positive
  • Na+ can enter the cell
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30
Q

Gated Channels

A

When gated channels are open:

  • Ions move quickly across the membrane
  • Movement is along their electrochemical gradients
  • An electrical current is created
  • Voltage changes across the membrane
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31
Q

Electrochemical gradient

A
  • Ions flow along their chemical gradient when they move from an area of high concentration to an area of low concentration
  • Ions flow along their electrical gradient when they move toward an area of opposite charge
  • Electrochemical gradient – the electrical and chemical gradients taken together
32
Q

Resting Membrane potential (Vr)

A
  • The potential difference (–70 mV) across the membrane of a resting neuron
  • It is generated by different concentrations of Na+, K+, Cl-, and protein anions (A-)
  • Ionic differences are the consequence of:
  • Differential permeability of the neurilemma to Na+ and K+
  • Operation of the sodium-potassium pump
33
Q

Membrane Potentials: Signals

A
  • Used to integrate, send, and receive information
  • Membrane potential changes are produced by:
  • Changes in membrane permeability to ions
  • Alterations of ion concentrations across the membrane
  • Types of signals – graded potentials and action potentials
34
Q

Changes in Membrane Potential

A

Changes are caused by three events

  • Depolarization – the inside of the membrane becomes less negative
  • Repolarization – the membrane returns to its resting membrane potential
  • Hyperpolarization – the inside of the membrane becomes more negative than the resting potential
35
Q

Graded Potentials

A
  • Short-lived, local changes in membrane potential
  • Decrease in intensity with distance
  • Magnitude varies directly with the strength of the stimulus
  • Sufficiently strong graded potentials can initiate action potentials
  • Voltage changes are decremental
  • Current is quickly dissipated due to the leaky plasma membrane
  • Only travel over short distances
  • Dendritic reception is a graded potential
36
Q

Action Potentials (APS)

A
  • A brief reversal of membrane potential with a total amplitude of 100 mV
  • Action potentials are only generated by muscle cells and neurons
  • They do not decrease in strength over distance
  • They are the principal means of neural communication
  • An action potential in the axon of a neuron is a nerve impulse
37
Q

Action Potential: Resting State

A
  • Na+ and K+ channels are closed
  • Leakage accounts for small movements of Na+ and K+
  • Each Na+ channel has two voltage-regulated gates
  • Activation gates – closed in the resting state
  • Inactivation gates – open in the resting state
38
Q

Action Potential:Depolarization Phase

A
  • Na+ permeability increases; membrane potential reverses
  • Na+ gates are opened; K+ gates are closed
  • Threshold – a critical level of depolarization (–55 to –50 mV)
  • At threshold, depolarization becomes self-generating
39
Q

Action Potential: Repolarization Phase

A
  • Sodium inactivation gates close
  • Membrane permeability to Na+ declines to resting levels
  • As sodium gates close, voltage-sensitive K+ gates open
  • K+ exits the cell and internal negativity of the resting neuron is restored
40
Q

Action Potential: Hyperpolarization

A
  • Potassium gates remain open, causing an excessive efflux of K+
  • This efflux causes hyperpolarization of the membrane (undershoot)
  • The neuron is insensitive to stimulus and depolarization during this time
41
Q

Action Potential: Role of the Sodium-Potassium Pump

A

Repolarization

  • Restores the resting electrical conditions of the neuron
  • Does not restore the resting ionic conditions
  • Ionic redistribution back to resting conditions is restored by the sodium-potassium pump
42
Q

Phases of the Action Potential

A

1 – resting state

2 – depolarization phase

3 – repolarization phase

4 – hyperpolarization

43
Q

Propagation of an Action Potential (Time = 0ms)

A
  • Na+ influx causes a patch of the axonal membrane to depolarize
  • Positive ions in the axoplasm move toward the polarized (negative) portion of the membrane
  • Sodium gates are shown as closing, open, or closed
44
Q

Propagation of an Action Potential (Time = 2ms)

A
  • Ions of the extracellular fluid move toward the area of greatest negative charge
  • A current is created that depolarizes the adjacent membrane in a forward direction
  • The impulse propagates away from its point of origin
45
Q
  • Propagation of an Action Potential (Time = 4ms)
A

The action potential moves away from the stimulus

Where sodium gates are closing, potassium gates are open and create a current flow

46
Q

Threshold and Action Potentials

A
  • Threshold – membrane is depolarized by 15 to 20 mV
  • Established by the total amount of current flowing through the membrane
  • Weak (subthreshold) stimuli are not relayed into action potentials
  • Strong (threshold) stimuli are relayed into action potentials
  • All-or-none phenomenon – action potentials either happen completely, or not at all
47
Q

Coding for Stimulus Intensity

A
  • All action potentials are alike and are independent of stimulus intensity
  • Strong stimuli can generate an action potential more often than weaker stimuli
  • The CNS determines stimulus intensity by the frequency of impulse transmission
48
Q

Conduction Velocity

A
  • Conduction velocities of neurons vary widely
  • Effect of axon diameter
    • Larger diameter fibers have less resistance to local current flow and have faster impulse conduction
  • Effect of myelination
    • Continuous conduction in unmyelinated axons is slower than saltatory conduction in myelinated axons
  • Effects of myelination
  • Myelin sheaths insulate and prevent leakage of charge
  • Saltatory conduction in myelinated axons is about 30 times faster
  • Voltage-gated Na+ channels are located at the nodes
  • APs appear to jump rapidly from node to node
49
Q

Multiple Sclerosis (MS)

A
  • An autoimmune disease that mainly affects young adults
  • Symptoms: visual disturbances, weakness, loss of muscular control, speech disturbances, and urinary incontinence
  • Myelin sheaths in the CNS become nonfunctional scleroses
  • Shunting and short-circuiting of nerve impulses occurs
  • Impulse conduction slows and eventually ceases
50
Q

Multiple Sclerosis: Treatment

A

Some immune system–modifying drugs, including interferons and *Copazone:

  • Hold symptoms at bay
  • Reduce complications
  • Reduce disability
51
Q

Nerve Fiber Classification

A

Nerve fibers are classified according to:

  • Diameter
  • Degree of myelination
  • Speed of conduction
  • Group A fibers
  • Large diameter, myelinated somatic sensory and motor fibers
  • Group B fibers
  • Intermediate diameter, lightly myelinated ANS fibers
  • Group C fibers
  • Smallest diameter, unmyelinated ANS fibers
52
Q

The Synapse

A

A junction that mediates information transfer from one neuron:

  • To another neuron, or
  • To an effector cell

Presynaptic neuron—conducts impulses toward the synapse

Postsynaptic neuron—transmits impulses away from the synapse

53
Q

Types of Synapses

A

Axodendritic—between the axon of one neuron and the dendrite of another

Axosomatic—between the axon of one neuron and the soma of another

Less common types:

  • Axoaxonic (axon to axon)
  • Dendrodendritic (dendrite to dendrite)
  • Dendrosomatic (dendrite to soma)
54
Q

Electrical Synapses

A
  • Less common than chemical synapses
  • Neurons are electrically coupled (joined by gap junctions)
  • Communication is very rapid, and may be unidirectional or bidirectional

Are important in:

Embryonic nervous tissue

Some brain regions

55
Q

Chemical Synapses

A

Specialized for the release and reception of neurotransmitters

Typically composed of two parts

Axon terminal of the presynaptic neuron, which contains synaptic vesicles

Receptor region on the postsynaptic neuron

56
Q

Synaptic Cleft

A
  • Fluid-filled space separating the presynaptic and postsynaptic neurons
  • Prevents nerve impulses from directly passing from one neuron to the next
  • Transmission across the synaptic cleft:
  • Is a chemical event (as opposed to an electrical one)
  • Involves release, diffusion, and binding of neurotransmitters
  • Ensures unidirectional communication between neurons
57
Q

Information Transfer

A
  • AP arrives at axon terminal of the presynaptic neuron and opens voltage-gated Ca2+ channels
  • Synaptotagmin protein binds Ca2+ and promotes fusion of synaptic vesicles with axon membrane
  • Exocytosis of neurotransmitter occurs
  • Neurotransmitter diffuses and binds to receptors (often chemically gated ion channels) on the postsynaptic neuron
  • Ion channels are opened, causing an excitatory or inhibitory event (graded potential)
58
Q

Termination of Neurotransmitter Effects

A

Within a few milliseconds, the neurotransmitter effect is terminated

  • Degradation by enzymes
  • Reuptake by astrocytes or axon terminal
  • Diffusion away from the synaptic cleft
59
Q

Synaptic Delay

A
  • Neurotransmitter must be released, diffuse across the synapse, and bind to receptors
  • Synaptic delay—time needed to do this (0.3–5.0 ms)
  • Synaptic delay is the rate-limiting step of neural transmission
60
Q

Postsynaptic Potentials

A
  • Graded potentials
  • Strength determined by:
  • Amount of neurotransmitter released
  • Time the neurotransmitter is in the area

Types of postsynaptic potentials

EPSP —excitatory postsynaptic potentials

IPSP —inhibitory postsynaptic potentials

61
Q

Excitatory Synapses and EPSPs

A
  • Neurotransmitter binds to and opens chemically gated channels that allow simultaneous flow of Na+ and K+ in opposite directions
  • Na+ influx is greater that K+ efflux, causing a net depolarization
  • EPSP helps trigger AP at axon hillock if EPSP is of threshold strength and opens the voltage-gated channels
62
Q

Inhibitory Synapses and IPSPs

A
  • Neurotransmitter binds to and opens channels for K+ or Cl–
  • Causes a hyperpolarization (the inner surface of membrane becomes more negative)
  • Reduces the postsynaptic neuron’s ability to produce an action potential
63
Q

Integration: Summation

A
  • A single EPSP cannot induce an action potential
  • EPSPs can summate to reach threshold
  • IPSPs can also summate with EPSPs, canceling each other out
  • Temporal summation
  • One or more presynaptic neurons transmit impulses in rapid-fire order
  • Spatial summation
  • Postsynaptic neuron is stimulated by a large number of terminals at the same time
64
Q

Integration: Synaptic Potentiation

A
  • Repeated use increases the efficiency of neurotransmission
  • Ca2+ concentration increases in presynaptic terminal and postsynaptic neuron
  • Brief high-frequency stimulation partially depolarizes the postsynaptic neuron
  • Chemically gated channels (NMDA receptors) allow Ca2+ entry
  • Ca2+ activates kinase enzymes that promote more effective responses to subsequent stimuli
65
Q

Integration: Presynaptic Inhibition

A
  • Release of excitatory neurotransmitter by one neuron may be inhibited by the activity of another neuron via an axoaxonic synapse
  • Less neurotransmitter is released and smaller EPSPs are formed
66
Q

Neurotransmitters

A
  • Most neurons make two or more neurotransmitters, which are released at different stimulation frequencies
  • 50 or more neurotransmitters have been identified
  • Classified by chemical structure and by function [We will look at 7 classes]
67
Q

Chemical Classes of Neurotransmitters

A
  • Acetylcholine (Ach)
    • Released at neuromuscular junctions and some ANS neurons
    • Synthesized by enzyme choline acetyltransferase
    • Degraded by the enzyme acetylcholinesterase (AChE)
  • Biogenic amines include:
    • Catecholamines
      • Dopamine, norepinephrine (NE), and epinephrine
    • Indolamines
      • Serotonin and histamine
  • Broadly distributed in the brain
  • Play roles in emotional behaviors and the biological clock
  • Amino acids include:
    • GABA—Gamma (y)-aminobutyric acid
    • Glycine
    • Aspartate
    • Glutamate
  • Peptides (neuropeptides) include:
    • Substance P
      • Mediator of pain signals
    • Endorphins
      • Act as natural opiates; reduce pain perception
    • Gut-brain peptides
      • Somatostatin and cholecystokinin
  • Purines such as ATP:
    • Act in both the CNS and PNS
    • Produce fast or slow responses
    • Induce Ca2+ influx in astrocytes
    • Provoke pain sensation
  • Gases and lipids
    • Nitric oxide (NO)
      • Synthesized on demand
      • Activates the intracellular receptor guanylyl cyclase to cyclic GMP
      • Involved in learning and memory
    • Carbon monoxide (CO) is a regulator of cGMP in the brain
  • Gases and lipids
    • Endocannabinoids
      • Lipid soluble; synthesized on demand from membrane lipids
      • Bind with G protein–coupled receptors in the brain
      • Involved in learning and memory
68
Q

Functional Classification of Neurotransmitters

A
  • Neurotransmitter effects may be excitatory (depolarizing) and/or inhibitory (hyperpolarizing)
    • Determined by the receptor type of the postsynaptic neuron
    • GABA and glycine are usually inhibitory
    • Glutamate is usually excitatory
    • Acetylcholine
      • Excitatory at neuromuscular junctions in skeletal muscle
      • Inhibitory in cardiac muscle
69
Q

Neurotransmitter Actions

A
  • Direct action
  • Neurotransmitter binds to channel-linked receptor and opens ion channels
  • Promotes rapid responses
  • Examples: ACh and amino acids
  • Indirect action
  • Neurotransmitter binds to a G protein-linked receptor and acts through an intracellular second messenger
  • Promotes long-lasting effects
  • Examples: biogenic amines, neuropeptides, and dissolved gases
70
Q

Neurotransmitter Receptors

A

Types

  • Channel-linked receptors
  • G protein-linked receptors
71
Q

Channel-Linked (Ionotropic) Receptors

A
  • Ligand-gated ion channels
  • Action is immediate and brief
  • Excitatory receptors are channels for small cations
  • Na+ influx contributes most to depolarization
  • Inhibitory receptors allow Cl– influx or K+ efflux that causes hyperpolarization
72
Q

G Protein-Linked (Metabotropic) Receptors

A
  • Transmembrane protein complexes
  • Responses are indirect, slow, complex, and often prolonged and widespread
  • Examples: muscarinic ACh receptors and those that bind biogenic amines and neuropeptides
73
Q

G Protein-Linked Receptors: Mechanism

A
  • Neurotransmitter binds to G protein–linked receptor
  • G protein is activated
  • Activated G protein controls production of second messengers, e.g., cyclic AMP, cyclic GMP, diacylglycerol or Ca2+
  • Second messengers
  • Open or close ion channels
  • Activate kinase enzymes
  • Phosphorylate channel proteins
  • Activate genes and induce protein synthesis
74
Q

Neural Integration: Neuronal Pools

A

Functional groups of neurons that:

  • Integrate incoming information
  • Forward the processed information to other destinations
  • Simple neuronal pool
    • Single presynaptic fiber branches and synapses with several neurons in the pool
    • Discharge zone—neurons most closely associated with the incoming fiber
    • Facilitated zone—neurons farther away from incoming fiber
75
Q

Types of Circuits in Neuronal Pools

A
  • Diverging circuit
    • One incoming fiber stimulates an ever-increasing number of fibers, often amplifying circuits
    • May affect a single pathway or several
    • Common in both sensory and motor systems
  • Converging circuit
    • Opposite of diverging circuits, resulting in either strong stimulation or inhibition
    • Also common in sensory and motor systems
  • Reverberating (oscillating) circuit
  • Chain of neurons containing collateral synapses with previous neurons in the chain
  • Parallel after-discharge circuit
  • Incoming fiber stimulates several neurons in parallel arrays to stimulate a common output cell
76
Q

Patterns of Neural Processing

A
  • Serial processing
    • Input travels along one pathway to a specific destination
    • Works in an all-or-none manner to produce a specific response
    • Example: reflexes—rapid, automatic responses to stimuli that always cause the same response
    • Reflex arcs (pathways) have five essential components: receptor, sensory neuron, CNS integration center, motor neuron, and effector
  • Parallel processing
    • Input travels along several pathways
    • One stimulus promotes numerous responses
    • Important for higher-level mental functioning
  • Example: a smell may remind one of the odor and associated experiences

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