Chapter 11 Flashcards
The Nervous System
Controlling and communication system of body
Cells communicate via electrical and chemical signals
-Rapid
-Specific
-Usually cause almost immediate responses
Functions of Nervous System
Sensory Input -Information Integration -Processing Motor Output -Activation
Divisions of Nervous System
Central Nervous System (CNS)
Peripheral Nervous System (PNS)
Central Nervous System (CNS)
Contents -Brain -Spinal Cord Location -Dorsal Body Cavity Function -Integration and control center --Interprets sensory input and dictates motor output
Peripheral Nervous System (PNS)
Contents -Spinal nerves to and from spinal cord -Cranial nerves to and from brain Location -Outside brain and spinal cord
2 Functional Divisions Peripheral Nervous System (PNS)
Sensory (afferent) division
Motor (efferent) division
Sensory (Afferent) Division of PNS
- Somatic sensory fibers- convey impulses from skin, skeletal muscles, and joints to CNS
- Visceral sensory fibers- convey impulses from visceral organs to CNS
Motor (Efferent) Division of PNS
Transmits impulses from CNS to effector organs
-Muscles and Glands
2 Divisions of Motor (Efferent) Division
Somatic Nervous System
Autonomic Nervous System
Motor Division of PNS: Somatic Nervous System of PNS
Somatic motor nerve fibers
- Conducts impulses from CNS to skeletal muscle
- Voluntary nervous system
- -Conscious control of skeletal muscles
Motor Division of PNS: Autonomic Nervous System
Visceral motor nerve fibers Smooth muscle, cardiac muscle, and glands Involuntary nervous systems Two functional Subdivisions -Sympathetic -Parasympathetic
Histology of Nervous Tissue
Higher cellular: little extracellular space
-tightly packed
Two principle cell types
-Neuroglia- Small cell that wraps delicate neurons
-Neurons (nerve cells)- Nerve cells, functional unit
Histology of Nervous Tissue: Neuroglia
Astrocytes (CNS) Microglial Cells (CNS) Satellite Cells (PNS) Schwann Cells (PNS)
Astrocytes
Most abundant, versatile, and highly branched glial cells
Cling to neurons, synaptic endings, and capillaries
Astrocytes Function
- Support and brace neurons
- Play role in exchanges
- Guide migration of young neurons
- Control chemical environment around neurons
- Respond to nerve impulses and neurotransmitters
- Influence neuronal functioning
Microglial Cells
- Small, ovoid cells with thorny processes that touch and monitor neurons
- Migrates toward injured neurons
- Can transform to phagocytize microorganisms and neuronal debris
Satellite Cells
Surround neuron cell bodies in PNS
Function to similar to astrocytes
Schwann Cells
Surround all peripheral nerve fibers and form myelin sheaths in thicker nerve fibers
Regeneration
Neurons
Definition -Structural unit of nervous system Function -Conduct impulses Extreme longevity -100 years or more Amitotic- with few exceptions High metabolic rate All have cell body and one or more processes
Neuron Cell Body (Soma)
Center of neuron
-Synthesizes proteins, membranes, and other chemicals
Spherical nucleus with nucleolus
Most neuron cell bodies in CNS
-Nuclei- clusters of neuron cell bodies in CNS
Ganglia- lie along nerves in PNS
-Most common in Spinal Cord
Neuron Processes
Armlike processes extend from body Tracts -Bundles of neuron processes in CNS Nerves -Bundles of neuron processes in PNS Two types of Processes -Dendrites -Axons
Dendrites
In motor neurons
-Hundreds of short, tapering, diffusely branched processes
Receptive (input) region of neuron
-Convey incoming messages toward cell body as graded potentials (short distance signals)
Axon: Structure
One axon per cell -In some axon short or absent -In other most of length of cell -Some 1 meter long Long axons called nerve fibers Branches profusely at end (terminus) Distal endings call axon terminals
Axon: Functional Characteristics
Conducting region of neuron
Generates nerve impulses
Transmits the Axon Terminal
-Secretory region
-Neurotransmitters released into extracellular space
Carries on many conversations with different neurons at same time
Lacks rough ER and Golgi Apparatus
-Relies on cell body to renew proteins and membrane
-Efficient transport mechanisms
-Quickly decay if cut or damaged
Transport Along the Axon
Molecules and organelles are moved along axons
- Anterograde
- Retrograde
Anterograde
Away from cell body Examples -Mitochondria -Cytoskeleton elements -Membrane components -Enzymes
Retrograde
Toward body cell Examples -Organelles to be degraded -Signal molecules -Viruses - Bacterial toxins
Myelin Sheath
Composed of myelin
-Whitish, protein-lipoid substance
Segmented sheath around most long or large-diameter axons
-Myelinated Fibers
Nonnyelinated fibers conduct impulses more slowly
2 Functions of Myelin
Protects and electrically insulates axon
Increases speed of nerve impulses transmission
Myelin in PNS
Formed by Schwann Cells
-Jelly roll
-One cell forms one segment of myelin sheath
Myelin Sheath
-Concentric layers of Schwann Cells around axon
Myelination in PNS
Myelin Sheath gaps
-Gaps between adjacent Schwann cells
-Sites where axon collaterals can emerge
Myelin sheath gaps between adjacent Schwann Cells
-Sites where axon collaterals can emerge
Nonmyelinated fibers
Myelination in CNS
Can wrap up to 60 axons at once
Myelin Sheath gap is present
No outer collar of perinuclear cytoplasm
Thinnest fibers are unmyelinated
White Matter
-Regions of brain and spinal cord with dense collections of myelinated fibers
-Usually fiber tracts
Gray Matter
-Mostly neuron cell bodies and nonmyelinated fibers
Structural Classification of Neurons
Multipolar- 3 or more processes
- 1 axon, other dendrites
- Most common: major neuron in CNS
Functional Classification of Neurons
Grouped by direction in which nerve impulse travels relative to CNS 3 Types -Sensory (Afferent) -Motor (Efferent) -Interneurons
Functional Classifications of Neurons: Sensory
- Transmit impulses from sensory receptors toward CNS
- Cell bodies in ganglia in PNS
Functional Classifications of Neurons: Motor
- Carry impulses from CNS to effectors
- Most cell bodies in CNS
Functional Classifications of Neurons: Interneurons
- Lie between motor and sensory neurons
- Shuttle signals through CNS pathways
- 99% of body’s neurons
- Most confined in CNS
Membrane Potential
- Excitability
- Respond to adequate stimulus by generating an action potential
- -Action Potential- Nerve Impulse
- Impulse is the same as each neuron
Voltage
A measure of potential energy generated by separated charge
- Volts (V) or Milivolts (mV)
- Called Potential Difference
- Greater charge difference between points = higher voltage
Current
Flow of electrical charge (ions) between two points
Resistance
Hindrance to charge flow
- Insulator
- Conductor
Ohm’s Law
The relationship of voltage, current, resistance
Current= voltage/resistance
-Current is directly proportional to voltage
-Current inversely related to resistance
-No net current flow between points with same potential
Role of Membrane Ion Channels
Large proteins: Ion channels
Two main types of ion channels
-Leakage (nongated) Channels
-Gated Channels
Gated Channels 3 Types
Chemically Gated Channels -Neurotransmitter Voltage-Gated Channels -Potentials Mechanically Gated Channels -Physical
Gated Channels
When open
- Ions diffuse quickly across membrane along electrochemical gradients
- -Chemical Gradients
- -Electric Gradients
The Resting Membrane Potential
Potential difference across membrane of resting cell
-Approximately -70mV in neurons
-Cytoplasmic side of membrane negatively charged relative to outside
-polarized
Generated by
-Differences in ionic makeup of ICF and ECF
ECF
Outside Neuron
Higher concentration of Na+
Balanced chiefly by chloride ions (Cl-)
ICF
Inside Neuron
Higher concentration of K+
Balanced by negatively charged proteins
K+
Plays most important role in membrane potential
Differences in Plasma Membrane Permeability
Impermeability
Slightly permeable to Na+
-through leakage channels
25 times more permeable to K+ than sodium
-more leakage channels
-potassium diffuses out of cell down concentration gradient
Very permeable to Cl-
Resting Membrane Potential
More potassium diffuses out than sodium diffuses in
-Cell more negative inside
-Establishes resting membrane potential
Sodium-Potassium pump stabilizes resting membrane potential
-Maintains concentration gradients for Na+ and K+
-3 Na+ pumped out of cell; 2 K+ pumped in
Membrane Potential Changes: Used as Communication Signals
Membrance potential changes when -Concentration of ions across membrane change -Membrane permeability to ions change Changes produce 2 types signal -Graded Potentials -Action Potentials
Graded Potentials
Incoming signals operating over short distances
Short-lived, localized changes in membrane potential
-Magnitude varies with stimulus strength
-Stronger stimulus- more voltage changes; farther current flows
Either depolarization or hyperpolarization
Current flows but dissipates quickly and decays
-Graded potentials are signals only over short distances
Action Potentials
Long-distance signals of axons
Principle way neurons send signals
Means of long-distance neural communication
Occur only in muscle cells and axons of neurons
Do not decay over distance as graded potentials do
Properties of Gated Channels: K
Each K+ channel has one voltage-sensitive gate
Closed at rest
Opens slowly with depolarization
Generation of an Action Potential: Resting State
All gated Na+ and K+ channels are closed
Only leakage channels for Na+ and K+ are open
-This maintains the resting membrane potential
Generation of an Action Potential: Depolarizing Phase
Depolarizing local currents open voltage-gated Na+ channels
-Na+ rushes into cell
Na+ influx causes more depolarization which opens more Na+ channels
Positive feedback causes opening of all Na+ channels- a reversal of membrane polarity to +30mV
-Spike of action potential
Generation of an Action Potential: Repolarizing Phase
Na+ channel gates close
Membrane permeability to Na+ declines to resting state
-Action Potential spike stops rising
Slow voltage-gated K+ channels open
-K+ exits the cell and internal negativity is restored
Role of Sodium-Potassium Pump (Na+/K+)
Repolarization resets electrical conditions
After repolarization Na+/K+ pumps (thousands of them in an axon) restore ionic conditions
Threshold
Not all depolarization events produce action potentials
For axon to “fire”, depolarization must reach threshold
-That voltage at which the action potential is triggered
At Threshold:
-Na+ permeability increases
-Na+ influx exceeds K+ efflux
-The positive feedback cycle begins
The All-or-None Phenomenon
An action potential either happens completely, or it does not happen at all
Coding for Stimulus Intensity
- All action potentials are alike and are independent of stimulus intensity
- Strong stimuli cause action potentials to occur more frequently
- Higher frequency means stronger stimulus
Absolute refractory Period
When voltage-gated Na+ channels open neurons cannot respond to another stimulus
- Time from opening of Na+ channels until resetting of the channels
- Ensures that each action potential is an all-or-none event
- Enforces one-way transmission of nerve impulses
Relative Refractory Period
Follows absolute refractory period
-Most Na+ channels have returned to their resting state
-Some K+ channels are still open
-Repolarization is occurring
Threshold for action potential generation is elevated
-Inside of membrane more negative than resting state
Only exceptionally strong stimulus could stimulate an action potential
Conduction Velocity
Rate of action potential propagation depends on
- Axon diameter (Large vs. Small)
- Degree of Myelination (Contains vs. nonmyelin)
Conduction Velocity: Effects of Myelination
Insulate and prevent leakage of charge
Saltatory conduction (possible only in myelinaed axons) is about 30 times faster
-Voltage-gated Na+ channels are located at myelin sheath gaps
-Action potential generated only at gaps
-Electrical signal appears to jump rapidly from gap to gap
Nerve Fiber Classification
- Diameter
- Degree of myelination
- Speed of conduction
Multiple Sclerosis (MS)
Autoimmune disease affecting primarily young adults
Myelin sheaths in CNS destroyed
-Immune system attacks myelin
-Impulse conduction slows and eventually ceases
-Demyelinated axons increase Na+ channels
Symptoms
-Visual disturbances
-Weakness
-Loss of muscular control
-Speech disturbance
-Urinary Incontinence
Treatment
-Drugs that modify immune system’s activity improve lives
Synapse
Nervous system works because information flows from neuron to neuron
Neurons functionally connected by synapses
-Junctions that mediate information transfer
–one neuron to another neuron
–one neuron to an effector cell
Presynaptic Neuron
- Neuron conducting impulses toward synapse
- Sends information
Postsynaptic Neuron
Neuron, Muscles cell, or Gland cell
-Neuron transmitting electrical signal away from synapse
-Receives the information
Most function as both
Chemical Synapses
Most Common
Specialized for release and reception of chemical neurotransmitters
Two Parts
Electrical impulse changed to chemical across synapse, then back into electrical
Two Parts of Neurotransmitter
Axon terminal of presynaptic neuron
-Contains synaptic vesicles filled with neurotransmitter
Neurotransmitter receptor region on postsynaptic neuron’s membrane
-Usually on dendrite or cell body
Synaptic Cleft
30-50 nm wide
Prevents nerve impulses from directly passing from one neuron to next
Transmission Across Synaptic Cleft
- Chemical event
- Depends on release, diffusion, and receptor binding of neurotransmitter
- Ensures unidirectional communication between neurons
Information Transfer Across Chemical Synapses
Watch Video
Action Potential arrives at axon terminal of presynaptic neuron
Causes voltage-gated Ca2+ channels to open
-Ca2+ floods into cell
Fusion of synaptic vesicles with axon membrane
Exocytosis of neurotransmitter into synaptic cleft occurs
-Higher impulse frequency- more released
Neurotransmitter diffuses across synapse
Binds to receptors on postsynaptic neuron
-Often chemically gated ion channels
Ion channels are opened
Causes an excitatory or inhibitory event
Neurotransmitter effects terminated
Termination of Neurotransmitter Effects
Within a few milliseconds neurotransmitter effect terminated in one of three ways
- Re-uptake
- Degradation
- Diffusion
Termination of Neurotransmitter Effects: Re-uptake
By astrocytes or axon terminal
Termination of Neurotransmitter Effects: Degradation
By enzymes
Termination of Neurotransmitter Effects: Diffusion
Away from synaptic cleft
Synaptic Delay
Time needed for neurotransmitter to be released, diffuse across synapse, and bind to receptors
-0.3-5.0 milliseconds
Postsynaptic Potentials
Neurotransmitter receptors cause graded potentials that vary in strength with
- Amount of neurotransmitter released
- Time neurotransmitter stays in area
Types if Postsynaptic Potentials
EPSP
IPSP
Not Action Potentials
EPSP
Excitatory Postsynaptic Potentials
IPSP
Inhibitory Postsynaptic Potentials
Excitatory Synapses and EPSPs
Neurotransmitter binding opens chemically gated channels
-Allows simultaneous flow of Na+ and K+ in opposite directions
Na+ influx greater than K+ efflux - net depolarization called EPSP not AP
EPSP help trigger AP if EPSP is of threshold strength
-Can trigger opening of voltage gated channels, and cause AP to be generated
Inhibitory Synapses and IPSPs
Reduces postsynaptic neuron’s ability to produce an action potential
-Makes membrane more permeable to K+ or Cl-
–If K+ channels open, it moves out of cell
–If Cl- channels open, it more into cell
Neurotransmitter hyperpolarizes cell
-Inner surface of membrane becomes more negative
-Action potential less likely to be generated
Synaptic Integration: Summation
A single EPSP cannot induce an action potential
EPSPs can summate to influence postsynaptic neuron
IPSPs can also summate
Most neurons receive both excitatory and inhibitory inputs from thousands of other neurons
-Only if EPSP’s predominate and bring to threshold- Action Potential
Integration: Synaptic Potentiation
Repeated use of synapse increases ability of presynaptic cell to excite postsynaptic neuron
-Ca2+ concentration increases in presynaptic terminal and postsynaptic neuron
Neurotransmitters
Language of nervous system
50 or more neurotransmitters have been identified
Most neurons make two or more neurotransmitter
-Neurons can exert several influences
Released at different stimulation frequencies
Classification of Neurotransmitters: Function
Great diversity of functions
Classified by
-Effects- excitatory vs. inhibitory
-Actions- direct vs. indirect
Effects
Neurotransmitter effects can be excitatory (depolarizing) and/or inhibitory (hyperpolarizing)
Effect determined by receptor to which it binds
Direct Action
Neurotransmitter binds to and opens ion channels
Promotes rapid responses by altering membrane potential
Indirect Action
Neurotransmitter acts through intracellular second messenger
Broader, longer-lasting effects similar to hormones
Channel Linked Receptors
Mediate fast synaptic transmission
Channel-Linked (Ionotropic) Receptors: Mechanism of Action
Ligand-gated ion channels
Action is immediate and brief
Basic Concepts of Neural Integration
Neurons function in groups
There are billions of neurons in CNS
-Must be integration so the individual parts fuse to make a smoothly operating whole
Organization of Neurons: Neuronal Pools
Functional groups of neurons
- Integrate incoming information
- Forward processed information to other destinations
Circuits
Patterns of synaptic connections in neuronal pools
4 Types of Circuits (Look at Diagrams)
Diverging
Converging
Reverberating
Parallel after Discharge
Diverging Circuit (Look at Diagrams)
One input, many outputs
An amplifying circuit
Converging Circuit (Look at Diagrams)
Many inputs, one output
A concentrating citcuit
Reverberating Circuit (Look at Diagrams)
Signal travels through a chain of neurons, each feeding back to previous neurons
An oscillating circuit
Controls rhythmic activity
Parallel after Discharge Circuit (Look at Diagrams)
Signal Stimulates neurons arranged in parallel arrays that eventually converge on a single output cell
Impulses reach output cell at different times, causing a burst of impulses call an after-discharge
Patterns of Neural Processing: Serial Processing
Input travels along one pathway to specific destination
System works in all-or-none manner to produce specific, anticipated response
Spinal Reflexes
Rapid, automatic responses to stimuli Particular stimulus always causes same response Occur over pathways called reflex arcs: 5 parts -Receptor -Sensory Neuron -CNS Integration Center -Motor Neuron -Effector
Patterns of Neural Processing: Parallel Processing
Input travels along several pathways
Different parts of circuitry deal simultaneously with the information
-One stimulus promotes numerous responses
Important for higher-level mental functioning
Developmental Aspects of Neurons
Nervous system originates from neural tube and neural crest formed from ectoderm
The neural tube becomes CNS
Cell Death
About 2/3 of neurons die before birth
- If do not form synapse with target
- Many cells also die due to apoptosis (programmed cell death) during development
