Nervous Tissue Lecture Notes

Nervous Tissue - Section 1: Cells of the Nervous System

• Neurons:
  • Receive information.
  • Process information.
  • Send information to other cells.
• Glia:
  • Support neurons in their functions.

Neuron Structure and Function

• Cell Body (Soma):
  • Handles eating, digesting, and protein synthesis.
  • Contains the cell's genetic material.
• Dendrites:
  • Specialized to receive information.
  • Receive stimuli like light, sound, touch, pain, and chemicals.
  • Receive neurotransmitters from other neurons.
• Trigger Zone (Axon Hillock):
  • Evaluates information from dendrites.
  • Decides whether to send information down the axon.
• Axons:
  • Carry large positive electrical signals (action potentials) over long distances to axon terminals.
• Axon Terminals:
  • Tips of the axon that release chemicals called neurotransmitters onto other cells.

Message Transmission by Neurons

• An action potential (large positive electrical signal) forms inside the neuron at the trigger zone.
• The action potential travels unchanged down the axon.
• Neurons don’t physically touch; they communicate chemically.
• The axon terminal releases a neurotransmitter, which:
  • Crosses the synapse (gap between cells).
  • Binds to the next neuron.
  • Either excites or inhibits the next neuron.
  • Is then destroyed or recycled.
• If the neurotransmitter excites the next cell, a new electrical charge travels down that cell's axon, and it releases neurotransmitters from its axon terminal.
• Thousands of neurons can be involved in a single thought or movement.
• Synapse: The space between cells filled with extracellular fluid.

Neuron Shapes and Functions

• Different neuron shapes correspond to different functions and locations.
• Multipolar Neuron:
  • Has many processes sticking out of the cell body.
  • Cell body located in the brain and spinal cord.
  • Used in thinking and sending information to muscles.
  • Motor Neuron: A type of multipolar neuron connected to muscle; makes you move.
  • Dendrites and cell body in the brain or spinal cord send information down a long axon to other parts of the brain or muscles and glands far away in the body.
• Unipolar Neuron:
  • Has one process sticking out of the cell body.
  • Sends information up to the spinal cord and brain.
  • Sensory Neuron: A type of unipolar neuron that brings sensations from the body to awareness.
  • An unipolar neuron axon can carry information from the toe to the spinal cord.
• Cell body of a sensory neuron is located in the spinal cord, not near its dendrites, to protect it from physical damage.
• Regeneration: If only the dendrite or axon is cut off or damaged in the PNS, it can grow back as long as the cell body is still alive and sending it nutrients, and Schwann cells are able to create a tube for the axon to follow.

Glial Cells

• Glia help support neurons.
• There are many more glia than neurons.
• Different kinds exist in the:
  • Central Nervous System (CNS): Brain and spinal cord.
  • Peripheral Nervous System (PNS): Neurons in all other parts of the body.

Types of Glial Cells and Their Functions

• PNS:
  • Satellite Glia: Cover PNS neuron cell bodies and control nutrient passage from blood to the neuron cell body.
  • Schwann Cells: Myelinate axons in the PNS.
• CNS:
  • Astrocytes: Part of the blood-brain barrier, which determines what blood contents can reach the neuron. Provide nutrition and support to epithelial cells.
  • Microglia: Mobile defensive cells that phagocytize cell debris and pathogens around neurons.
  • Oligodendrocytes: Myelinate axons in the CNS.
  • Ependymal Cells: Line cavities in the brain to make and circulate cerebrospinal fluid (CSF).
• Blood-Brain Barrier (BBB):
  • Epithelial cells connected by tight junctions keep most pathogens in the blood, preventing them from entering brain tissue.
  • Microglia catch viruses and bacteria that do enter the CNS.
  • The protective BBB can make it difficult to get large antibiotics like penicillin into brain tissue for treating disorders like bacterial meningitis.
• Special glial cells wrap parts of the axon to speed electrical conduction and insulate the axon from neighbors, forming a myelin sheath.
• Myelin Sheath Formation:
  • In the PNS, Schwann cells individually wrap small parts of each axon.
  • The Schwann cell rotates around the axon, wrapping its plasma membrane around it.
  • The Schwann cell cytoplasm is forced into the outer layer.
  • The empty membrane wrappings around the axon form the myelin sheath.
  • In the CNS, oligodendrocytes each wrap hundreds of different axons.
• The gap between myelin sections is called a node.

Neuron Voltage and Ion Concentration

• Neurons, like all cells, exist in an extracellular fluid 'salt water bath' containing sodium ($Na^+$, chloride ($Cl^-$), calcium ($Ca^{++}$), and potassium ($K^+$) ions.
• Cells are filled with water containing the same ions, but in different concentrations.
• Ion Concentrations:
  • Outside the Cell: High sodium, high chloride, high calcium, low potassium.
  • Inside the Cell: Low sodium, low chloride, low calcium, high potassium.
• Maintaining ion gradients requires energy and mechanisms like membrane pumps.

Membrane Pumps and Ion Gradients

• Millions of membrane pumps maintain low intracellular sodium and high intracellular potassium.
• Each pump uses ATP to move 3 sodium ions out for every 2 potassium ions in.
  $3Na^+ \text{ out for every } 2K^+ \text{ in}$
• The pump makes the cell more negative inside by moving more positive ions out than in. This charge difference is the membrane potential.

Membrane Potential

• The sodium-potassium pump packs potassium into the cell, like pumping water into a water tower. The cell then uses this stored potassium to make the resting membrane potential negative.
• The inside of the cell becomes more negative, around -10mV, as positive charges leave.
• Sodium is lower inside, and potassium is higher inside.
• To achieve a neuron's highly negative state (-70mV), positive ions must leave the cell through channels.
• Types of Channels:
  • Leaky channels (always open).
  • Chemical-gated channels (open when neurotransmitters bind).
  • Voltage-gated channels (open in response to voltage changes).
• Ion shape and charge determine which ions can pass through a channel.

Resting Membrane Potential (RMP)

• Neurons have many leaky potassium channels.
• Potassium flows out, down its concentration gradient, making the cell more negative inside.
• The resting membrane potential is now very low (-70mV).
• The electrical values can be measured with a voltmeter.
• Voltmeters confirm that neurons at rest are more negative inside than outside.
• In neurons, the RMP is -70mV.

Summary: Establishing -70mV RMP

• The sodium/potassium pump establishes an ion gradient with higher potassium inside and lower sodium inside; it runs on ATP.
• Neurons have very permeable (leaky) potassium ion channels.
• When lots of potassium leaves the cell, the cell becomes more negative.

Maintaining RMP for Patient Care

• Patients need a diet including sugars, proteins, fats (to run the pump), and electrolytes (ions).
• Food energy is transferred to ATP, which powers the sodium and potassium pump.
• If a patient doesn’t eat:
  • ATP is not made.
  • The pump fails.
  • Ions equalize.
  • Membrane potential goes to zero.
  • The patient dies.

Importance of -70mV RMP

• Like you, neurons need a quiet state that can quickly become a very active state!
• Neurons need a -70mV RMP for quick activation.
• Sodium is stored outside the cell, contributing to a negative resting membrane potential inside.
• When sodium channels open abruptly, sodium ions flow in quickly, making the cell positive and active.

Ion Movement and Charge

• Calcium would move from outside the cell to inside the cell.
• Sodium would move from outside the cell to inside the cell.
• Potassium would move from inside the cell to outside the cell.
• Chloride would move from outside the cell to inside the cell.
• Calcium flowing in makes the inside of the cell more positive.
• Sodium flowing in makes the inside of the cell more positive.
• Chloride flowing in makes the inside of the cell more negative.
• Potassium flowing out makes the inside of the cell more negative.

Summary: Mechanisms for a Large Negative RMP

• Many pumps move sodium and potassium ions against their concentration gradients; more positive ions are pumped out than enter, making the inside negative.
• Many leaky channels allow potassium to move down its concentration gradient; positive potassium leaving the cell makes the inside more negative.

Why Cells Need a Large Negative RMP

• If the cell is very negative, opening sodium ion channels and letting positive sodium flow in easily “wakes it up”.
• This positive change signals the cell to become active.
• For a neuron, this means processing information and determining whether to send it to another cell.

How Cells Use RMP to Communicate

• -70mV RMP
• Ion gradients for all 4 ions
• Neurotransmitters (chemicals)
• Neurotransmitter receptors
• Chemical-gated channels

Neurotransmitters

• Axon terminals store neurotransmitters, which are released into the synapse and attach to the dendrites of the next cell.
• NT is released from the pre-synaptic cell.
• NT attaches to RECEPTORS of the right shape on the dendrites of post-synaptic cell
• Different cells release different neurotransmitters.
  • Some excite(turn on) the next cell
  • Some inhibit (quiet) the next cell
• Inside the receptors are channels opened by the neurotransmitter. Only certain ions can fit through each channel
  • Excitatory NT opens $Na^+$ or $Ca^{+}$ channels, making cell more +
  • Inhibitory NT opens $Cl^-$ or $K^+$ channels making cell more -
• A particular NT opens a particular shape of channel, so only a particular ion type can flow through. Ions can only flow DOWN their concentration gradient

Excitatoty and Inhibitory Neurotransmitters

• When an excitatory NT binds to a receptor, a chemical gated $Na^+$ or $Ca^{++}$ channel opens in the receptor, and ions flow. $Na^+$ and $Ca^{++}$ have higher concentrations outside the cell. When positive ions flow thru the open channel into the cell, the cell becomes more positive, excited.
  • Excitatory--post-synaptic-potential- Cell voltage becomes more positive more likely to pass current down the axon
• When an inhibitory NT binds to a receptor, chemical gated $K^+$ or $Cl^-$ channels open in the receptor, and ions flow down their concentration gradients.
  • If negative $Cl^-$ flows IN, it brings a negative charge, quieting the cell, preventing electrical activity.(inhibition)
  • If positive $K^+$ flows OUT, the cell becomes more negative, quieting the cell, preventing electrical activity.(inhibition)
  • Inhibitory-post-synaptic-potential- Cell voltage becomes more negative. Less likely to pass current down the axon
• Why do we need both EXCITATORY and INHIBITORY NT? the cell adds up the + and - input like a vote. The cell membrane potential goes up and down, depending on which ions are flowing in or out. But a decision to make an action potential, a signal down the axon, must be made. What is the deciding point? Hint! when the cell reaches -55mv, threshold! voltage gated $Na^+$ channels opened. many neurons drop NT on a single neuron

Action Potential

• If enough excitatory NT is present…. the Trigger Zone voltage rises from the RMP of -70mV to a more positive -55mV and a new type of channel opens in the trigger zone
• Voltage gated sodium channels are not opened by chemicals. They are opened by a voltage change from -70mV to -55mV. Many sodium ions enter the cell very quickly.
• The trigger zone reaches -55mv
  • Many Voltage gated $Na^+$ channels open in the trigger zone
  • $Na^+$ pours into the cell, forcing the voltage up to +30mV (depolarizes)
  • VG $Na^+$ channels close at +30mV
  • At +30, VG $K^+$ channels open $K^+$ rushes out, cell returns to -70mV (repolarizes)
  • VG $K^+$ channels close
• The positive wave of sodium created at the Trigger zone runs out long before it reaches the end of the cell. A wire cannot carry electricity infinitely, and neither can an axon. So, how does this work?
• The axon has many BOOSTER STATIONS all the way down the cell, and keeps boosting the voltage at periodic intervals so that current can travel the long distance down an axon from brain to muscle. The booster stations are the same VOLTAGE GATED $Na^+$ channels used at the TRIGGER ZONE to boost the cell voltage from -55mV t0 +30 mV.
• Each time an action potential occurs at a set of VG $Na^+$ channels enough positive voltage is sent forward to open the next set of voltage gated $Na^+$ channels. All the way down the axon to the axon terminals!
  • $Na^+$ IN, makes the cell more +. This depolarization is called an Action Potential.
  • After the + signal occurs, the cell must return to baseline so that another AP can occur when needed. So, VG $K^+$ channels on the axon membrane open at +30.
    • $K^+$ flows OUT of the cell, and returns the cell to the resting membrane potential of -70mV.
• NT RELEASE
  • Summary: If excitatory NT brings the TZ to -55mV, Voltage gated sodium channels open first in the trigger zone and then all along the axon. Sodium enters the cell through sodium channels and creates large positive action potentials all the way down the axon. At the end of the axon, NT is released.
• Arrival of the AP at the axon terminal causes NT release.
  • Action potential arrives at the axon terminal
  • VG calcium channels open in the terminal
  • Calcium flows INTO the axon terminal to release NT from vesicle into synapse
  • NT attaches to the next cell
  • If the NT is excitatory, the next cell is excited toward threshold. IF the NT is inhibitory, the next cell is moved away from threshold.
• ion movement into the cell takes precious time

Myelin and Information Transfer

• This is where myelin comes in
• Myelin (white) covers much of the axon, and ions cannot pass through myelin
• TINY SPACES between the myelin wrappings, are called nodes (blue) Nodes contain the Voltage gated sodium channels (booster stations) Action potentials are only boosted at the NODES.
• Rather than boosting action potentials all the way down the neuron, we boost at fewer spots. Boosting takes time because ions need to move across the membrane, so fewer booster stations means more time is saved. Booster stations are close enough together so the positive charge does not “run out”, or get smaller than -55mV, before the next node is reached.
• Myelin actually makes transmission 30 x faster.a myelin covered axon transmits the action potential 30x quicker than a naked axon!

Axon Speed and Pain

• During pain, conduction must be fast to warn your body. Which shape axon do pain carrying neurons have?
• Fast axons (larger diameter and myelinated serve pathways where speed is essential such as skeletal reflexes and pain. Slower (smaller diameter and unmyelinated) serve internal organs (viscera) that work at a fairly constant rate.

Clinical Notes

• Lidocaine patches decrease pain by blocking VG $Na^+$ channels so they cannot open How does lidocaine decrease pain? If $Na^+$ channels do not open, the action potentials cannot occur to carry information down the axon.
• What you put in an IV bag flows into BLOOD and from the blood to EXTRACELLULAR FLUID What would happen to the patient if a bag with abnormally high sodium was hung by accident? . If a nurse hangs an IV bag with TOO MUCH $Na^+$, the blood will have high sodium, and the extracellular fluid will have too much sodium. (HYPERNATREMIA). IF the extracellular fluid is too salty, water will flow OUT of the cell, down its concentration gradient, and the cell will dehydrate. This cellular dehydration can cause cell damage and death.