Chapter 12: Neural Tissue

Transcript Chapter 12: Neural Tissue

Chapter 12: Neural Tissue

Neural Tissue

• 3% of body mass • Cellular, ~20% extracellular space • Two categories of cells: 1.

Neurons: conduct nervous impulses - cells that send and receive signals Neuroglia/glial cells: - Supporting Cells - Protect neurons “nerve glue” 2

Organs of the Nervous System

• • • Brain and spinal cord Sensory receptors (eyes, ears, etc.) of sense organs Nerves connect nervous system with other systems 3

Nervous Systems

Central Nervous System (CNS) Spinal cord, brain Functions: integrate, process, coordinate sensory input and motor output Peripheral Nervous System (PNS) All neural tissue outside of CNS Functions: Carry info to/from the CNS via nerves Nerves: Bundle of axons (nerve fibers) with blood vessels and CT Carry sensory information and motor commands in PNS Cranial nerves brain Spinal nerves spinal cord 5

Division of PNS

Sensory/Afferent Division: carries sensory information - Sensory receptors  A.

CNS Somatic afferent division - From skin, skeletal muscles, and joints B.

Visceral afferent division - From internal organs 6

Division of PNS

2. Motor/Efferent Division: carries motor commands A.

CNS  effectors Somatic Nervous System: Controls skeletal muscle contractions “voluntary nervous system” To skeletal muscles  contractions Autonomic Nervous System (ANS) “involuntary nervous system” To smooth and cardiac muscle, glands  1. Sympathetic Division: stimulating effect - “fight or flight” 2. Parasympathetic Division: relaxing effect - “rest and digest” contractions **Tend to be Antagonistic to Each Other** 7

Receptors and Effectors

• Receptors : – detect changes or respond to stimuli – neurons and specialized cells – complex sensory organs (e.g., eyes, ears) • Effectors : – respond to efferent signals – cells and organs 8

What would damage to the afferent division of the PNS affect?

ability to learn new facts ability to experience motor stimuli ability to experience sensory stimuli ability to remember past events

The structure of a typical neuron, and the function of each component.

Histology of Nervous System

• Neuron/Nerve Cell – Function: conduct nervous impulses (message) – Characteristics: 1. Extreme longevity 2. Amitotic - Direct cell division by simple cleavage of the nucleus without spindle formation or appearance of chromosomes - exceptions: hippocampus, olfactory receptors 3. High metabolic rate: need O2 and glucose 11

The Structure of Neurons

12 Figure 12–1

The Structure of Neurons

• Large soma/perikaryon(cytoplasm) • Large nucleus, large nucleolus (rRNA) • • • Many mitochondria, ribosomes, RER, Golgi – Increase ATP, increase protein synthesis to produce neurotransmitters Nissl bodies: visible RER and ribosomes, gray Neurofilaments: internal structure – Neurofibrils, neurotubules • • No centrioles 2 types of processes 1. Dendrite 2. Axon (cell extension): 13

Regions of a Neuron

1. Dendrites: - Receive info - Carry a graded potential toward soma - Contain same organelles as soma - Short, branched - End in dendritic spines 14

Regions of a Neuron

Axon: - single, long - Carry an action potential away from soma - Release neurotransmitters at end to signal next cell - Long ones = “nerve fibers” - Contains: - Neurofibrils and neurotubules (abundant) - Vesicles of neurotransmitter - Lysosomes, mitochondria, enzymes No nissl bodies, no golgi (no protein synthesis in axon) 15

Regions of a Neuron

2. Axon - Connects to soma at axon hillock - Covered in axolemma (cytoplasm) (membrane) -- - May branch: axon collaterals Axoplasm - End in synaptic terminals or knobs - May have myelin sheath : protein+lipid - Function: - Protection, Insulation, and Increase speed of impulse - CNS: myelin from Oligodendrocytes - PNS: myelin from Schwann cells 16

Axoplasmic Transport

• Move materials between soma and terminal • Large molecules synthesized in the cell body, such as vesicles and mitochondria are unable to move via simple diffusion • Large molecules are transported by motor proteins called kinesins , which walk along neurotubule tracks to their destinations. • Anterograde transport = soma  terminal – neurotransmitters from soma • Retrograde transport = terminal neurotransmitters  soma – Recycle breakdown products from used • Some viruses use retrograde transport to gain access to CNS (polio, herpes, rabies) 17

Synapse

• Site where neuron communicates with another cell: – neuron or effector • Presynaptic cell sends message along axon to axon terminal • Postsynaptic cell receives message as neurotransmitter Neurotransmitter = chemical, transmits signal from pre to postsynaptic cell across synaptic cleft Synaptic knob = small, round, when postsynaptic cell is neuron, synapse on dendrite or soma Synaptic terminal = complex structure, at neuromuscular or neuroglandular junction 18

Structural Classification of Neurons

Anaxonic neurons : Dendrites and axon look same Brain and special sense organs Bipolar neurons : 1 dendrite, 1 axon Soma in middle Rare: special sense organs, relay from receptor to neuron Unipolar neurons : 1 long axon, dendrites at one end, soma off side (T shape) Most sensory neurons Multipolar neurons : 2 or more dendrites 1 long axon 99% of all neurons Most CNS 19

A tissue sample shows unipolar neurons. Are these more likely to be sensory neurons or motor neurons?

sensory neurons motor neurons

Functional Classification of Neurons

Sensory/Afferent Neurons - Transmit info from sensory receptors to CNS - Mostly unipolar neurons - Soma in peripheral sensory ganglia - Ganglia = collection of cell bodies in PNS A.

Somatic Sensory Neurons - Receptors monitor outside conditions Visceral Sensory Neurons - Receptors monitor internal conditions 21

Functional Classification of Neurons

Motor/Efferent Neurons - Transmit commands from CNS to effectors - Mostly multipolar neurons A.

Somatic Motor Neurons - Innervate skeletal muscle - Innervation = distribution of sensory/motor nerves to a specific region/organ - Conscious control or reflexes B.

Visceral/Autonomic Motor Neurons - Innervate effectors on smooth muscle, cardiac muscle, glands, and adipose 22

Functional Classification of Neurons

Interneurons/Association Neurons - Distribute sensory info and coordinate motor activity - Between sensory and motor neurons - In brain, spinal cord, autonomic ganglia - Most are multipolar 23

The locations and functions of neuroglia.

Neuroglia

• Neuroglia = supporting cells • Neuroglia in CNS – Outnumber neurons 10:1 – Half mass of brain • Neuroglia Cell in the CNS 1.

Ependymal cells Astrocytes Oligodendrocytes Microglia 25

Neuroglia Cells of the CNS

Ependymal Cells Line central canal of spinal cord and ventricles of the brain Secrete cerebrospinal fluid (CSF) Have cilia to circulate CSF CSF: cushion brain, nutrient and gas exchange Astrocytes Most abundant CNS neuroglia Varying functions: 1. Blood brain barrier Processes wrap capillaries Control chemical exchange between blood and interstitial fluid of the brain 2. Framework of CNS 3. Repair damaged neural tissue 4. Guide neuron development in embryo 5. Control interstitial environment: Regulate conc. Ions, gasses, nutrients, neurotransmitters 26

Neuroglia Cells of the CNS

Oligodendrocytes Wide flat processes wrap around local axons = myelin sheath 1 cell contributes myelin to many neighboring axons Lipid in membrane insulates axon for faster action potential conductance Gaps on axon between processes/myelin = nodes of Ranvier, necessary to conduct impulse White, myelinated axons = “white matter” Microglia Phagocytic Wander CNS Engulf debris, pathogens Important CNS defense No immune cells or antibodies 27

Neuroglia of the CNS

28 Figure 12–4

Neuroglia in PNS

Satellite Cells Surround somas in ganglia Isolate PNS cells Regulate interstitial environment of ganglia Ganglia = mass of neuronal soma and dendrites 2.

Schwann cells Myelinate axon in PNS Whole cells wraps axon, many layers Neurilemma: bulge of schwann cell, contains organelles Nodes of Ranvier between cells 29

Neuroglia in PNS

Schwann Cells cont.

- Some hold bundles of unmyelinated axon - Vital to repair of peripheral nerve fibers after injury - Guide growth to original synapse 30

Which type of neuroglia would occur in larger than normal numbers in the brain tissue of a person with a CNS infection?

astrocytes microglial cells ependymal cells oligodendrocytes

Neural Responses to Injuries

32 Figure 12–6 (1 of 2)

Neural Responses to Injuries

33 Figure 12–6 (2 of 2)

KEY CONCEPT

• Neurons perform all communication, information processing, and control functions of the nervous system • Neuroglia preserve physical and biochemical structure of neural tissue, and are essential to survival and function of neurons 34

How the resting potential is created and maintained.

5 Main Membrane Processes in Neural Activities

36 Figure 12–7 (Navigator)

•

Neurophysiology

Neurons : conduct electrical impulse – Requires transmembrane potential = electrical difference across the cell membrane – Cells: positive charge outside (pump cations out) and negative charge inside (protein) Voltage = measure of potential energy generated by separation of opposite charges Current = flow of electrical charges (ions) Cell can produce current (nervous impulse) when ions move to eliminate the potential difference (volts) across the membrane Resistance = Restricts ion movement (current) • High resistance = low current • Membrane has resistance, restricts ion flow/current 37

Neurophysiology

• Ohm’s Law : current = voltage ÷ resistance • Current is highest when voltage is high and resistance is low • Cell voltage set at -70mV but membrane resistance can be altered to create current • Membrane resistance depends on permeability to ions: open or closed ion channels • Cell must always have some resistance or ions would equalize, voltage = zero – No current generated = no nervous impulse 38

Membrane Ion Channels

• Allow ion movement (alter resistance) • Each channel is specific to one ion type 1.

Passive Channels (leaky channels) 2.

Active Channels A. Chemically regulated/ligand-gated B. Voltage regulated channels C. Mechanically regulated channels 39

Membrane Ion Channels

1. Passive Channels (leaky channels) - Resting Potential - Always open, free flow - Sets resting membrane potential at -70mV 40

Active Channels: Gated Channels

41 Figure 12–10

Membrane Ion Channels

Active Channels - open/close in response to signal A.

Chemically regulated/ligand-gated - Open in response to chemical binding - Located on any cell membrane - Dendrites and soma 42

Membrane Ion Channels

2. Active Channels B.

Voltage regulated channels - open/close in response to shift in transmembrane potential - excitable membrane only: conduct action potentials - axolemma, sarcolemma 43

Membrane Ion Channels

2. Active Channels C.

Mechanically Regulated Channels - Open in response to membrane distortion - On dendrites of sensory neurons for: - touch, pressure, vibration 44

Membrane Ion Channels

• When channel opens, ions flow along electrochemical gradient: – Diffusion (high conc. to low) – Electrical attraction/repulsion 45

Sodium-Potassium Pump

• • • • •

Sodium-Potassium Pump

– Uses ATP to move 3 Na+ out and 2 K+ in 70% of neurons use ATP for this Runs anytime the cell is not conducting an impulse Creates high [K+] inside and high [Na+] outside – When Na+ cell opens Na+ flows into cell: 1. Favored by diffusion gradient 2. Favored by electrical gradient Open channel = decr. Resistance = incr. ion flow/current – When K+ channel opens K+ flows out of cell: 1. Favored by diffusion gradient only 2. Electrical gradient repels K+ exit - Thus less current than Na+ 47

• Channels open = resistance low = ions move until equilibrium potential: depends on – Diffusion gradient – Electrical gradient • Equilibrium Potential 48

Electrical vs. Chemical Gradients

• The electrical gradient opposes the chemical gradient – K+ inside and outside of the cell are attracted to the negative charges on the inside of the cell membrane, and repelled by the positive charges on the outside of the cell membrane • indicated in white on the next slide – Chemical gradient is strong enough to overpower the electrical gradient, but this weakens the force driving K+ out of the cell • Net driving force indicated in grey on the next slide – The Electrochemical Gradient 49

Electrochemical Gradients

50 Figure 12–9c, d

Summary: Resting Potential

51 Table 12-1

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 • Membrane permeability to Na + and K + determines transmembrane potential • Sodium and potassium channels are either passive or active 52

Graded Potentials : The Resting State

• Opening sodium channel produces a current which causes graded potential 53 Figure 12–11 (Navigator)

Graded Potential

• Graded potential: – localized shift in transmembrane potential due to movement of charges into/out of cell • Na+ channel opens = Na+ flows – depolarization ( cell less negative ) in • K+ channel opens = K+ flows out – hyperpolarization ( cell more negative ) 54

Graded Potentials

• Occur on any membrane : dendrites and somas • Can be depolarizing or hyperpolarizing • Amount of depolarization or hyperpolarization depends on the intensity of stimulus – Incr. channels open = Incr. voltage change • Passive spread from site of stimulation over short distance • Effect on membrane potential decreases with distance from stimulation site • Repolarization occurs as soon as stimulus is removed – Leaky channels and Na+/K+ pump reset resting potential 55

Graded Potentials

• Localized change in transmembrane potential, not nervous impulse (message) • If big enough depolarization – Action potential = nervous impulse = transmission to next cell 56

Graded Potentials : Step 1

• Resting membrane exposed to chemical • Sodium channel opens • Sodium ions enter the cell • Transmembrane potential rises • Depolarization occurs – A shift in transmembrane potential toward 0 mV 57 Figure 12–11 (Step 1)

Graded Potentials : Step 2

• Movement of Na + through channel • Produces local current • Depolarizes nearby cell membrane ( graded potential ) • Change in potential is proportional to stimulus 58 Figure 12–11 (Step 2)

Characteristics of Graded Potentials

59 Table 12-2

Action Potential

• Occur on excitable membranes only – Axolemma, sarcolemma • Always depolarizing • Must depolarize to threshold (-55mV) before action potential begins – Voltage gated channels on excitable membrane open at threshold to propagate action potential • “all-or-none” – All stimuli that exceed threshold will produce identical action potentials • Action potential at one site depolarizes adjacent sites to threshold • Propagated across entire membrane surface without decrease in strength 60

Generating the Action Potential

61 Figure 12–13 (Navigator)

Generation of an Action Potential

Depolarization to threshold: A graded potential depolarizes local membrane and flows toward the axons If threshold is met (-55mV) at the hillock, an action potential will be triggered 2.

Activation of sodium channels and rapid depolarization: At threshold (-55mV) , voltage-regulated sodium channels on the excitable membrane open Na+ flows into the cell depolarizing it The transmembrane potential rapidly changes from 55mV to +30 mV 62

Generation of an Action Potential

3. Inactivation of sodium channels and activation of potassium channels: At +30mV Na+ channels close and K+ channels open K+ flows out of the cell repolarizing it 4. Return to normal permeability: At -70mV K+ channels begin to close The cell hyperpolarizes to -90mV until all channels finish closing Leak channels restore the resting membrane potential to -70mV 63

Generation of an Action Potential

• • Restimulation only when Na+ channels closed: – Influx of Na+ necessary for action potential Absolute Refractory Period: – Threshold (-55mV) to +30mV, Na+ channels open, membrane cannot respond to additional stimulus • Relative Refractory Period: – +30mV to -70mV (return to resting potential) – Na+ channels closed, membrane capable of second action potential but requires larger/longer stimulus (threshold elevated) • Cell has ions for thousands of action potentials • Eventually must run Sodium-Potassium pump (burn ATP) to reset high [K+] inside and high [Na+] outside – Death = no ATP, but stored ions can generate action potentials for awhile 64

65 Table 12-3

How would a chemical that blocks the sodium channels in neuron cell membranes affect a neuron’s ability to depolarize?

It would enhance depolarization.

It would inhibit depolarization completely.

It would slow depolarization.

It would have no effect on depolarization.

What effect would decreasing the concentration of extracellular potassium ions have on the transmembrane potential of a neuron?

repolarization hypopolarization decreased transmembrane potential hyperpolarization

Propagation of Action Potential

• Once generated must be transmitted along the length of the axon hillock to terminal • Speed depends on: 1. Degree of myelination 2. Axon diameter 68

2 Methods of Propagating Action Potentials

Continuous propagation : – unmyelinated axons 2.

Saltatory propagation : – myelinated axons 69

Propagation of Action Potential

1. Myelination A.

Continuous Propagation: - Unmyelinated axons - Whole membrane depolarizes and repolarizes sequentially hillock to terminal - Only forward movement - Membrane behind always in absolute refractory period 70

Continuous Propagation

• Of action potentials along an unmyelinated axon • Affects 1 segment of axon at a time 71 Figure 12–14

Continuous Propagation: Step 1

• Action potential in segment 1 • Depolarizes membrane to +30 mV 72 Figure 12–14 (Step 1)

Continuous Propagation: Step 2

• Local current • Depolarizes second segment to threshold 73 Figure 12–14 (Step 2)

Continuous Propagation: Step 3

• Second segment develops action potential • First segment enters refractory period 74 Figure 12–14 (Step 3)

Continuous Propagation: Step 4

• Local current depolarizes next segment • Cycle repeats • Action potential travels in 1 direction (1 m/sec) 75 Figure 12–14 (Step 4)

Propagation of Action Potential

1. Myelination A.

Saltatory Propagation: - Myelinated axons - Depolarization only on exposed membrane at nodes - Myelin insulates covered membrane from ion flow - Action potential jumps from node to node - Faster and requires less energy to reset 76

Saltatory Propagation

• Of action potential along myelinated axon 77 Figure 12–15

Saltatory Propagation

78 Figure 12–15 (Steps 1, 2)

Saltatory Propagation

79 Figure 12–15 (Steps 3, 4)

Graded Potentials and Action Potentials

80 Table 12–4

Axon Diameter and Propagation Speed

• Ion movement is related to cytoplasm concentration • Axon diameter affects action potential speed • The larger diameter, the lower the resistance 81

Propagation of Action Potentials

2. Axon Diameter – - Larger axon  flow  less resistance faster action potential  easier ion – Axons are classified by: • Diameter, myelination, speed of action potentials • Three types of axons: Type A , Type B , and Type C fibers 82

Axon Diameter

Type A Fibers 4-20µm diameter Myelinated (saltatory propagation) Action potential 140m/sec Carry somatic motor and somatic sensory info Type B Fibers 2-4µm diameter Myelinated (saltatory propagation) Action potential 18m/sec Carry autonomic motor and visceral sensory info Type C Fibers < 2µm diameter Unmyelinated (continuous propagation) Action potential 1m/sec Carry autonomic motor and visceral sensory info 83

KEY CONCEPT

• “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 84

Myelination

• Requires space, metabolically expensive • Only important fibers large and myelinated • Occurs in early childhood • Results in improved coordination •

Multiple Sclerosis:

– Genetic disorder, myelin on neurons in PNS destroyed  numbness, paralysis 85

Synapse

• Synapse – Junction between transmitting neuron (presynaptic cell) and receiving cell (postsynaptic cell), where nerve impulse moves from one cell to the next • Two types: 1.

Electrical Synapse: - Direct contact via gap junctions - Ion flow directly from pre to post cell - Less common synapse - In brain (conscious perception) Chemical Synapse: - Most common 86

2. Chemical Synapse Most common Pre and post cells separated by synaptic cleft Presynaptic neuron releases neurotransmitter to trigger effect on post synaptic cell Dynamic: facilitate or inhibit transmission, depends on neurotransmitter 1. Excititory Neurotransmitters = - Depolarization (shift from resting potential toward 0 mV) - Propagate Action Potential 2. Inhibitory Neurotransmitters = - Hyperpolarization (shift from resting potential to -80 mV) - Suppress Action Potential Propagation across chemical synapse always slow but allow variability 87

The events that occur at a chemical synapse.

The Effect of a Neurotransmitter

• On a postsynaptic membrane: – depends on the receptor – not on the neurotransmitter • e.g., acetylcholine (ACh) : – usually promotes action potentials – but inhibits cardiac neuromuscular junctions 89

Synaptic Activity

90 Figure 12–16 (Navigator)

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 91

Events at a Cholinergic Synapse : Step 1

• Action potential arrives, depolarizes synaptic knob 92 Figure 12–16 (Step 1)

Events at a Cholinergic Synapse : Step 2

• Calcium ions enter synaptic knob, trigger exocytosis of ACh 93 Figure 12–16 (Step 2)

Events at a Cholinergic Synapse : Step 3

• ACh binds to receptors, depolarizes postsynaptic membrane 94 Figure 12–16 (Step 3)

Events at a Cholinergic Synapse : Step 4

• AChE breaks ACh into acetate choline and 95 Figure 12–16 (Step 4)

Events at a Cholinergic Synapse

96 Table 12-5

What effect would blocking voltage regulated calcium channels at a cholinergic synapse have on synaptic communication?

Communication would cease.

Communication would be enhanced.

Communication would be misdirected.

Communication would continue as before.

Neurotransmitter Mechanism of Action

Direct effect on membrane potential - Open or close ion channels upon binding to the post synaptic cell - Provides a rapid response - E.g. Ach ( cholinergic synapse ) 98

Neurotransmitter Mechanism of Action

Indirect effect on membrane potential - Binds a receptor that activates a G protein the post synaptic cell - Active G protein activates a second messenger - cAMP, cGMP, diacylglyceride, Ca++ in - The second messenger opens ion channels or activates enzymes - Provides slower but longer lasting effects - E.g. Norepinephrine (Adrenergic synapse) 99

Neurotransmitter Mechanism of Action

Indirect effect on membrane potential - Example of indirect action: 1. Neurotransmitter binds receptor 2. Receptor activates G protein 3. G Protein activates adenylate cyclase 4. Adenylate cyclase converts ATP into cyclic AMP 5. cAMP opens ion channels 100

Post Synaptic Potential

• Graded potential caused by a neurotransmitter due to opening or closing of ion channels on post synaptic cell membrane • Two types: 1.

Excititory Post Synaptic Potential (EPSP) - Causes depolarization 2.

Inhibitory Post Synaptic Potential (IPSP) - Causes hyperpolarization - Inhibits postsynaptic cell - Need larger stimulus to reach threshold 101

Post Synaptic Potential

• Multiple EPSPs needed to trigger action potential in post cell axon • EPSP summation: – Temporal and Spatial Summation 1.

Temporal Summation - Single synapse fires repeatedly - String of EPSPs in one spot - Each EPSP depolarizes more until threshold reached at hillock 102

Post Synaptic Potential

• EPSP summation: – Temporal and Spatial Summation 1.

Spatial Summation - Multiple synapses fire stimultaneously - Collective depolarization reaches threshold 103

EPSP/IPSP Interactions

104 Figure 12–19

•

Post Synaptic Potential

Facilitated = Depolarized – Brought closer to threshold by some sort of stimulus – Less stimulus now required to reach threshold – E.g. Caffeine • Post Synaptic Potentiation: – Repeat stimulation of the same synapse conditions synapse, pre cell more easily stimulated, allowing the post cell to reach the threshold (repetition) • Most nervous system activities results from interplay of EPSPs and IPSPs to promote differing degrees of facilitation or inhibition to allow constant fine tuning of response • Neuromodulators: – Chemicals that influence synthesis, release, or degradation of neurotransmitters thus altering normal response of the synapse 105

Common Neurotransmitters

Acetycholine: cholinergic synapses Excititory Direct effect Skeletal neuromuscular junctions, many CNS synapses, all neuron to neuron PNS, all parasympathetic ANS Norepinephrine – adrenergic synapses Excititory Second messengers Many brain synapses, all sympathetic ANS effector junctions 106

Common Neurotransmitters

Dopamine Excititory or inhibitory Second messengers Many brain synapses Cocaine: inhibits removal = ‘high’ Parkinson’s disease: damage neurons = ticks, jitters Serotonin Inhibitory Direct or second messenger Brain stem for emotion Anti-depression/anti-anxiety drugs block uptake Gamma aminobytyric acid (GABA) Inhibitory Direct effect Brain: anxiety control, motor coordination Alcohol: augments effects = loss of coordination 107

Presynaptic Facilitation

• Activity at an axoaxonal synapse increases the amount of neurotransmitter released when an action potential arrives at the synaptic knob. • This increase enhances and prolongs the neurotransmitter’s effect on the Postsynaptic membrane 108 Figure 12–20a

Presynaptic Inhibition

• Activity at an axoaxonal synapse via the release of GABA inhibits the opening of voltage-regulated calcium channels in the synaptic knob.

• Results in a reduced amount of neurotransmitters released when an action potential arrives there • Thus, reducing the effects of synaptic activity on the postsynaptic membrane 109 Figure 12–20a

Factors that Disrupt Neural Function

ph: normal = 7.4

At pH 7.8  At pH 7.0  spontaneous action potentials = convulsions no action potentials = unresponsive Ion concentration High extracellular [K+]  arrest depolarize membrane = death, cardiac Temperature: normal = 37°C higher: neurons more excitable  Lower: neurons non-responsive  Fever = hallucinations Hypothermia = lethargy, confusion Nutrients neurons: no reserves, use a lot of ATP Require constant and abundant glucose Glucose only Oxygen Aerobic respiration only for ATP No ATP = neuron damage/death 110

SUMMARY

• Neural tissue and the neuron • Anatomical divisions of the nervous system • Central and peripheral nervous systems • Nerves and axons • Functional divisions of the nervous system • Afferent division and receptors and Efferent division and effectors • Somatic and autonomic nervous systems • Structure of neurons: – organelles of neuron: neurofilaments, neurotubules, neurofibrils – structures of axon: axon hillock, initial segment, axoplasm – synapse and neurotransmitters • Classification of neurons: – structural classifications: anaxonic, bipolar, unipolar, and multipolar – functional classifications: sensory, motor, and interneurons 111

SUMMARY

• 4 types of neuroglia: – ependymal, astrocytes, and oligodendrocytes, microglia • Ganglia and neurons of PNS: – satellite cells, Schwann cells • Repair of neurons in the PNS • Transmembrane potential: – electrochemical gradient – passive and active channels • Gated channels: – chemically regulated, voltage-regulated, mechanically regulated • Action potentials: – threshold – refractory period – continuous and saltatory propagation – 3 types of axons (A, B, and C fibers) 112

SUMMARY

• Transmission of nerve impulses across a synapse: – presynaptic and postsynaptic neurons – electrical and chemical synapses – excitatory and inhibitory neurotransmitters – cholinergic synapses (ACh) – other neurotransmitters (NE, dopamine, seratonin, GABA) • Graded potentials: – depolarization and hyperpolarization • Neuromodulators: – direct, indirect, and lipid-soluble gases • Rate of generation of action potentials Information processing: – integration of postsynaptic potentials – EPSPs and IPSPs – spatial and temporal summation – presynaptic inhibition and facilitation 113