Neurons

Neuron

A neuron is a type of cell that has all the normal cellular components, including the soma (cell body) and various extensions

What are the main parts of a neuron?

The main parts of a neuron are the cell body, dendrites, and axon.

What is the function of dendrites?

Dendrites receive signals from other neurons and transmit them to the cell body.

What is the function of an axon?

An axon transmits electrical impulses away from the cell body to other neurons or muscles.

Soma (Cell Body)

The soma is the central part of the neuron where the cell's genetic material is located. It is responsible for maintaining the cell's basic functions.

Types of Neurons

Multipolar, Bipolar, Unipolar

Myeline Sheath

fatty, insulating layer that surrounds the axon, allowing electrical impulses to transmit more quickly.

Myelin Structure

Oligodendrocytes (CNS) and Schwann Cells (PNS)

Nodes of Ranvier

The myelin sheath does not cover the entire axon, leaving gaps called nodes of Ranvier. These gaps allow the electrical impulse to "jump" from node to node, a process known as saltatory conduction.

saltatory conduction.

Electrical impulse jumps from node to node through gaps

Axon Structure and compoments

The axon is the extension of the neuron that carries signals away from the soma.

• Axon Hillock

• Axon Terminus

• Terminal buttons (boutons)

Axon hillock

the initial segment of the axon, where the action potential is generated

Axon Terminus

the end of the axon, where the signal is transmitted to other neurons or cells

Terminal buttons (boutons)

the small endings of the axon terminus that make connections with other cells

What is resting membrane potential?

The electrical potential difference across a membrane when a neuron is not actively transmitting a signal, usually around negative 70 millivolts (-70mV).

What is the all-or-none principle in action potentials?

The all-or-none principle states that once the threshold is reached, an action potential occurs fully, with no variation in intensity.

sodium-potassium pump (Na+/K+ ATPase)

maintains the resting potential by pumping sodium ions out of the cell and potassium ions into the cell.

The sodium-potassium pump helps maintainion imbalance by pumping 3 sodium ions out of the cell and 2 potassium ions into the cell, resulting in a net loss of one positive ion.

Na+

• concentration

  • inside cell: low

  • outside cell: high

K+

• concentration

  • inside cell: high

  • outside cell: low

action potential

the electrical impulse that travels down the axon, allowing the neuron to transmit a signal.

resting potential

state of the neuron when it is not transmitting a signal.

Leak Channels

channels that allow ions to slowly leak out of the cell, helping to maintain the ion imbalance. They are important for maintaining the resting membrane potential.

Voltage-Gated Channels

channels that only open at a certain membrane potential, called the threshold. They are important for generating action potentials.

Voltage-Gated Sodium Channels

Threshold: -50mV

State at Resting: Closed

Fast, opening quickly to allow sodium ions to flow into the cell.

Voltage-Gated Potassium Channels

Threshold: -50mV

State at Resting: Closed

Slow, opening later to allow potassium ions to flow out of the cell.

Depolarization

"Depolarization means the charge difference is becoming smaller, aka going from negative 70 towards 0, but we also just count that for when it's going above 2. So depolarization is positive."

Polarization

term that refers to the orientation of electric dipoles in a cell membrane, resulting in a difference in electrical potential between the inside and outside of the cell.

Why Do Cells Maintain a Resting Membrane Potential?

• Utilize diffusion and electrical potential for various cellular processes

• Establish a concentration gradient and electrical gradient that can be used for action potentials and other cellular processes

• Provide a store of energy that can be used to do work

Examples of Resting Membrane Potential in Other Cells

• Muscle cells: Calcium gradients help create contraction

• Neurons: Resting membrane potential helps generate action potentials## Action Potentials

Repolarization

"Repolarization is returning to the resting potential from either direction, so to speak."

K channels open

Rehyperpolarization

Rehyperpolarization is moving away from the rest potential negative direction. You're making that polarization of negative 70 even more."

Equilibrium potential

potential where there is no driving force

Action Potential Process

Resting Potential	The cell is at its resting potential, typically around -70mV.
Depolarization	The cell becomes depolarized, reaching the threshold potential of around -50mV.
Sodium Channels Open	Sodium channels open, allowing positive ions to flow into the cell.
Repolarization	Potassium channels open, allowing positive ions to flow out of the cell, repolarizing the cell.
Rehyperpolarization	The cell becomes rehyperpolarized, moving away from the rest potential negative direction.

Ion Movement: Sodium Ions

Move into the cell during depolarization, making the inside of the cell more positive.

Ion Movement: Potassium Ions

Move out of the cell during repolarization, making the inside of the cell more negative.

What is the significance of the action potential's all-or-none principle?

The all-or-none principle states that once the threshold is reached, an action potential will occur fully; there are no partial action potentials.

How Action Potentials Spread

The action potential is a rapid change in the membrane potential of a neuron, caused by the movement of ions across the cell membrane. This change in potential triggers the opening of voltage-gated sodium channels, allowing sodium ions to rush into the cell.

Refractory Period

The refractory period is a critical component of action potential propagation. It is the time during which the neuron is unable to fire another action potential.

Absolute Refractory Period

The time during which it is absolutely impossible to fire another action potential. This occurs when the sodium channels are fully inactivated and the cell is too positive.

Relative Refractory Period

The time during which it is possible to fire another action potential, but only with a stronger stimulus.

Equilibrium Potential

point at which there is no driving force for an ion to move across the cell membrane.

Na = +60mV

K = -90mV

Action Potential Key Points

• Action potentials are rapid changes in membrane potential caused by the movement of ions across the cell membrane.

• The refractory period is the time during which the neuron is unable to fire another action potential.

• The equilibrium potential is the point at which there is no driving force for an ion to move across the cell membrane.

• The action potential graph shows the changes in membrane potential over time.## Refractory Periods and Electrical Synapses

Electrical Synapses

electrical synapse is a type of synapse where the neurons are physically connected through gap junctions. This allows for the direct transfer of ions between the neurons.

Characteristics of Electrical Synapses

	CharacteristicDescription
Physically Connected	Neurons are connected through gap junctions
Always Excitatory	Electrical synapses always cause action potentials in the postsynaptic cell
Bidirectional	Both cells can be the pre- or postsynaptic cell
Unregulated	Electrical synapses do not have a regulatory mechanism to control the flow of ions

Gap Junctions

Gap junctions are specialized channels that connect the cytoplasm of two adjacent cells, allowing for the direct transfer of ions and small molecules.

play a crucial role in cardiac muscle cells, allowing for the coordinated contraction of the heart muscle.

Chemical Synapses

a type of synapse where the neurons are not physically connected, but instead communicate through the release of neurotransmitters.

• structure

  • Presynaptic Neuron: The neuron that releases the neurotransmitter

  • Postsynaptic Dendrite: The dendrite that receives the neurotransmitter

  • Synaptic Cleft: The space between the presynaptic neuron and the postsynaptic dendrite

Neurotransmitters

Neurotransmitters are chemical messengers that are released by the presynaptic neuron and bind to receptors on the postsynaptic dendrite.

• Examples of Neurotransmitters: Serotonin, dopamine, epinephrine, norepinephrine, GABA, glutamate

• Receptors: Proteins on the postsynaptic dendrite that bind to the neurotransmitter

how do neurotransmitters get released?

The release of neurotransmitters is a complex process that involves the following steps:

• Vesicle Formation: The presynaptic neuron forms vesicles that contain the neurotransmitter

• Vesicle Release: The vesicles are released into the synaptic cleft

• Binding to Receptors: The neurotransmitter binds to receptors on the postsynaptic dendrite

Regulation of Neurotransmitter Release

The release of neurotransmitters is regulated by a variety of mechanisms, including:

• Calcium Channels: Calcium ions play a crucial role in the release of neurotransmitters

• Synapsin: A protein that helps to anchor and protect the neurotransmitter vesicles

• Cytoskeleton Filaments: Filaments that help to regulate the movement of the neurotransmitter vesicles## 🌐 Voltage-Gated Calcium Channels

Voltage-Gated Calcium Channel

A type of ion channel that opens in response to changes in the electrical potential of the cell membrane, allowing calcium ions to flow into the cell.

Ligand-Gated Ion Channel

A type of ion channel that opens in response to the binding of a specific molecule, such as a neurotransmitter, allowing ions to flow into or out of the cell.

Types Ligand-Gated Ion Channel

	Type of Ion ChannelEffect on Postsynaptic Neuron
Sodium Channel	Excitatory
Potassium Channel	Inhibitory
Chloride Channel	Inhibitory
Calcium Channel	Excitatory

Regulation of Synaptic Transmission

Synaptic transmission is regulated by the type of neurotransmitter released, the type of receptor on the postsynaptic neuron, and the type of ion channel opened. This allows for a high degree of specificity and control over the transmission of signals between neurons.

Keypoints on Neurons and neurotransmtiters

• Neurons can only make one type of neurotransmitter, but they can respond to many different types of neurotransmitters.

• Dopamine is a type of neurotransmitter that is involved in reward pathways and motor control.

• Parkinson's disease is caused by the loss of dopaminergic neurons in the basal ganglia

Types of neurotransmitters

• A neuron can respond to many types of neurotransmitters, but it can only produce one type.

• Neurotransmitters can be excitatory, inhibitory, or have other effects on the postsynaptic cell.

Neurotransmitter Recycling and Breakdown

• Neurotransmitters can be recycled or broken down in the synapse.

• Medicines can be used to change the amount of time a neurotransmitter spends in the synapse, adjusting the response.

Selective Serotonin Reuptake Inhibitors (SSRIs)

• SSRIs are a type of medication that prevents serotonin from being taken back up into the presynaptic neuron.

• This allows serotonin to act longer in the synapse, increasing its effect.

Monoamine Oxidase Inhibitors (MAOIs)

• MAOIs are a type of medication that blocks the enzyme that breaks down serotonin in the synapse.

• This allows serotonin to act longer in the synapse, increasing its effect.

Postsynaptic response

Dependence on Receptors

• single neurotransmitter can have different effects on different cells, depending on the receptors present.

Action Potential Threshold

• Takes more than one vesicle to have significant effect = to prevent acidental relase of single vesicle

Myasthenia Gravis

An autoimmune disease in which antibodies block the receptors for acetylcholine, leading to flaccid paralysis.

Lambert-Eaton Syndrome

A rare autoimmune disorder in which antibodies attack the presynaptic neuron, affecting the release of neurotransmitters.

What distinguishes excitatory neurotransmitters from inhibitory ones?

Excitatory neurotransmitters promote action potentials in the postsynaptic cell, while inhibitory neurotransmitters decrease the likelihood of action potentials.

What does the term 'diencephalon' refer to?

The diencephalon is a region of the brain that includes the thalamus, hypothalamus, and epithalamus.

What is the difference between EPSP and IPSP

EPSP (excitatory postsynaptic potential) depolarizes the postsynaptic membrane, while IPSP (inhibitory postsynaptic potential) hyperpolarizes it.

How does the concept of summation affect neuronal firing?

Summation integrates multiple synaptic inputs, determining whether the postsynaptic neuron reaches the threshold for firing an action potential.

What is spatial summation in the context of synaptic inputs?

Spatial summation refers to the additive effect of multiple simultaneous excitatory and inhibitory inputs from different presynaptic neurons.

What is temporal summation in neuronal signaling?

Temporal summation is the process where frequent impulses from a single presynaptic neuron combine to increase the likelihood of reaching the action potential threshold in the postsynaptic neuron.

Functions of the Nervous System

The nervous system has three main functions:

• Sensory Input: receiving information from the environment through sensory receptors

• Integration: processing and interpreting the information in the central nervous system

• Motor Output: sending commands to the body to respond to the information

Reflex

rapid integrations that avoid potential injury. They involve a rapid response to a stimulus, often without conscious thought. Reflexes can be thought of as a "shortcut" through the nervous system, allowing for a quick response to a stimulus without the need for processing in the brain.

Reflex Arc pathway

pahway:

• Sensory Neuron: a neuron that detects a stimulus and sends a signal to the spinal cord

• Interneuron: a neuron that integrates the signal from the sensory neuron and sends a signal to the motor neuron

• Motor Neuron: a neuron that receives the signal from the interneuron and stimulates a muscle to contract

Knee Jerk Reflex

a common reflex that occurs when the tendon below the kneecap is tapped. This causes the quadriceps muscle to contract and the hamstring muscle to relax, resulting in a sudden extension of the knee.