ch. 12 nervous tissue. overview of the nervous system objectives list the structures and basic...
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CH. 12
Nervous Tissue
Overview of the Nervous System
Objectives List the structures and basic functions of the nervous
system. Describe the organization of the nervous system.
Structures
Nervous system Smallest and most
complex system Billions of neurons Includes:
Brain Cranial nerves Nerves – bundle of axons Spinal cord – connects to
brain Spinal nerves
Functions
Sensory – detect internal and external stimuli Sensory neurons carry information to brain
Integrative – analyzes and stores information; makes appropriate responses Perception – conscious awareness of sensory stimuli Interneurons – participate in integration
Function
Motor – causes movement or gland secretion in response to stimuli Motor neurons – carry information to muscle or gland
(effectors)
CNS – Central NS
PNS – Peripheral NS
Contains brain and spinal cord (axial)
Processes sensory info
Source of thoughts, emotions, memories
All tissue outside the CNS (appendicular)
Includes cranial nerves, branches, spinal nerves, sensory receptors
Divided further SNS – somatic NS (body) ANS – autonomic NS (self) ENS – enteric NS
(intestines)
Nervous System
Nervous System
Peripheral NS Branches
Somatic NS – voluntary Consists of:
Sensory neurons in head, body, limbs, vision, hearing, taste, and smell that send messages to CNS
Motor neurons that conduct messages from CNS to skeletal muscles only
Autonomic NS – involuntary Consists of:
Sensory neurons in organs that send messages to CNS Motor neurons that conduct messages from CNS to
smooth muscle, cardiac muscle, and glands
Sympathetic divisionParasympathetic division
Supports exercise or emergency actions
“fight or flight”Ex: increases heart
rate
“rest and digest” Ex: decreases heart
rate
Usually work in opposing actions
ANS Branches
ENS
“brain of the gut”Controls all activities associated with
digestion and the gastrointestinal (GI) tractInvoluntary
Overall Organization
Organization
Review
What are the components of the CNS and PNS?
What kinds of problems would result from damage of sensory neurons, interneurons, and motor neurons?
What are the components and functions of the SNS, ANS, and ENS?
Which subdivisions of the PNS control voluntary actions? Involuntary actions?
Histology of Nervous Tissue
Objectives Contrast the histological characteristics and the
functions of neurons. Contrast the functions of neuroglia.
Neurons Neuroglia
Provide unique functions
Sensing, thinking, remembering, controlling muscle activity, regulating glandular secretions
Support, nourish, and protect the neurons
Maintain homeostasis in the interstitial fluid that bathes them
Neurons vs. Neuroglia
Neurons
Vocabulary: Neuron – nerve cell Electrical excitability
the ability to respond to a stimulus and convert it into an action potential
Stimulus any change in the environment that is strong enough to
initiate an action potential Action potential – nerve impulse
An electrical signal that propogates (travels) along the surface of the membrane of a neuron
Can travel up to 280 mph
Parts of a Neuron
Three parts Cell body
Main part of the cell Includes organelles, nucleus, and cytoplasm
Dendrites Receiving parts of the neuron Short, tapered, and highly branched
Axon Transmitting parts of the neuron Long, thin, cylindrical
Parts of a Neuron
Parts of a Neuron
Synapse – site of communication between 2 neurons or a neuron and an effector cell
Synaptic end bulb – swollen end of an axon where synaptic vesicles hold neurotransmitters
Neural Diversity
Multipolar neurons Several dendrites, one axon Found in brain and spinal cord
Bipolar neurons One main dendrite, one axon Eye, ear, olfactory of brain
Unipolar neurons Axon and dendrite fuse at beginning and then branch Occurs as an embryo
Neural Diversity
Others
Purkinje cells – cerebellum
Pyramidal cells – cerebral cortex of brain
Neuroglia
Actively participate in nervous tissue functioning
Do not generate action potentials
Can multiply and divide – neurons cannot
Types of Neuroglia
CNS Astrocytes – create blood-brain barrier, strength Oligodendrocytes – create myelin sheath around CNS
axons Microglia – remove cellular debris during neural
development Ependymal cells – assist with circulation of cerebrospinal
fluidPNS
Schwann cells – create myelin sheath around PNS axons Satellite cells – support, regulate exchange of materials
Types of Neuroglia
Types of Neuroglia
Myelination
Myelin sheath – multilayered lipid and protein covering around some axons
Provides insulationIncreases speed of nerve impulseIf a cell has myelin we say that it is
myelinatedGaps in the myelin sheath are called nodes of
Ranvier
Review
Describe the parts of a neuron and the functions of each.
Give examples of the structural diversity of neurons.
Give examples of the different types of neuroglia. Where are each found? What do they do?
What is myelin?
Electrical Signals in Neurons
Objectives Describe the cellular properties that permit
communication among neurons and effectors. Compare the basic type of ion channels, and explain
how they relate to action potentials and graded potentials.
Describe the factors that maintain a resting membrane potential.
Mini-Physics Lesson
Potential energy – energy stored in a system (the body) as a result of its position or chemical composition
Kinetic energy – energy being used for motion or force
Physics to Anatomy
Neurons are excitable because of a voltage difference across the membrane - potential
“Potential” will initiate an impulse that can travel through the nervous system
Graded potentials – used for short-distance communication
Action potentials – allow communication over short and long distance within the body
Potential
Membrane potential – electrical voltage difference across the membrane
Resting membrane potential – membrane potential in an excitable cell Like voltage stored in a battery If + and – ends connect, electrons flow in a current
Current Flow of charged particles For the body – these are ions instead of elecrons
Ion Channels
Gradient – differenceElectrochemical gradient – difference in
charge and concentrationPositive cations move toward negative areas,
negative anions move toward positive areasIon location can be controlled with gates that
can open or close the pore
Ion Channels
Leakage channelsVoltage-gated channelsLigand-gated channelsMechanically gated channels
Ion Channels
Leakage channels Randomly open and close most cells leak more potassium (K+) than sodium
(Na+) voltage-gated channels
Opens in response to a change in voltage (membrane potential)
Generate and conduct action potentials
Ion Channels
Ligand-gated channels Opens and closes in response to chemical stimulus
(nts, hormones, other ions – ligand molecule) Ex: Ach opens channels that allow Na+ and Ca2+ to
go in and K+ to go out Work in 2 ways
ligand molecule can open or close the channel itself by binding
Ligand molecule activates another chemical messenger to open the channel
Ion Chanenls
Mechanically gated Opens or closes in response to stimulation by:
Vibration: sound waves Pressure: touch Tissue stretching
The channel is physically disrupted and opens
Ion Channels
Resting Membrane Potential
Exists due to negative ions in cytosol (ICF) and equal positive ions in ECF
The greater the difference in charge, the larger the potential
Example: 5 Na+ on outside, 4 Cl- on inside – small potential 25 Na+ on outside, 4 Cl- on insdie – great potential
RMP
Most cells have potential between -40 to -90 mV; typical is -70mV
Minus sign means the cell is negative – not negative potential!
Any cell with potential is polarized The potential varies between +5 to -100 mV
How does the potential get there?
ECF Rich in Na+ and Cl-
ICF Rich in K+ Also has P-, amino acids,
RMP
Ion interaction There are many K+ leakage channels, so K+ diffuse
out + ions exit, inside becomes more negative - ions can’t leave because they are bound to molecules - charges attract the K+ back in toward the cell Few Na+ ions leak inward This would destroy the membrane potential, so there
are pumps that take care of this
Na+/K+ pump
To keep the RMP Pump out Na+ as it leaks in Pump in K+ as it leaks out 3 Na+ for each 2 K+ - this still maintains a negative
charge in the cell
RMP - Draw
Review
Define resting membrane potential.Describe each type of ion channel.Describe the cellular processes that create
the resting membrane potential.
Potentials
Objectives Describe what causes a graded potential. Understand the process for creating an action
potential. Explain depolarization and repolarization, including
the relationship between them.
Graded Potentials
When a stimulus causes a channel to open or close in an excitable cell
Causes more polarization (more - inside)Causes less polarization (less – inside)Hyperpolarized (much more – inside)Depolarized (much less – inside)Graded signals – vary in size
Spread out along plasma membrane and die out Only used for short distance communication
Generation of Action Potentials
Action potential (AP) or impulse – sequence of rapidly occurring events that take place in two phases Depolarizing phase – negative membrane potential
becomes Less negative Reaches 0 positive
Repolarizing phase – membrane potential is Restored to the resting state of -70 mV
Action Potential
Depol. vs. Repol.
Na+ channels openNa+ rushes into cell
All or none principle when depolarization reaches the threshold, the AP
occurs AP is always the same size Like dominos – no matter how hard you push the first
domino they will all fall or they won’t – same goes for the AP
K+ channels openK+ flows out of cell
Depolarizing Phase
Na+ channels open rapidly – Na+ rushes into the cell
Potential goes from -55 to +30 (inside is 30 more positive than the outside)
The more Na+ there is, the more channels open – this is a ____________-feedback mechanism
Repolarizing Phase
K+ channels open as the Na+ channels are closing
Na+ inflow slows, K+ outflow increasesPotential goes from +30 to -70
Depol. and Repol.
Painkillers
Local anesthetics – block pain and other body sensations
Novacaine or LidocaineBlock the opening of Na+ channelsThe nerve impulse can’t go past the blocked
region – CNS never gets the message that there is pain
Review
What causes a graded potential?Describe what happens to cause an action
potential.What happens during depolarization? What
does this cause?What happens during repolarization? What
does this cause?
Signal Transmission Synapses
Objectives Explain the events of signal transmission at a
chemical synapse.
Synaptic vocabulary
Presynaptic neuron – the one sending the signal
Postsynaptic neuron – the one receiving the signal
3 types Axodendritic – axon to dendrite Axosomatic – axon to body cell Axoaxonic – axon to axon
Electrical Synapse
Action potentials conduct between cells through gap junctions Tunnels connect the two cells together and the
electrical impulse can pass places:
Smooth muscle, cardiac muscle, developing embryo advantages:
Faster – don’t have to “jump the gap” Synchronization – allows groups of cell’s to work
together – heart beating
Electrical Synapse
Chemical Synapse
Pre- and post-synapses do not touch – separated by synaptic cleft
Transfer of signal must occur Pre-synaptic neuron turns electrical signal into
chemical signal (neurotransmitter) Post-synaptic neuron turns chemical signal into
electrical signal this transfer takes more time than electrical
signals
Chemical Synapse
1. Nerve impulse arrives2. Depolarizing phase
opens Ca++ channels, Ca++ flows in
3. Ca++ causes release of NT from vesicles
4. NT binds to receptors on postsynaptic neuron
5. Action potential continues (Na+, K+, Cl-)
Review
What are the differences between a chemical and electrical synapse?
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