C h a p t e r 12 Neural Tissue PowerPoint® Lecture Slides prepared by Jason LaPres Lone Star College - North Harris Copyright © 2009 Pearson Education,

C h a p t e r 12 Neural Tissue PowerPoint® Lecture Slides prepared by Jason LaPres Lone Star College - North Harris Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

An Introduction to the Nervous System
Includes all neural tissue in the body Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neural Tissue Contains two kinds of cells Neurons: cells that send and receive signals Neuroglia (glial cells): cells that support and protect neurons Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Organs of the Nervous System Brain and spinal cord Sensory receptors of sense organs (eyes, ears, etc.) Nerves connect nervous system with other systems Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Divisions of the Nervous System
Anatomical Divisions of the Nervous System Central nervous system (CNS) Peripheral nervous system (PNS) Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Peripheral Nervous System (PNS) Includes all neural tissue outside the CNS Functions of the PNS Deliver sensory information to the CNS Carry motor commands to peripheral tissues and systems Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Functional Divisions of the PNS Afferent division Carries sensory information From PNS sensory receptors to CNS Efferent division Carries motor commands From CNS to PNS muscles and glands Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Functional Divisions of the PNS The efferent division Somatic nervous system (SNS): controls skeletal muscle contractions: voluntary and involuntary (reflexes) muscle 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 Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neurons Neurons The basic functional units of the nervous system
Most neurons have a large cell body (soma) that is connected to a single, elongate axon and several short, branched dendrites. Most neurons lack centrioles and cannot divide. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neurons The Structure of Neurons The Cell Body (soma) Consists of:
nucleus with a prominent nucleolus perikaryon (cytoplasm) Perikaryon contains neurofilaments and neurotubules. Contains organelles that provide energy and synthesize organic materials such as the neurotransmitters necessary for cell-to-cell communication Some areas contain clusters of rough endoplasmic reticulum and free ribosomes that are called Nissl Bodies Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neurons Major Organelles of the Cell Body Dendrites Axons
extend out from the cell body and are highly branched. They pick up stimuli from other neurons Axons a long cytoplasmic process capable of propagating an action potential (electrical impulse) to a target. Axon hillock Thick section of the cell body that attaches to initial segment Initial segment attaches to axon hillock Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neurons Structures of the Axon Collaterals Telodendria
Branches of a single axon Telodendria Fine extensions of distal axon Synaptic terminals Tips of telodendria Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neurons The Structure of Neurons The synapse
Area where a neuron communicates with another cell Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neurons The Structure of Neurons The synapse Presynaptic cell:
neuron that sends message Postsynaptic cell: cell that receives message The synaptic cleft: the small gap that separates the presynaptic membrane and the postsynaptic membrane Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neurons The Synapse The synaptic knob
Is expanded area of axon of presynaptic neuron Contains synaptic vesicles of neurotransmitters Neurotransmitters: are chemical messengers are released at presynaptic membrane affect receptors of postsynaptic membrane are broken down by enzymes are reassembled at synaptic knob Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neurons Types of Synapses Neuromuscular junction
Synapse between neuron and muscle Neuroglandular junction Synapse between neuron and gland Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neurons Figure 12–2 The Structure of a Typical Synapse.
Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neurons Four Structural Classifications of Neurons a. anaxonic
neurons are small and have no anatomical features that distinguish dendrites from axons located in the brain and in special sense organs function is poorly understood b. bipolar neurons has two distinct processes (one axon and one dendrite) with a cell body between the two rare but occur in special sense organs (sight, hearing, smell) Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neurons c. unipolar neuron d. multipolar neuron
the dendrites and axon are continuous with the cell body lying off to one side Most sensory neurons of the peripheral nervous system are unipolar d. multipolar neuron have two or more dendrites and a single axon most common neuron in the CNS all motor neurons that control skeletal muscles are multipolar Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neurons Figure 12–3 A Structural Classification of Neurons.

Neurons Three Functional Classifications of Neurons Sensory neurons
Afferent neurons of PNS Motor neurons Efferent neurons of PNS Interneurons Association neurons Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neurons Sensory Neurons also called afferent neurons
they make the afferent division of the PNS deliver information from sensory receptors to the CNS The cell bodies of sensory neurons are located in peripheral sensory ganglia (a collection of neuron cell bodies in the PNS) They are unipolar neurons with afferent fibers Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neurons Motor Neurons Also known as efferent neurons
They make the efferent division of the PNS Carry instructions from the CNS to peripheral effectors in a peripheral tissue, organ, or organ system Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neurons Interneurons also known as association neurons
Outnumber all other types of neurons Most are located in the brain and spinal cord Responsible for the distribution of sensory information and the coordination of motor activity One or more interneurons are located between sensory and motor neurons Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neuroglia Neuroglia Half the volume of the nervous system
Many types of neuroglia in CNS and PNS Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neuroglia Four Types of Neuroglia in the CNS Ependymal cells
Astrocytes Oligodendrocytes Microglia Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neuroglia Four Types of Neuroglia in the CNS Ependymal cells
cells with highly branched processes Form epithelium called ependyma 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 Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neuroglia Four Types of Neuroglia in the CNS Astrocytes
large cell bodies with many processes Maintain blood–brain barrier (isolates CNS) Create three-dimensional framework for CNS Repair damaged neural tissue Guide neuron development Control interstitial environment Oligodendrocytes Processes contact other neuron cell bodies Wrap around axons to form myelin sheaths Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neuroglia Four Types of Neuroglia in the CNS Oligodendrocytes
Myelination increases speed of action potentials myelin insulates myelinated axons makes nerves appear white Nodes and internodes internodes: myelinated segments of axon nodes (also called nodes of Ranvier) gaps between internodes where axons may branch Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neuroglia Figure 12–6a Schwann Cells and Peripheral Axons.

Neuroglia Four Types of Neuroglia in the CNS Oligodendrocytes
Myelination White matter: regions of CNS with many myelinated nerves Gray matter: unmyelinated areas of CNS Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neuroglia Four Types of Neuroglia in the CNS Microglia
Possess many fine branches Migrate through neural tissue Clean up cellular debris, waste products, and pathogens Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neuroglia Figure 12–5b Neuroglia in the CNS.

Neuroglia of PNS Ganglia Masses of neuron cell bodies
Surrounded by neuroglia Found in the PNS Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neuroglia Neuroglia of the Peripheral Nervous System Satellite cells
Surround ganglia Regulate environment around neuron Schwann cells Form myelin sheath (neurilemma) around peripheral axons One Schwann cell sheaths one segment of axon: many Schwann cells sheath entire axon Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neuroglia Figure 12–4 An Introduction to Neuroglia.

Transmembrane Potential
Figure 12–8 An Overview of Neural Activities. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Ion Movements and Electrial Signals Transmembrane potential is the voltage difference (or electrical potential difference) between the interior and exterior of a cell. All cells have a transmembrane potential that varies from moment to moment depending on the activities of the cell. Resting potential is the transmembrane potential of a resting cell. All neural activities begin with a change in the resting potential of a neuron that is caused by ion movement. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Three important concepts about transmembrane potential: Extracellular fluid contains high concentrations of Na+ and Cl- while the cytosol has high concentrations of K+ and negatively charged particles. Equilibrium does not exist across cell membranes. they have selectively permeable membranes At resting potential, ion movement occurs through leak channels (channels that are always open). Since membrane permeability varies by ions, there is not an equal distribution of charges across the membrane. Inner surface has an excess of negative charges with respect to the outer surface. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Forces acting across the membrane The passive forces acting across the membrane are both chemical and electrical in nature. Chemical Gradients Intracellular concentration of K+ is high K+ ions move out of the cell through open K+ channels Extracellular concentration of Na+ is high Na+ ions move into the cell. Electrical Gradient Cell membrane is more permeable to K+ than to Na+ causes potassium ions to leave the cytoplasm more rapidly than sodium ions enter. cytosol along the interior of the cell becomes negatively charged extracellular fluid becomes positively charged. This causes a potential difference to occur (when positive and negative ions are held apart). Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The size of a potential difference is measured in millivolts (mV). Average resting potential for most cells is -70mV. The minus sign signifies that the inner surface of the cell membrane is negatively charged with respect to the exterior. Chemical Gradient + Electrical Gradient = Electrochemical Gradient If the cell membrane were freely permeable to K+ but impermeable to other positively charged ions, potassium ions would continue to leave the cell until the electrical gradient was as strong as the chemical gradient. Equilibrium would occur around -90 mV. If the cell membrane were freely permeable to Na+ ions equilibrium would be reached at +66 mV. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Then WHY is the resting potential -70 mV?? Na+ has only a small effect on the normal resting potential. 1. Although the electrochemical gradient for sodium is very large, the membrane’s permeability to these ions is very low. 2. The Sodium-Potassium Exchange Pump (Active Force) It actively pumps out three sodium ions and pumps in two potassium ions. Powered by ATP Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Changes in Transmembrane Potential Transmembrane potential rises or falls In response to temporary changes in membrane permeability Resulting from opening or closing specific membrane channels Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Sodium and Potassium Channels Membrane permeability to Na+ and K+ determines transmembrane potential Are either passive or active Passive channels (also called leak channels): are always open permeability changes with conditions Active channels (also called gated channels): open and close in response to stimuli at resting potential, most gated channels are closed Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Three Conditions of Gated Channels Closed, but capable of opening Open (activated) Closed, not capable of opening (inactivated) Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Three Classes of Gated Channels Chemically gated channels Open in presence of specific chemicals (ex, ACh) at binding site Found on neuron cell body and dendrites Ex) receptors that bind Ach at neuromuscular junction Voltage-gated channels Respond to changes in transmembrane potential Have activation gates (opens) and inactivation gates (closes) Characteristic of excitable membrane (make action potential) Found in neural axons, skeletal muscle sarcolemma, cardiac muscle Ex) voltage regulated sodium, potassium, and calcium channels Mechanically gated channels Respond to membrane distortion Found in sensory receptors (touch, pressure, vibration) Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Graded Potentials Also called local potentials Changes in transmembrane potential That cannot spread far from site of stimulation Any stimulus that opens a gated channel Produces a graded potential Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Graded Potentials Step One :Depolarization Sodium ions enter the cell and are attracted to the negative charges along the inner surface of the membrane. The arrival of positive charges shifts the transmembrane potential toward 0 mV. This is called depolarization. What happens to the resting membrane when chemicals open sodium channels? Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Graded Potentials Step Two: Local Current As the cell membrane depolarizes, sodium ions are released from its outer surface and move toward the open channels, replacing ions that have already entered the cell. This is called local current. (As sodium ions move into the cell, other sodium ions fill in the gaps.) This causes adjacent portions of the cell membrane to become depolarized. The degree of depolarization decreases with distance away from the stimulation site and because some of the sodium ions move back across the membrane through the sodium leak channels. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Graded Potentials Step Three: Repolarization When a chemical stimulus is removed and normal membrane permeability is restored, the transmembrane potential soon returns to resting levels. This is called repolarization. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

What happens to the resting membrane when chemical open potassium channels? Opening a gated potassium channel would have the opposite effect on the transmembrane potential as opening a gated sodium channel. Step One: The rate of potassium outflow would increase, and the interior of the cell would lose positive ions. This is called hyperpolarization. When this happens an increase in the negativity of the resting potential occurs. Step Two: The local current will distribute the effect to adjacent portions of the cell membrane, and the effect would decrease with distance from the open channel or channels. Step Three: Repolarization occurs when the chemical stimulus is removed. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Action Potential Action Potentials
Propagated changes in transmembrane potential Affect an entire excitable membrane Link graded potentials at cell body with motor end plate actions Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Action Potential Initiating Action Potential Initial stimulus
The stimulus that initiates an action potential is a depolarization large enough to open voltage-regulated sodium channels. That opening occurs at a transmembrane potential known as the threshold. The threshold for a typical axon is between -60mV and -55 mV. If the stimulus does not shift the resting membrane potential at least 10-15mV you will only produce a graded depolarization and not an action potential. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Action Potential Initiating Action Potential All-or-none principle
If a stimulus exceeds threshold amount: the action potential is the same no matter how large the stimulus Action potential is either triggered, or not Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Action Potential Four Steps in the Generation of Action Potentials
Step One: Depolarization to Threshold. An area of excitable membrane must be depolarized to its threshold by local currents before an action potential can begin. Step 2: Activation of Na+ channels Rapid depolarization Na+ ions rush into cytoplasm Inner membrane changes from negative to positive Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Step 3: Inactivation of Na+ channels, activation of K+ channels At +30 mV Inactivation gates close (Na+ channel inactivation) K+ channels open Repolarization begins Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Step 4: Return to normal permeability K+ channels begin to close: when membrane reaches normal resting potential (–70 mV) Na+ channels regain normal properties K+ channels finish closing: membrane is hyperpolarized to -90 mV transmembrane potential returns to resting level: action potential is over Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Action Potential The Refractory Period The time period
From beginning of action potential To return to resting state During which membrane will not respond normally to additional stimuli Absolute refractory period Sodium channels open or inactivated No action potential possible Relative refractory period Membrane potential almost normal Very large stimulus can initiate action potential Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Action Potential Powering the Sodium-Potassium Exchange Pump
To maintain concentration gradients of Na+ and K+ over time Requires energy (1 ATP for each 2K+ pumped in and 3 Na+ pumped out) Without ATP Neurons stop functioning Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Action Potential Propagation of Action Potentials Propagation
Moves action potentials generated in axon hillock Along entire length of axon A series of repeated actions, not passive flow Two methods of propagating action potentials Continuous propagation: unmyelinated axons Saltatory propagation: myelinated axons Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Action Potential Continuous Propagation
Of action potentials along an unmyelinated axon Affects one segment of axon at a time Steps in a propagation Step 1: Action potential in segment 1: depolarizes membrane to +30 mV local current Step 2: Depolarizes second segment to threshold: second segment develops action potential Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Action Potential Figure 12–15 Continuous Propagation of an Action Potential along an Unmyelinated Axon (Steps 1 and 2). Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Action Potential Continuous Propagation Steps in propagation
Step 3: First segment enters refractory period Step 4: Local current depolarizes next segment Cycle repeats Action potential travels in one direction (1 m/sec) Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Action Potential Figure 12–15 Continuous Propagation of an Action Potential along an Unmyelinated Axon (Steps 3 and 4). Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Action Potential Saltatory Propagation
Action potential along myelinated axon Faster and uses less energy than continuous propagation Myelin insulates axon, prevents continuous propagation Local current “jumps” from node to node Depolarization occurs only at nodes Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Three Groups of Axons Type A fibers Type B fibers Type C fibers These groups are classified by Diameter Myelination Speed of action potentials Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Type A Fibers Myelinated Large diameter High speed (140 m/sec) Carry rapid information to/from CNS For example, position, balance, touch, and motor impulses Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Type B Fibers Myelinated Medium diameter Medium speed (18 m/sec) Carry intermediate signals For example, sensory information, peripheral effectors Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Type C Fibers Unmyelinated Small diameter Slow speed (1 m/sec) Carry slower information For example, involuntary muscle, gland controls Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

“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 Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Synapses Synaptic Activity Action potentials (nerve impulses)
Are transmitted from presynaptic neuron To postsynaptic neuron (or other postsynaptic cell) Across a synapse Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Synapses Two Types of Synapses Electrical synapses Chemical synapses
Direct physical contact between cells Chemical synapses Signal transmitted across a gap by chemical neurotransmitters Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Synapses Electrical Synapses
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 Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Synapses Chemical Synapses
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 Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Synapses The Effect of a Neurotransmitter On a postsynaptic membrane
Depends on the receptor Not on the neurotransmitter For example, acetylcholine (ACh) Usually promotes action potentials But inhibits cardiac neuromuscular junctions Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Synapses Cholinergic Synapses Any synapse that releases ACh
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 Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Synapses Events at a Cholinergic Synapse
Action potential arrives, depolarizes synaptic knob Calcium ions enter synaptic knob, trigger exocytosis of Ach ACh binds to receptors, depolarizes postsynaptic membrane AChE breaks ACh into acetate and choline Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Synapses Synaptic Delay
A synaptic delay of 0.2–0.5 msec occurs between Arrival of action potential at synaptic knob And effect on postsynaptic membrane Fewer synapses mean faster response Reflexes may involve only one synapse Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Synapses Synaptic Fatigue
Occurs when neurotransmitter cannot recycle fast enough to meet demands of intense stimuli Synapse inactive until ACh is replenished Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neurotransmitters and Neuromodulators
Important Neurotransmitters Other than acetylcholine Norepinephrine (NE) Dopamine Serotonin Gamma aminobutyric acid (GABA) Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Norepinephrine (NE) Released by adrenergic synapses Excitatory and depolarizing effect Widely distributed in brain and portions of ANS Dopamine A CNS neurotransmitter May be excitatory or inhibitory If neurons that release dopamine are damaged or destroyed, Parkinson’s disease can occur Cocaine inhibits the removal of dopamine from synapses causing the “high” experienced by users Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Serotonin Another CNS neurotransmitter Inadequate supplies can have widespread effects on a person’s attention and emotional states and may be responsible for severe chronic depression Prozac, Paxil, Zoloft and related antidepressant drugs inhibit the reabsorption of serotonin by synaptic knobs Gamma Aminobutyric Acid (GABA) Inhibitory effect Functions in CNS Not well understood Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Other chemicals released by synaptic knobs 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 Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Opioids Neuromodulators called opioids have effects similar to those of opium and morphine. Their primary function is the release of pain. Inhibit release of the neurotransmitter “substance P” at synapses that relay pain sensations. There are four classes of opioids in the CNS: 1. endorphins 2. enkephalins 3. endomorphins 4. dynomorphins Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

How Neurotransmitters and Neuromodulators Work Three categories: Direct effects on membrane channels Indirect effects via G proteins Indirect effects via intracellular enzymes Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Direct Effects open and close gated ion channels Ex) ACh, glycine, aspartate Called ionotropic because they alter ion movement across the membrane Indirect Effects: G Proteins work through intermediaries known as second messengers the neurotransmitter is the first messenger because it delivers the message to receptors on the cell membrane or within the cell Ex of indirect neurotransmitters - epinephrine, norepinephrine, dopamine, serotonin, histamine, GABA second messengers are ions or molecules that are produced or released inside the cell when a first messenger binds to a receptor The link between the first messenger and the second messenger involves a G protein (an enzyme complex coupled to a membrane receptor) One possible result of binding is the activation of the enzyme adenylate cyclase (converts ATP to cyclic-AMP (cAMP). cAMP is a second messenger that may open membrane channels, activate intracellular enzymes, or both. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Indirect Effects: Intracellular receptors Ex) nitric oxide and carbon monoxide They can enter the cell and bind to enzymes on the inner surface of the membrane or elsewhere in the cytoplasm. They promote the appearance of second messengers Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Information Processing
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 Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Postsynaptic Potentials Graded potentials developed in a postsynaptic cell In response to neurotransmitters Two Types of Postsynaptic Potentials Excitatory postsynaptic potential (EPSP) Graded depolarization of postsynaptic membrane Ex) binding of ACh Inhibitory postsynaptic potential (IPSP) Graded hyperpolarization of postsynaptic membrane Ex) opening of K+ channels Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Inhibition A neuron that receives many IPSPs Is inhibited from producing an action potential Because the stimulation needed to reach threshold is increased Summation To trigger an action potential One EPSP is not enough EPSPs (and IPSPs) combine through summation: temporal summation spatial summation Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Temporal Summation Multiple times Rapid, repeated stimuli at one synapse Spatial Summation Multiple locations Many stimuli, arrive at multiple synapses Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Facilitation A neuron whose transmembrane potential shifts closer to threshold is said to be facilitated. The larger the degree of facilitation, the smaller the additional stimulus needed to trigger an action potential. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Axoaxonic Synapses Synapses between the axons of two neurons Presynaptic inhibition The release of GABA inhibits the opening of voltage-regulated calcium channels in the synaptic knob This reduces the amount of neurotransmitter released when an action potential arrives there, and reduces the effects of synaptic activity on the postsynaptic membrane. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Axoaxonic Synapses Synapses between the axons of two neurons Presynaptic facilitation The release of serotonin causes voltage-regulated calcium channels to remain open longer. This increases the amount of neurotransmitter released when an action potential arrives at the synaptic knob, and enhances and prolongs the neurotransmitter’s effects on the postsynaptic membrane. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

C h a p t e r 12 Neural Tissue PowerPoint® Lecture Slides prepared by Jason LaPres Lone Star College - North Harris Copyright © 2009 Pearson Education,

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C h a p t e r 12 Neural Tissue PowerPoint® Lecture Slides prepared by Jason LaPres Lone Star College - North Harris Copyright © 2009 Pearson Education,

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Presentation on theme: "C h a p t e r 12 Neural Tissue PowerPoint® Lecture Slides prepared by Jason LaPres Lone Star College - North Harris Copyright © 2009 Pearson Education,"— Presentation transcript:

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