Neuroscience: Neurons, Action Potentials and Synapses

Dendrites: receive signals Cell body (soma): integrates signals, holds the nucleus Axon hillock: where action potentials start Axon: carries the signal Axon terminals: release neurotransmitter

Sensory (afferent): carry signals to the CNS. Motor (efferent): carry signals from the CNS to muscles and glands. Interneurons: connect neurons within the CNS.

Fatty insulation around axons, made by oligodendrocytes in the CNS and Schwann cells in the PNS; speeds conduction.

Gaps in the myelin sheath packed with voltage-gated Na⁺ channels; the impulse jumps from node to node (saltatory conduction).

Myelination and a larger axon diameter.

Astrocytes: support neurons, help form the blood–brain barrier, balance ions Oligodendrocytes: CNS myelin Microglia: immune defense Ependymal cells: make cerebrospinal fluid Schwann cells: PNS myelin Satellite cells: support PNS ganglia

About −70 mV (the inside is negative relative to the outside).

These gradients are what drive the action potential.

The Na⁺/K⁺ pump (3 Na⁺ out, 2 K⁺ in per ATP) and the membrane's greater permeability to K⁺, which leaks out through leak channels.

E_{ion} = \frac{RT}{zF}\ln\frac{[\text{ion}]_{out}}{[\text{ion}]_{in}}

About −55 mV. Reaching it opens enough voltage-gated Na⁺ channels to fire an action potential.

Depolarization: voltage-gated Na⁺ channels open, Na⁺ rushes in Repolarization: Na⁺ channels inactivate, K⁺ channels open, K⁺ flows out Hyperpolarization: K⁺ channels close slowly, overshooting rest Return to resting potential

An action potential fires fully or not at all. Stimulus strength is coded by firing frequency, not by the size of the action potential.

Absolute: no new action potential is possible (Na⁺ channels are inactivated). Relative: a stronger-than-normal stimulus can trigger one (during hyperpolarization).

Action potential reaches the terminal → voltage-gated Ca²⁺ channels open → Ca²⁺ triggers vesicle fusion → neurotransmitter is released by exocytosis → it binds postsynaptic receptors → it is cleared by reuptake, enzymes or diffusion

A small depolarization (often from Na⁺ entering) that makes the postsynaptic neuron more likely to fire.

A small hyperpolarization (Cl⁻ entering or K⁺ leaving) that makes the postsynaptic neuron less likely to fire.

Temporal: inputs from one synapse arriving in quick succession add up. Spatial: inputs from many synapses arriving at the same time add up.

Ionotropic: the receptor is itself a ligand-gated ion channel (fast). Metabotropic: a G protein-coupled receptor acting through second messengers (slower, longer-lasting).

Neurotransmitter at the neuromuscular junction and in the parasympathetic system; involved in memory. Broken down by acetylcholinesterase; reduced in Alzheimer disease.

Neurotransmitter for reward, motivation and movement. Loss of dopamine neurons in the substantia nigra causes Parkinson disease.

Neurotransmitter that regulates mood, sleep and appetite; low activity is linked to depression, and SSRIs target it.

Neurotransmitter for alertness and arousal; released by most sympathetic postganglionic neurons.

The main excitatory neurotransmitter in the brain; key for learning and memory through NMDA receptors.

The main inhibitory neurotransmitter in the brain; its effects are enhanced by benzodiazepines and alcohol.

A major inhibitory neurotransmitter in the spinal cord and brainstem.

The body's own opioid peptides; reduce pain and produce feelings of well-being.

They block the serotonin reuptake transporter, so serotonin stays in the synaptic cleft longer.

A lasting strengthening of a synapse after repeated high-frequency stimulation; a cellular basis of learning and memory, well studied in the hippocampus.

Multiple sclerosis: autoimmune destruction of myelin in the CNS. Myasthenia gravis: antibodies against acetylcholine receptors at the neuromuscular junction, causing muscle weakness that worsens with use.