Neurons and glial cells form the foundation of our nervous system. Neurons transmit electrical signals, while glial cells provide support and protection. Understanding their structure and function is crucial for grasping how our brains process information and control our bodies.

Action potentials are the electrical signals that neurons use to communicate. These all-or-nothing events involve rapid changes in membrane potential, triggered by ion movements. Synapses then allow neurons to pass these signals to each other, using chemical neurotransmitters to bridge the gap between cells.

Neuronal Structure and Function

Structure and function of neural cells

• Neurons
  • Cell body (soma) houses nucleus and organelles orchestrates cellular processes and protein synthesis
  • Dendrites branch out like tree limbs receive and integrate incoming signals from other neurons
  • Axon extends long distances transmits electrical signals called action potentials
  • Axon terminal releases neurotransmitters into synaptic cleft facilitating communication with other neurons
• Glial cells
  • Astrocytes provide structural support regulate neurotransmitter uptake maintain blood-brain barrier
  • Oligodendrocytes produce myelin insulation in central nervous system enhancing signal transmission
  • Schwann cells form myelin sheaths in peripheral nervous system enabling rapid saltatory conduction
  • Microglia act as immune defenders in the brain phagocytose cellular debris and pathogens

Mechanisms of action potentials

• Resting membrane potential maintained at -70 mV by ion concentration gradients (Na+, K+, Cl-)
• Depolarization triggered by stimulus opens voltage-gated sodium channels
• Action potential threshold reached at -55 mV initiates all-or-nothing response
• Rapid depolarization occurs as voltage-gated sodium channels activate membrane potential rises to +40 mV
• Repolarization follows as sodium channels inactivate potassium channels open
• Hyperpolarization drops membrane potential below resting level due to continued potassium efflux
• Refractory period
  1. Absolute: No new action potential can be generated sodium channels inactive
  2. Relative: Stronger stimulus needed for new action potential some sodium channels recover
• Propagation occurs via saltatory conduction in myelinated axons jumping between nodes of Ranvier or continuous conduction in unmyelinated axons

Synaptic Transmission and Neuronal Communication

Process of synaptic transmission

• Synaptic vesicles store neurotransmitters in presynaptic terminal
• Action potential arrival triggers calcium influx through voltage-gated calcium channels
• Vesicle fusion with presynaptic membrane releases neurotransmitters into synaptic cleft via exocytosis
• Neurotransmitters diffuse across synaptic cleft (~20-40 nm wide)
• Binding to receptors on postsynaptic membrane
  • Ionotropic receptors directly gate ion channels (AMPA, NMDA)
  • Metabotropic receptors activate second messenger systems (G protein-coupled)
• Postsynaptic potential generation alters membrane potential of postsynaptic neuron
• Neurotransmitter clearance occurs through reuptake by transporters or enzymatic degradation

Excitatory vs inhibitory synapses

• Excitatory synapses
  • Use neurotransmitters like glutamate and acetylcholine
  • Increase postsynaptic neuron's membrane potential generating EPSPs
  • Open sodium or calcium channels depolarizing the cell
• Inhibitory synapses
  • Employ neurotransmitters such as GABA and glycine
  • Decrease postsynaptic neuron's membrane potential producing IPSPs
  • Open chloride or potassium channels hyperpolarizing the cell
• Spatial summation integrates multiple synaptic inputs across neuron's dendritic tree
• Temporal summation combines rapidly occurring inputs over time
• Threshold for action potential generation determined by net effect of excitatory and inhibitory inputs at axon hillock