Cellular Neurophysiology

• Neurophysiology is the study of the function of the nervous system
• It incorporates techniques from fields like biophysics, molecular biology, and anatomy to understand how the nervous system works
• Its scope ranges from studying single ion channels to cognitive functions

Cellular Neurophysiology

• Neurons communicate through electrical and chemical signals
• Electrical signals are based on changes in the neuron's membrane potential
• The resting membrane potential is typically around -70 mV
• This potential is maintained by ion gradients and selective permeability of the membrane to ions like sodium (Na+) and potassium (K+)
• Ion channels are transmembrane proteins that allow specific ions to pass through the membrane
• Voltage-gated ion channels open or close in response to changes in membrane potential
• Ligand-gated ion channels open or close in response to the binding of a specific neurotransmitter or other molecule
• Action potentials are rapid, transient changes in membrane potential that propagate along the neuron's axon
• An action potential is initiated when the membrane potential reaches a threshold, typically around -55 mV
• Depolarization occurs as Na+ channels open, allowing Na+ to flow into the cell and making the membrane potential more positive
• Repolarization occurs as Na+ channels inactivate and K+ channels open, allowing K+ to flow out of the cell and restoring the negative membrane potential
• Hyperpolarization may occur due to the prolonged opening of K+ channels, making the membrane potential more negative than the resting potential
• The refractory period is a period after an action potential during which another action potential cannot be generated (absolute refractory period) or requires a stronger stimulus (relative refractory period)
• Myelination increases the speed of action potential propagation
• Action potentials "jump" between the Nodes of Ranvier, which contain a high concentration of voltage-gated Na+ channels; this is called saltatory conduction

Synaptic Transmission

• Synapses are specialized junctions through which neurons communicate with each other or with target cells like muscle or gland cells
• Electrical synapses involve direct electrical coupling between cells through gap junctions, allowing for very fast transmission
• Chemical synapses involve the release of neurotransmitters from the presynaptic neuron
• When an action potential reaches the presynaptic terminal, voltage-gated Ca2+ channels open, allowing Ca2+ to enter the cell
• Influx of Ca2+ triggers the fusion of neurotransmitter-containing vesicles with the presynaptic membrane, releasing neurotransmitter into the synaptic cleft
• Neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane
• Neurotransmitter receptors can be ionotropic receptors (ligand-gated ion channels) or metabotropic receptors (G protein-coupled receptors)
• Ionotropic receptors mediate fast postsynaptic responses by directly opening or closing ion channels
• Metabotropic receptors activate intracellular signaling pathways that can modulate ion channels or other cellular processes, leading to slower but longer-lasting postsynaptic effects
• Excitatory neurotransmitters (e.g., glutamate) cause depolarization of the postsynaptic membrane, leading to an excitatory postsynaptic potential (EPSP)
• Inhibitory neurotransmitters (e.g., GABA, glycine) cause hyperpolarization or stabilization of the postsynaptic membrane, leading to an inhibitory postsynaptic potential (IPSP)
• Postsynaptic potentials are graded potentials, meaning their amplitude depends on the amount of neurotransmitter released and the number of receptors activated
• Synaptic integration is the process by which postsynaptic potentials are summed to determine whether the postsynaptic neuron will fire an action potential
• Neurotransmitters are removed from the synaptic cleft by diffusion, enzymatic degradation, or reuptake into the presynaptic terminal or glial cells

Sensory Neurophysiology

• Sensory systems detect and process information from the environment
• Sensory receptors are specialized cells that transduce different forms of energy (e.g., light, sound, pressure, chemicals) into electrical signals
• Sensory transduction involves converting the stimulus energy into a change in membrane potential
• Receptor potentials are graded potentials that are produced in response to sensory stimuli
• Sensory neurons encode information about the intensity, duration, and location of the stimulus
• Different sensory modalities (e.g., vision, hearing, touch) are processed in different brain regions
• Somatosensory system detects touch, temperature, pain, and proprioception
• Mechanoreceptors detect touch and pressure
• Thermoreceptors detect temperature changes
• Nociceptors detect pain
• Proprioceptors detect body position and movement
• Visual system detects light
• Photoreceptors (rods and cones) in the retina transduce light into electrical signals
• Rods are sensitive to low light levels and are responsible for night vision
• Cones are responsible for color vision and require higher light levels
• Auditory system detects sound
• Hair cells in the cochlea transduce sound vibrations into electrical signals
• Vestibular system detects head position and movement
• Hair cells in the vestibular organs transduce head movements into electrical signals
• Olfactory system detects odors
• Olfactory receptor neurons in the nasal epithelium detect odor molecules
• Gustatory system detects tastes
• Taste receptor cells in taste buds detect taste molecules

Motor Neurophysiology

• Motor systems control movement
• The motor system involves a hierarchy of brain regions, including the motor cortex, basal ganglia, cerebellum, and brainstem
• The motor cortex plans and initiates voluntary movements
• The basal ganglia are involved in the selection and initiation of movements
• The cerebellum coordinates movements and maintains balance
• The brainstem contains motor nuclei that control basic motor functions
• Upper motor neurons in the motor cortex project to lower motor neurons in the brainstem and spinal cord
• Lower motor neurons directly innervate muscles
• The neuromuscular junction is the synapse between a motor neuron and a muscle fiber
• Acetylcholine is the neurotransmitter released at the neuromuscular junction
• Muscle contraction is initiated by the binding of acetylcholine to receptors on the muscle fiber, leading to depolarization and the release of calcium ions

Higher-Order Neurophysiology

• Cognitive neurophysiology studies the neural basis of cognitive functions such as learning, memory, attention, and decision-making
• Learning is the process by which experiences change the nervous system and behavior
• Memory is the ability to store and retrieve information
• Long-term potentiation (LTP) is a long-lasting increase in synaptic strength that is thought to be a cellular mechanism of learning and memory
• Long-term depression (LTD) is a long-lasting decrease in synaptic strength
• The hippocampus is a brain region that is critical for the formation of new declarative memories (memories for facts and events)
• The amygdala is a brain region that is involved in processing emotions, particularly fear
• Attention is the process of selectively focusing on certain information while ignoring other information
• Decision-making is the process of selecting a course of action from a set of alternatives
• The prefrontal cortex is a brain region that is involved in executive functions such as planning, working memory, and decision-making