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The nervous system is a complex network responsible for transmitting and processing information throughout the body, comprising neurons and glial cells that work together to ensure proper functioning.
Neurotransmitters, essential for synaptic transmission, are removed from the synaptic cleft through several mechanisms: enzymatic breakdown (e.g., acetylcholine), reuptake into the presynaptic neuron via specific carriers (e.g., serotonin, dopamine, norepinephrine), and diffusion out of the synaptic cleft (e.g., nitric oxide).
Neurons maintain a negative internal environment primarily through selective permeability to ions and the Na+/K+ ATPase pump. This pump expels 3 Na+ ions from the cell for every 2 K+ ions it imports, maintaining the resting membrane potential of approximately -70 mV.
The Na+/K+ ATPase pump also restores the resting potential and sodium-potassium gradients depleted during action potential propagation, essential for neuronal function.
Dendrites are neuronal appendages that receive incoming signals from other cells, initiating neuronal communication.
Nerves are bundles of neurons in the peripheral nervous system (PNS), classified as sensory, motor, or mixed depending on their function.
Nuclei refer to clusters of cell bodies of neurons within the central nervous system (CNS), organized into tracts that facilitate communication.
Glial cells, including astrocytes, ependymal cells, microglia, oligodendrocytes, and Schwann cells, support neurons by providing structural support, nutrition, and regulating the extracellular environment.
Sodium channels in neurons exist in three states: closed (resting state), open (during depolarization), and inactive (after depolarization, until reset to resting potential).
The spinal cord is divided into four regions: cervical, thoracic, lumbar, and sacral, each responsible for specific sensory and motor functions.
In neurons, potassium (K+) concentrations are higher inside, while sodium (Na+) concentrations are higher outside, crucial for generating and propagating action potentials.
Axon terminals of neurons communicate with target structures such as glands, muscles, or other neurons, facilitating coordinated responses.
Refractory periods following action potentials include absolute (when a neuron cannot fire another action potential) and relative (when a stronger stimulus can initiate an action potential).
Tracts are bundles of axons within the CNS, carrying specific types of information between regions.
Gray matter consists of unmyelinated cell bodies and dendrites, while white matter comprises myelinated axons that facilitate rapid signal conduction.
The myelin sheath, produced by oligodendrocytes in the CNS and Schwann cells in the PNS, insulates axons and accelerates signal conduction.
The speed of action potential propagation depends on the axon's length and cross-sectional area; larger diameters decrease resistance, facilitating faster transmission.
Ganglia are clusters of neuron cell bodies in the PNS, whereas nuclei are similar structures found in the CNS.
Spatial summation integrates signals based on the number and location of inputs, while temporal summation integrates signals over time.
The axon hillock integrates incoming signals and plays a crucial role in action potential initiation.
The resting membrane potential of neurons is approximately -70 mV, maintained by the balance of ion concentrations across the cell membrane.
Supraspinal circuits involve input from the brain or brainstem, coordinating complex responses and behaviors.
In summary, the nervous system's organization and function are critical for sensory perception, motor control, and maintaining homeostasis. Understanding its components, from neurons and neurotransmitters to glial cells and neuronal pathways, provides insights into how information is processed and transmitted throughout the body, essential knowledge for the MCAT.