Lecture Materials for Amy Warenda Czura, Ph.D. Suffolk …€¦ · -contains:-neuroﬁbrils & neurotubules (abundant)-vesicles of neurotransmitter-lysosomes, mitochondria, enzymes-no

Nervous System: Nervous Tissue (Chapter 12)

Lecture Materials

for

Amy Warenda Czura, Ph.D.

Suffolk County Community College

Eastern Campus

Primary Sources for figures and content:

Marieb, E. N. Human Anatomy & Physiology 6th ed. San Francisco: Pearson Benjamin Cummings, 2004.

Martini, F. H. Fundamentals of Anatomy & Physiology 6th ed. San Francisco: Pearson

Benjamin Cummings, 2004.

Amy Warenda Czura. Ph.D. 1 SCCC BIO130 Chapter 12 Lecture Slides

Neural Tissue -3% of body mass -cellular, ~20% extracellular space -two categories of cells: 1. Neurons: conduct nervous impulses 2. Neuroglia / glial cells: “nerve glue”, supporting cells

Organization of Nervous System


1. Central Nervous System (CNS) -spinal cord, brain -function: integrate, process, coordinate sensory input and motor output 2. Peripheral Nervous System (PNS) -all neural tissue outside CNS -function: carry info to/from CNS via nerves Nerve = bundle of axons (nerve fibers) with blood vessels and CT -cranial nerves ↔ brain -spinal nerves ↔ spinal cord

Divisions of PNS: 1. Sensory/Afferent Division -sensory receptors → CNS A. Somatic afferent division -from skin, skeletal muscles, joints B. Visceral afferent division -from internal organs


2. Motor/Efferent Division -CNS → effectors A. Somatic Nervous System -“voluntary nervous system” -to skeletal muscles B. Autonomic Nervous System (ANS) -“involuntary nervous system” -to smooth & cardiac muscle, glands 1. Sympathetic Division - “fight or flight” 2. Parasympathetic Division - “rest and digest” (tend to be antagonistic to each other) Histology of Nervous System Neuron -function:conduct nervous impulses (message) -characteristics: 1. Extreme longevity 2. Amitotic (exceptions: hippocampus, olfactory receptors) 3. High metabolic rate: need O2 and glucose


Structure:

-large soma / perikaryon -large nucleus, large nucleolus (rRNA) -many mitochondria, ribosomes, RER, Golgi: (↑ATP, ↑protein synthesis to produce neurotransmitters) -Nissl bodies: visible RER & ribosomes, gray -neurofilaments = neurofibrils, neurotubules (internal structure) -no centrioles -2 types of processes: (cell extensions)


1. Dendrites: -receive info -carry a graded potential toward soma -contain same organelles as soma -short, branched -end in dendritic spines

2. Axon: -single, long -carry an action potential away from soma -release neurotransmitters at end to signal next cell -long ones = “nerve fibers”


-contains: -neurofibrils & neurotubules (abundant) -vesicles of neurotransmitter -lysosomes, mitochondria, enzymes -no Nissl bodies, no Golgi (no protein synthesis in axon) -connects to soma at axon hillock -covered in axolemma (membrane) -may branch: axon collaterals -end in synaptic terminals or knobs -may have myelin sheath: protein+lipid -protection -insulation -increase speed of impulse CNS: myelin from oligodendrocytes PNS: myelin from Schwann cells/ neurilemma cells


Axoplasmic transport -move materials between soma and terminal -along neurotubules on kinesins -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)


-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

Synapse -site where neuron communicates with another cell: neuron or effector


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% all neurons -most CNS


Functional Classification of Neurons: 1. Sensory/Afferent neurons -transmit info from sensory receptors to CNS

-most unipolar -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 2. Motor/Efferent neurons -transmit commands from CNS to effectors -most multipolar A. Somatic motor neurons -innervate skeletal muscle -conscious control or reflexes B. Visceral/Autonomic motor neurons -innervate effectors on smooth muscle, cardiac muscle, glands, adipose


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

Neuroglia =supporting cells

Neuroglia in CNS -outnumber neurons 10:1 -half mass of brain


1. Ependymal cells -line central canal of spinal cord and ventricles of brain -secrete cerebrospinal fluid (CSF) -have cilia to circulate CSF -CSF: cushion brain, nutrient & gas exchange

2. Astrocytes -most abundant CNS neuroglia -varying functions: a. blood brain barrier: processes wrap capillaries, control chemical exchange between blood and interstitial fluid of brain b. framework of CNS c. repair damaged neural tissue d. guide neuron development in embryo e. control interstitial environment: regulate conc. ions, gasses, nutrients, neurotransmitters


3. Oligodendrocytes -wide flat processes wrap 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)


Cells in the CNS


Neuroglia in PNS 1. Satellite cells -surround somas in ganglia -isolate PNS cells -regulate interstitial environment of ganglia

2. Schwann cells / Neurilemma cells -myelinate axons in PNS -whole cells wraps axon, many layers, organelles compressed in superficial layer (neurilemma) -Nodes (of Ranvier) between cells


-vital to repair of peripheral never fibers after injury: guide growth to original synapse


+ + + +

+

+ + + + +

+ + + +

+ + + +

+ + + + +

+ + + +

- -

- - - -

- - -

- - - - - - - - - - - -

Neurophysiology Neurons: conduct electrical impulse -requires transmembrane potential = electrical difference across cell membrane -cells: positive charge outside (pump cations out) and negative charge inside (proteins)

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


Ohm’s Law: current = voltage ÷ resistance Current highest when voltage high and resistance low Cell voltage set at -70mV but membrane resistance can be altered to create current Membrane resistance depends on permeability to ions: open or close ion channels Cell must always have some resistance or ions would equalize, voltage = zero, no current generated = no nervous impulse Membrane ion channels: -allow ion movement (alter resistance) -each channel specific to one ion type 1. Passive channels (leak channels) -always open, free flow -sets resting membrane potential at -70mV


2. 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, soma)

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


C. Mechanically regulated channels -open in response to membrane distortion -on dendrites of sensory neurons for touch, pressure, vibration

When channel opens, ions flow along electrochemical gradient: -diffusion (high conc. to low) -electrical attraction/repulsion


Sodium-Potassium Pump: -uses ATP to move 3 Na+ out 2 K+ in (70% of neuron ATP for this)

-runs anytime cell not conducting impulse -creates high [K+] inside and high [Na+]outside When Na+ channel opens: - Na+ flows into cell: 1. Favored by diffusion gradient 2. Favored by electrical gradient open channel = ↓resistance = ↑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+


Channels open = resistance low = ions move until equilibrium potential: depends on -diffusion gradient -electrical gradient Equilibrium Potential For K+ = -90mV

For Na+ = +66mV

Open channel → current → graded potential Graded potential = localized shift in transmembrane potential due to movement of charges in to /out of cell


Na+ channel opens = Na+ flows in, depolarization (cell less negative) K+ channel opens = K+ flows out, hyperpolarization (cell more negative)

Graded potentials: -occur on any membrane: dendrites and somas -can be depolarizing or hyperpolarizing -amount of depolarization or hyperpolarization depends on intensity of stimulus: ↑ channels open = ↑ 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: leak channels & Na+/K+ pump reset resting potential

Graded potential = localized change in transmembrane potential, not nervous impulse (message)


If big enough depolarization = action potential = nervous impulse = transmission to next cell

Action potentials: -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


The Generation of an Action Potential

-55 mV

1. Depolarization to threshold: - a graded potential depolarizes local membrane and flows toward the axon - 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 axolemma membrane open - Na+ flows into the cell depolarizing it - the transmembrane potential rapidly changes from -55mV to +30mV

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

(Handout)


Restimulation only when Na+ channels closed: influx of Na+ necessary for action potential

Absolute Refractory Period = -55mV (threshold) 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) Propagation of Action Potentials -once generated must be transmitted length of axon: hillock to terminal -speed depends on: 1. Degree of myelination (yes or no) 2. Axon diameter 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


B. 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

Continuous Propagation


Saltatory Propagation


2. Axon diameter -larger axon → less resistance → easier ion

flow → faster action potential A. Type A Fibers/Axon - 4-20µm diameter - myelinated (saltatory propagation) - action potentials 140m/sec - carry somatic motor and somatic sensory info B. Type B Fibers/Axon - 2-4µm diameter - myelinated (saltatory propagation) - action potentials 18m/sec - carry autonomic motor and visceral sensory info C. Type C Fibers/Axon - < 2µm diameter - unmyelinated (continuous propagation) - action potentials 1m/sec - carry autonomic motor and visceral sensory info


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

Synapse = junction between transmitting neuron (presynaptic cell) and receiving cell (postsynaptic cell), where nerve impulse moves from one cell to next Two types: 1. Electrical Synapse -direct contact via gap junctions -ions flow directly from pre to post cell -less common synapse -in brain (conscious perception) 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. Excitatory Neurotransmitters = -depolarization -propagate action potential 2. Inhibitory Neurotransmitters = -hyperpolarization -suppress action potential Propagation across chemical synapse always slow but allows variability


Events at a Synapse: e.g.Cholinergic Synapse

(Acetylcholine as neurotransmitter) (Handout)


Neurotransmitter Mechanism of Action

1. Direct effect on membrane potential

2. Indirect effect on membrane potential

(Handout)

(Handout)


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. Excitatory Post Synaptic Potential (EPSP) -causes depolarization 2. Inhibitory Post Synaptic Potential (IPSP) -causes hyperpolarization -inhibits postsynaptic cell (need larger stimulus to reach threshold) Multiple EPSPs needed to trigger action potential in post cell axon EPSP summation: 1. Temporal summation -single synapse fires repeatedly: string of EPSPs in one spot -each EPSP depolarizes more until threshold reached at hillock


2. Spatial summation -multiple synapses fire simultaneously -collective depolarization reaches threshold

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 same synapse conditions synapse, pre cell more easily stimulates post cell to threshold (repetition)


Most nervous system activity 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 Common Neurotransmitters: 1. Acetycholine- cholinergic synapses -excitatory -direct effect -skeletal neuromuscular junctions, many CNS synapses, all neuron to neuron PNS, all parasympathetic ANS 2. Norepinephrine- adrenergic synapses -excitatory -second messengers -many brain synapses, all sympathetic ANS effector junctions


3. Dopamine -excitatory or inhibitory -second messengers -many brain synapses, many functions -responsible for reward feeling -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 aminobutyric acid (GABA) -inhibitory -direct effect -brain: anxiety control, motor coordination -alcohol: augments effects = loss of coordination


Factors that disrupt neural function: 1. pH: normal = 7.4 @ pH 7.8 → spontaneous action potentials = convulsions @ pH 7.0 → no action potentials = unresponsive 2. Ion concentrations high extracellular [K+] → depolarize membranes = 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


Documents

Lecture Materials for Amy Warenda Czura, Ph.D. Suffolk …€¦ · -contains:-neuroﬁbrils & neurotubules (abundant)-vesicles of neurotransmitter-lysosomes, mitochondria, enzymes-no