Functions of the Nervous System 1 Sensory input

Functions of the Nervous System 1. Sensory input • Information gathered by sensory receptors about internal and external changes 2. Integration • Interpretation of sensory input 3. Motor output • Activation of effector organs (muscles and glands) produces a response

Sensory input Integration Motor output Figure 11. 1

Divisions of the Nervous System • Central nervous system (CNS) • Brain and spinal cord • Integration and command center • Peripheral nervous system (PNS) • Paired spinal and cranial nerves carry messages to and from the CNS

Peripheral Nervous System (PNS) • Two functional divisions 1. Sensory (afferent) division • Somatic afferent fibers—convey impulses from skin, skeletal muscles, and joints • Visceral afferent fibers—convey impulses from visceral organs 2. Motor (efferent) division • Transmits impulses from the CNS to effector organs

Motor Division of PNS 1. Somatic (voluntary) nervous system • Conscious control of skeletal muscles

Motor Division of PNS 2. Autonomic (involuntary) nervous system (ANS) • Visceral motor nerve fibers • Regulates smooth muscle, cardiac muscle, and glands • Two functional subdivisions • Sympathetic • Parasympathetic

Peripheral nervous system (PNS) Central nervous system (CNS) Cranial nerves and spinal nerves Communication lines between the CNS and the rest of the body Brain and spinal cord Integrative and control centers Sensory (afferent) division Somatic and visceral sensory nerve fibers Conducts impulses from receptors to the CNS Somatic sensory fiber Motor (efferent) division Motor nerve fibers Conducts impulses from the CNS to effectors (muscles and glands) Somatic nervous system Somatic motor (voluntary) Conducts impulses from the CNS to skeletal muscles Skin Visceral sensory fiber Stomach Skeletal muscle Motor fiber of somatic nervous system Sympathetic division Mobilizes body systems during activity Sympathetic motor fiber of ANS Structure Function Sensory (afferent) division of PNS Motor (efferent) division of PNS Parasympathetic motor fiber of ANS Autonomic nervous system (ANS) Visceral motor (involuntary) Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands Parasympathetic division Conserves energy Promotes housekeeping functions during rest Heart Bladder Figure 11. 2

Histology of Nervous Tissue • Two principal cell types 1. Neurons—excitable cells that transmit electrical signals

Histology of Nervous Tissue 2. Neuroglia (glial cells)—supporting cells: • Astrocytes (CNS) • Microglia (CNS) • Ependymal cells (CNS) • Oligodendrocytes (CNS) • Satellite cells (PNS) • Schwann cells (PNS)

Astrocytes • Most abundant, versatile, and highly branched glial cells • Cling to neurons, synaptic endings, and capillaries • Support and brace neurons

Astrocytes • Help determine capillary permeability • Guide migration of young neurons • Control the chemical environment • Participate in information processing in the brain

Capillary Neuron Astrocyte (a) Astrocytes are the most abundant CNS neuroglia. Figure 11. 3 a

Microglia • Small, ovoid cells with thorny processes • Migrate toward injured neurons • Phagocytize microorganisms and neuronal debris

Neuron Microglial cell (b) Microglial cells are defensive cells in the CNS. Figure 11. 3 b

Ependymal Cells • Range in shape from squamous to columnar • May be ciliated • Line the central cavities of the brain and spinal column • Separate the CNS interstitial fluid from the cerebrospinal fluid in the cavities

Fluid-filled cavity Ependymal cells Brain or spinal cord tissue (c) Ependymal cells line cerebrospinal fluid-filled cavities. Figure 11. 3 c

Oligodendrocytes • Branched cells • Processes wrap CNS nerve fibers, forming insulating myelin sheaths

Myelin sheath Process of oligodendrocyte Nerve fibers (d) Oligodendrocytes have processes that form myelin sheaths around CNS nerve fibers. Figure 11. 3 d

Satellite Cells and Schwann Cells • Satellite cells • Surround neuron cell bodies in the PNS • Schwann cells (neurolemmocytes) • Surround peripheral nerve fibers and form myelin sheaths • Vital to regeneration of damaged peripheral nerve fibers

Satellite cells Cell body of neuron Schwann cells (forming myelin sheath) Nerve fiber (e) Satellite cells and Schwann cells (which form myelin) surround neurons in the PNS. Figure 11. 3 e

Neurons (Nerve Cells) • Special characteristics: • Long-lived ( 100 years or more) • Amitotic—with few exceptions • High metabolic rate—depends on continuous supply of oxygen and glucose • Plasma membrane functions in: • Electrical signaling • Cell-to-cell interactions during development

Cell Body (Perikaryon or Soma) • Biosynthetic center of a neuron • Spherical nucleus with nucleolus • Well-developed Golgi apparatus • Rough ER called Nissl bodies (chromatophilic substance)

Cell Body (Perikaryon or Soma) • Network of neurofibrils (neurofilaments) • Axon hillock—cone-shaped area from which axon arises • Clusters of cell bodies are called nuclei in the CNS, ganglia in the PNS

Dendrites (receptive regions) Cell body (biosynthetic center and receptive region) Nucleolus Axon (impulse generating and conducting region) Nucleus Nissl bodies Axon hillock (b) Impulse direction Node of Ranvier Schwann cell Neurilemma (one inter. Terminal node) branches Axon terminals (secretory region) Figure 11. 4 b

Processes • Dendrites and axons • Bundles of processes are called • Tracts in the CNS • Nerves in the PNS

Dendrites • Short, tapering, and diffusely branched • Receptive (input) region of a neuron • Convey electrical signals toward the cell body as graded potentials

The Axon • One axon per cell arising from the axon hillock • Long axons (nerve fibers) • Occasional branches (axon collaterals)

The Axon • Numerous terminal branches (telodendria) • Knoblike axon terminals (synaptic knobs or boutons) • Secretory region of neuron • Release neurotransmitters to excite or inhibit other cells

Axons: Function • Conducting region of a neuron • Generates and transmits nerve impulses (action potentials) away from the cell body

Axons: Function • Molecules and organelles are moved along axons by motor molecules in two directions: • Anterograde—toward axonal terminal • Examples: mitochondria, membrane components, enzymes • Retrograde—toward the cell body • Examples: organelles to be degraded, signal molecules, viruses, and bacterial toxins

Dendrites (receptive regions) Cell body (biosynthetic center and receptive region) Nucleolus Axon (impulse generating and conducting region) Nucleus Nissl bodies Axon hillock (b) Impulse direction Node of Ranvier Schwann cell Neurilemma (one inter. Terminal node) branches Axon terminals (secretory region) Figure 11. 4 b

Myelin Sheath • Segmented protein-lipoid sheath around most long or large-diameter axons • It functions to: • Protect and electrically insulate the axon • Increase speed of nerve impulse transmission

Myelin Sheaths in the PNS • Schwann cells wraps many times around the axon • Myelin sheath—concentric layers of Schwann cell membrane • Neurilemma—peripheral bulge of Schwann cell cytoplasm

Myelin Sheaths in the PNS • Nodes of Ranvier • Myelin sheath gaps between adjacent Schwann cells • Sites where axon collaterals can emerge

Schwann cell plasma membrane Schwann cell cytoplasm Axon A Schwann cell envelopes an axon. 1 Schwann cell nucleus 2 The Schwann cell then rotates around the axon, wrapping its plasma membrane loosely around it in successive layers. Neurilemma Myelin sheath (a) Myelination of a nerve fiber (axon) 3 The Schwann cell cytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath. Figure 11. 5 a

Unmyelinated Axons • Thin nerve fibers are unmyelinated • One Schwann cell may incompletely enclose 15 or more unmyelinated axons

Myelin Sheaths in the CNS • Formed by processes of oligodendrocytes, not the whole cells • Nodes of Ranvier are present • No neurilemma • Thinnest fibers are unmyelinated

Myelin sheath Process of oligodendrocyte Nerve fibers (d) Oligodendrocytes have processes that form myelin sheaths around CNS nerve fibers. Figure 11. 3 d

White Matter and Gray Matter • White matter • Dense collections of myelinated fibers • Gray matter • Mostly neuron cell bodies and unmyelinated fibers

Structural Classification of Neurons • Three types: 1. Multipolar— 1 axon and several dendrites • Most abundant • Motor neurons and interneurons 2. Bipolar— 1 axon and 1 dendrite • Rare, e. g. , retinal neurons

Structural Classification of Neurons 3. Unipolar (pseudounipolar)—single, short process that has two branches: • Peripheral process—more distal branch, often associated with a sensory receptor • Central process—branch entering the CNS

Table 11. 1 (1 of 3)

Table 11. 1 (2 of 3)

Functional Classification of Neurons • Three types: 1. Sensory (afferent) • Transmit impulses from sensory receptors toward the CNS 2. Motor (efferent) • Carry impulses from the CNS to effectors

Functional Classification of Neurons 3. Interneurons (association neurons) • Shuttle signals through CNS pathways; most are entirely within the CNS

Table 11. 1 (3 of 3)

Neuron Function • Neurons are highly irritable • Respond to adequate stimulus by generating an action potential (nerve impulse) • Impulse is always the same regardless of stimulus

Principles of Electricity • Opposite charges attract each other • Energy is required to separate opposite charges across a membrane • Energy is liberated when the charges move toward one another • If opposite charges are separated, the system has potential energy

Definitions • Voltage (V): measure of potential energy generated by separated charge • Potential difference: voltage measured between two points • Current (I): the flow of electrical charge (ions) between two points

Definitions • Resistance (R): hindrance to charge flow (provided by the plasma membrane) • Insulator: substance with high electrical resistance • Conductor: substance with low electrical resistance

Role of Membrane Ion Channels • Proteins serve as membrane ion channels • Two main types of ion channels 1. Leakage (nongated) channels—always open

Role of Membrane Ion Channels 2. Gated channels (three types): • Chemically gated (ligand-gated) channels—open with binding of a specific neurotransmitter • Voltage-gated channels—open and close in response to changes in membrane potential • Mechanically gated channels—open and close in response to physical deformation of receptors

Receptor Neurotransmitter chemical attached to receptor Na+ Na+ Chemical binds K+ Closed Na+ Membrane voltage changes K+ Open (a) Chemically (ligand) gated ion channels open when the appropriate neurotransmitter binds to the receptor, allowing (in this case) simultaneous movement of Na+ and K+. Closed Open (b) Voltage-gated ion channels open and close in response to changes in membrane voltage. Figure 11. 6

Gated Channels • When gated channels are open: • Ions diffuse quickly across the membrane along their electrochemical gradients • Along chemical concentration gradients from higher concentration to lower concentration • Along electrical gradients toward opposite electrical charge • Ion flow creates an electrical current and voltage changes across the membrane

Resting Membrane Potential (Vr) • Potential difference across the membrane of a resting cell • Approximately – 70 m. V in neurons (cytoplasmic side of membrane is negatively charged relative to outside) • Generated by: • Differences in ionic makeup of ICF and ECF • Differential permeability of the plasma membrane

Voltmeter Plasma membrane Ground electrode outside cell Microelectrode inside cell Axon Neuron Figure 11. 7

Resting Membrane Potential • Differences in ionic makeup • ICF has lower concentration of Na+ and Cl– than ECF • ICF has higher concentration of K+ and negatively charged proteins (A–) than ECF

Resting Membrane Potential • Differential permeability of membrane • Impermeable to A– • Slightly permeable to Na+ (through leakage channels) • 75 times more permeable to K+ (more leakage channels) • Freely permeable to Cl–

Resting Membrane Potential • Negative interior of the cell is due to much greater diffusion of K+ out of the cell than Na+ diffusion into the cell • Sodium-potassium pump stabilizes the resting membrane potential by maintaining the concentration gradients for Na+ and K+

The concentrations of Na+ and K+ on each side of the membrane are different. The Na+ concentration is higher outside the cell. The K+ concentration is higher inside the cell. Outside cell K+ (5 m. M ) Na+ (140 m. M ) K+ (140 m. M ) Na+ (15 m. M ) Inside cell The permeabilities of Na+ and K+ across the membrane are different. Suppose a cell has only K+ channels. . . K+ loss through abundant leakage channels establishes a negative membrane potential. K+ leakage channels K+ K+ K+ Na+ K+ K+ Na+-K+ ATPases (pumps) maintain the concentration gradients of Na+ and K+ across the membrane. Cell interior – 90 m. V Now, let’s add some Na+ channels to our cell. . . Na+ entry through leakage channels reduces the negative membrane potential slightly. Cell interior – 70 m. V Na+-K+ pump Finally, let’s add a pump to compensate for leaking ions. Na+-K+ ATPases (pumps) maintain the concentration gradients, resulting in the resting membrane potential. Cell interior – 70 m. V Figure 11. 8

Membrane Potentials That Act as Signals • Membrane potential changes when: • Concentrations of ions across the membrane change • Permeability of membrane to ions changes • Changes in membrane potential are signals used to receive, integrate and send information

Membrane Potentials That Act as Signals • Two types of signals • Graded potentials • Incoming short-distance signals • Action potentials • Long-distance signals of axons

Changes in Membrane Potential • Depolarization • A reduction in membrane potential (toward zero) • Inside of the membrane becomes less negative than the resting potential • Increases the probability of producing a nerve impulse

Depolarizing stimulus Inside positive Inside negative Depolarization Resting potential Time (ms) (a) Depolarization: The membrane potential moves toward 0 m. V, the inside becoming less negative (more positive). Figure 11. 9 a

Changes in Membrane Potential • Hyperpolarization • An increase in membrane potential (away from zero) • Inside of the membrane becomes more negative than the resting potential • Reduces the probability of producing a nerve impulse

Hyperpolarizing stimulus Resting potential Hyperpolarization Time (ms) (b) Hyperpolarization: The membrane potential increases, the inside becoming more negative. Figure 11. 9 b

Graded Potentials • Short-lived, localized changes in membrane potential • Depolarizations or hyperpolarizations • Graded potential spreads as local currents change the membrane potential of adjacent regions

Stimulus Depolarized region Plasma membrane (a) Depolarization: A small patch of the membrane (red area) has become depolarized. Figure 11. 10 a

(b) Spread of depolarization: The local currents (black arrows) that are created depolarize adjacent membrane areas and allow the wave of depolarization to spread. Figure 11. 10 b

Graded Potentials • Occur when a stimulus causes gated ion channels to open • E. g. , receptor potentials, generator potentials, postsynaptic potentials • Magnitude varies directly (graded) with stimulus strength • Decrease in magnitude with distance as ions flow and diffuse through leakage channels • Short-distance signals

Membrane potential (m. V) Active area (site of initial depolarization) – 70 Resting potential Distance (a few mm) (c) Decay of membrane potential with distance: Because current is lost through the “leaky” plasma membrane, the voltage declines with distance from the stimulus (the voltage is decremental ). Consequently, graded potentials are short-distance signals. Figure 11. 10 c

Action Potential (AP) • Brief reversal of membrane potential with a total amplitude of ~100 m. V • Occurs in muscle cells and axons of neurons • Does not decrease in magnitude over distance • Principal means of long-distance neural communication

The big picture 1 Resting state 3 Repolarization Membrane potential (m. V) 2 Depolarization 3 4 Hyperpolarization 2 Action potential Threshold 1 4 1 Time (ms) Figure 11. 11 (1 of 5)

Generation of an Action Potential • Resting state • Only leakage channels for Na+ and K+ are open • All gated Na+ and K+ channels are closed

Properties of Gated Channels • Properties of gated channels • Each Na+ channel has two voltage-sensitive gates • Activation gates • Closed at rest; open with depolarization • Inactivation gates • Open at rest; block channel once it is open

Properties of Gated Channels • Each K+ channel has one voltage-sensitive gate • Closed at rest • Opens slowly with depolarization

Depolarizing Phase • Depolarizing local currents open voltagegated Na+ channels • Na+ influx causes more depolarization • At threshold (– 55 to – 50 m. V) positive feedback leads to opening of all Na+ channels, and a reversal of membrane polarity to +30 m. V (spike of action potential)

Repolarizing Phase • Repolarizing phase • Na+ channel slow inactivation gates close • Membrane permeability to Na+ declines to resting levels • Slow voltage-sensitive K+ gates open • K+ exits the cell and internal negativity is restored

Hyperpolarization • Hyperpolarization • Some K+ channels remain open, allowing excessive K+ efflux • This causes after-hyperpolarization of the membrane (undershoot)

3 2 Action potential Na+ permeability K+ permeability 1 4 1 Relative membrane permeability Membrane potential (m. V) The AP is caused by permeability changes in the plasma membrane Time (ms) Figure 11. 11 (2 of 5)

Role of the Sodium-Potassium Pump • Repolarization • Restores the resting electrical conditions of the neuron • Does not restore the resting ionic conditions • Ionic redistribution back to resting conditions is restored by the thousands of sodiumpotassium pumps

Propagation of an Action Potential • Na+ influx causes a patch of the axonal membrane to depolarize • Local currents occur • Na+ channels toward the point of origin are inactivated and not affected by the local currents

Propagation of an Action Potential • Local currents affect adjacent areas in the forward direction • Depolarization opens voltage-gated channels and triggers an AP • Repolarization wave follows the depolarization wave • (Fig. 11. 12 shows the propagation process in unmyelinated axons. )

Voltage at 0 ms Recording electrode (a) Time = 0 ms. Action potential has not yet reached the recording electrode. Resting potential Peak of action potential Hyperpolarization Figure 11. 12 a

Voltage at 2 ms (b) Time = 2 ms. Action potential peak is at the recording electrode. Figure 11. 12 b

Voltage at 4 ms (c) Time = 4 ms. Action potential peak is past the recording electrode. Membrane at the recording electrode is still hyperpolarized. Figure 11. 12 c

Threshold • At threshold: • Membrane is depolarized by 15 to 20 m. V • Na+ permeability increases • Na influx exceeds K+ efflux • The positive feedback cycle begins

Threshold • Subthreshold stimulus—weak local depolarization that does not reach threshold • Threshold stimulus—strong enough to push the membrane potential toward and beyond threshold • AP is an all-or-none phenomenon—action potentials either happen completely, or not at all

Coding for Stimulus Intensity • All action potentials are alike and are independent of stimulus intensity • How does the CNS tell the difference between a weak stimulus and a strong one? • Strong stimuli can generate action potentials more often than weaker stimuli • The CNS determines stimulus intensity by the frequency of impulses

Action potentials Threshold Stimulus Time (ms) Figure 11. 13

Absolute Refractory Period • Time from the opening of the Na+ channels until the resetting of the channels • Ensures that each AP is an all-or-none event • Enforces one-way transmission of nerve impulses

Absolute refractory period Relative refractory period Depolarization (Na+ enters) Repolarization (K+ leaves) After-hyperpolarization Stimulus Time (ms) Figure 11. 14

Relative Refractory Period • Follows the absolute refractory period • Most Na+ channels have returned to their resting state • Some K+ channels are still open • Repolarization is occurring • Threshold for AP generation is elevated • Exceptionally strong stimulus may generate an AP

Conduction Velocity • Conduction velocities of neurons vary widely • Effect of axon diameter • Larger diameter fibers have less resistance to local current flow and have faster impulse conduction • Effect of myelination • Continuous conduction in unmyelinated axons is slower than saltatory conduction in myelinated axons

Conduction Velocity • Effects of myelination • Myelin sheaths insulate and prevent leakage of charge • Saltatory conduction in myelinated axons is about 30 times faster • Voltage-gated Na+ channels are located at the nodes • APs appear to jump rapidly from node to node

Stimulus Size of voltage (a) In a bare plasma membrane (without voltage-gated channels), as on a dendrite, voltage decays because current leaks across the membrane. Voltage-gated Stimulus ion channel (b) In an unmyelinated axon, voltage-gated Na+ and K+ channels regenerate the action potential at each point along the axon, so voltage does not decay. Conduction is slow because movements of ions and of the gates of channel proteins take time and must occur before voltage regeneration occurs. Stimulus Myelin sheath (c) In a myelinated axon, myelin keeps current in axons (voltage doesn’t decay much). APs are generated only in the nodes of Ranvier and appear to jump rapidly from node to node. Node of Ranvier 1 mm Myelin sheath Figure 11. 15

The Synapse • A junction that mediates information transfer from one neuron: • To another neuron, or • To an effector cell

The Synapse • Presynaptic neuron—conducts impulses toward the synapse • Postsynaptic neuron—transmits impulses away from the synapse

Types of Synapses • Axodendritic—between the axon of one neuron and the dendrite of another • Axosomatic—between the axon of one neuron and the soma of another • Less common types: • Axoaxonic (axon to axon) • Dendrodendritic (dendrite to dendrite) • Dendrosomatic (dendrite to soma)

Axodendritic synapses Dendrites Axosomatic synapses Cell body Axoaxonic synapses (a) Axon Axosomatic synapses (b) Cell body (soma) of postsynaptic neuron Figure 11. 16

Electrical Synapses • Less common than chemical synapses • Neurons are electrically coupled (joined by gap junctions) • Communication is very rapid, and may be unidirectional or bidirectional • Are important in: • Embryonic nervous tissue • Some brain regions

Chemical Synapses • Specialized for the release and reception of neurotransmitters • Typically composed of two parts • Axon terminal of the presynaptic neuron, which contains synaptic vesicles • Receptor region on the postsynaptic neuron

Synaptic Cleft • Fluid-filled space separating the presynaptic and postsynaptic neurons • Prevents nerve impulses from directly passing from one neuron to the next

Synaptic Cleft • Transmission across the synaptic cleft: • Is a chemical event (as opposed to an electrical one) • Involves release, diffusion, and binding of neurotransmitters • Ensures unidirectional communication between neurons

Information Transfer • AP arrives at axon terminal of the presynaptic neuron and opens voltage-gated Ca 2+ channels • Synaptotagmin protein binds Ca 2+ and promotes fusion of synaptic vesicles with axon membrane • Exocytosis of neurotransmitter occurs

Information Transfer • Neurotransmitter diffuses and binds to receptors (often chemically gated ion channels) on the postsynaptic neuron • Ion channels are opened, causing an excitatory or inhibitory event (graded potential)

Chemical synapses transmit signals from one neuron to another using neurotransmitters. Presynaptic neuron Postsynaptic neuron 1 Action potential arrives at axon terminal. 2 Voltage-gated Ca 2+ channels open and Ca 2+ enters the axon terminal. Mitochondrion Ca 2+ 3 Ca 2+ entry causes neurotransmittercontaining synaptic vesicles to release their contents by exocytosis. Axon terminal Ca 2+ Synaptic cleft Synaptic vesicles 4 Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane. Postsynaptic neuron Ion movement Enzymatic degradation Graded potential Reuptake Diffusion away from synapse 5 Binding of neurotransmitter opens ion channels, resulting in graded potentials. 6 Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse. Figure 11. 17

Chemical synapses transmit signals from one neuron to another using neurotransmitters. Presynaptic neuron Postsynaptic neuron 1 Action potential arrives at axon terminal. Mitochondrion Ca 2+ Axon terminal Ca 2+ Synaptic cleft Synaptic vesicles Postsynaptic neuron Figure 11. 17, step 1

Chemical synapses transmit signals from one neuron to another using neurotransmitters. Presynaptic neuron Postsynaptic neuron 1 Action potential arrives at axon terminal. 2 Voltage-gated Ca 2+ channels open and Ca 2+ enters the axon terminal. Mitochondrion Ca 2+ Axon terminal Ca 2+ Synaptic cleft Synaptic vesicles Postsynaptic neuron Figure 11. 17, step 2

Chemical synapses transmit signals from one neuron to another using neurotransmitters. Presynaptic neuron Postsynaptic neuron 1 Action potential arrives at axon terminal. 2 Voltage-gated Ca 2+ channels open and Ca 2+ enters the axon terminal. Mitochondrion Ca 2+ 3 Ca 2+ entry causes neurotransmittercontaining synaptic vesicles to release their contents by exocytosis. Axon terminal Ca 2+ Synaptic cleft Synaptic vesicles Postsynaptic neuron Figure 11. 17, step 3

Chemical synapses transmit signals from one neuron to another using neurotransmitters. Presynaptic neuron Postsynaptic neuron 1 Action potential arrives at axon terminal. 2 Voltage-gated Ca 2+ channels open and Ca 2+ enters the axon terminal. Mitochondrion Ca 2+ 3 Ca 2+ entry causes neurotransmittercontaining synaptic vesicles to release their contents by exocytosis. 4 Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane. Axon terminal Ca 2+ Synaptic cleft Synaptic vesicles Postsynaptic neuron Figure 11. 17, step 4

Ion movement Graded potential 5 Binding of neurotransmitter opens ion channels, resulting in graded potentials. Figure 11. 17, step 5

Enzymatic degradation Reuptake Diffusion away from synapse 6 Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse. Figure 11. 17, step 6

Chemical synapses transmit signals from one neuron to another using neurotransmitters. Presynaptic neuron Postsynaptic neuron 1 Action potential arrives at axon terminal. 2 Voltage-gated Ca 2+ channels open and Ca 2+ enters the axon terminal. Mitochondrion Ca 2+ 3 Ca 2+ entry causes neurotransmittercontaining synaptic vesicles to release their contents by exocytosis. Axon terminal Ca 2+ Synaptic cleft Synaptic vesicles 4 Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane. Postsynaptic neuron Ion movement Enzymatic degradation Graded potential Reuptake Diffusion away from synapse 5 Binding of neurotransmitter opens ion channels, resulting in graded potentials. 6 Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse. Figure 11. 17

Termination of Neurotransmitter Effects • Within a few milliseconds, the neurotransmitter effect is terminated • Degradation by enzymes • Reuptake by astrocytes or axon terminal • Diffusion away from the synaptic cleft

Synaptic Delay • Neurotransmitter must be released, diffuse across the synapse, and bind to receptors • Synaptic delay—time needed to do this (0. 3– 5. 0 ms) • Synaptic delay is the rate-limiting step of neural transmission

Postsynaptic Potentials • Graded potentials • Strength determined by: • • Amount of neurotransmitter released • Time the neurotransmitter is in the area Types of postsynaptic potentials 1. EPSP—excitatory postsynaptic potentials 2. IPSP—inhibitory postsynaptic potentials

Table 11. 2 (2 of 4)

Table 11. 2 (3 of 4)

Table 11. 2 (4 of 4)

Neurotransmitters • Most neurons make two or more neurotransmitters, which are released at different stimulation frequencies • 50 or more neurotransmitters have been identified • Classified by chemical structure and by function

Chemical Classes of Neurotransmitters • Acetylcholine (Ach) • Released at neuromuscular junctions and some ANS neurons • Synthesized by enzyme choline acetyltransferase • Degraded by the enzyme acetylcholinesterase (ACh. E)

Chemical Classes of Neurotransmitters • Biogenic amines include: • Catecholamines • Dopamine, norepinephrine (NE), and epinephrine • Indolamines • Serotonin and histamine • Broadly distributed in the brain • Play roles in emotional behaviors and the biological clock

Chemical Classes of Neurotransmitters • Amino acids include: • GABA—Gamma ( )-aminobutyric acid • Glycine • Aspartate • Glutamate

Chemical Classes of Neurotransmitters • Peptides (neuropeptides) include: • Substance P • Mediator of pain signals • Endorphins • Act as natural opiates; reduce pain perception • Gut-brain peptides • Somatostatin and cholecystokinin

Chemical Classes of Neurotransmitters • Purines such as ATP: • Act in both the CNS and PNS • Produce fast or slow responses • Induce Ca 2+ influx in astrocytes • Provoke pain sensation

Chemical Classes of Neurotransmitters • Gases and lipids • Nitric oxide (NO) • Synthesized on demand • Activates the intracellular receptor guanylyl cyclase to cyclic GMP • Involved in learning and memory • Carbon monoxide (CO) is a regulator of c. GMP in the brain

Chemical Classes of Neurotransmitters • Gases and lipids • Endocannabinoids • Lipid soluble; synthesized on demand from membrane lipids • Bind with G protein–coupled receptors in the brain • Involved in learning and memory

Functional Classification of Neurotransmitters • Neurotransmitter effects may be excitatory (depolarizing) and/or inhibitory (hyperpolarizing) • Determined by the receptor type of the postsynaptic neuron • GABA and glycine are usually inhibitory • Glutamate is usually excitatory • Acetylcholine • Excitatory at neuromuscular junctions in skeletal muscle • Inhibitory in cardiac muscle