The neuron

How many neurons does the human brain contain?

Approximately 80–90 billion.

How many synapses does the brain have

Approximately 1 quadrillion (1,000,000,000,000,000)

What percentage of the body's energy does the brain use?

About 20%

What is the neuron doctrine?

Neurons are discrete, structurally independent cells that communicate across synapses rather than being fused into one continuous network.

Who is considered the father of neuroscience?

Santiago Ramón y Cajal (1852–1934).

What are the four functional zones of a neuron?

Input zone (dendrites)

Integration zone (cell body/axon hillock)

Conduction zone (axon)

Output zone (axon terminals).

What is the function of dendrites?

Receive information from other cells through synaptic inputs.

What are dendritic spines?

Small projections on dendrites that provide additional sites for synaptic connections.

How does dendritic structure affect a neuron?

More dendritic surface area generally allows more potential synaptic connections and more incoming information.

What is the soma?

The cell body, which contains the nucleus and most organelles, maintains the cell, and integrates incoming signals.

What is the axon hillock?

The region where the axon begins and incoming signals are integrated to determine whether an action potential will be initiated.

What is the function of an axon?

Carries electrical signals away from the cell body and transports materials along the neuron.

What are axon collaterals?

Branches of an axon that allow one neuron to communicate with multiple targets

What is anterograde axonal transport?

Movement of materials from the cell body toward the axon terminals.

What is retrograde axonal transport?

Movement of materials and signals from the axon terminals back toward the cell body.

What is the function of axon terminals?

Release neurotransmitters to communicate with target cells across synapses.

What are the three main neuron types by shape?

Multipolar, bipolar, and unipolar neurons.

What is a multipolar neuron?

A neuron with many dendrites and one axon; it is the most common structural type.

What is a bipolar neuron?

a type of nerve cell that has two extensions (processes) emerging from opposite ends of its cell body: a single dendrite that receives signals and a single axon that transmits them

What is a unipolar neuron?

A neuron with a single process extending from the cell body; these neurons commonly transmit sensory information such as touch and pain.

What are the three main neuron types by function?

Sensory neurons, motor neurons, and interneurons.

What do sensory neurons do?

Detect internal or external stimuli and transmit sensory information toward the central nervous system.

What do motor neurons do?

Carry commands from the nervous system to muscles and glands.

What do interneurons do?

Process and relay information between neurons, supporting complex functions such as thinking, memory, and decision-making.

What are the three main components of a synapse?

The presynaptic membrane, synaptic cleft, and postsynaptic membrane.

What is the presynaptic membrane?

The membrane of the sending cell, typically at the axon terminal.

What is the postsynaptic membrane?

The membrane of the receiving cell, often on a dendrite or cell body.

What is the synaptic cleft?

The tiny gap separating the presynaptic and postsynaptic membranes; the lecture gives a width of about 20–40 nanometers.

How does chemical synaptic transmission work?

An action potential reaches the terminal; vesicles release neurotransmitters; neurotransmitters cross the cleft and bind postsynaptic receptors, changing the receiving cell's activity

What are synaptic vesicles?

Small membrane-bound sacs containing neurotransmitters.

What is neural plasticity?

The ability of neural connections to change in structure or function; it supports learning and memory.

What are glial cells?

Cells that support neurons and also contribute to neural signaling, information processing, and nervous system maintenance

What is the difference between oligodendrocytes and Schwann cells?

Oligodendrocytes form myelin in the CNS and can myelinate segments on multiple axons. Schwann cells form myelin in the PNS, typically one cell per axon segment.

What are nodes of Ranvier?

Gaps between myelin segments where the axon membrane is exposed.

What is myelin, and what does it do?

A fatty insulating sheath around axons that increases conduction speed and improves energy efficiency.

What is saltatory conduction?

The rapid propagation of an action potential from node to node along a myelinated axon.

What is multiple sclerosis (MS)?

An autoimmune disease in which myelin is damaged, potentially slowing or blocking neural signals.

What are the main functions of astrocytes?

Help regulate local blood flow, maintain the blood-brain barrier, influence synaptic transmission, and participate in synapse formation and elimination.

Why can microglia be a double-edged sword?

They protect the brain by removing harmful material, but excessive activation can contribute to inflammation and disease.

What are the main functions of microglia?

Immune surveillance, removal of cellular debris and pathogens, and responses to injury and infection.

A neuron has damaged dendrites but an intact axon. What function would be most directly affected?

Receiving information from other neurons. Dendrites are the input zone, so damage would reduce the neuron's ability to receive synaptic signals, even if its axon could still conduct signals.

A drug prevents the cell body from producing essential proteins. Which neuronal function would be affected, and why?

Cell maintenance and normal functioning would be impaired. The soma contains the nucleus and most organelles and is a major site of protein production.

A neuron receives many excitatory signals, but they are too weak to reach threshold at the axon hillock. What happens?

The neuron does not generate an action potential. Excitatory inputs must produce enough combined depolarization to reach threshold.

A neuron reaches threshold at the axon hillock. What happens next?

An action potential is initiated and propagates along the axon toward its terminals.

A disease damages the axon but leaves the dendrites and soma intact. How might communication be affected?

The neuron may still receive and integrate inputs, but conducting electrical signals to its terminals may be impaired.

One neuron needs to send information to several different target cells. Which structural feature helps it do this?

Axon collaterals. These branches allow one axon to distribute signals to multiple targets.

A toxin prevents synaptic vesicles from releasing neurotransmitters. What would happen to chemical communication at the synapse?

Neurotransmitters would not be released normally into the synaptic cleft, disrupting communication with the postsynaptic cell.

A drug blocks neurotransmitter receptors on a postsynaptic neuron. The presynaptic neuron still releases neurotransmitter. Why might communication fail?

The neurotransmitter cannot activate its postsynaptic receptors effectively, so the receiving neuron may not respond normally.

A neuron has many dendrites and one axon. How would you classify it by shape?

Multipolar. Multipolar neurons have many dendrites and a single axon.

A sensory neuron carries pain information from the skin toward the spinal cord. What functional category does it belong to?

Sensory neuron. It detects a stimulus and carries information toward the central nervous system.

A neuron in the spinal cord receives sensory information and helps determine the appropriate motor response. Which type of neuron is it likely to be?

An interneuron. Interneurons process information and connect neural pathways between sensory and motor systems.

A motor neuron is damaged, but the sensory neurons supplying the same muscle remain intact. What might happen?

Sensory information from the muscle may still reach the CNS, but motor commands to the muscle may be disrupted, weakening or preventing movement.

A neuron has unusually elaborate dendritic branches. What advantage might this provide?

Greater dendritic surface area and potentially more synaptic connections, allowing the neuron to receive input from more sources.

A researcher discovers that brain cells communicate across tiny gaps instead of being fused into one continuous network. Which principle does this support?

The neuron doctrine. It holds that neurons are discrete cells that communicate across synapses.

A chemical is transported from the cell body toward the axon terminals to support neurotransmitter release. What process is involved?

Anterograde axonal transport. It moves materials from the cell body toward the terminals.

A damaged axon terminal sends information back to the cell body about its condition. Which transport direction is involved?

Retrograde transport, which moves materials and signals from axon terminals toward the cell body.

A patient has damage to oligodendrocytes in the brain and spinal cord. What neuronal function may be impaired?

Damage to oligodendrocytes in the brain and spinal cord impairs myelin sheath formation and causes demyelination, which slows down, disrupts, or completely blocks saltatory conduction (the rapid electrical transmission of nerve impulses along axons

A neuron receives excitatory input from five other neurons. Four inputs arrive far apart in time, while all five arrive together in a second trial. Why might the second trial be more likely to trigger an action potential?

The inputs arriving together may produce greater combined depolarization at the axon hillock. Their effects can summate, making threshold more likely to be reached. The exact outcome depends on the strength and location of each input.

A disease damages astrocytes and oligodendrocytes. Explain two different problems that could result.

Damaged oligodendrocytes can impair CNS myelination, slowing or disrupting signal conduction. Damaged astrocytes can interfere with local blood-flow regulation, synaptic support, and maintenance of the blood-brain barrier.

A neuron has healthy dendrites, soma, and axon, but its axon terminals cannot release neurotransmitter. At which stage of communication is the problem occurring?

The problem is occurring at the chemical transmission stage (or synaptic transmission).

While the electrical signaling stage (the action potential) can travel safely down the healthy dendrites, soma, and axon, the neuron fails at the final step: converting that electrical signal into a chemical one to pass the message to the next cell.

Why might damage to interneurons affect complex behavior even if sensory neurons and motor neurons still function?

— process and integrate information between sensory input and motor output. Their damage can disrupt the circuits that support appropriate responses, memory, decision-making, and other complex functions.

A drug increases the number of functional synapses a neuron can form. Explain how this might influence information processing, and why more synapses do not automatically mean better performance.

More functional synapses can provide more potential sources of input or output connections. However, the effects depend on connection strength, the information carried, and the balance of excitation and inhibition. Additional connections are not necessarily beneficial if they disrupt appropriate circuit activity.