lecture 2 - neurophysiology pt 2

Nervous tissue has 2 types of cells:

Neurons

Glial Cells

Neurons

• Nerve cells that transmit information

• Made of:

  • Cell body → integrates incoming and outgoing information

  • Dendrites → information moves towards the cell body

  • Axon → information moves away from the cell body

  • Capacity to generate action potential and convey information 

Multipolar

•  mainly in the CNS

Pseudounipolar

• mainly in the PNS

Bipolar

• mainly in sensory organs

• Sensory (afferent)

• Carry information from PNS → CNS

• Motor (efferent)

• Carry information from CNS → muscles and glands

• Interneurons (association)

• Relay information between neurons within the CNS

• Specialized receptors

• Transducers = convert stimuli into signals

Glial Cells

• Non-neuronal cells

• About 10× more abundant than neurons

• Types include:

  • Oligodendrocytes

  • Astrocytes

  • Ependymal cells

  • Microglia

Functions

• Provide structural support

• Oligodendrocytes → form myelin

• Astrocytes → secrete glutamate and can affect neuronal excitation

• Microglia → can perform phagocytosis

• Contact blood vessels and neurons

  • Help transport nutrients to neurons

  • Neurons do not store glucose or O₂, so they need a constant supply

Grey Matter

• Mainly contains cell bodies

White Matter

• Contains bundles of neuron processes

• Appears white because of myelin

Nerves

• Bundles of axons

• Run to or from the CNS

Ganglia

• Clusters of sensory neuron cell bodies

• Located outside the CNS

Motor Neurons

• Cell bodies are located in specific areas of the CNS

• CNS = brain and spinal cord

Myelin

•  white lipid surrounding nerve fibers

Myelin Sheaths

• Myelin = white lipid surrounding nerve fibers

• Made when glial cells wrap around an axon

• Cytoplasm is lost, leaving layers of lipids

• Found in white matter

• Not all nerve fibers are myelinated

Function; Myelin Sheaths

• Acts as an electrical insulator

• Makes action potential transmission faster

Nodes of Ranvier

• Gaps in the myelin sheath

• Occur about every 1–2 mm

• Exposed areas of the axon allow depolarization

• Allow transmission of the action potential (AP)

Membrane Potential

• Every cell has a membrane potential

• Resting Membrane Potential (RMP) = difference in electrical charge across the cell membrane

• Inside of the cell is negative relative to outside

• RMP varies depending on:

  • Amount of charges

  • Ion channels

  • Membrane thickness

Nerve Cells

• Average RMP = about –70 to –90 mV

What Maintains the RMP?

Three main factors:

1. Selective permeability

   • Ions passively move through channels by diffusion

2. Na⁺/K⁺ pump

   • Pumps 3 Na⁺ out

   • Brings 2 K⁺ in

3. Large anions

   • Negative molecules trapped inside the cell

Selective Permeability

• Ions can passively leak through channels according to their concentration gradient

• At rest, the membrane is:

  • Very permeable to K⁺

  • Barely permeable to Na⁺, Ca²⁺, and Cl⁻

• Positive charges therefore tend to accumulate outside

Na⁺/K⁺ Pump

• Ion concentrations need to stay relatively constant

• The pump compensates for ion leakage

• Moves:

  • 3 Na⁺ OUT

  • 2 K⁺ IN

• Moves ions against their concentration gradients

• Requires a lot of energy

  • Can use up to 40% of ATP availability

Important

Neurons do not store glucose or O₂ → they need a constant supply.

Excitable Cells

• Cells that can generate electrical impulses (action potentials)

• Must first be stimulated

Stimuli can be:

• Chemical

• Electrical

• Physical

A stimulus changes the membrane potential.

If the membrane reaches threshold:
→ voltage-gated ion channels open
→ action potential occurs

Depolarization

• Na⁺ channels open

• Na⁺ rushes into the cell

• Membrane becomes less negative

• Can eventually become positive

• This is called depolarization

In some nerve endings and smooth/cardiac muscle cells, Ca²⁺ can be involved instead.

Repolarization

• K⁺ channels open

• K⁺ flows out of the cell

• Membrane potential returns toward the RMP

• This is called repolarization

Generation of an Action Potential

cont

1. Initial Depolarization

• A stimulus causes depolarization

• Must reach threshold

2. Na⁺ Channels Open

• Voltage-gated Na⁺ channels open

• Na⁺ rushes into the cell

• Causes rapid depolarization

3. Na⁺ Channels Close

After about 0.5 ms, Na⁺ channels rapidly close

4. K⁺ Channels Open

• K⁺ voltage-gated channels open

• K⁺ flows out

• Causes repolarization

5. Hyperpolarization

• K⁺ channels close gradually

• K⁺ continues leaving after reaching the RMP

• Membrane becomes more negative than the RMP

• This is hyperpolarization

6. Return to RMP

• Gated channels close

• Ions return to their proper compartments through:

  • Diffusion

  • Na⁺/K⁺ pumps

Refractory Period

• Neuron cannot be re-stimulated until the RMP is restored.

Ion-Gated Channels

Several types of gated ion channels:

• Voltage-gated channels

• Open in response to changes in membrane potential

• Ligand-gated channels

• Have binding sites for neurotransmitters

• Open when a neurotransmitter binds

Channel Structure

• Each channel is made of several subunits

• Channels have different levels of specificity

All-or-None Rule

• Nerve cells follow the all-or-none rule

• If threshold is reached → an action potential (AP) is generated

• If threshold is not reached → no AP

• The amplitude of the AP is fixed for that neuron

Important

The intensity of a signal is coded by the frequency of APs, NOT their amplitude.

Stronger stimulus → more frequent APs

Conduction of Action Potential

• Depolarization and repolarization propagate along the cell membrane

• The change in membrane potential must reach threshold in the nearby area

• This triggers the opening of gated channels

• The AP then continues along the axon

Nerve Velocity Depends On:

• Thickness of myelin

• Diameter of the fiber

  • Thicker fiber → faster conduction

Unmyelinated Axons

• AP occurs continuously along the cell membrane

Myelinated Axons

• AP occurs only at the Nodes of Ranvier

• Myelin prevents ion leakage

• Electrical current jumps from node to node

• This is called saltatory conduction

Advantages of Saltatory Conduction

• Faster transmission

• Less membrane is affected

• Requires less energy to transport ions

Conduction Speeds

• Can range from about 0.5–100 m/sec

• About 250–2500 impulses/sec

Synaptic Transmission

• Allows a signal to pass:

  • From one neuron → another neuron

  • From a neuron → target cell, such as a skeletal muscle

Synaptic Gap

• Cell membranes are made of phospholipids

  • Act as an electrical insulator

• There is a gap between the two cell membranes

• This is called the:

  • Synaptic gap

  • Synaptic cleft

Electrical Synapses

• Sometimes there is direct continuity of the electrical impulse through gap junctions

• Found in:

  • Cardiac muscle

  • Some smooth muscle

Chemical Synapses

• In vertebrates, neuronal synapses are mostly chemical

Neurotransmitters

• Molecules that transmit information from a neuron

• Convert an electrical signal (AP) → chemical signal

Process

1. Released by the presynaptic neuron

2. Enter the synaptic gap

3. Bind to specific receptors on the postsynaptic membrane

4. Cause a response

Types; Neurotransmitters

cont

Small molecules

• Made in the nerve terminals

• Made using specific enzymes

• Include:

  • Amino acid derivatives

  • Biogenic amines

Neuropeptides

• Made of 3–40 amino acids

• Synthesized in the cell body

• Packaged into secretory vesicles

• Transported to the site of release

Neuromuscular Synapse

• Acetylcholine (ACh) = neurotransmitter at the neuromuscular synapse

• Postsynaptic membrane has folds

  • Increases surface area

  • Common at neuromuscular synapses

  • Not found in interneurons

Steps

1. Action potential (AP)
↓
2. AP opens voltage-gated Ca²⁺ channels
↓
3. Ca²⁺ enters the cell
↓
4. Ca²⁺ triggers exocytosis of neurotransmitter
↓
5. Neurotransmitter diffuses across the cleft
↓
6. Neurotransmitter binds to specific receptors
↓
7. Ion channels open on postsynaptic membrane → depolarization
↓
8. Neurotransmitter is inactivated → signal ends

Small Molecules

Can be terminated by:

• Being taken back up by the presynaptic neuron

  • Recycled for future use

• Being broken down in the cleft by enzymes

  • Example: Acetylcholinesterase (AChE)

Neuropeptides

Can be terminated by:

• Being taken into the postsynaptic cell through endocytosis

• Being broken down by cellular enzymes

• Being broken down by extracellular peptidases in the gap

• The receptor can become desensitized

Integration of Multiple Synapses

cont

Neuromuscular Synapse

• 1 neuron → AP → muscle cell depolarization

Neuron-Neuron Synapse

• One neuron can receive signals from many other neurons

• Synapses can be:

  • Excitatory

  • Inhibitory

• One impulse does not always cause a response

• The neuron must reach threshold to generate an AP

Excitatory Synapse

• Causes depolarization

• Usually involves Na⁺ entering

Inhibitory Synapse

• Causes hyperpolarization

• Can involve:

  • Cl⁻ entering

  • K⁺ leaving