1 Chapter 12: Neural Tissue. 2 Neural Tissue 3% of body mass Cellular, ~20% extracellular space Two categories of cells: 1.Neurons: conduct nervous impulses.

1

Chapter 12: Neural Tissue

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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

2. Neuroglia/glial cells: “nerve glue”- Supporting Cells- Protect neurons

3

Organs of the Nervous System

• Brain and spinal cord• Sensory receptors of sense organs

(eyes, ears, etc.)• Nerves connect nervous system

with other systems

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5

Nervous Systems1. Central Nervous System (CNS)

- Spinal cord, brain- Functions:

- integrate, process, coordinate sensory input and motor output

2. 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

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Division of PNS

1. Sensory/Afferent Division: carries sensory information

- Sensory receptors CNSA. Somatic afferent division

- From skin, skeletal muscles, and joints

B. Visceral afferent division- From internal organs

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Division of PNS2. Motor/Efferent Division: carries motor

commands- CNS effectorsA. Somatic Nervous System: Controls skeletal

muscle contractions- “voluntary nervous system”- To skeletal muscles contractions

B. Autonomic Nervous System (ANS)- “involuntary nervous system”- To smooth and cardiac muscle, glands

contractions1. Sympathetic Division: stimulating effect

- “fight or flight”2. Parasympathetic Division: relaxing effect

- “rest and digest” **Tend to be Antagonistic to Each Other**

8

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

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

1. ability to learn new facts

2. ability to experience motor stimuli

3. ability to experience sensory stimuli

4. ability to remember past events

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The structure of a typical neuron, and the function

of each component.

11

Histology of Nervous System

• Neuron/Nerve Cell– Function: conduct nervous impulses

(message)– Characteristics:

1. Extreme longevity2. 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

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The Structure of Neurons

Figure 12–1

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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 (cell extension):

1. Dendrite2. Axon

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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

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Regions of a Neuron2. 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)

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Regions of a Neuron2. Axon

- Connects to soma at axon hillock- Covered in axolemma (membrane) --- Axoplasm

(cytoplasm) - May branch: axon collaterals- 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

17

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 soma– Recycle breakdown products from used

neurotransmitters• Some viruses use retrograde transport to

gain access to CNS (polio, herpes, rabies)

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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 neurotransmitterNeurotransmitter = chemical, transmits signal from

pre- to postsynaptic cell across synaptic cleftSynaptic knob = small, round, when postsynaptic

cell is neuron, synapse on dendrite or somaSynaptic terminal = complex structure, at

neuromuscular or neuroglandular junction

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Structural Classification of Neurons

1. Anaxonic neurons:- Dendrites and axon look same- Brain and special sense organs

2. Bipolar neurons:- 1 dendrite, 1 axon- Soma in middle- Rare: special sense organs, relay from receptor to neuron

3. Unipolar neurons:- 1 long axon, dendrites at one end, soma off side (T shape)- Most sensory neurons

4. Multipolar neurons:- 2 or more dendrites- 1 long axon- 99% of all neurons- Most CNS

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

neurons?

1. sensory neurons2. motor neurons

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Functional Classification of Neurons

1. 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

B. Visceral Sensory Neurons- Receptors monitor internal conditions

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2. Motor/Efferent Neurons- Transmit commands from CNS to effectors- Mostly multipolar neuronsA. 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

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3. Interneurons/Association Neurons- Distribute sensory info and

coordinate motor activity- Between sensory and motor neurons- In brain, spinal cord, autonomic

ganglia- Most are multipolar

24

The locations and functions of

neuroglia.

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Neuroglia

• Neuroglia = supporting cells• Neuroglia in CNS

– Outnumber neurons 10:1– Half mass of brain

• Neuroglia Cell in the CNS1. Ependymal cells2. Astrocytes3. Oligodendrocytes4. Microglia

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Neuroglia Cells of the CNS1. 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

2. Astrocytes- Most abundant CNS neuroglia- Varying functions:

1. Blood brain barrier- Processes wrap capillaries- Control chemical exchange between

blood and interstitial fluid of the brain2. Framework of CNS3. Repair damaged neural tissue4. Guide neuron development in embryo5. Control interstitial environment:

- Regulate conc. Ions, gasses, nutrients, neurotransmitters

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Neuroglia Cells of the CNS3. 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”

4. Microglia- Phagocytic- Wander CNS- Engulf debris, pathogens- Important CNS defense

- No immune cells or antibodies

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Neuroglia of the CNS

Figure 12–4

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Neuroglia in PNS1. 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

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Neuroglia in PNS2. Schwann Cells cont.

- Some hold bundles of unmyelinated axon

- Vital to repair of peripheral nerve fibers after injury- Guide growth to original synapse

31

Which type of neuroglia would occur in larger than normal

numbers in the brain tissue of a person with a CNS infection?

1. astrocytes2. microglial cells3. ependymal cells4. oligodendrocytes

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Neural Responses to Injuries

Figure 12–6 (1 of 2)

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Neural Responses to Injuries

Figure 12–6 (2 of 2)

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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

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How the resting potential is

created and maintained.

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5 Main Membrane Processes in Neural Activities

Figure 12–7 (Navigator)

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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

38

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

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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-gatedB. Voltage regulated channelsC. Mechanically regulated channels

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

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Active Channels: Gated Channels

Figure 12–10

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2. Active Channels- open/close in response to signalA. Chemically regulated/ligand-gated

- Open in response to chemical binding- Located on any cell membrane

- Dendrites and soma

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2. Active ChannelsB. Voltage regulated channels

- open/close in response to shift in transmembrane potential- excitable membrane only: conduct action

potentials- axolemma, sarcolemma

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2. Active ChannelsC. Mechanically Regulated Channels

- Open in response to membrane distortion

- On dendrites of sensory neurons for:- touch, pressure, vibration

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

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Sodium-Potassium Pump

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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 gradient2. Favored by electrical gradientOpen channel = decr. Resistance = incr. ion flow/current

• When K+ channel opens– K+ flows out of cell:

1. Favored by diffusion gradient only2. Electrical gradient repels K+ exit

- Thus less current than Na+

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

• Equilibrium Potential

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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

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Electrochemical Gradients

Figure 12–9c, d

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Summary: Resting Potential

Table 12-1

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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

53

Graded Potentials: The Resting State

• Opening sodium channel produces a current which causes graded potential


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• Graded potential:– localized shift in transmembrane

potential due to movement of charges into/out of cell

• Na+ channel opens = Na+ flows in– depolarization (cell less negative)

• K+ channel opens = K+ flows out– hyperpolarization (cell more negative)

Graded Potential

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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

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Graded Potentials

• Localized change in transmembrane potential, not nervous impulse (message)

• If big enough depolarization– Action potential = nervous impulse =

transmission to next cell

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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

Figure 12–11 (Step 1)

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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


59

Characteristics of Graded Potentials

Table 12-2

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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

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Generating the Action Potential


62

Generation of an Action Potential

1. 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

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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

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• 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

65Table 12-3

66

How would a chemical that blocks the sodium channels in

neuron cell membranes affect a neuron’s ability to depolarize?

1. It would enhance depolarization.

2. It would inhibit depolarization completely.

3. It would slow depolarization.4. It would have no effect on

depolarization.

67

What effect would decreasing the concentration of extracellular potassium ions have on the

transmembrane potential of a neuron?

1. repolarization2. hypopolarization3. decreased transmembrane

potential4. hyperpolarization

68

Propagation of Action Potential

• Once generated must be transmitted along the length of the axon hillock to terminal

• Speed depends on:1. Degree of myelination2. Axon diameter

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2 Methods of Propagating Action Potentials

1. Continuous propagation:– unmyelinated axons

2. Saltatory propagation:– myelinated axons

70


1. MyelinationA. Continuous Propagation:

- Unmyelinated axons- Whole membrane depolarizes and repolarizes

sequentially hillock to terminal- Only forward movement

- Membrane behind always in absolute refractory period

71Figure 12–14

Continuous Propagation

• Of action potentials along an unmyelinated axon

• Affects 1 segment of axon at a time

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Continuous Propagation: Step 1

• Action potential in segment 1 • Depolarizes membrane to +30 mV


73


• Local current • Depolarizes second segment to

threshold


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• Second segment develops action potential

• First segment enters refractory period


75


• Local current depolarizes next segment

• Cycle repeats• Action potential travels in 1

direction (1 m/sec)


76


1. MyelinationA. 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

77

Saltatory Propagation

• Of action potential along myelinated axon

Figure 12–15

78


Figure 12–15 (Steps 1, 2)

79


Figure 12–15 (Steps 3, 4)

80

Graded Potentials and Action Potentials

Table 12–4

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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

82

Propagation of Action Potentials

2. Axon Diameter- Larger axon less resistance easier

ion flow faster action potential– Axons are classified by:

• Diameter, myelination, speed of action potentials

– Three types of axons: • Type A, Type B, and Type C fibers

83

Axon Diameter1. Type A Fibers

- 4-20µm diameter- Myelinated (saltatory propagation)- Action potential 140m/sec- Carry somatic motor and somatic sensory info

2. Type B Fibers- 2-4µm diameter- Myelinated (saltatory propagation)- Action potential 18m/sec- Carry autonomic motor and visceral sensory info

3. Type C Fibers- < 2µm diameter- Unmyelinated (continuous propagation)- Action potential 1m/sec- Carry autonomic motor and visceral sensory info

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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

85

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

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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)

2. Chemical Synapse:- Most common

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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

neurotransmitter1. Excititory Neurotransmitters =

- Depolarization (shift from resting potential toward 0 mV)

- Propagate Action Potential2. Inhibitory Neurotransmitters =

- Hyperpolarization (shift from resting potential to -80 mV)

- Suppress Action Potential

Propagation across chemical synapse always slow but allow variability

88

The events that occur at a chemical synapse.

89

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

90

Synaptic Activity


91

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

92Figure 12–16 (Step 1)

Events at a Cholinergic Synapse: Step 1

• 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

96

Events at a Cholinergic Synapse

Table 12-5

97

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

communication?

1. Communication would cease.

2. Communication would be enhanced.

3. Communication would be misdirected.

4. Communication would continue as before.

98

Neurotransmitter Mechanism of Action

1. 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)

99


2. Indirect effect on membrane potential- Binds a receptor that activates a G

protein in the post synaptic cell - Active G protein activates a second

messenger- cAMP, cGMP, diacylglyceride, Ca++

- The second messenger opens ion channels or activates enzymes

- Provides slower but longer lasting effects- E.g. Norepinephrine (Adrenergic synapse)

100


2. Indirect effect on membrane potential- Example of indirect action:

1. Neurotransmitter binds receptor2. Receptor activates G protein3. G Protein activates adenylate cyclase4. Adenylate cyclase converts ATP into cyclic AMP5. cAMP opens ion channels

101

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

102

Post Synaptic Potential• Multiple EPSPs needed to trigger action

potential in post cell axon• EPSP summation:

– Temporal and Spatial Summation1. Temporal Summation

- Single synapse fires repeatedly- String of EPSPs in one spot

- Each EPSP depolarizes more until threshold reached at hillock

103

Post Synaptic Potential

• EPSP summation: – Temporal and Spatial Summation1. Spatial Summation

- Multiple synapses fire stimultaneously- Collective depolarization reaches threshold

104

EPSP/IPSP Interactions

Figure 12–19

105

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

106

Common Neurotransmitters

1. Acetycholine: cholinergic synapses- Excititory- Direct effect- Skeletal neuromuscular junctions, many CNS

synapses, all neuron to neuron PNS, all parasympathetic ANS

2. Norepinephrine – adrenergic synapses- Excititory- Second messengers- Many brain synapses, all sympathetic ANS

effector junctions

107

Common Neurotransmitters3. Dopamine

- Excititory or inhibitory- Second messengers- Many brain synapses

- Cocaine: inhibits removal = ‘high’- Parkinson’s disease: damage neurons = ticks, jitters

4. Serotonin- Inhibitory- Direct or second messenger- Brain stem for emotion

- Anti-depression/anti-anxiety drugs block uptake

5. Gamma aminobytyric acid (GABA)- Inhibitory- Direct effect- Brain: anxiety control, motor coordination

- Alcohol: augments effects = loss of coordination

108

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 Figure 12–20a

109

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

Figure 12–20a

110

Factors that Disrupt Neural Function

1. ph: normal = 7.4- At pH 7.8 spontaneous action potentials = convulsions- At pH 7.0 no action potentials = unresponsive

2. Ion concentration- High extracellular [K+] depolarize membrane = death,

cardiac arrest

3. Temperature: normal = 37°C- higher: neurons more excitable Fever = hallucinations- Lower: neurons non-responsive Hypothermia = lethargy,

confusion

4. Nutrients- neurons: no reserves, use a lot of ATP- Require constant and abundant glucose- Glucose only

5. Oxygen- Aerobic respiration only for ATP- No ATP = neuron damage/death

111

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

112

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)

113

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

1 Chapter 12: Neural Tissue. 2 Neural Tissue 3% of body mass Cellular, ~20% extracellular space Two categories of cells: 1.Neurons: conduct nervous impulses.

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