C11 - Nervous System & Nervous Tissue Pt.1

Define Nervous System

The master controlling and communicating system of body

How do cells communicate?

• Via electrical and chemical signals

  1. Rapid and specific

  2. Usually cause almost immediate responses

List and explain the basic functions of the nervous system

1. Sensory input

   • information gathered by sensory receptors about internal and external changes

2. Integration

   • Processing and interpretation of sensory input

3. Motor output

   • Activation of effector organs (muscles and glands) produces a response

Explain the structural and functional divisions of the nervous system

1. Central Nervous System (CNS)

   • Brain and spinal cord of dorsal body cavity

   • Integration and control center

   • Interprets sensory input and dictates motor output

2. Peripheral Nervous System (PNS)

   • The portion of nervous system outside CNS

   • Consists mainly of nerves that extend from brain and spinal cord

     • Spinal nerves to and from spinal cord

     • Cranial nerves to and from brain

Explain the functional sub-divisions of the PNS

1. Sensory (afferent) division

   • Somatic sensory fibers → convey impulses from skin, skeletal muscles, and joints to CNS

   • Visceral sensory fibers → convey impulses from visceral organs to CNS

2. Motor (efferent) division

   • Transmits impulses from CNS to effector organs

     1. Muscles

     2. Glands

   • Two divisions

     1. Somatic nervous system

     2. Autonomic nervous system

Explain the functional sub-sub-divisions of the Motor (Efferent) Division

1. Somatic Nervous System

   • Somatic motor nerve fibers conduct impulses from CNS to skeletal muscle

   • Voluntary nervous system → conscious control of skeletal muscles

2. Autonomic Nervous System

   • Consists of visceral motor nerve fibers

   • Regulates smooth muscle, cardiac muscle, and glands

   • Involuntary nervous system

   • Two functional subdivisions

     1. Sympathetic

     2. Parasympathetic

Explain the functional sub-sub-sub-divisions of the Autonomic Nervous System

1. Sympathetic

   • Puts tour body’s systems on alert

2. Parasympathetic

   • Carries signals that relax those systems

Overview of Structural and Functional Divisions of the Nervous System

List and define the two principal cell types in Nervous Tissue

1. Neuroglia (glial cells)

   • Small cells that surround and wrap delicate neurons

2. Neurons (nerve cells)

   • Excitable cells that transmit electrical signal

List the types of neuroglia and their functions of CNS

1. Astrocytes

2. Microglial cells

3. Ependymal cells

4. Oligodendrocytes

Explain the function of Astrocytes

Neuroglia (glial cells)

• STRUCTURE

  • Most abundant, versatile, and highly branched of glial cells

  • Cling to neurons, synaptic endings, and capillaries

• FUNCTION

  1. Support and brace neurons

  2. Play role in exchanges between capillaries and neurons

  3. Participate in information processing in brain

Explain the function of Microglial cells

Neuroglia (glial cells)

• STRUCTURE

  • Small, ovoid cells with thorny processes that touch and monitor neurons

  • Migrate toward injured neurons

• FUNCTION

  1. Can transform to phagocytize microorganism and neuronal debris

Explain the function of Ependymal cells

Neuroglia (glial cells)

• STRUCTURE

  • Range in shape from squamous to columnar

  • Line the central cavities of the brain and spinal column

  • May be ciliated → cilia beat to circulate CSF

• FUNCTION

  1. Produce cerebrospinal fluids (CSF)

  2. Form permeable barrier between CSF in cavities and tissue fluid bathing CNS cells

Explain the function of Oligodendrocytes

Neuroglia (glial cells)

• STRUCTURE

  • Branched cells

• FUNCTION

  1. Processes wrap CNS nerve fibers, forming insulating myelin sheaths in thicker nerve fibers

Name and describe two major neuroglia seen in PNS

Surround neurons in PNS

1. Satellite cells

   • STRUCTURE

     • Surround neuron cell bodes in PNS

   • FUNCTION

     • Similar to astrocytes of CNS

2. Schwann cells (neurolemmocytes)

   • STRUCTURE

     • Surround all peripheral nerve fibers and form myelin sheaths in thicker nerve fibers

   • FUNCTION

     • Similar functions as oligodendrocytes

     • Vital to regeneration of damaged peripheral nerve fibers

Define Neuron

• aka Nerve cell→ Structural units of nervous system

• Large, highly specialized cells that conducts impulses

• All have cell body and one or more processes

List the special characteristics of Neurons

1. Extreme longevity (lasts a person’s lifetime)

2. Amitotic, with few exceptions

3. High metabolic rate → requires continuous supply of oxygen and glucose

Describe Neurons important structural components, and relate each to a functional role

1. Cell body (soma)

   • Biosynthetic center of a neuron

2. Dendrites (Neuron process)

   • Branching neuron process that serves as a receptive, or input region

   • Transmits an electrical signal TOWARD the cell body

3. Axon (Neuron process)

   • Carries action potentials AWAY from the neuron cell body

   • Efferent process

   • The conduction portion of a neuron

Explain the structure and function of Neuron Cell Body

aka Perikaryon or Soma

• STRUCTURE

  • Contains spherical nucleus with nucleolus

  • Some contain pigments

  • In most, plasma membrane is part of receptive region that receives input into from other neurons

• FUNCTION

  • Biosynthetic center of neuron

    • Synthesizes proteins, membranes, chemicals

    • Rough ER (chromatophliic substances, or Nissi bodies)

Differentiate between a Nucleus and Ganglion

• Most neuron cell bodies are located in CNS

  1. Nuclei → clusters of neuron cell bodies in CNS

  2. Ganglia → clusters of neuron cell bodies in PNS

Explain the structure and function of Neuron Processes

• STRUCTURE

  • Armlike processes that extend from cell body

• LOCATION

  • CNS contains both neuron cell bodies and their processes

  • PNS contains chiefly neuron processes

• FUNCTION

  1. Dendrites

  2. Axon

Differentiate Nerve and a tract 

• Tracts

  • Bundles of neuron processes in CNS

• Nerves

  • Bundles of neuron processes in PNS

Explain the structure and function of Dendrites

• STRUCTURE

  • Motor neuron can contain 100s of these short, tapering, diffusely branched processes

  • Contain same organelles as in cell body

  • In many brain areas, finer dendrites are highly specialized to collect information

    • Contain dendritic spines, appendages with bulbous or spiky ends

• FUNCTION

  • Receptive (input) region of neuron

  • Convey incoming messages TOWARD cell body as graded potentials (short distance signals)

Explain the structure and function of Axon

• STRUCTURE

  • Each neuron has one axon that starts at cone-shaped area are called axon hillock

  • In some neuron axons are short or absent; in others, axon comprises almost entire length of cell

    • Some axons can be over 1 meter long

  • Nerve fibers → Long axons

  • Axon collaterals → Occasional branches

    • Can number as many as 10,000 terminal branches

  • Axon terminals (or terminal boutons) → Distal endings

• FUNCTION

  • Axon is the conducting region of neuron

  • Generates nerve impulses and transmits them AWAY from axolemma (neuron cell membrane) to axon terminal

    • Terminal → region that secretes neurotransmitters, which are released into extracellular space

    • Can excite or inhibit neurons it contracts

  • Carries conversations with different neurons at same time

  • Axons rely on cell bodies to renew proteins and membranes

  • Quickly decay if cut or damaged

Axon Transport

• Axons have efficient internal transport mechanisms

  • Molecules and organelles are moved along axons by motor proteins and cytoskeletal elements

• Movement occurs in both directions

  • Anterograde → AWAY from the cell body

    • EX: mitochondria, cytoskeletal elements, membrane components (vesicles) used to renew the axon plasma membrane, and enzymes needed to synthesize certain neurotransmitters

  • Retrograde → TOWARD cell body

    • EX: organelles to be degraded, signal molecules, viruses, and bacterial toxins

Axonal Transport - Clinical Homeostatic Imbalance

• Certain viruses and bacterial toxins damage neural tissues by using retrograde axonal transport

• EX: polio, rabies, and herpes simplex viruses, and tentanus toxin

Define Myelin sheath

• STUCTURE

  • Composed of myelin, a whitish, protein-lipid substance

• FUNCTION

  • Protect and electrically insulate axon

  • Increase speed of nerve impulse transmission

Myelinated vs Non-myelinated fibers

1. Myelinated fibers

   • Segmented sheath surrounds most long or large-diameter axons

2. Non-myelinated fibers

   • Do not contain sheath

   • Conduct impulses more slowly

Explain the importance of the myelin sheath and describe how it is formed in the peripheral and central nervous systems

1. Myelination in PNS

   • Formed by Schwann cells

     • Wraps around axon in jelly roll fashion

     • One cell forms one segment of myelin sheath

   • Plasma membranes have less protein

     • No channels or carriers, so good electrical insulators

   • Myelin sheath gaps

     • Gaps between adjacent Schwann cells

     • Sites where axon can emerge

   • Non-myelinated fibers

     • Thin fibers not wrapped in myelin; surrounded by Schwann cells but no coiling; one cell may surround 15 different fibers

2. Myelination in CNS

   • Formed by processes of oligodendrocytes, not whole cells

     • Each cell can wrap up to 60 axons at once

   • Myelin sheath gap present

   • Thinnest fibers are unmyelinateud

     • But covered by long extensions of adjacent neuroglia

White matter vs Gray matter

1. White matter

   • Regions of brain and spinal cord with dense collections of myelinated fibers

     • Usually fiber tracts

2. Gray matter

   • Mostly neuron cell bodies and non-myelinated fibers

Classify neurons by structure and by function 

1. Multipolar

   • STRUCTURE

     • 1 axon, other dendrites

     • Three or more processes

   • FUNCTION

     • Most common and major neuron type in CNS

2. Bipolar

   • STRUCTURE

     • 1 axon, one dendrites

     • Two processes

   • FUNCTION

     • Rare

     • EX: retina and olfactory mucosa

3. Unipolar (pseudo-unipolar)

   • STRUCTURE

     • Two axons

     • One T-like processes

   • FUNCTION

     • Peripheral (distal) process: associated with sensory receptor

     • Proximal (central) process: enters CNS

List the types of neurons grouped by direction in which nerve impulse travels relative to CNS

1. Sensory

   • Transmit impulses from sensory receptors TOWARD CNS

   • Almost all are unipolar

   • Cell bodies are located in ganglia in PNS

2. Motor

   • Carry impulses FROM CNS to effectors

   • Multipolar

   • Most cell bodies are located in CNS (except some autonomic neurons

3. Interneurons

   • Shuttle signals THROUGH CNS pathways

   • Most are entirely within CNS

   • Lie between motor and sensory neurons

   • 99% of body’s neurons are interneurons

T/F: Neurons can change their resting membrane potential

True

• Like all cells, neurons have a resting membrane

• Unlike most other cells, neurons can rapidly change resting membrane potential

• Neurons are highly excitable

Explain the Basic Principles of Electricity

• Opposite charges are attracted to each other

• Energy is required to keep opposite charges separated across a membrane

• Energy is liberated when the charges move toward one another

• When opposite charges are separated, the system has potential energy

Define Voltage

• A measure of potential energy generated by separated charge

  • Measured between two points in volts (V) or millivolts (mV)

Define Current

Flow of electrical charge (ions) between two points

Role of membrane ion channels

Large proteins serve as selective membrane ion channels

Identify different types of membrane ion channels 

1. Leakage (non-gated) channels

   • Always open

2. Gated channels

   • Part of the protein changes shape to open/close the channel

     1. Chemically gated

     2. Voltage-gated

     3. Mechanically gated

List and describe the three main Gated Channels

1. Chemically gated

   • Open in response to binding of the appropriate neurotransmitter

2. Voltage-gated

   • Open in response to changes in membrane potential

3. Mechanically gated

   • Open and close in response to physical deformation of receptors, as in sensory receptors

Explain what happens when gated channels are open.

• Ions diffuse quickly:

  1. Along chemically concentration gradients from HIGHER concentration to LOWER concentration

  2. Along electrical gradients toward OPPOSITE electrical charge

Define Electrochemical Gradient

• Electrical and chemical gradients combined

  • Ion flow creates an electrical current, and voltage changes across membrane

Describe the relationship between current, voltage and resistance 

• Current: flow of electrical charge (ions) between two points

• Voltage: a measure of potential energy generated by separated charge

  • Called potential difference or potential

    • Charge difference across plasma membrane results in potential

  • Greater charge difference between points = higher voltage

• Electrochemical gradient: electrical and chemical gradients combined

  • Ion flow creates an electrical current, and voltage changes across membrane

Define resting membrane potential and describe its electrochemical basis 

• Resting membrane potential of a resting neuron is approximately -70mV

  • The cytoplasmic side of membrane is negatively charged relative to the outside

  • The actual voltage difference varies form -40mV to -90mV

  • The membrane is said to be polarized → one side having postive charge and other having negative

Define Voltmeter

Can measure potential (charge) difference across membrane of resting cell

How is Potential generated?

1. Differences in ionic composition of intracellular fluid (ICF) and extracellular fluid (ECF)

2. Differences in plasma membrane permeability

Explain Differences in Ionic Composition

Generating the resting membrane potential

• ECF has HIGHER concentration of Na+ than ICF

  • Balanced chiefly by chloride ions (Cl-)

• ICF has HIGHER concentration of K+ than ECF

  • Balanced by negatively charged proteins

• K+ plays most important role in membrane potential

Explain Differences in Plasma Membrane Permeability

Generating the resting membrane potential

• Impermeable to large anionic proteins

• Slightly permeable to Na+ (through leakage channels)

  • Na+ diffuses into cell down concentration gradient

• 25 times more permeable to K+ than Na+ (more leakage channels)

  • K diffuses out of cell down concentration gradient

• Quite permeable to Cl-

• More potassium diffuses out than sodium diffuses in

  • RESULT INSIDE OF THE CELL MORE NEGATIVE

    • Negative membrane potenital

Role of Sodium-Potassium Pump (Na+/K+ ATPase)

• Stabilizes resting membrane potential

• Maintains concentration gradients for Na+ and K+

• Three Na+ are pumped out of cell while two K+ are pumped back in

How is the Resting Membrane Potential Changed?

1. Concentrations of ions across membrane change

2. Membrane permeability to ions changes

Changing the Resting Membrane Potential produces what two types of signals?

Changes in membrane potential are used as signals to receive, integrate, and send information

1. Graded potential

   • Incoming signals operating over short distances

2. Action potentials

   • Long-distance signals of axons

Depolarization vs Hyperpolarization

• Depolarization: Decrease in membrane potential (moves toward zero and above)

  • Inside of membrane becomes LESS NEGATIVE than resting membrane potential

  • Probability of producing impulse increases

• Hyper-polarization: Increases in membrane potential (away from zero)

  • Increase of membrane becomes MORE NEGATIVE than resting membrane potential

  • Probability of producing impulse decrease

Compare and contrast graded potentials and action potentials 

SIMILAR

• Brief reversal of membrane potential with a change in voltage of ~100mV

• In neurons, also referred to as nerve impulses

• Involves opening of specific voltage-gated channels

DIFFERENCE

• Action potentials do not decay over distance as graded potentials do

  • Principle way neurons send signals - means of long distance neural communication

Overview of Voltage channels

Explain how action potential are generated and propagated along neurons 

1. Resting state: all voltage-gated Na+ and K+ channels are closed

   • Only leakage channels for Na+ and K+ are open

   • Maintains the resting membrane potential (-70mV)

2. Depolarization: Threshold stimulus → Na+ channels open, allowing Na+ entry

   • Inside less negative

3. Repolarization: Na+ channels are inactivating and K+ voltage-gated channels open

   • Na+ channel inactivation gates close

     • Membrane permeability to Na+ declines to resting state

     • AP spike stops rising

   • Voltage-gated K+ channels open

     • K+ exists cell down its electrochemical gradient

   • Repolarization → membrane returns to resting membrane

     • Resets electrical conditions, not ionic conditions

4. Hyperpolarization: Some K+ channels remain open and Na+ channels reset

   • Na+/K+ pumps (thousands of them in an axon) restore ionic conditions

T/F: All depolarization events produce APs

False

• Not all depolarization events produce APs

• For an axon to “fire,” depolarization must reach threshold voltage to trigger AP

Explain Threshold and the All-Or-Nothing Phenomenon

• At threshold:

  1. Membrane is depolarized by 15-20 mV

  2. Na+ permeability increases

  3. Na+ influx exceeds K+ efflux

  4. The positive feedback cycle begins

• All-or-None

  • An AP either happens completely, or does not happen at all

Explain Propagation of an Action Potential

• Propagation allows AP to be transmitted from origin down entire axon length toward terminals

  1. Na+ influx through voltage gates in one membrane area local currents that cause opening of Na+ voltage gates in adjacent membrane area

     • Leads to depolarization of the area → which in turn causes depolarization in the next area

  2. One initiated an AP is self-propagating

  3. Since Na+ channels closer to the AP origin are still inactivated, no new AP is generated there

     • AP occurs in a FOWARD DIRECTION

Define Refractory period

• Time in which neuron cannot trigger another AP

• Voltage-gated Na+ channels are open, so neuron respond to another stimulus

Define absolute and relative refractory periods 

• Absolute refractory period: neuron cannot respond to another stimulus, no matter how strong

• Relative refractory period: An exceptionally strong stimulus can reopen the Na+ channels that have already returned to their resting state and generate another AP

Define Continuous Conduction

Slow conduction that occurs in non-myelinated axons

Define saltatory conduction and explain how it differs from continuous conduction 

• Saltatory conduction: occurs in myelinated axons is about 30x faster

  • Myelin sheaths insulate and prevent leakage of charge

  • Voltage-gated Na+ channels are located at myelin sheath gaps

  • APs generated only at gaps

  • Electrical signals appear to jump rapidly from gap to gap

Clinical Example: know the cause of multiple sclerosis

• Multiple sclerosis (MS) is an autoimmune disease that affects primarily young adults

• Myelin sheaths in CNS are destroyed when immune system attacks myelin

  • Turns myelin into harden lesions called scleroses

  • Impulse condition slows and eventually ceases

  • Demyelinated axons increase Na+ channels, causing cycles of relapse and remission

Symptoms and Treatment of Multiple Sclerosis

• Symptoms

  • Visual disturbances

  • Weakness

  • Loss of muscular control

  • Speech disturbances

  • Incontinence

• Treatment

  • Drugs that modify immune system activity

List examples of impaired AP impulse propagation

• Impaired AP impulse propagation can be caused by a number chemical and physical factors

  1. Local anesthetics act by blocking voltage-gated Na+ channels

  2. Cold temperatures or continuous pressure interrupt blood circulation and delivery of oxygen to neurons

     • Cold fingers get numb, or foot “goes to sleep”

Define synapse

• Junctions that mediate information transfer, connects neurons

  • From one neuron to another neuron

  • Or from one neuron an effector cell

• Nervous system works because information flows from neuron to neuron

Describe the Structure of the Synapse

Most function as both

1. Presynaptic neuron

   • Neuron conducting impulses TOWARD synapse (sends information)

2. Postsynaptic neuron

   • Neuron transmitting electrical signal AWAY from synapse (receives information)

     • In PNS may be a neuron, muscle cell, or gland cell

List the Main Types of Synapses

1. Chemical synapse

   • Most common type of synapse

   • Specialized for release and reception of chemical neurotransmitters

2. Electrical synapse

Distinguish between electrical and chemical synapses by structure and by the way they transmit information

1. Chemical synapses

   • STRUCTURE

     • Axon terminal of presynaptic neuron → contains synaptic vesicles filled of synaptic vesicles filled with neurotransmitter

     • Receptor region on postsynaptic neuron’s membrane → receives neurotransmitter

       • Usually on dendrite or cell body

     • Two parts separated by fluid filled synaptic cleft

   • FUNCTION

     • Electrical impulse changed to chemical across synapse, then back into electrical

   • Transmission across synaptic cleft

     1. Synaptic cleft prevents nerve impulses from directly passing from one neuron to next

     2. Chemical event (as opposed to an electrical one)

     3. Depends on release, diffusion, and receptor binding of neurotransmitters

     4. Ensures unidirectional communication between neurons

Events at the Chemical Synapse

1. AP arrives at axon terminal of presynaptic neuron

2. Voltage-gated Ca2+ channels open, and Ca2+ enters axon terminal

   • Ca2+ flows down electrochemical gradient from ECF to inside of axon terminal

3. Ca2+ entry causes synaptic vesicles to release neurotransmitter

   • Exocytosis of neurotransmitter into synaptic cleft

   • The higher the impulse frequency, the more vesicles exocytose, leading to a greater effect on the postsynaptic cell

4. Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane

   • Often chemically gated ion channels

5. Binding of neurotransmitter opens ion channels, creating graded potentials

   • Binding causes receptor protein to change shape, which causes ion channels to open

   • Causes a graded potential in postsynaptic cell

   • Can be excitatory or inhibitory event

   • Some receptor proteins are also ion channels

6. Neurotransmitter effects are terminated

   • As long as neurotransmitter is binding to receptor, potentials will continue, so process needs to be regulated

   • Within a few milliseconds, neurotransmitter effect is terminated in one of three ways

     1. Re-uptake → by astrocytes or axon terminal

     2. Degradation → by enzymes

     3. Diffusion → away from synaptic cleft

Define Synaptic delay

• limiting step of neural transmission

  • Transmission of AP down axon can be very quick, but synapse slows transmission to postsynaptic neuron down significantly

  • Not noticeable,, because these are still very fast

• Time needed for neurotransmitter to be release diffuse across synapse, and bind to receptors

  • Can take anywhere from 0.3 to 5.0 ms

Clinical - Examples of disorders linked to issues in the synaptic region

• ADHD

• Autism

• Schizophrenia

Define Neutransmitters receptors

• Cause graded potentials that vary in strength based on:

  1. Amount of neurotransmitter released

  2. Time neurotransmitter stays in cleft

List the two types of Postsynaptic potentials

1. EPSP → Excitatory Postsynaptic Potentials

2. IPSP → Inhibitory Postsynaptic Potenitals

Distinguish between excitatory and inhibitory postsynaptic potentials

Depending on effect of chemical synapse

• Excitatory (EPSP)

  • Depolarization that spreads to initial segment of axon → moves membrane potential TOWARD threshold for generating an AP

  • Opens chemically gated channels that allow both Na+ and K+ to move

• Inhibitory (IPSP)

  • Hyper-polarization that spreads to initial segment of axon → moves membrane potential AWAY from threshold for generating an AP

  • Opens chemically gated K+ or Cl- channels

Define Neurotransmitter

• A signaling molecule secreted by a neuron to affect another cell across a synapse

• ~50 neurotransmitters have been identified

• Most neurons make two or more neurotransmitters

  • Neurons can exert several influences

• Usually released at different stimulation frequencies

• Classified by:

  1. Chemical structure

  2. Function

Classify neurotransmitters by Chemical Structure

1. Acetylcholine (ACh)

   • First identified and best understood

   • Released at neuromuscular junctions

     • Also used by many autonomic nervous system (ANS)

   • Degraded by enzyme acetylcholinesterase (AChE)

2. Biogenic amines

   • Catecholamines → made of amino acid tyrosine

     • Dopamine

     • Norepinephrine (NE)

     • Epinephrine

3. Peptides

   • Neuropeptides → strings of amino acids that have diverse functions

     • Endorphins: acts as natural opiates; reduce pain perception

4. Endocannabinoids

   • Act as same receptors as THC (active ingredient in marijuna)

   • Lipid soluble

   • Synthesized on demand

   • Regulate sleep, mood, appetite, suppress nausea, learning, memory, body temperature, pain immune functions and fertility

Classify neurotransmitters by Function

1. Effects → determined by receptor to which is binds

   • Excitatory

     • Depolarizing → decrease in membrane potential; Inside membrane becomes less negative than resting

   • Inhibitory

     • Hyper-polarizing → increase in membrane potential; Inside membrane becomes more negative than resting

   • EX: Acetylcholine and NE bind to at least two receptors types with opposite effects

     • ACH is excitatory at neuromuscular junctions in skeletal muscle

     • ACh is inhibitory in cardiac muscle

2. Actions

   • Direct

     • Fast

     • EX: ACh and the amino acid neurotransmitters

   • Indirect

     • Slow; Broader, longer-lasting effects

     • EX: biogenic amines, neuropeptides, and dissolved gases are indirect neurotransmitters