Transcript Document

BIOL 2401
Fundamentals of Anatomy and Physiology
Mrs. Willie Grant
[email protected]
(210) 486-2780
© 2012 Pearson Education, Inc.
© 2012 Pearson Education, Inc.
An Introduction to the Nervous System
Learning Outcomes
12-1 Describe the anatomical and functional divisions of the nervous system.
12-2 Sketch and label the structure of a typical neuron, describe the functions of
each component, and classify neurons on the basis of their structure and
function.
12-3 Describe the locations and functions of the various types of neuroglia.
12-4 Explain how the resting potential is created and maintained.
12-5 Describe the events involved in the generation and propagation of an action
potential.
12-6 Discuss the factors that affect the speed with which action potentials are
propagated.
12-7 Describe the structure of a synapse, and explain the mechanism involved in
synaptic activity.
12-8 Describe the major types of neurotransmitters and neuromodulators, and
discuss their effects on postsynaptic membranes.
12-9 Discuss the interactions that enable information processing to occur in neural
tissue.
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An Introduction to the Nervous System
The Nervous System
Includes all neural tissue in the body
Neural tissue contains two kinds of cells
Neurons
Cells that send and receive signals
Neuroglia (glial cells)
Cells that support and protect neurons
Organs of the Nervous System
Brain and spinal cord
Sensory receptors of sense organs (eyes, ears, etc.)
Nerves connect nervous system with other systems
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12-1 Divisions of the Nervous System
The Central Nervous System (CNS)
Consists of the spinal cord and brain
Contains neural tissue, connective tissues, and blood vessels
Functions of the CNS are to process and coordinate:
Sensory data from inside and outside body
▪ Motor commands control
activities of peripheral organs (e.g., skeletal muscles ▪ Higher functions of
brain intelligence, memory, learning, emotion
The Peripheral Nervous System (PNS)
Nerves (also called peripheral nerves)
Bundles of axons with connective tissues and blood vessels ▪ Carry sensory
information and motor commands in PNS
Cranial nerves — connect to brain ▪
Spinal nerves — attach to spinal cord
Includes all neural tissue outside the CNS
Functions of the PNS
Deliver sensory information to the CNS ▪
peripheral tissues and systems
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Carry motor commands to
12-1 Divisions of the Nervous System
Functional Divisions of the PNS
Afferent division
Carries sensory information from PNS sensosry receptors to CNS
Efferent division
Carries motor commands from CNS to PNS muscles and glands
Receptors and effectors of afferent division
Receptors
Detect changes or respond to stimuli
Neurons and specialized cells
Complex sensory organs (e.g., eyes, ears)
Effectors
Respond to efferent signals
Cells and organs
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12-1 Divisions of the Nervous System
Functional Divisions of the PNS
The efferent division
Somatic nervous system (SNS)
Controls voluntary and involuntary (reflexes) muscle skeletal
contractions
Autonomic nervous system (ANS)
Controls subconscious actions, contractions of smooth muscle
and cardiac muscle, and glandular secretions
Sympathetic division has a stimulating effect
Parasympathetic division has a relaxing effect
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12-2 Neurons
Neurons
The basic functional units of the nervous system
The structure of neurons: Multipolar neuron common in the CNS has a cell body (soma),
short, branched dendrites, and a long, single axon.
The Cell Body
Large nucleus and nucleolus
Perikaryon (cytoplasm)
Mitochondria (produce energy)
RER and ribosomes (produce neurotransmitters)
Cytoskeleton
Neurofilaments and neurotubules in place of microfilaments and microtubules
Neurofibrils: bundles of neurofilaments that provide support for dendrites and axon
Nissl bodies
Dense areas of RER and ribosomes
Make neural tissue appear gray (gray matter)
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12-2 Neurons
Dendrites
Highly branched
Many fine processes (dendritic spines)
Receive information from other neurons
80–90% of neuron surface area
The axon
Is long and carries electrical signal (action potential) to target
Axon structure is critical to function
Structures of the Axon
Axoplasm
Cytoplasm of axon (Contains neurofibrils, neurotubules, enzymes,
organelles)
Axolemma—specialized cell membrane that covers the axoplasm
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cell membrane that covers the axoplasm
12-2 Neurons
Structures of the Axon
Axon hillock
Thick section of cell body that attaches to initial segment
Initial segment
Attaches to axon hillock
Collaterals
Branches of a single axon
Telodendria
Fine extensions of distal axon
Synaptic terminals
Tips of telodendria
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1 What roles do the dendrites, cell body, and axon play in communication of signals?
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12-2 Neurons
The Structure of Neurons
The synapse—area where a neuron communicates with another cell
The synapse
Presynaptic cell (neuron that sends message)
Postsynaptic cell (cell that receives message)
The synaptic cleft (small gap that separates the presynaptic
membrane and the postsynaptic)
Types of Synapses
Neuromuscular junction
Synapse between neuron and muscle
Neuroglandular junction
Synapse between neuron and gland
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Figure 12-2 The Structure of a Typical Synapse
Telodendrion
Synaptic terminal
Endoplasmic
reticulum
Mitochondrion
Synaptic
vesicles
Presynaptic
Cell—sends a
message
--Contains the neurotransmitter
Presynaptic
Membrane
Postsynaptic Cell—
receives the message
Postsynaptic
membrane
Synaptic
the
cleft --separates
two cells
.
Neurotransmitter—chemical (Ach)
http://www.youtube.com/watch?v=HXx9qlJetSU
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Figure 12-3 A Structural Classification of Neurons
Anaxonic neuron
Bipolar neuron
Unipolar neuron
Multipolar neuron
Dendrites
Dendrites
Dendritic
branches
Initial
segment
Cell
body
Axon
Dendrite
Cell body
Cell
body
Axon
Cell
body
Synaptic
terminals
Axon
Axon
Synaptic
terminals
Synaptic
terminals
2 Which type of neuron is the most abundant type of neuron in the CNS?
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12-2 Neurons
Functions of Sensory Neurons
Monitor internal environment (visceral sensory neurons)
Monitor effects of external environment (somatic sensory neurons)
Three Types of Sensory Receptors
Interoceptors
Monitor internal systems (digestive, respiratory, cardiovascular,
urinary, reproductive)
Internal senses (taste, deep pressure, pain)
Exteroceptors
External senses (touch, temperature, pressure)
Distance senses (sight, smell, hearing)
Proprioceptors
Monitor position and movement (skeletal muscles and joints)
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12-2 Neurons
Motor Neurons
Two major efferent systems
Somatic nervous system (SNS)
Includes all somatic motor neurons that innervate skeletal muscles
Autonomic (visceral) nervous system (ANS)
Visceral motor neurons innervate all other peripheral effectors
Smooth muscle, cardiac muscle, glands, adipose tissue
Two groups of efferent axons
Signals from CNS motor neurons to visceral effectors pass synapses at autonomic
ganglia dividing axons into:
Preganglionic fibers
Postganglionic fibers
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12-2 Neurons
Interneurons
Most are located in brain, spinal cord, and autonomic ganglia
Between sensory and motor neurons
Are responsible for:
Distribution of sensory information
Coordination of motor activity
Are involved in higher functions
Memory, planning, learning
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© 2012 Pearson Education, Inc.
© 2012 Pearson Education, Inc.
© 2012 Pearson Education, Inc.
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Figure 12-6a Schwann Cells and Peripheral Axons
Axon hillock
Nucleus
Axon
Myelinated
internode
Initial
segment
(unmyelinated)
Nodes
Schwann
cell nucleus
Axon
Neurilemma
Axon
Myelin
covering
internode
Axolemma
A myelinated axon, showing the
organization of Schwann cells along
the length of the axon. Also shown
are stages in the formation of a
myelin sheath by a single Schwann
cell along a portion of a single axon.
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Dendrite
12-3 Neuroglia
Neural Responses to Injuries (response is limited)
Wallerian degeneration (Process of repairing damaged nerves)
Axon distal to injury degenerates
Schwann cells form path for new growth and wrap new axon in myelin
Nerve Regeneration in CNS (limited)
Limited by chemicals released by astrocytes that block growth and
produce scar tissue
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© 2012 Pearson Education, Inc.
12-4 Transmembrane Potential
Ion Movements and Electrical Signals
All plasma (cell) membranes produce electrical signals by ion movements
Transmembrane potential is particularly important to neurons
Three important concepts
The extracellular fluid (ECF) and intracellular fluid (cytosol) differ
greatly in ionic composition
Concentration gradient of ions (Na+, K+)
Cells have selectively permeable membranes
Membrane permeability varies by ion
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12-4 Transmembrane Potential
Five Main Membrane Processes in Neural Activities
Resting potential is the transmembrane potential of a resting cell
Graded potential is a temporary localized change in the resting potential caused by a
stimulus
Action potential is an electrical impulse produced by a graded potential that spreads
along the surface of an axon to synapse.
Synaptic activity releases neurotransmitters at presynaptic membrane and produces
graded potentials in postsynaptic membrane
Information processing is the response (integration of stimuli) of the postsynaptic cell
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Resting Membrane Potential
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Electrochemical Gradients for Potassium and Sodium Ions
The electrochemical gradient for a specific ion is the sum of the chemical
and electrical forces acting on the ion across the plasma membrane.
The electrochemical gradients for K+ and Na+ are the primary factors
affecting the resting membrane potential of most cells.
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Gated Channels
Membrane channels control the movement of ions across the plasma membrane.
There are Passive channels or leak channels (always open).
There are Active channels or gated channels (open and close in response to specific
stimuli.
Gated channels can be chemically gated or ligand-gated channels.
Gated channels can be voltage-gated channels.
Gated channels can be mechanically gated channels.
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Graded Potentials
Graded potentials, or local potentials are changes in the membrane potential that cannot spread
Far from the site of stimulation.
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12-4 Transmembrane Potential
Graded Potentials (local potentials)
Changes in transmembrane potential that cannot spread far from site of stimulation—local current.
The resting state
Opening sodium channel produces graded potential when resting membrane is exposed to chemical
Sodium channel opens/Sodium ions enter the cell/Transmembrane potential rises
Depolarization is a shift in the transmembrane potential toward 0mV, toward a more
positive potential.
Repolarization is the restoration to the normal resting potential after depolarization.
Hyperolarization is an increase in the negativity of the resting potential
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Figure 12-13 Depolarization, Repolarization, and Hyperpolarization
Chemical
stimulus
applied
Chemical
stimulus
removed
Transmembrane
potential (mV)
Repolarization
Resting potential
Depolarization
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Chemical Chemical
stimulus stimulus
applied removed
Hyperpolarization
Return to
resting potential
12-5 Action Potential
Action Potentials (Propagated changes in transmembrane, that once initiated, affect the
excitable membrane. These electrical events are known as nerve impulses.
The membrane potential at which an action potential begins is called the
threshold. This is between -60 mV and -55 mV.
The All-or-None Principle
This concept says that because a given stimulus either triggers a typical
action potential, or none at all. The all-or-none principle applies to all
excitable membranes.
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At the Resting Potential—the activation gates of the voltage-gated sodium channels are closed.
At the Refractory Period—the membrane does not respond to additional depolarizing stimuli from the time
an action potential begins until the normal resting membrane potential has stabilized.
At the Absolute Refractory Period—the first part of the refractory period
At the Relative Refractory Period—at the point when the sodium channels regain their normal resting condition,
and continues until the membrane potential stabilizes at resting levels.
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Powering the Sodium–Potassium Exchange Pump
To maintain concentration gradients of Na+ and K+ over time
Requires energy (1 ATP for each 2 K+/3 Na+ exchange)
Without ATP
Neurons stop functioning
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12-5 Action Potential
Propagation of Action Potentials
Propagation—movement of action potentials generated in axon
hillock along entire length of axon. It is either continuous or
saltatory.
Continuous propagation along an unmyelinated axons that
affects one segment of axon at a time.
In an unmyelinated axon, an action potential movesl along by continuous propagation. The action potential
Spreads by depolarizing the adjacent region of the axon membrane. The process continues to spread as a
Chain reaction down the axon.
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12-5 Action Potential
Saltatory Propagation along a myelinated axon. It is faster and uses less
energy than a continuous propagation
Myelin insulates axon, prevents continuous propagation
Local current “jumps” from node to node
Because myelin limits the movement of ions across the axon membrane, the action potential
Must “jump” from node to node during propagation. This results in much faster propagation along
the axon.
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12-6 Axon Diameter and Speed
Axon Diameter and Propagation Speed
Ion movement is related to cytoplasm concentration
Axon diameter affects action potential speed
The larger the diameter, the lower the resistance
Three Groups of Axons classed by diameter, myelination and speed:
Type A
Type B
Type C
4-20 μm
2-4 μm
2 μm
Myelinated
Myelinated
Unmyelinated
120 m/sec (268 mph)
18 m/sec (40 mph)
1m/sec (2 mph)
5 What is the functional advantage of myelination?
6 What factors determine the speed of propagation of an action potential?
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12-6 Axon Diameter and Speed
Information
“Information” travels within the nervous system
As propagated electrical signals (action potentials)
The most important information (vision, balance, motor commands)
Is carried by large-diameter, myelinated axons
Synaptic Activity
Action potentials (nerve impulses)
Are transmitted from presynaptic neuron
To postsynaptic neuron (or other postsynaptic cell)
Across a synapse
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12-7 Synapses
Two Types of Synapses
Electrical synapses (Direct physical contact between cells)
Are locked together at gap junctions (connexons)
Allow ions to pass between cells
Produce continuous local current and action potential propagation
Are found in areas of brain, eye, ciliary ganglia
Chemical synapses (Signal transmitted across a gap by chemical
neutrotransmitters)
Are found in most synapses between neurons and all synapses between neurons and
other cells
Cells not in direct contact
Action potential may or may not be propagated to postsynaptic cell, depending on
Amount of neurotransmitter released
Sensitivity of postsynaptic cell
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12-7 Synapses
Two Classes of Neurotransmitters
Excitatory neurotransmitters
Cause depolarization of postsynaptic membranes
Promote action potentials
Inhibitory neurotransmitters
Cause hyperpolarization of postsynaptic membranes
Suppress action potentials
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12-7 Synapses
Cholinergic Synapses
Any synapse that releases ACh at:
All neuromuscular junctions with skeletal muscle fibers
Many synapses in CNS
All neuron-to-neuron synapses in PNS
All neuromuscular and neuroglandular junctions of ANS
parasympathetic division
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12-7 Synapses
Synaptic Delay
A synaptic delay of 0.2–0.5 msec occurs between:
Arrival of action potential at synaptic terminal
And effect on postsynaptic membrane
Fewer synapses mean faster response
Reflexes may involve only one synapse
Synaptic Fatigue
Occurs when neurotransmitter cannot recycle fast enough to meet
demands of intense stimuli
Synapse inactive until ACh is replenished
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12-8 Neurotransmitters and Neuromodulators
Neurotransmitters Other Than Acetylcholine
Norepinephrine (NE) (Released by adrenergic synapses)
Excitatory and depolarizing effect
Widely distributed in brain and portions of ANS
Biogenic Amines
Dopamine (a CNS neurotransmitter)
May be excitatory or inhibitory
Involved in Parkinson’s disease and cocaine use
Serotonin (a CNS neurotransmitter)
Affects attention and emotional states
Gamma Aminobutyric Acid (GABA) (a CNS neutrotransmitter)
Amino Acid
Inhibitory effect but is not well understood
Nitric oxide and Carbon monoxide (gasses that are PNS and CNS
neurotransmitters)
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Gases
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12-8 Neurotransmitters and Neuromodulators
Neuromodulators
Other chemicals released by synaptic terminals
Similar in function to neurotransmitters
Characteristics of neuromodulators
Effects are long term, slow to appear
Responses involve multiple steps, intermediary compounds
Affect presynaptic membrane, postsynaptic membrane, or both
Released alone or with a neurotransmitter
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12-8 Neurotransmitters and Neuromodulators
Neuropeptides
Neuromodulators that bind to receptors and activate enzymes
Opioids
Neuromodulators in the CNS
Bind to the same receptors as opium or morphine
Relieve pain
Four Classes of Opioids (effects similar to drugs opium and morphine)
Endorphins
Enkephalins
Endomorphins
Dynorphins
Function: to relieve pain
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12-9 Information Processing
Information Processing is the integration process which determines the
rate of action potential generation at the initial segment. The excitatory and
inhibitory stimuli are integrated through interactions between postsynaptic
potentials. These are graded potentials that develop in the postsynaptic
Membrane in response to a neurotransmitter.
The two major types of postsynaptic potentials develop at neuron-to-neuron
synapses are: Excitatory Postsynaptic potential (EPSP)
Inhibitory Postsynaptic potential (IPSP)
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12-9 Information Processing
Frequency of Action Potentials
In the Nervous System
A change in transmembrane potential that determines whether or not
action potentials are generated is the simplest form of information
processing.
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Clinical Case
Clinical Case—Did Franklin D. Roosevelt Really Have Polio?
Do you think the polio virus attacks sensory neurons, motor neurons or neuroglia of the spinal cord (CNS)?
If Mr. Roosevelt suffered from Guillain-Barre’Syndrome, where in the nervous system was the pathology
taking place?
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Neural Activities
1
The transmembrane potential of a resting cell
2 A typical stimulus produces a temporary localized change
in the resting potential.
3
An electrical impulse that is spread along the surface of an axon.
4
Produces graded potentials in the plasma membrane of the postsynaptic cell.
5
Integration of stimuli at the level of the individual cell.
2
3
4
1
5
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12-8 Neurotransmitters and Neuromodulators
Chemical Synapse
The synaptic terminal releases a neurotransmitter that binds to the
postsynaptic plasma membrane
Produces temporary, localized change in permeability or function of
postsynaptic cell
Changes affect cell, depending on nature and number of stimulated
receptors
Many Drugs affect nervous system by stimulating receptors that respond to
neurotransmitters and can have complex effects on perception, motor
control, and emotional states.
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