Chapter 12 Neural Tissue 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. An Introduction to the Nervous System Learning Outcomes 12-4 Explain how the resting membrane 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. An Introduction to the Nervous System Learning Outcomes 12-7 Describe the structure of a synapse, and explain the mechanism involved in synaptic activity. 12-8 Describe the major types of © 2015 Pearson Education, Inc.
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Chapter 12
Neural Tissue
An Introduction to the Nervous System• Learning Outcomes
o 12-1 Describe the anatomical and functional divisions of the nervous system.
o 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.
o 12-3 Describe the locations and functions of the various types of neuroglia.
An Introduction to the Nervous System• Learning Outcomes
o 12-4 Explain how the resting membrane potential is created and maintained.
o 12-5 Describe the events involved in the generation and propagation of an action potential.
o 12-6 Discuss the factors that affect the speed with which action potentials are propagated.
An Introduction to the Nervous System• Learning Outcomes
o 12-7 Describe the structure of a synapse, and explain the mechanism involved in synaptic activity.
o 12-8 Describe the major types of neurotransmitters and neuromodulators, and discuss their effects on postsynaptic membranes.
o 12-9 Discuss the interactions that enable information processing to occur
in neural tissue.
An Introduction to the Nervous System• The Nervous System
o Includes all neural tissue in the bodyo Neural tissue contains two kinds of cells
1. Neurons o Cells that send and receive signals
2. Neuroglia (glial cells)
© 2015 Pearson Education, Inc.
o Cells that support and protect neurons
An Introduction to the Nervous System• Organs of the Nervous System
o Brain and spinal cordo Sensory receptors of sense organs (eyes, ears, etc.)o Nerves connect nervous system with other systems
12-1 Divisions of the Nervous System• Anatomical Divisions of the Nervous System
o Central nervous system (CNS)o Peripheral nervous system (PNS)
12-1 Divisions of the Nervous System• The Central Nervous System (CNS)
o Consists of the spinal cord and brain o Contains neural tissue, connective tissues, and blood vesselso 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
12-1 Divisions of the Nervous System• The Peripheral Nervous System (PNS)
o Includes all neural tissue outside the CNSo Functions of the PNS
Deliver sensory information to the CNS Carry motor commands to peripheral tissues and systems
12-1 Divisions of the Nervous System• The Peripheral Nervous System (PNS)
o Nerves (also called peripheral nerves) Bundles of axons with connective tissues and blood vessels Carry sensory information and motor commands in PNS
o Cranial nerves – connect to braino Spinal nerves – attach to spinal cord
12-1 Divisions of the Nervous System• Functional Divisions of the PNS
© 2015 Pearson Education, Inc.
o Afferent division Carries sensory information From PNS sensory receptors to CNS
o Efferent division Carries motor commands From CNS to PNS muscles and glands
12-1 Divisions of the Nervous System• Functional Divisions of the PNS
o Receptors and effectors of afferent division Receptors
o Detect changes or respond to stimulio Neurons and specialized cells o Complex sensory organs (e.g., eyes, ears)
Effectorso Respond to efferent signals o Cells and organs
12-1 Divisions of the Nervous System• Functional Divisions of the PNS
o The efferent division Somatic nervous system (SNS)
o Controls voluntary and involuntary (reflexes) skeletal muscle contractions
12-1 Divisions of the Nervous System• Functional Divisions of the PNS
o The efferent division Autonomic nervous system (ANS)
o Controls subconscious actions, contractions of smooth muscle and cardiac muscle, and glandular secretions
o Sympathetic division has a stimulating effecto Parasympathetic division has a relaxing effect
12-2 Neurons• Neurons
o The basic functional units of the nervous systemo The structure of neurons
The multipolar neurono Common in the CNS
Cell body (soma) Short, branched dendrites
© 2015 Pearson Education, Inc.
Long, single axon
12-2 Neurons• The Cell Body
o Large nucleus and nucleolus o Perikaryon (cytoplasm)o Mitochondria (produce energy)o RER and ribosomes (produce neurotransmitters)
12-2 Neurons• The Cell Body
o Cytoskeleton Neurofilaments and neurotubules in place of microfilaments and
microtubules Neurofibrils: bundles of neurofilaments that provide support for
dendrites and axono Nissl bodies
Dense areas of RER and ribosomes Make neural tissue appear gray (gray matter)
12-2 Neurons• Dendrites
o Highly branched o Dendritic spines
Many fine processes Receive information from other neurons 80–90 percent of neuron surface area
12-2 Neurons• The axon
o Is longo Carries electrical signal (action potential) to targeto Axon structure is critical to functiono
12-2 Neurons• Structures of the Axon
o Axoplasm Cytoplasm of axon Contains neurofibrils, neurotubules, enzymes, organelles
o Axolemma
© 2015 Pearson Education, Inc.
Specialized cell membrane Covers the axoplasm
12-2 Neurons• Structures of the Axon
o Axon hillock Thick section of cell body Attaches to initial segment
o Initial segment Attaches to axon hillock
12-2 Neurons• Structures of the Axon
o Collaterals Branches of a single axon
o Telodendria Fine extensions of distal axon
o Axon terminals Tips of telodendria
12-2 Neurons• The Structure of Neurons
o The synapse Area where a neuron communicates with another cell
12-2 Neurons• The Structure of Neurons
o The synapse Presynaptic cell
o Neuron that sends message Postsynaptic cell
o Cell that receives message The synaptic cleft
o The small gap that separates the presynaptic membrane and the postsynaptic membrane
12-2 Neurons• The Synapse
o The synaptic terminal Is expanded area of axon of presynaptic neuron Contains synaptic vesicles of neurotransmitters
© 2015 Pearson Education, Inc.
12-2 Neurons• Neurotransmitters
o Are chemical messengerso Are released at presynaptic membraneo Affect receptors of postsynaptic membrane o Are broken down by enzymeso Are reassembled at axon terminal
12-2 Neurons• Recycling Neurotransmitters
o Axoplasmic transport Neurotubules within the axon Transport raw materials Between cell body and axon terminal Powered by mitochondria, kinesin, and dynein
12-2 Neurons• Types of Synapses
o Neuromuscular junction Synapse between neuron and muscle
o Neuroglandular junction Synapse between neuron and gland
12-2 Neurons• Structural Classification of Neurons
o Anaxonic neurons Found in brain and sense organs
o Bipolar neurons Found in special sensory organs (sight, smell, hearing)
o Unipolar neurons Found in sensory neurons of PNS
o Multipolar neurons Common in the CNS Include all skeletal muscle motor neurons
12-2 Neurons• Anaxonic Neurons
o Smallo All cell processes look alike
• Bipolar Neuronso Are small
© 2015 Pearson Education, Inc.
o One dendrite, one axon
12-2 Neurons• Unipolar Neurons
o Also called pseudounipolar neuronso Have very long axonso Fused dendrites and axon o Cell body to one side
• Multipolar Neuronso Have very long axonso Multiple dendrites, one axon
12-2 Neurons• Three Functional Classifications of Neurons
1. Sensory neurons Afferent neurons of PNS
2. Motor neurons Efferent neurons of PNS
3. Interneurons Association neurons
12-2 Neurons• Functions of Sensory Neurons
o Monitor internal environment (visceral sensory neurons)o Monitor effects of external environment (somatic sensory neurons)
• Structures of Sensory Neuronso Unipolaro Cell bodies grouped in sensory gangliao Processes (afferent fibers) extend from sensory receptors to CNS
12-2 Neurons• Three Types of Sensory Receptors
1. Interoceptors Monitor internal systems (digestive, respiratory, cardiovascular, urinary,
reproductive) Internal senses (taste, deep pressure, pain)
2. Exteroceptors External senses (touch, temperature, pressure) Distance senses (sight, smell, hearing)
3. Proprioceptors Monitor position and movement (skeletal muscles and joints)
© 2015 Pearson Education, Inc.
12-2 Neurons• Motor Neurons
o Carry instructions from CNS to peripheral effectors o Via efferent fibers (axons)
12-2 Neurons• Motor Neurons
o Two major efferent systems1. Somatic nervous system (SNS)
o Includes all somatic motor neurons that innervate skeletal muscles2. Autonomic (visceral) nervous system (ANS)
o Visceral motor neurons innervate all other peripheral effectors Smooth muscle, cardiac muscle, glands, adipose tissue
12-2 Neurons• Motor Neurons
o Two groups of efferent axons Signals from CNS motor neurons to visceral effectors pass synapses
at autonomic ganglia dividing axons into:o Preganglionic fibers o Postganglionic fibers
12-2 Neurons• Interneurons
o Most are located in brain, spinal cord, and autonomic ganglia Between sensory and motor neurons
o Are responsible for: Distribution of sensory information Coordination of motor activity
o Are involved in higher functions Memory, planning, learning
12-3 Neuroglia• Neuroglia
o Half the volume of the nervous systemo Many types of neuroglia in CNS and PNS
12-3 Neuroglia• Four Types of Neuroglia in the CNS
1. Ependymal cells Cells with highly branched processes; contact neuroglia directly
© 2015 Pearson Education, Inc.
2. Astrocytes Large cell bodies with many processes
3. Oligodendrocytes Smaller cell bodies with fewer processes
4. Microglia Smallest and least numerous neuroglia with many fine-branched
processes
12-3 Neuroglia• Ependymal Cells
o Form epithelium called ependymao Line central canal of spinal cord and ventricles of brain
Secrete cerebrospinal fluid (CSF) Have cilia or microvilli that circulate CSF Monitor CSF Contain stem cells for repair
12-3 Neuroglia• Astrocytes
o Maintain blood–brain barrier (isolates CNS)o Create three-dimensional framework for CNSo Repair damaged neural tissueo Guide neuron developmento Control interstitial environment
12-3 Neuroglia• Oligodendrocytes
o Myelination Increases speed of action potentials Myelin insulates myelinated axons Makes nerves appear white
12-3 Neuroglia• Oligodendrocytes
o Nodes and internodes Internodes – myelinated segments of axon Nodes (also called nodes of Ranvier)
o Gaps between internodeso Where axons may branch
12-3 Neuroglia
© 2015 Pearson Education, Inc.
• Myelinationo White matter
Regions of CNS with many myelinated nerves o Gray matter
Unmyelinated areas of CNS
12-3 Neuroglia• Microglia
o Migrate through neural tissueo Clean up cellular debris, waste products, and pathogens
12-3 Neuroglia• Neuroglia of the Peripheral Nervous System
o Ganglia Masses of neuron cell bodies Surrounded by neuroglia Found in the PNS
12-3 Neuroglia• Neuroglia of the Peripheral Nervous System
o Satellite cells Also called amphicytes Surround ganglia Regulate environment around neuron
12-3 Neuroglia• Neuroglia of the Peripheral Nervous System
o Schwann cells Also called neurilemma cells Form myelin sheath (neurilemma) around peripheral axons One Schwann cell sheaths one segment of axon
o Many Schwann cells sheath entire axon
12-3 Neuroglia• Neurons and Neuroglia
o Neurons perform: All communication, information processing, and control functions of the
nervous system o Neuroglia preserve:
Physical and biochemical structure of neural tissueo Neuroglia are essential to:
© 2015 Pearson Education, Inc.
Survival and function of neurons
12-3 Neuroglia• Neural Responses to Injuries
o Wallerian degeneration Axon distal to injury degenerates
o Schwann cells Form path for new growth Wrap new axon in myelin
12-3 Neuroglia• Nerve Regeneration in CNS
o Limited by chemicals released by astrocytes that: Block growth Produce scar tissue
12-4 Membrane Potential• Ion Movements and Electrical Signals
o All plasma (cell) membranes produce electrical signals by ion movements o Membrane potential is particularly important to neurons
12-4 Membrane Potential• Five Main Membrane Processes in Neural Activities
1. Resting potential The membrane potential of resting cell
2. Graded potential Temporary, localized change in resting potential Caused by stimulus
12-4 Membrane Potential• Five Main Membrane Processes in Neural Activities
3. Action potential Is an electrical impulse Produced by graded potential Propagates along surface of axon to synapse
12-4 Membrane Potential• Five Main Membrane Processes in Neural Activities
4. Synaptic activity Releases neurotransmitters at presynaptic membrane Produces graded potentials in postsynaptic membrane
© 2015 Pearson Education, Inc.
5. Information processing Response (integration of stimuli) of postsynaptic cell
12-4 Membrane Potential• The Membrane Potential
o Three important concepts1. The extracellular fluid (ECF) and intracellular fluid (cytosol) differ
greatly in ionic compositiono Concentration gradient of ions (Na+, K+)
2. Cells have selectively permeable membranes3. Membrane permeability varies by ion
12-4 Membrane Potential• Passive Forces Acting across the Plasma Membrane
o Chemical gradients Concentration gradients (chemical gradient) of ions (Na+, K+)
o Electrical gradients Separate charges of positive and negative ions Result in potential difference
12-4 Membrane Potential• Electrical Currents and Resistance
o Electrical current Movement of charges to eliminate potential difference
o Resistance The amount of current a membrane restricts
12-4 Membrane Potential• The Electrochemical Gradient
o For a particular ion (Na+, K+) is: The sum of chemical and electrical forces
o Acting on the ion across a plasma membrane A form of potential energy
12-4 Membrane Potential• Equilibrium Potential
o The membrane potential at which there is no net movement of a particular ion across the cell membrane
o Examples: K+ 90 mV Na+ 66 mV
© 2015 Pearson Education, Inc.
12-4 Membrane Potential• Active Forces across the Membrane
o Sodium–potassium ATPase (exchange pump) Is powered by ATP Carries 3 Na+ out and 2 K+ in Balances passive forces of diffusion Maintains resting potential (70 mV)
12-4 Membrane Potential• The Resting Potential
o Because the plasma membrane is highly permeable to potassium ions: The resting potential of approximately 70 mV is fairly close to 90 mV,
the equilibrium potential for K+
o The electrochemical gradient for sodium ions is very large, but the membrane’s permeability to these ions is very low Na+ has only a small effect on the normal resting potential, making it
just slightly less negative than the equilibrium potential for K+
12-4 Membrane Potential• The Resting Potential
o The sodium–potassium exchange pump ejects 3 Na+ ions for every 2 K+ ions that it brings into the cell It serves to stabilize the resting potential when the ratio of Na+ entry to
K+ loss through passive channels is 3:2o At the normal resting potential, these passive and active mechanisms are
in balance The resting potential varies widely with the type of cell A typical neuron has a resting potential of approximately 70 mV
12-4 Membrane Potential• Changes in the Membrane Potential
o Membrane potential rises or falls In response to temporary changes in membrane permeability Resulting from opening or closing specific membrane channels
12-4 Membrane Potential• Sodium and Potassium Channels
o Membrane permeability to Na+ and K+ determines membrane potentialo They are either passive or active
12-4 Membrane Potential
© 2015 Pearson Education, Inc.
• Passive Channels (Leak Channels)o Are always openo Permeability changes with conditions
• Active Channels (Gated Channels)o Open and close in response to stimulio At resting potential, most gated channels are closed
12-4 Membrane Potential• Three States of Gated Channels
1. Closed, but capable of opening2. Open (activated)3. Closed, not capable of opening (inactivated)
12-4 Membrane Potential• Three Classes of Gated Channels
1. Chemically gated channels2. Voltage-gated channels3. Mechanically gated channels
12-4 Membrane Potential• Chemically Gated Channels
o Open in presence of specific chemicals (e.g., ACh) at a binding siteo Found on neuron cell body and dendrites
12-4 Membrane Potential• Voltage-gated Channels
o Respond to changes in membrane potentialo Have activation gates (open) and inactivation gates (close)o Characteristic of excitable membraneo Found in neural axons, skeletal muscle sarcolemma, cardiac muscle
12-4 Membrane Potential• Mechanically Gated Channels
o Respond to membrane distortion o Found in sensory receptors (touch, pressure, vibration)
12-4 Membrane Potential• Membrane Potential Exists across Plasma Membrane
o Because: Cytosol and extracellular fluid have different chemical/ionic balance
© 2015 Pearson Education, Inc.
The plasma membrane is selectively permeable • Membrane Potential
o Changes with plasma membrane permeabilityo In response to chemical or physical stimuli
12-4 Membrane Potential• Graded Potentials
o Also called local potentialso Changes in membrane potential
That cannot spread far from site of stimulationo Any stimulus that opens a gated channel
Produces a graded potential
12-4 Membrane Potential• Graded Potentials
o The resting state Opening sodium channel produces graded potential
o Resting membrane exposed to chemicalo Sodium channel openso Sodium ions enter the cello Membrane potential riseso Depolarization occurs
12-4 Membrane Potential• Graded Potentials
o Depolarization A shift in membrane potential toward 0 mV
o Movement of Na+ through channel o Produces local current o Depolarizes nearby plasma membrane (graded potential)o Change in potential is proportional to stimulus
12-4 Membrane Potential• Graded Potentials
o Whether depolarizing or hyperpolarizing, share four basic characteristics1. The membrane potential is most changed at the site of stimulation, and
the effect decreases with distance2. The effect spreads passively, due to local currents
12-4 Membrane Potential• Graded Potentials
© 2015 Pearson Education, Inc.
o Whether depolarizing or hyperpolarizing, share four basic characteristics
3. The graded change in membrane potential may involve either depolarization or hyperpolarization The properties and distribution of the membrane channels involved
determine the nature of the changeo For example, in a resting membrane, the opening of sodium
channels causes depolarization, whereas the opening of potassium channels causes hyperpolarization The change in membrane potential reflects
whether positive charges enter or leave the cell
12-4 Membrane Potential• Graded Potentials
o Whether depolarizing or hyperpolarizing, share four basic characteristics4. The stronger the stimulus, the greater the change in the membrane
potential and the larger the area affected
12-4 Membrane Potential• Graded Potentials
o Repolarization When the stimulus is removed, membrane potential returns to normal
o Hyperpolarization Increasing the negativity of the resting potential Result of opening a potassium channel Opposite effect of opening a sodium channel Positive ions move out, not into cell
12-4 Membrane Potential• Graded Potentials
o Effects of graded potentials At cell dendrites or cell bodies
o Trigger specific cell functionso For example, exocytosis of glandular secretions
At motor end plateo Release ACh into synaptic cleft
12-5 Action Potential• Action Potentials
o Propagated changes in membrane potentialo Affect an entire excitable membraneo Link graded potentials at cell body with motor end plate actions
© 2015 Pearson Education, Inc.
12-5 Action Potential• Initiating Action Potential
o Initial stimulus A graded depolarization of axon hillock large enough (10 to 15 mV) to
change resting potential (70 mV) to threshold level of voltage-gated sodium channels (60 to 55 mV)
12-5 Action Potential• Initiating Action Potential
o All-or-none principle If a stimulus exceeds threshold amount
o The action potential is the sameo No matter how large the stimulus
Action potential is either triggered, or not
12-5 Action Potential• Four Steps in the Generation of Action Potentials
o Step 1: Depolarization to thresholdo Step 2: Activation of Na channelso Step 3: Inactivation of Na channels and activation of K channelso Step 4: Return to normal permeability
12-5 Action Potential• Step 1: Depolarization to Threshold• Step 2: Activation of Na Channels
o Rapid depolarizationo Na+ ions rush into cytoplasmo Inner membrane changes from negative to positive
12-5 Action Potential• Step 3: Inactivation of Na Channels and Activation of K Channels
o At 30 mVo Inactivation gates close (Na channel inactivation)o K channels openo Repolarization begins
12-5 Action Potential• Step 4: Return to Normal Permeability
o K+ channels begin to close When membrane reaches normal resting potential (70 mV)
© 2015 Pearson Education, Inc.
o K+ channels finish closing Membrane is hyperpolarized to 90 mV Membrane potential returns to resting level Action potential is over
12-5 Action Potential• The Refractory Period
o The time period: From beginning of action potential To return to resting state During which membrane will not respond normally to additional stimuli
12-5 Action Potential• Absolute Refractory Period
o Sodium channels open or inactivatedo No action potential possible
• Relative Refractory Periodo Membrane potential almost normalo Very large stimulus can initiate action potential
12-5 Action Potential• Powering the Sodium–Potassium Exchange Pump
o To maintain concentration gradients of Na+ and K+ over time Requires energy (1 ATP for each 2 K+/3 Na+ exchange)
o Without ATP Neurons stop functioning
12-5 Action Potential• Propagation of Action Potentials
o Propagation Moves action potentials generated in axon hillock Along entire length of axon
o Two methods of propagating action potentials1. Continuous propagation (unmyelinated axons)2. Saltatory propagation (myelinated axons)
12-5 Action Potential• Continuous Propagation
o Of action potentials along an unmyelinated axono Affects one segment of axon at a timeo Steps in propagation
© 2015 Pearson Education, Inc.
Step 1: Action potential in segment 1 o Depolarizes membrane to 30 mVo Local current
Step 2: Depolarizes second segment to threshold o Second segment develops action potential
12-5 Action Potential• Continuous Propagation
o Steps in propagation Step 3: First segment enters refractory period Step 4: Local current depolarizes next segment
o Cycle repeats Action potential travels in one direction (1 m/sec)
12-5 Action Potential• Saltatory Propagation
o Action potential along myelinated axono Faster and uses less energy than continuous propagationo Myelin insulates axon, prevents continuous propagationo Local current “jumps” from node to nodeo Depolarization occurs only at nodes
12-6 Axon Diameter and Speed• Axon Diameter and Propagation Speed
o Ion movement is related to cytoplasm concentrationo Axon diameter affects action potential speedo The larger the diameter, the lower the resistance
12-6 Axon Diameter and Speed• Three Groups of Axons
1. Type A fibers2. Type B fibers3. Type C fibers
These groups are classified by:o Diametero Myelinationo Speed of action potentials
12-6 Axon Diameter and Speed• Type A Fibers
o Myelinated
© 2015 Pearson Education, Inc.
o Large diametero High speed (140 m/sec)o Carry rapid information to/from CNSo For example, position, balance, touch, and motor impulses
12-6 Axon Diameter and Speed• Type B Fibers
o Myelinatedo Medium diametero Medium speed (18 m/sec)o Carry intermediate signalso For example, sensory information, peripheral effectors
12-6 Axon Diameter and Speed• Type C Fibers
o Unmyelinatedo Small diametero Slow speed (1 m/sec)o Carry slower informationo For example, involuntary muscle, gland controls
12-6 Axon Diameter and Speed• Information
o “Information” travels within the nervous system As propagated electrical signals (action potentials)
o The most important information (vision, balance, motor commands) Is carried by large-diameter, myelinated axons
12-7 Synapses• Synaptic Activity
o Action potentials (nerve impulses) Are transmitted from presynaptic neuron To postsynaptic neuron (or other postsynaptic cell) Across a synapse
12-7 Synapses• Two Types of Synapses
1. Electrical synapses Direct physical contact between cells
2. Chemical synapses Signal transmitted across a gap by chemical neurotransmitters
© 2015 Pearson Education, Inc.
12-7 Synapses• Electrical Synapses
o Are locked together at gap junctions (connexons)o Allow ions to pass between cellso Produce continuous local current and action potential propagationo Are found in areas of brain, eye, ciliary ganglia
12-7 Synapses• Chemical Synapses
o Are found in most synapses between neurons and all synapses between neurons and other cells
o Cells not in direct contacto Action potential may or may not be propagated to postsynaptic cell,
depending on: Amount of neurotransmitter released Sensitivity of postsynaptic cell
12-7 Synapses• Two Classes of Neurotransmitters
1. Excitatory neurotransmitters Cause depolarization of postsynaptic membranes Promote action potentials
2. Inhibitory neurotransmitters Cause hyperpolarization of postsynaptic membranes Suppress action potentials
12-7 Synapses• The Effect of a Neurotransmitter
o On a postsynaptic membrane Depends on the receptor Not on the neurotransmitter
o For example, acetylcholine (ACh) Usually promotes action potentials But inhibits cardiac neuromuscular junctions
12-7 Synapses• Cholinergic Synapses
o Any synapse that releases ACh at:1. All neuromuscular junctions with skeletal muscle fibers2. Many synapses in CNS3. All neuron-to-neuron synapses in PNS4. All neuromuscular and neuroglandular junctions of ANS
© 2015 Pearson Education, Inc.
parasympathetic division
12-7 Synapses• Events at a Cholinergic Synapse
1. Action potential arrives, depolarizes synaptic terminal2. Calcium ions enter synaptic terminal, trigger exocytosis of ACh3. ACh binds to receptors, depolarizes postsynaptic membrane4. ACh removed by AChE
AChE breaks ACh into acetate and choline
12-7 Synapses• Synaptic Delay
o A synaptic delay of 0.2–0.5 msec occurs between: Arrival of action potential at synaptic terminal And effect on postsynaptic membrane
o Fewer synapses means faster responseo Reflexes may involve only one synapse
12-7 Synapses• Synaptic Fatigue
o Occurs when neurotransmitter cannot recycle fast enough to meet demands of intense stimuli
o Synapse inactive until ACh is replenished
12-8 Neurotransmitters and Neuromodulators• Other Neurotransmitters
o At least 50 neurotransmitters other than ACh, including: Biogenic amines Amino acids Neuropeptides Dissolved gases
12-8 Neurotransmitters and Neuromodulators• Important Neurotransmitters
o Other than acetylcholine Norepinephrine (NE) Dopamine Serotonin Gamma aminobutyric acid (GABA)
© 2015 Pearson Education, Inc.
12-8 Neurotransmitters and Neuromodulators• Norepinephrine (NE)
o Released by adrenergic synapseso Excitatory and depolarizing effecto Widely distributed in brain and portions of ANS
• Dopamineo A CNS neurotransmittero May be excitatory or inhibitoryo Involved in Parkinson’s disease and cocaine use
12-8 Neurotransmitters and Neuromodulators• Serotonin
o A CNS neurotransmittero Affects attention and emotional states
• Gamma Aminobutyric Acid (GABA)o Inhibitory effecto Functions in CNS
Not well understood
12-8 Neurotransmitters and Neuromodulators• Chemical Synapse
o The synaptic terminal releases a neurotransmitter that binds to the postsynaptic plasma membrane
o Produces temporary, localized change in permeability or function of postsynaptic cell
o Changes affect cell, depending on nature and number of stimulated receptors
12-8 Neurotransmitters and Neuromodulators• Many Drugs
o Affect nervous system by stimulating receptors that respond to neurotransmitters
o Can have complex effects on perception, motor control, and emotional states
12-8 Neurotransmitters and Neuromodulators• Neuromodulators
o Other chemicals released by synaptic terminalso Similar in function to neurotransmitters o Characteristics of neuromodulators
Effects are long term, slow to appear
© 2015 Pearson Education, Inc.
Responses involve multiple steps, intermediary compounds Affect presynaptic membrane, postsynaptic membrane, or both Released alone or with a neurotransmitter
12-8 Neurotransmitters and Neuromodulators• Neuropeptides
o Neuromodulators that bind to receptors and activate enzymes• Opioids
o Neuromodulators in the CNSo Bind to the same receptors as opium or morphineo Relieve pain
12-8 Neurotransmitters and Neuromodulators• Four Classes of Opioids
1. Endorphins2. Enkephalins3. Endomorphins4. Dynorphins
12-8 Neurotransmitters and Neuromodulators• How Neurotransmitters and Neuromodulators Work
o Direct effects on membrane channels For example, ACh, glycine, aspartate
o Indirect effects via G proteins For example, E, NE, dopamine, histamine, GABA
o Indirect effects via intracellular enzymes For example, lipid-soluble gases (NO, CO)
12-8 Neurotransmitters and Neuromodulators• Direct Effects
o Ionotropic effectso Open/close gated ion channels
12-8 Neurotransmitters and Neuromodulators• Indirect Effects – G Proteins
o Work through second messengerso Enzyme complex that binds GTPo Link between neurotransmitter (first messenger) and second messengero Activate enzyme adenylyl cyclase
Which produces second messenger cyclic-AMP (cAMP)
© 2015 Pearson Education, Inc.
12-8 Neurotransmitters and Neuromodulators• Indirect Effects – Intracellular Receptors
o Lipid-soluble gases (NO, CO)o Bind to enzymes in brain cells
12-9 Information Processing• Information Processing
o At the simplest level (individual neurons) Many dendrites receive neurotransmitter messages simultaneously Some excitatory, some inhibitory Net effect on axon hillock determines if action potential is produced
12-9 Information Processing• Postsynaptic Potentials
o Graded potentials developed in a postsynaptic cell In response to neurotransmitters
• Two Types of Postsynaptic Potentials1. Excitatory postsynaptic potential (EPSP)
Graded depolarization of postsynaptic membrane2. Inhibitory postsynaptic potential (IPSP)
Graded hyperpolarization of postsynaptic membrane
12-9 Information Processing• Inhibition
o A neuron that receives many IPSPs: Is inhibited from producing an action potential Because the stimulation needed to reach threshold is increased
• Summationo To trigger an action potential:
One EPSP is not enough EPSPs (and IPSPs) combine through summation
1. Temporal summation2. Spatial summation
12-9 Information Processing• Temporal Summation
o Multiple timeso Rapid, repeated stimuli at one synapse
• Spatial Summationo Multiple locationso Many stimuli, arrive at multiple synapses
© 2015 Pearson Education, Inc.
12-9 Information Processing• Facilitation
o A neuron becomes facilitated As EPSPs accumulate Raising membrane potential closer to threshold Until a small stimulus can trigger action potential
12-9 Information Processing• Summation of EPSPs and IPSPs
o Neuromodulators and hormones Can change membrane sensitivity to neurotransmitters Shifting balance between EPSPs and IPSPs
12-9 Information Processing• Axoaxonic Synapses
o Synapses between the axons of two neurons o Presynaptic inhibition
Action of an axoaxonic synapse at a synaptic terminal that decreases the neurotransmitter released by presynaptic membrane
o Presynaptic facilitation Action of an axoaxonic synapse at a synaptic terminal that increases
the neurotransmitter released by presynaptic membrane
12-9 Information Processing• Frequency of Action Potentials
o Information received by a postsynaptic cell may be simply the frequency of action potentials received
• Rate of Generation of Action Potentialso Frequency of action potentials depends on degree of depolarization above
threshold o Holding membrane above threshold level
Has same effect as a second, larger stimulus Reduces relative refractory period
12-9 Information Processing• In the Nervous System
o A change in membrane potential that determines whether or not action potentials are generated is the simplest form of information processing
12-9 Information Processing• Summary
© 2015 Pearson Education, Inc.
o Information is relayed in the form of action potentials In general, the degree of sensory stimulation or the strength of the
motor response is proportional to the frequency of action potentialso The neurotransmitters released at a synapse may have either excitatory or
inhibitory effects The effect on the axon’s initial segment reflects a summation of the
stimuli that arrive at any moment The frequency of generation of action potentials is an indication of the
degree of sustained depolarization at the axon hillock
12-9 Information Processing• Summary
o Neuromodulators Can alter either the rate of neurotransmitter release or the response of
a postsynaptic neuron to specific neurotransmitterso Neurons
May be facilitated or inhibited by extracellular chemicals other than neurotransmitters or neuromodulators
12-9 Information Processing• Summary
o The response of a postsynaptic neuron to the activation of a presynaptic neuron can be altered by: 1. The presence of neuromodulators or other chemicals that cause
facilitation or inhibition at the synapse2. Activity under way at other synapses affecting the postsynaptic cell3. Modification of the rate of neurotransmitter release through presynaptic
facilitation or presynaptic inhibition
© 2015 Pearson Education, Inc.
Related Documents
