BIO II Ch 48: Neurons, Synapses, and Signaling

Ganglia

Simple clusters of neurons where processing of information takes place

Neuron

A cell that exemplifies the close fit between form and function

Glia/Glial cells

Cells that insulate/nourish neurons

Route from how information is transmitted:

1. Presynaptic cell (neuron) to a postsynaptic cell (neuron, muscle cell, or gland cell)

What three stages (in order) do nervous systems process information?

1. Sensory input

2. Integration

3. Motor output

Central Nervous System (CNS)

A section of the nervous system where integration takes place (the brain and the nerve/spinal cord)

Peripheral Nervous System (PNS)

A section of the nervous system that carries information into and out of the CNS

-The neurons in this system, when bundled together, form nerves

Membrane potential

The voltage (difference in electrical charge) every cell has across its plasma membrane

Resting potential

The membrane potential of a neuron not currently sending signals

Action potentials

Changes in membrane potential

Concentration of Sodium and Potassium at neurons resting potential:

-Concentration of Potassium (K+) is high INSIDE CELL

-Concentration of Sodium (Na+) is high OUTSIDE CELL

** Therefore: Sodium wants to move in, potassium wants to move out

Sodium-potassium pumps

Use the energy of ATP to maintain these K+ and Na+ gradients across the plasma membrane

-As the ions float across the membrane, the charges change

-The concentration gradients represent chemical potential energy and converts to electrical potential energy.

Ion channels

Open in the plasma membrane and converts chemical potential to electrical potential

Does a neuron at resting potential have more Potassium channels open or Sodium channels open? Does Potassium diffuse into or out of the cell?

-More Potassium channels open (higher inside the cell)- fewer sodium channels open

-Potassium diffuses out of the cell

What are the general charges inside and outside of the neuron cell- at resting potential in terms of the sodium-potassium pump? Why are these the charges?

-It is negative inside the cell (higher in potassium) because it also has proteins inside the cell, which cause the overall negative charge

-It is positive outside the cell (higher in sodium)

-Actively transports 3 Potassium ions out for every 2 Sodium ions brought in, establishing the charge difference.

-Maintains a steady -70 charge (mV)

Resting potential model (artificial membrane that separates two chambers)

-Concentration of KCl is higher in inner chamber, lower in the outer chamber

-K+ diffuses down gradient to the outer chamber

-Negative charge (Cl-) builds in inner chamber

***At equillibrium, both electrical and chemical gradients are balanced

Gated ion channels

Open or close in response to stimuli

Why do changes in membrane potential occur?

The neurons contain gated ion channels that open or close in response to stimuli

Voltage-gated ion channels 

Open or close in response to a change in voltage across the plasma membrane of a neuron

Hyperpolarization

When gated K+ channels open, K+ diffuses out- makes inside of cell more negative

An increase in the magnitude of the membrane potential

-increase membrane permeability to K+

-Resting potential decreases and goes away from the threshold

Depolarization

When gated Na+ channels open, Na+ enters the cell, making the inside more positive

A decrease in the magnitude of the membrane potential

-increase membrane permeability to Na+

-Resting potential increases towards hitting the threshold  

Order of events in Membrane Potential on an axon

1. Resting state: Potassium high inside the cell (negative), Sodium high outside cell (positive).

2. Depolarization occurs, Na+ channels open, inside of the cell becomes more positive.

3. Rising phase of action potential: Gated sodium channels close due to the charge inside the cell becoming positive. Resting potential moves closer towards the threshold.

4. Action potential: Once the threshold is reached, an action potential is triggered, causing a rapid depolarization

5. Falling phase of action potential: Gated potassium channels open, allowing K+ to flow out, returning the inside of the cell to a more negative state.

6. Undershoot: Both channels push both ions out of the cell (rebound effect) and then officially restore back to the resting state. 

Refractory period

-After an action potential, a second action potential cannot be initiated 

-Result of a temporary inactivation of the Na+ channels

What occurs at the axon hillock?

An electrical current depolarizes the neighboring region of the axon membrane

What insulates axons in vertebrates? Does it also cause action potential speed to increase?

Myelin sheath; yes

**Made by glial cells (Oligodendrocytes in CNS and Schwann cells in PNS)

Nodes of Ranvier

-Gaps in the myelin sheath

-Restrict the voltage-gated sodium channels

-Action potentials in myelinated axons jump between these gaps

Salatory conduction

The process by which action potentials hop from one node of Ranvier to the next in myelinated axons, increasing the speed of signal transmission.

How does the electrical current flow from one neuron to another at electrical synapses?

Through gap junctions

How does the chemical current flow from one neuron to another at chemical synapses?

A chemical neurotransmitter carries information between neurons

***Most synapses are chemical synapses

Ligand-gated ion channels

Neurotransmitters bind to these in the postsynaptic cell, which allows for direct synaptic transmission

-Neurotransmitter binding causes ion channels to open, generating a postsynaptic potential

What are the two categories for postsynaptic potentials?

1. Excitatory postsynaptic potentials (EPSPs)

2. Inhibitory postsynaptic potentials (IPSPs)

Excitatory postsynaptic potentials (EPSPs)

-Make the receiving neuron more likely to generate an action potential

Inhibitory postsynaptic potentials (IPSPs)

-make the receiving neuron less likely to generate an action potential

How do the two kinds of postsynaptic potentials work with each other to follow through with sending signals through the cell?

-Together, EPSPs and IPSPs work to determine the overall excitability of the neuron. If the sum of EPSPs exceeds the threshold, an action potential is generated; if IPSPs dominate, the neuron remains inactive.

Acetylcholine

-Common neurotransmitter in vertebrates and invertebrates

-Involved in muscle stimulation, memory formation, and learning (EX: diaphragm)

-Vertebrates have two major classes of this neurotransmitter receptor: Ligand and metabotropic.

Glycine

An amino acid that acts at inhibitory synapses in parts of the CNS outside the brain

Gamma-aminobutyric (GABA)

An amino acid that serves as a major inhibitory neurotransmitter in the brain, playing a key role in regulating neuronal excitability.

Biogenic Amines

A class of neurotransmitters derived from amino acids that includes dopamine, norepinephrine, epinephrine, and serotonin, known for their role in regulating mood, cognition, and autonomic functions.

-Play a central role in a lot of nervous system disorders

• EX: Parkinson’s disease is associated with a lack of dopamine in the brain

Neuropeptides

—Small, several, relatively short chains of amino acids that function as neurotransmitters

-Include substance P and endorphins (perception of pain)

-Opiates bind to same receptors to act as painkillers

-Pain, reward, and stress*****

Gases

-Local regulators in the PNS, which act as signaling molecules in the nervous system, modulating neurotransmission and vascular functions

-Such as nitric oxide (NO) and carbon monoxide (CO)

-*Unlike most neurotransmitters, NO is synthesized on demand as opposed to being stored in cytoplasmic vesicles and is broken down immediately, and plays a role in vasodilation and neurotransmission.

-NO is broken down within a few seconds of production