Neurons, Active, graded, and resting potentials, and neurotransmitters

sensory neurons

transmit information from sensory cells in the body called receptors to the brain either directly or by way of the spinal cord.

interneurons

they recieve the output and are nerve cells that connect other neurons with one another.

Motor neurons

transmit commands from interneurons to the glands and muscles of the body, most often through the spinal cord. Motor neurons carry out both voluntary actions, such as grabbing a glass of water, and vital bodily functions, such as digestion and heartbeat.

what is a resting potential?

The resting potential is the polarised state of a neuron when it is not firing. The inside of the cell is negatively charged, and the outside is positively charged, due to the uneven distribution of ions. This creates a difference of approximately −70 millivolts (mV) across the membrane.

Why is a neuron polarised at rest

Because the membrane is selectively permeable: Na⁺ (sodium) and Cl⁻ (chloride) ions concentrate outside the cell. K⁺ (potassium) and negatively charged proteins are inside the cell. The membrane blocks Na⁺ from entering and traps proteins inside, creating a negative internal charge

what ions are involved in resting potential?

Sodium (Na⁺) high concentration outside

Chloride (Cl⁻) high concentration outside

Potassium (K⁺) more inside

Negatively charged proteins inside, and cannot leave

How is the resting potential maintained?

The sodium potassium pump actively transports 3 Na⁺ out and 2 K⁺ in, using ATP. The membrane is impermeable to large negative proteins inside the cell. Passive ion channels also help maintain the ionic balance.

why is the term potential used in resting potential

Because the neuron stores electrical potential energy at rest. This energy can be used to generate an action potential when the neuron is stimulated.

What is a graded potential

A graded potential is a small, local change in membrane potential that occurs in the dendrites or cell body of a neuron in response to stimulation from another neuron.

What happens during depolarisation

membrane becomes less negative -70mv to -55mv caused by an influx of sodium ions Na+, excites the neuron. Makes it more likely to fire.

what is hyperpolarisation

Membrane becomes more negative from -70mv to -80mv caused by an enflux of potassium ions Ka+ and a influx off chloride ions (CI), inhibiting the neuron, causing it to be less likely to fire.

where do graded potentials occur

Graded potentials usually occur at synapses along the dendrites and cell body of the neuron.

two key features of a graded potential?

• Decremental – They weaken as they move away from the source (like ripples).

• Additive (Cumulative) – Multiple graded potentials can add up or cancel out.

What happens when graded potentials are added together?

They summate:

• A depolarisation and hyperpolarisation of equal size cancel out.

• Multiple depolarisations can combine to bring the neuron closer to firing.

what is an action potential

An action potential is a rapid, all-or-none electrical impulse that occurs when the membrane potential reaches a threshold (about −50 mV), causing a sharp depolarisation and then repolarisation of the neuron’s membrane.

what triggers an action potential

When graded potentials depolarise the axon hillock from −70 mV to about −50 mV, voltage-gated sodium channels open, causing a sudden influx of Na⁺ ions and the start of an action potential.

what are the steps of an action potential?

• Depolarisation: Na⁺ ions rush in → membrane potential rises to about +50 mV.

• Repolarisation: K⁺ ions flow out → membrane returns to negative.

• Entire process takes <2 milliseconds.

What is the all or none principle, what does it mean in relation to an action potential

An action potential either happens fully or not at all. If threshold is not reached, no firing occurs; if it is, the full spike happens regardless of stimulus strength.

why are graded potentials important

They integrate multiple signals at the cellular level to initiate action potentials, playing a central role in processing and generating new neural information.

How is a nerve impulse transmitted between neurons?

(a) An action potential travels down the axon and reaches synaptic vesicles, which release neurotransmitters into the synaptic cleft.
(b) Neurotransmitters bind to postsynaptic receptors, triggering a graded potential. These receptors detect neurotransmitters, send signals to the nucleus, and regulate ion channels to control ion flow in or out of the cell.

How do neurotransmitters affect the likelihood of a neuron firing?

Excitatory neurotransmitters cause depolarisation, making the postsynaptic neuron more likely to fire. Inhibitory neurotransmitters cause hyperpolarisation, making it less likely to fire. Some neurons release multiple neurotransmitters, influencing connected cells in varied ways. Modulatory neurotransmitters or neuromodulators affect multiple neurons over time, enhancing or dampening the impact of other neurotransmitters in the area.

what is glutamate and what does it do

• Glutamate is the brain’s main excitatory neurotransmitter.

• It is involved in learning, memory formation, and synaptic plasticity.

• High levels (excitotoxicity) are linked to neurodegenerative diseases like Huntington’s and Alzheimer’s.

what is GABA and whats its role

• GABA (gamma-aminobutyric acid) is the brain’s main inhibitory neurotransmitter.

• Used by about one-third of all neurons in the brain.

• Crucial for reducing anxiety and calming neural activity.

• Drugs like Valium and alcohol bind to GABA receptors to reduce anxiety.

what are the functions of dopamine in the brain

• Dopamine is a neurotransmitter involved in motivation, emotion, pleasure, reward, movement, and cognition.

• It plays a key role in reward learning and explains why we enjoy activities like eating or music.

• Linked to positive affect, addiction, and emotional arousal.

• Abnormal dopamine levels are associated with schizophrenia and addictive behaviour.

how is dopamine related to parkinsons disease

• Parkinson’s disease is caused by degeneration of dopamine-producing neurons in the substantia nigra.

• Symptoms: tremors, difficulty initiating/stopping movement, slowed thinking, and depression.

• Treated with L-Dopa, a chemical that converts to dopamine.

• Dopamine itself cannot cross the blood–brain barrier, but L-Dopa can (partially).

• Side effects of L-Dopa include hallucinations and movement disorders due to widespread dopamine activity.

What is the role of serotonin in the brain

• Serotonin is a neurotransmitter involved in regulating mood, sleep, appetite, arousal, and pain.

• Often inhibitory, helping regulate aggression and social behaviour.

• Low serotonin is linked to depression; treated with medications like Prozac (fluoxetine) that boost serotonin.

• Non-drug ways to increase serotonin: exercise, diet, sunlight, and social connection.

What is acetylcholine and how is it produced?

Acetylcholine (ACh) is a neurotransmitter, a chemical messenger that transmits signals in the nervous system. It is produced in neurons by combining choline and acetyl coenzyme A through the action of the enzyme choline acetyltransferase

In which parts of the nervous system does acetylcholine function?

Acetylcholine functions in both the central nervous system (CNS) and the peripheral nervous system (PNS), including the somatic and autonomic branches

Acetylcholine main cognitive functions

Acetylcholine is involved in attention, arousal, learning, memory, motivation, and REM sleep

Acetylcholine main functions, and how does it trigger muscle movment?

Acetylcholine is crucial for voluntary muscle movement, smooth muscle contraction, glandular secretion, heart rate regulation, and cognitive functions such as memory, attention, learning, and arousal. it triggers muscle movement at the neuromuscular junction, acetylcholine binds to nicotinic receptors on muscle cells, triggering an influx of sodium ions that leads to muscle contraction

What are the effects of acetylcholine deficiency in the brain?

A deficiency of acetylcholine in the brain is associated with memory loss and cognitive decline, as seen in Alzheimer’s disease, where ACh levels are abnormally low.

Which medical conditions are linked to acetylcholine dysfunction?

Conditions such as Alzheimer’s disease (memory loss), myasthenia gravis (muscle weakness), and the effects of certain toxins (e.g., black widow spider venom, which causes excessive ACh release and muscle paralysis) are linked to acetylcholine dysfunction