BSCI 1511 - Neurons, synapses and signaling

Sensory input

Receives information – sensing the external environment (e.g., light) or internal conditions (e.g., blood pressure)

PNS

Integration

Processes information – processing sensory input in context

CNS

Motor input

Transmits information directing a physiological or behavioral response – e.g, activation of muscle, gland, etc

Learning & Memory

Provides a mechanism for using experience to modify the response

CNS

How do neurons transmit information?

It uses pulses of electrical current to receive and transmit information over long distances and chemical signals to transfer information over very short distances between cells

Name the key structures of a neuron and their functions.

Cell body: contains the nucleus, integrates incoming cell signals from dendrites, and decides whether to transmit an action potential via the axon.

Dendrites: Receive signals from other neurons.

    Axon hillock: The region where the signal travels before entering the axon.

    ◦ Axon: A single, long extension that transmits signals in one direction.

    ◦ Synaptic terminals: Branched ends of an axon that transmit info to other cells.

    ◦ Synapse: The junction where a neuron communicates with another cell

Describe the three functional types of neurons.

Sensory neurons: Transmit information regarding internal and external stimuli.

    ◦ Interneurons: Integrate information and form circuits within the brain or ganglia.

    ◦ Motor neurons: Trigger muscle or gland activity

What is the difference between the CNS and the PNS?

Central Nervous System (CNS) - The brain and the spinal cord, neurons carry out integration

Peripheral Nervous System (PNS) - All nerves and ganglia located outside the brain and spinal cord, Neurons that carry information into and out of the CNS

Ganglia

Describe the step-by-step process of how a neuron receives, conducts, and transmits information.

1. Neuron receives info from the external environment or other neurons. The input is collected by the cell body and dendrites

2. The neuron processes this incoming sensory input in context. This integration determines whether the signal is strong enough to be passed along. All information travels toward the axon hillock, which acts as the trigger zone for the next step

3. If the stimulus is strong enough to reach a specific threshold (typically -55mV in mammals), the neuron generates an action potential

4. The action potential travels like a pulse of electrical current down the neuron's single, long axon

5. When the signal reaches the synaptic terminals, it must cross a junction called a synapse to reach the next cell.

   • Chemical Signal: The electrical impulse triggers the release of chemical messengers called neurotransmitters.

   • Response: These chemicals cross the gap and bind to receptors on the postsynaptic cell (which could be another neuron, a muscle, or a gland), directing a specific physiological or behavioral response

Central Nervous System

Neurons that carry out integration

Consist of the brain and spinal cord

Peripheral Nervous System

Neurons that carry information into and out of the CNS

Consist of nerves and ganglia

Splits into autonomic and somatic nervous system

Nerves

Axons of neurons bundled together

What are glial cells? and what are there functions?

The neuron’s supporting cells

• Nourish neurons

• Insulate axons

• Immune protection

• Regulate the extracellular fluid surrounding neurons

List the major types of glial cells and their specific roles

Ependymal cells: Line brain ventricles.

    ◦ Astrocytes: Nourish neurons, regulate extracellular fluid, and facilitate information transfer.

    ◦ Oligodendrocytes: Myelinate axons in the CNS.

    ◦ Schwann cells: Myelinate axons in the PNS.

    ◦ Microglia: Serve as immune cells in the CNS

What is membrane potential, and what is the specific value for a resting neuron?

Membrane potential is the charge difference (voltage) across the plasma membrane caused by the attraction of opposite charges. For a resting neuron, this is between -60 and -80 mV

Describe the concentration gradients of Potassium (K+) and Sodium (Na+) in a resting neuron.

K+ concentration is higher inside the cell (140 mM vs 5 mM outside). Na+ concentration is higher outside the cell (150 mM vs 15 mM inside)

What does resting potential result from?

ATP-dependent sodium-potassium ion pumps create concentration gradients across the plasma membrane: more K+ inside, more Na+ outside 2. The plasma membrane, because it has more leak channels for K+ than for Na+ , is more permeable to K+ than Na+A

How are ions distributed between the interior of a cell and its surrounding fluid, and what is the resulting electrical charge?

Ions are unequally distributed between the interior of cells and the surrounding fluid. Specifically, K+ concentrations are higher inside the cell, while Sodium (Na+) and Chloride (Cl−) concentrations are higher outside.

The inside of a cell is negatively charged relative to the outside

How does a neuron maintain its chemical gradients?

ATP-dependent sodium-potassium pumps, which pump three Na+ out for every two K+ they pump in.

Why is the resting potential of a cell closer to the equilibrium potential of K+ than Na+?

The plasma membrane has many K+ leak channels but very few Na+ leak channels, making it more permeable to K+. While EK​ is -90 mV, the few Na+ ions that leak in keep the actual resting potential between -60 and -80 mV

Equilibrium Potential (E)

The magnitude of a cell’s membrane voltage at equilibrium and is calculated using the Nernst equation

Nernst Equation

Contrast hyperpolarization and depolarization in terms of ion movement.

Hyperpolarization increases membrane potential magnitude (more negative) by opening gated K+ channels. Depolarization decreases membrane potential magnitude (more positive) by opening gated Na+ channels

Action potential

A consequence of the sequential opening and closing of voltage-gated ion channels for sodium and potassium

List the five phases of an action potential.

1.Neuron is at a resting state, 2. Depolarization- iflux of sodium into the cell, which changes the membrane potential 3. Rising phase, 4. Falling phase, and 5. Undershoot

What are graded potentials?

These are small changes in membrane potential that either depolarize or hyperpolarize the cell. By themselves, they only travel a few millimeters before dying out

Conduction Mechanism of an Action Potential

An action potential is generated as Na+ flows inward across the membrane; this influx depolarizes the neighboring region, reaching the threshold and reinitiating the action potential there

Continuous conduction

Invertebrate animals: insects etc

• Axons are not insulated

• Voltage-gated channels throughout

• Transfer speed: 5 cm to 30 m per sec

• To increase speed of conduction, axons are made thicker

Saltatory conduction

Vertebrate animals

Schwann cells in the PNS and oligodendrocytes in the CNS insulate the axons using myelin

• Voltage-gated ion channels are only in the gaps between the myelin sheaths, called nodes of Ranvier

• Transfer speed: 100 m/sec or faster

• Faster conduction – fewer ion channels need to be activated and deactivated

Extra note: increasing the diameter of the axon increases speed

Chemical synapses

A junction between two neurons where Chemical neurotransmitters released by the presynaptic neuron are received by the postsynaptic cell

• Most common

Electrical synapses

Special junctions between neurons, where electric currents flow from one neuron to another

Use for rapid behaviors such as heart contractions

Not common, probably wont be on exam

How do chemical synapses work?

1. An action potential arrives, depolarizing the membrane

2. Depolarization causes voltage-gated channels to open, triggering an influx of Ca2+

3. Elevated calcium concentrations cause neurotransmitters to be released from vesicles into the synaptic cleft

4. A neurotransmitter binds to ligand-gated ion channels in post synpatic membrane

Ionotropic receptors: Excitatory and Inhibitory

It is a receptor that is a ligand gated ion channel

Binding results in a graded potential called a postsynaptic potential

Excitatory: When the channel is permeable to both K + and Na + , it depolarizes • Excitatory postsynaptic potential (EPSP)

Inhibitory: When the channel is permeable to only K + or only Cl - , it hyperpolarizes • Inhibitory postsynaptic potential (IPSP), inhibits transfer of info

Metabotropic receptor

the receptor is a GPCR

Receptor activates a signal transduction cascade that involves a second messenger and activates ion channels

Metabotropic receptors are slower, but have a longer response

Temporal summation, spatial summation, Spatial summation of IPSP and EPSP

Temporal summation: Two rapid EPSPs at the same synapse add up to trigger an action potential

Spatial summation: Two simultaneous EPSPs at different synapses add up to trigger an action potential

Spatial summation of EPSP and IPSP: IPSPs cancel out the reception of an EPSP, can even go below the threshold

How to stop the signaling of neurotransmitters?

Enzymatic hydrolysis of the neurotransmitter

Recapture by the presynaptic neuron for reuse

Simple diffusion: diffuses out of the synapse

Acetylcholine

Muscle stimulation, memory formation, and learning

• At the neuromuscular junction, it binds an ionotropic receptor and induces skeletal muscle contraction; it is degraded by acetylcholinesterase

• In cardiac muscle, it binds a metabotropic receptor and reduces the heart rate

Glutamate

In CNS

Formation of long-term memory

Dopamine and Serotonin

Affect sleep, mood, attention, and learning

Nitric oxide

Relaxes smooth muscle