Nervous Tissue Fundamentals of the Nervous P A

Nervous Tissue Fundamentals of the Nervous P A R and T A System Nervous Tissue 1

Nervous System The master controlling and communicating system of the body ¡ Functions l Sensory input –the stimuli goes to CNS l Integration – interpretation of sensory input l Motor output – response to stimuli coming from CNS ¡ 2

Nervous System 3 Figure 11. 1

Organization of the Nervous System Central nervous system (CNS) l Brain and spinal cord l Integration and command center ¡ Peripheral nervous system (PNS) l Paired spinal and cranial nerves l Carries messages to and from the spinal cord and brain ¡ 4

Peripheral Nervous System (PNS): Two Functional Divisions ¡ Sensory (afferent) division l Somatic sensory fibers – carry impulses from skin, skeletal muscles, and joints to the brain l Visceral sensory fibers – transmit impulses from visceral organs to the brain 5

Motor Division: Two Main Parts ¡ Motor (efferent) division l Somatic motor nervous system (voluntary)- carries impulses from the CNS to skeletal muscles. Conscious control 6

Motor Division: Two Main Parts ¡ Visceral motor nervous system or Autonomic nervous system (ANS) (involuntary) - carry impulses from the CNS to smooth muscle, cardiac muscle, and glands l Sympathetic and l Parasympathetic l Enteric Nervous System (ENS) 7

Histology of Nerve Tissue ¡ The two principal cell types of the nervous system are: l Neurons – excitable cells that transmit electrical signals l Neuroglias (glial) – cells that surround and wrap neurons 8

Supporting Cells: Neuroglia ¡ Neuroglia l Provide a supportive scaffolding for neurons l Segregate and insulate neurons l Guide young neurons to the proper connections l Promote health and growth 9

Neuroglia of the CNS Astrocytes ¡ Most abundant, versatile, and highly branched glial cells ¡ Maintain blood-brain barrier ¡ They cling to neurons and their synaptic endings, ¡ They wrap around capillaries l Regulate their permeability ¡ 10

Neuroglia of the CNS Provide structural framework for the neuron ¡ Guide migration of young neurons ¡ Control the chemical environment ¡ Repair damaged neural tissue ¡ 11

Astrocytes 12 Figure 11. 3 a

Neuroglia of the CNS Microglia – small, ovoid cells with spiny processes l Phagocytes that monitor the health of neurons ¡ Ependymal cells – range in shape from squamous to columnar l They line the central cavities of the brain and spinal column ¡ 13

Microglia and Ependymal Cells 14 Figure 11. 3 b, c

Neuroglia of the CNS ¡ Oligodendrocytes – branched cells l Myelin ¡ Wraps of oligodendrocytes processes around nerve fibers ¡ Insulates the nerve fibers 15

Neuroglia of the PNS Schwann cells (neurolemmocytes) – l Myelin ¡ Wraps itself around nerve fibers ¡ Insulates the nerve fibers ¡ Satellite cells surround neuron cell bodies located within the ganglia l Regulate the environment around the neurons ¡ 16

Oligodendrocytes, Schwann Cells, and Satellite Cells 17 Figure 11. 3 d, e

Neurons (Nerve Cells) Structural units of the nervous system l Composed of a body, axon, and dendrites l Long-lived, amitotic, and have a high metabolic rate ¡ Their plasma membrane function in: l Electrical signaling l Cell-to-cell signaling during 18 development ¡

Neurons (Nerve Cells) 19 Figure 11. 4 b

Nerve Cell Body (Perikaryon or Soma) Contains the nucleus and a nucleolus ¡ Is the major biosynthetic center ¡ Is the focal point for the outgrowth of neuronal processes ¡ Has no centrioles (hence its amitotic nature) ¡ Has well-developed Nissl bodies (rough ER) ¡ Contains an axon hillock – coneshaped area from which axons arise 20 ¡

Processes Arm like extensions from the soma ¡ There are two types of processes: axons and dendrites ¡ Myelinated axons are called tracts in the CNS and nerves in the PNS ¡ 21

Dendrites: Structure Short, tapering processes ¡ They are the receptive, or input, regions of the neuron ¡ Electrical signals are conveyed as graded potentials (not action potentials) ¡ 22

Neurons (Nerve Cells) 23 Figure 11. 4 b

Axons: Structure Slender processes of uniform diameter arising from the hillock ¡ Long axons are called nerve fibers ¡ Usually there is only one unbranched axon per neuron ¡ Axon collaterals ¡ Telodendria ¡ Axonal terminal or synaptic knobs ¡ 24

Axons: Function Generate and transmit action potentials ¡ Secrete neurotransmitters from the axonal terminals ¡ Movement along axons occurs in two ways l Anterograde — toward the axon terminal l Retrograde — toward the cell body 25 ¡

Myelin Sheath Whitish, fatty (protein-lipoid), segmented sheath around most long axons ¡ It functions to: l Protect the axon l Electrically insulate fibers from one another l Increase the speed of nerve impulse transmission ¡ 26

Myelin Sheath and Neurilemma: Formation Formed by Schwann cells in the PNS ¡ A Schwann cell: l Envelopes an axon l Encloses the axon with its plasma membrane l Has concentric layers of membrane that make up the myelin sheath ¡ 27

Myelin Sheath and Neurilemma: Formation 28 Figure 11. 5 a–c

Nodes of Ranvier (Neurofibral Nodes) Gaps in the myelin sheath between adjacent Schwann cells ¡ They are the sites where axon collaterals can emerge ¡ 29

Unmyelinated Axons A Schwann cell surrounds nerve fibers but coiling does not take place ¡ Schwann cells partially enclose 15 or more axons ¡ Conduct nerve impulse slowly ¡ 30

Axons of the CNS Both myelinated and unmyelinated fibers are present ¡ Myelin sheaths are formed by oligodendrocytes ¡ Nodes of Ranvier are widely spaced ¡ 31

Regions of the Brain and Spinal Cord White matter – dense collections of myelinated fibers ¡ Gray matter – mostly soma, dendrites, glial cells and unmyelinated fibers ¡ 32

Neuron Classification ¡ Structural: l Multipolar — three or more processes l Bipolar — two processes (axon and dendrite) l Unipolar — single, short process 33

A Structural Classification of Neurons 34 Figure 12. 4

Neuron Classification ¡ Functional: l Sensory (afferent) — transmit impulses toward the CNS l Motor (efferent) — carry impulses toward the body surface l Interneurons (association neurons) — any neurons between a sensory and a motor neuron 35

Comparison of Structural Classes of Neurons 36 Table 11. 1. 1

Comparison of Structural Classes of Neurons 37 Table 11. 1. 2

Neurophysiology Neurons are highly irritable ¡ Action potentials, or nerve impulses, are: l Electrical impulses carried along the length of axons l Always the same regardless of stimulus ¡ 38

Electricity 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 between two points Resistance (R) – hindrance to charge flow 39

Electricity Definitions ¡ ¡ Insulator – substance with high electrical resistance Conductor – substance with low electrical resistance 40

Electrical Current and the Body Reflects the flow of ions rather than electrons ¡ There is a potential on either side of membranes when: l The number of ions is different across the membrane l The membrane provides a resistance to ion flow ¡ 41

Role of Ion Channels ¡ Types of plasma membrane ion channels: l Nongated, or leakage channels – always open l Chemically gated channels – open with binding of a specific neurotransmitter 42

Role of Ion Channels Voltage-gated channels – open and close in response to membrane potential. ¡ 2 gates l Mechanically gated channels – open and close in response to physical deformation of receptors l 43

Gated Channels 44 Figure 12. 13

Operation of a Chemically Gated Channel Example: Na+-K+ gated channel ¡ Closed when a neurotransmitter is not bound to the extracellular receptor l Na+ cannot enter the cell and K+ cannot exit the cell ¡ Open when a neurotransmitter is attached to the receptor l Na+ enters the cell and K+ exits the cell ¡ 45

Operation of a Voltage-Gated Channel Example: Na+ channel ¡ Closed when the intracellular environment is negative l Na+ cannot enter the cell ¡ Open when the intracellular environment is positive l Na+ can enter the cell ¡ 46

Gated Channels ¡ When gated channels are open: l Ions move quickly across the membrane l Movement is along their electrochemical gradients l An electrical current is created l Voltage changes across the membrane 47

Electrochemical Gradient Ions flow along their chemical gradient when they move from an area of high concentration to an area of low concentration ¡ Ions flow along their electrical gradient when they move toward an area of opposite charge ¡ Electrochemical gradient – the electrical and chemical gradients taken together ¡ 48

Resting Membrane Potential (Vr) The potential difference (– 70 m. V) across the membrane of a resting neuron ¡ It is generated by different concentrations of Na+, K+, Cl , and protein anions (A ) ¡ 49

Resting Membrane Potential (Vr) ¡ Ionic differences are the consequence of: l Differential permeability of the neurilemma to Na+ and K+ l Operation of the sodiumpotassium pump 50

Measuring Membrane Potential 51 Figure 11. 7

Resting Membrane Potential (Vr) 52 Figure 11. 8

Nervous Tissue Fundamentals P ANervous RT B of the System and Nervous Tissue 53

Membrane Potentials: Signals Used to integrate, send, and receive information ¡ Membrane potential changes are produced by: l Changes in membrane permeability to ions l Alterations of ion concentrations across the membrane ¡ Types of signals – graded potentials and action potentials 54 ¡

Changes in Membrane Potential ¡ Changes are caused by three events l Depolarization – the inside of the membrane becomes less negative l Repolarization – the membrane returns to its resting membrane potential l Hyperpolarization – the inside of the membrane becomes more negative than the resting 55 potential

Depolarization, Repolarization and Hyperpolarization 56 Figure 12. 15

Graded Potentials Short-lived, local changes in membrane potential ¡ Decrease in intensity with distance ¡ Magnitude varies directly with the strength of the stimulus ¡ Depolarization or hyperpolarization ¡ Sufficiently strong graded potentials can initiate action potentials ¡ 57

Graded Potentials 58 Figure 11. 10

Graded Potentials Voltage changes are decremental ¡ Current is quickly dissipated due to the leaky plasma membrane ¡ Only travel over short distances ¡ 59

Action Potentials (APs) A brief reversal of membrane potential with a total amplitude of 100 m. V ¡ Action potentials are only generated by muscle cells and neurons ¡ They do not decrease in strength over distance ¡ They are the principal means of neural communication ¡ An action potential in the axon of a neuron is a nerve impulse 60 ¡

Action Potential: Resting State ¡ ¡ ¡ Na+ and K+ voltage gated channels are closed Leakage accounts for small movements of Na+ and K+ Each Na+ channel has two voltageregulated gates l Activation gates l Inactivation gates 61 Figure 11. 12. 1

Action Potential: Resting State Na channel is closed, but capable of opening 62

Action Potential: Depolarization Phase Na+ permeability increases; membrane potential reverses ¡ Na+ gates are opened; K+ gates are closed ¡ Threshold – a critical level of depolarization (-55 to -50 m. V) ¡ At threshold, depolarization becomes self-generating ¡ 63 Figure 11. 12. 2

Action Potential: Depolarization Phase Both Na channels are opened 64

Action Potential: Repolarization Phase Sodium inactivation gates close ¡ Membrane permeability to Na+ declines to resting levels ¡ As sodium gates close, voltagesensitive K+ gates open ¡ K+ exits the cell and internal negativity of the resting neuron is restored ¡ 65 Figure 11. 12. 3

Action Potential: Repolarization Phase Na activation channel is opened and inactivation is closed. 66

Action Potential: Hyperpolarization Potassium gates remain open, causing an excessive efflux of K+ ¡ This efflux causes hyperpolarization of the membrane (undershoot) ¡ 67 Figure 11. 12. 4

Action Potential: Hyperpolarization 68

Action Potential: Role of the Sodium-Potassium Pump Repolarization l Restores the resting electrical conditions of the neuron l Does not restore the resting ionic conditions ¡ Ionic redistribution back to resting conditions is restored by the sodiumpotassium pump ¡ 69

Phases of the Action Potential • 1 – resting state • 2 – depolarization phase • 3 – repolarization phase • 4 –hyperpolarization 70 Figure 11. 12

Action Potential 71

Propagation of an Action Potential along an Unmyelinated Axon 72

Propagation of an action potential Continuous propagation l Unmyelinated axon ¡ Saltatory propagation l Myelinated axon ¡ 73

Propagation of an Action Potential (Time = 0 ms)- continuous propagation Na gates open ¡ Na+ influx causes a patch of the axonal membrane to depolarize ¡ Positive Na ions in the axoplasm move toward the polarized (negative) portion of the membrane ¡ 74

Propagation of an Action Potential (Time = 2 ms)- continuous propagation A local current is created that depolarizes the adjacent membrane in a forward direction ¡ Local voltage-gated Na channels are opened ¡ The action potential moves away from the stimulus ¡ 75

Propagation of an Action Potential (Time = 4 ms)- continuous propagation Sodium gates close ¡ Potassium gates open ¡ Repolarization ¡ Hyperpolarization l Sluggish K channels are kept opened ¡ Resting membrane potential is restored in this region ¡ 76

Saltatory Conduction Current passes through a myelinated axon only at the nodes of Ranvier ¡ Voltage-gated Na+ channels are concentrated at these nodes ¡ Action potentials are triggered only at the nodes and jump from one node to the next ¡ Much faster than conduction along unmyelinated axons 77 ¡

Saltatory Conduction 78 Figure 11. 16

Types of stimuli ¡ ¡ Threshold stimulus l Stimulus strong enough to bring the membrane potential to a threshold voltage causing an action potential Subthreshold stimulus l weak stimuli that cause depolarization (graded potentials) but not action potentials 79

Coding for Stimulus Intensity ¡ ¡ ¡ Action potential are All-or-none phenomenon Strong stimuli can generate an action potential more often than weaker stimuli The CNS determines stimulus intensity by the frequency of impulse transmission 80

Stimulus Strength and AP Frequency 81 Figure 11. 14

Absolute Refractory Period Time from the opening of the Na+ activation gates until the closing of inactivation gates ¡ The absolute refractory period: l Prevents the neuron from generating an action potential l Ensures that each action potential is separate l Enforces one-way transmission of nerve impulses 82 ¡

Absolute and Relative Refractory Periods 83 Figure 11. 15

Relative Refractory Period The interval following the absolute refractory period when: l Sodium gates are closed l Potassium gates are open l Repolarization is occurring ¡ Stimulus stronger than the original one cause a new action potential ¡ 84

Conduction Velocities of Axons Conduction velocities vary widely among neurons ¡ Rate of impulse propagation is determined by: l Axon diameter – the larger the diameter, the faster the impulse l Presence of a myelin sheath – myelination dramatically increases impulse speed ¡ 85

Nerve Fiber Classification ¡ Nerve fibers are classified according to: l Diameter l Degree of myelination l Speed of conduction 86

Axon classification Type A fibers l Diameter of 4 to 20 µm and myelinated ¡ Type B fibers l Diameter of 2 to 4 µm and myelinated ¡ Type C fibers l Diameter less than 2 µm and unmyelinated ¡ 87

Multiple Sclerosis (MS) An autoimmune disease that mainly affects young adults ¡ Symptoms: visual disturbances, weakness, loss of muscular control, and urinary incontinence ¡ Nerve fibers are severed and myelin sheaths in the CNS become nonfunctional scleroses ¡ Shunting and short-circuiting of nerve impulses occurs 88 ¡

Synapses A junction that mediates information transfer from one neuron: l To another neuron l To an effector cell ¡ Presynaptic neuron – conducts impulses toward the synapse ¡ Postsynaptic neuron – transmits impulses away from the synapse ¡ 89

Synapses 90 Figure 11. 17

Types of Synapses Axodendritic – synapses between the axon of one neuron and the dendrite of another ¡ Axosomatic – synapses between the axon of one neuron and the soma of another ¡ Others: Axoaxonic, etc ¡ 91

Electrical Synapses ¡ Electrical synapses: l Are less common than chemical synapses l Correspond to gap junctions found in other cell types l Very fast propagation of action potentials 92

Electrical Synapses l Are important in the CNS in: ¡ Arousal from sleep ¡ Mental attention ¡ Emotions and memory ¡ Ion and water homeostasis ¡ Light transmitting (eye) 93

Chemical Synapses Specialized for the release and reception of neurotransmitters ¡ Typically composed of: ¡ Presynaptic neuron l Contains synaptic vesicles ¡ Postsynaptic neuron l The receptors are located typically on dendrites and soma ¡ 94

Chemical Synapses Synaptic Cleft l Fluid-filled space separating the presynaptic and postsynaptic neurons ¡ Transmission across the synaptic cleft: l Is a chemical event (as opposed to an electrical one) l Ensures unidirectional communication between neurons ¡ 95

Synaptic Cleft: Information Transfer Nerve impulses reach the axonal terminal of the presynaptic neuron and open Ca 2+ channels ¡ Neurotransmitter is released into the synaptic cleft via exocytosis ¡ 96

Synaptic Cleft: Information Transfer Neurotransmitter crosses the synaptic cleft and binds to receptors on the postsynaptic neuron ¡ Postsynaptic membrane permeability changes, causing an excitatory or inhibitory effect ¡ 97

Synaptic Cleft: Information Transfer n tio ial Ac tent po Ca 2+ 1 Neurotransmitter Axon terminal of presynaptic neuron Postsynaptic membrane Mitochondrion Axon of presynaptic neuron Na+ Receptor Postsynaptic membrane Ion channel open Synaptic vesicles containing neurotransmitter molecules 5 Degraded neurotransmitter 2 Synaptic cleft Ion channel (closed) 3 4 Ion channel closed Ion channel (open) 98 Figure 11. 18

Nervous Tissue Fundamentals ART C of the. PNervous System and Nervous Tissue 99

Termination of Neurotransmitter Effects ¡ Neurotransmitter bound to a postsynaptic neuron: l Produces a continuous postsynaptic effect l Blocks reception of additional “messages” l Must be removed from its receptor 100

Termination of Neurotransmitter Effects ¡ Removal of neurotransmitters occurs when they: l Are degraded by enzymes l Are reabsorbed by astrocytes or the presynaptic terminals l Diffuse from the synaptic cleft 101

Synaptic Delay Neurotransmitter must be released, diffuse across the synapse, and bind to receptors ¡ Synaptic delay – time needed to do this ¡ Synaptic delay is the rate-limiting step of neural transmission ¡ 102

Postsynaptic Potentials ¡ Neurotransmitter receptors mediate changes in membrane potential according to: l The amount of neurotransmitter released l The amount of time the neurotransmitter is bound to receptors 103

Postsynaptic Potentials ¡ The two types of postsynaptic potentials are: l EPSP – excitatory postsynaptic potentials l IPSP – inhibitory postsynaptic potentials 104

Excitatory Postsynaptic Potentials EPSPs are graded potentials that can initiate an action potential in an axon l Use only chemically gated channels l Na+ and K+ flow in opposite directions at the same time ¡ Postsynaptic membranes do not generate action potentials ¡ 105

Excitatory Postsynaptic Potential (EPSP) 106 Figure 11. 19 a

Inhibitory Synapses and IPSPs ¡ Neurotransmitter binding to a receptor at inhibitory synapses: l Causes the membrane to become more permeable to potassium and chloride ions l Leaves the charge on the inner surface negative l Reduces the postsynaptic neuron’s ability to produce an action potential 107

Inhibitory Postsynaptic (IPSP) 108 Figure 11. 19 b

Summation A single EPSP cannot induce an action potential ¡ EPSPs must summate temporally or spatially to induce an action potential ¡ Temporal summation – presynaptic neurons transmit impulses in rapid-fire order ¡ 109

Temporal Summation 110

Summation Spatial summation – postsynaptic neuron is stimulated by a large number of terminals at the same time ¡ IPSPs can also summate with EPSPs, canceling each other out ¡ 111

Spatial Summation 112

EPSP – IPSP Interactions 113

Neurotransmitters Chemicals used for neuronal communication with the body and the brain ¡ 50 different neurotransmitters have been identified ¡ Classified chemically and functionally ¡ 114

Chemical Neurotransmitters Acetylcholine (ACh) ¡ Biogenic amines ¡ Amino acids ¡ Peptides ¡ Novel messengers: ATP and dissolved gases NO and CO ¡ 115

Neurotransmitters: Acetylcholine First neurotransmitter identified, and best understood ¡ Released at the neuromuscular junction ¡ Synthesized and enclosed in synaptic vesicles ¡ 116

Neurotransmitters: Acetylcholine Degraded by the enzyme acetylcholinesterase (ACh. E) ¡ Released by: l All neurons that stimulate skeletal muscle l Some neurons in the autonomic nervous system ¡ 117

Neurotransmitters: Biogenic Amines Include: l Catecholamines – dopamine, norepinephrine (NE), and epinephrine l Indolamines – serotonin and histamine ¡ Broadly distributed in the brain ¡ Play roles in emotional behaviors and our biological clock ¡ 118

Synthesis of Catecholamines Enzymes present in the cell determine length of biosynthetic pathway ¡ Norepinephrine and dopamine are synthesized in axonal terminals ¡ Epinephrine is released by the adrenal medulla ¡ 119 Figure 11. 21

Synthesis of Catecholamines 120

Neurotransmitters: Amino Acids Include: l GABA – Gamma ( )-aminobutyric acid l Glycine l Aspartate l Glutamate ¡ Found only in the CNS ¡ 121

Neurotransmitters: Peptides Include: l Substance P – mediator of pain signals l Beta endorphin, dynorphin, and enkephalins ¡ Act as natural opiates; reduce pain perception ¡ Bind to the same receptors as opiates and morphine ¡ Gut-brain peptides – somatostatin, 122 and cholecystokinin ¡

Neurotransmitters: Novel Messengers ¡ ATP l Is found in both the CNS and PNS l Produces excitatory or inhibitory responses depending on receptor type l Induces Ca 2+ wave propagation in astrocytes l Provokes pain sensation 123

Neurotransmitters: Novel Messengers Nitric oxide (NO) l Activates the intracellular receptor guanylyl cyclase l Is involved in learning and memory ¡ Carbon monoxide (CO) is a main regulator of c. GMP in the brain ¡ 124

Functional Classification of Neurotransmitters ¡ Two classifications: excitatory and inhibitory l Excitatory neurotransmitters cause depolarizations (e. g. , glutamate) l Inhibitory neurotransmitters cause hyperpolarizations (e. g. , GABA and glycine) 125

Functional Classification of Neurotransmitters ¡ Some neurotransmitters have both excitatory and inhibitory effects l Determined by the receptor type of the postsynaptic neuron l Example: acetylcholine ¡ Excitatory at neuromuscular junctions with skeletal muscle ¡ Inhibitory in cardiac muscle 126

Neurotransmitter Receptor Mechanisms Direct: neurotransmitters that open ion channels l Promote rapid responses l Examples: ACh and amino acids ¡ Indirect: neurotransmitters that act through second messengers l Promote long-lasting effects l Examples: biogenic amines, peptides, and dissolved gases ¡ 127

Channel-Linked Receptors Composed of integral membrane protein ¡ Mediate direct neurotransmitter action ¡ Action is immediate, brief, simple, and highly localized ¡ 128

Channel-Linked Receptors Ligand binds the receptor, and ions enter the cells ¡ Excitatory receptors depolarize membranes ¡ Inhibitory receptors hyperpolarize membranes ¡ 129

Channel-Linked Receptors 130 Figure 11. 22 a

G Protein-Linked Receptors Responses are indirect, slow, complex, prolonged, and often diffuse ¡ These receptors are transmembrane protein complexes ¡ First and Second messengers. ¡ 131

Neurotransmitter Receptor Mechanism Ions flow Blocked ion flow (a) Channel closed Adenylate cyclase Channel open Neurotransmitter (ligand) released from axon terminal of presynaptic neuron 3 1 PPi 4 GTP ATP 5 c. AMP 5 3 Changes in membrane permeability and potential GTP 2 GDP Protein synthesis Enzyme activation GTP Receptor G protein (b) Nucleus Activation of specific genes 132 Figure 11. 22 b

Neural Integration: Neuronal Pools ¡ Functional groups of neurons that: l Integrate incoming information l Forward the processed information to its appropriate destination 133

Neural Integration: Neuronal Pools ¡ Simple neuronal pool l Input fiber – presynaptic fiber l Discharge zone – neurons most closely associated with the incoming fiber l Facilitated zone – neurons farther away from incoming fiber 134

Simple Neuronal Pool 135 Figure 11. 23

Types of Neuronal Pools 136

Types of Circuits in Neuronal Pools ¡ Divergent – information spreads from one neuron to several neurons or from one pool to several pools. l One motor neuron of the brain stimulates many muscle fibers 137 b Figure 11. 24 a,

Types of Circuits in Neuronal Pools ¡ Convergent – input from many neurons is funneled to one neuron or neuronal pools l Motor neurons of the diaphragm is controlled by different areas of the brain for subconscious and conscious functioning 138 d Figure 11. 24 c,

Types of Circuits in Neuronal Pools Reverberating –collateral branches of axons extend back toward the source of the impulse ¡ Involved in rhythmic activities l Breathing, sleep-wake cycle, etc ¡ Figure 139 11. 24 e

Types of Circuits in Neuronal Pools Parallel - incoming neurons stimulate several neurons in parallel arrays ¡ Divergence must take place before the parallel processing l Response to a painfull stimuli causes ¡ Withdrawal of the limb ¡ Shift of the weight ¡ Fell pain ¡ Shout “ouch!” ¡ Figure 140 11. 24 f

Patterns of Neural Processing ¡ Serial l Transmitting of the impulse from one neuron to another or from one pool to another l Spinal reflexes 141

Development of Neurons The nervous system originates from the neural tube and neural crest ¡ The neural tube becomes the CNS ¡ There is a three-phase process of differentiation: l Proliferation of cells needed for development l Migration – neuroblasts become amitotic and move externally 142 l Differentiation into neurons ¡

Axonal Growth ¡ Guided by: l Scaffold laid down by older neurons l Orienting glial fibers l Release of nerve growth factor by astrocytes l Neurotropins released by other neurons l Repulsion guiding molecules l Attractants released by target cells 143