2.1 Structure and Function of Biological Neurons
6 min read•july 30, 2024
Neurons are the building blocks of our nervous system, acting as tiny information processors. They come in different shapes and sizes, each with a specific job in our brain and body. Understanding their structure and function is key to grasping how our nervous system works.
Neurons communicate through electrical and chemical signals, allowing our brain to process information and control our body. This complex network of cells forms the basis for learning, memory, and behavior, making neurons essential for all aspects of our daily lives.
Neuron Structure and Function
Key Components and Their Roles
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- The soma contains the nucleus and other organelles essential for cellular function and metabolism
- are branched extensions of the soma that receive incoming signals from other neurons (e.g., in the cerebral cortex)
- The is a long, thin fiber that conducts electrical impulses away from the soma to other neurons or effector cells (e.g., innervating muscles)
- The axon hillock is the junction between the soma and the axon where action potentials are generated
- The insulates the axon to increase the speed of signal transmission
- Formed by Schwann cells in the peripheral nervous system
- Formed by oligodendrocytes in the central nervous system
- The axon terminals are the endpoints of the axon that release neurotransmitters into the to communicate with other neurons (e.g., at neuromuscular junctions)
Cellular Processes and Metabolism
- The nucleus contains the neuron's genetic material (DNA) and is the site of RNA synthesis
- Mitochondria generate ATP through oxidative phosphorylation to meet the neuron's energy demands
- The endoplasmic reticulum and Golgi apparatus are involved in the synthesis, modification, and transport of proteins and lipids
- Lysosomes contain digestive enzymes that break down cellular waste and damaged organelles
- The cytoskeleton, composed of microtubules, microfilaments, and neurofilaments, provides structural support and enables intracellular transport
Neural Communication: Electrical and Chemical Signals
Membrane Potential and Graded Potentials
- Neurons maintain a resting membrane potential of around -70mV due to the unequal distribution of ions across the cell membrane
- Graded potentials are localized changes in the membrane potential that can be excitatory (depolarizing) or inhibitory (hyperpolarizing)
- Used to integrate incoming signals from dendrites and soma
- Decay over distance and do not propagate along the axon
- The membrane potential is determined by the relative permeability of the membrane to different ions (primarily Na+, K+, and Cl-) and the activity of ion pumps (e.g., Na+/K+ ATPase)
Action Potentials and Their Propagation
- Action potentials are all-or-none electrical impulses generated when the membrane potential reaches a threshold value, typically around -55mV
- The consists of , overshoot, repolarization, and a brief
- Depolarization: Na+ channels open, causing rapid Na+ influx and membrane potential rise
- Overshoot: membrane potential briefly becomes positive due to continued Na+ influx
- Repolarization: K+ channels open, causing K+ efflux and return of membrane potential to resting level
- Refractory period: Na+ channels are inactivated, preventing the generation of another action potential
- Action potentials are propagated along the axon without attenuation due to the regenerative opening of voltage-gated Na+ channels
- Voltage-gated sodium and potassium channels are responsible for the generation and propagation of action potentials
- Myelin sheaths and the clustering of Na+ channels at the nodes of Ranvier facilitate rapid saltatory conduction of action potentials
Neurotransmitters and Chemical Signaling
- Neurotransmitters are chemical messengers released from the presynaptic neuron that bind to receptors on the postsynaptic neuron
- Binding of neurotransmitters causes either excitatory or inhibitory postsynaptic potentials (EPSPs or IPSPs)
- EPSPs depolarize the postsynaptic membrane, increasing the likelihood of action potential generation
- IPSPs hyperpolarize the postsynaptic membrane, decreasing the likelihood of action potential generation
- Common neurotransmitters include glutamate (excitatory), GABA (inhibitory), acetylcholine, dopamine, serotonin, and norepinephrine
- Neurotransmitters are synthesized in the presynaptic neuron, stored in synaptic vesicles, and released via calcium-dependent exocytosis
Synapses: Communication and Plasticity
Types of Synapses
- Chemical synapses are the most common type, involving the release of neurotransmitters from the presynaptic neuron and their binding to receptors on the postsynaptic neuron
- Neurotransmitters diffuse across the synaptic cleft, a narrow space between the pre- and postsynaptic membranes
- Binding of neurotransmitters to receptors causes ion channels to open or close, resulting in EPSPs or IPSPs
- Electrical synapses are gap junctions that allow direct electrical coupling between neurons
- Enable faster signal transmission compared to chemical synapses
- Found in neural circuits requiring precise timing and synchronization (e.g., in the inferior olive)
Synaptic Plasticity and Learning
- refers to the ability of synapses to strengthen or weaken over time in response to activity
- Forms the basis for learning and memory in the nervous system
- Involves changes in the number and sensitivity of postsynaptic receptors, as well as structural modifications of dendritic spines
- Long-term potentiation (LTP) is a persistent increase in synaptic strength that results from high-frequency stimulation of the presynaptic neuron
- Involves the activation of NMDA receptors and calcium influx, leading to the insertion of additional AMPA receptors in the postsynaptic membrane
- Observed in the hippocampus and other brain regions involved in learning and memory (e.g., during spatial learning tasks)
- Long-term depression (LTD) is a persistent decrease in synaptic strength that results from low-frequency stimulation or the absence of correlated activity between pre- and postsynaptic neurons
- Involves the removal of AMPA receptors from the postsynaptic membrane and structural changes in dendritic spines
- Plays a role in synaptic pruning and the refinement of neural circuits during development
Neuron Types and Roles
Functional Classification
- (afferent neurons) transmit information from sensory receptors to the central nervous system
- Detect stimuli such as light, sound, touch, temperature, and chemicals
- Examples include retinal photoreceptors, cochlear hair cells, and cutaneous mechanoreceptors
- Motor neurons (efferent neurons) transmit signals from the central nervous system to effector cells such as muscles or glands
- Control voluntary and involuntary movements, as well as endocrine and exocrine secretion
- Examples include spinal motor neurons innervating skeletal muscles and autonomic neurons regulating smooth muscle and glands
- form connections between other neurons within the central nervous system
- Enable complex processing and integration of information
- Found in all regions of the brain and spinal cord, forming local circuits and long-range connections
Morphological Classification
- Bipolar neurons have two processes extending from the soma
- Found in sensory systems such as the retina (retinal bipolar cells) and olfactory epithelium (olfactory receptor neurons)
- Typically receive input from sensory receptors and transmit signals to other neurons in the sensory pathway
- Unipolar neurons have a single process that splits into two branches
- One branch extends to the periphery (e.g., skin, muscles) and the other to the central nervous system
- Typical of sensory neurons in the dorsal root ganglia and cranial nerve ganglia
- Multipolar neurons have multiple dendrites and a single axon extending from the soma
- Include motor neurons, interneurons, and many types of neurons in the brain
- Allow for the integration of multiple inputs and the transmission of signals to multiple targets
- are a type of multipolar neuron found in the cerebral cortex and hippocampus
- Characterized by a triangular-shaped soma, a single apical dendrite, and multiple basal dendrites
- Play crucial roles in cognitive functions such as perception, memory, and decision-making (e.g., in the prefrontal cortex)
Key Terms to Review (17)
Action Potential: An action potential is a rapid and temporary change in the electrical charge of a neuron's membrane, allowing it to transmit signals along its axon. This electrical impulse is crucial for communication between neurons and facilitates the transfer of information throughout the nervous system. The action potential is characterized by depolarization, repolarization, and a refractory period, which together ensure the unidirectional flow of electrical signals.
Axon: An axon is a long, slender projection of a neuron that conducts electrical impulses away from the cell body. This specialized structure is critical for transmitting signals over distances to other neurons, muscles, or glands, thereby facilitating communication within the nervous system. The axon enables the rapid propagation of action potentials and is often insulated by a myelin sheath, which increases the speed of signal transmission.
Dendrites: Dendrites are the branched projections of a neuron that receive signals from other neurons and transmit them toward the cell body. They play a crucial role in integrating synaptic inputs, allowing neurons to process information and communicate effectively within the nervous system. Dendrites have specialized structures called dendritic spines, where synapses form, enhancing their ability to receive and process signals.
Depolarization: Depolarization is the process in which a neuron's membrane potential becomes less negative, moving closer to zero. This occurs when sodium channels open and sodium ions flow into the neuron, reducing the electrical difference across the membrane and triggering an action potential. Understanding depolarization is crucial for grasping how neurons communicate and transmit signals throughout the nervous system.
Glial Cells: Glial cells, or neuroglia, are non-neuronal cells in the nervous system that provide support, protection, and nourishment to neurons. They play crucial roles in maintaining homeostasis, forming myelin, and participating in signal transmission, ultimately ensuring the proper functioning of the nervous system.
Interneurons: Interneurons are a type of neuron that act as a bridge between sensory and motor neurons within the central nervous system. They play a crucial role in processing information, integrating signals, and facilitating communication between different neurons, enabling complex reflexes and higher cognitive functions.
Motor Neurons: Motor neurons are specialized nerve cells that transmit signals from the central nervous system to muscles and glands, facilitating movement and response to stimuli. These neurons play a crucial role in controlling voluntary and involuntary movements, making them essential for everyday activities such as walking, talking, and breathing.
Myelin sheath: The myelin sheath is a protective, insulating layer that surrounds the axons of many neurons, allowing for faster transmission of electrical impulses. This fatty substance is composed primarily of lipids and proteins, which enhance the speed of nerve signal conduction through a process called saltatory conduction. The presence of myelin sheath not only improves communication between neurons but also plays a role in maintaining the overall health and efficiency of neural function.
Neurotransmission: Neurotransmission is the process by which signaling molecules, known as neurotransmitters, are released from the neuron and travel across the synaptic cleft to bind to receptors on a neighboring neuron. This process is crucial for communication within the nervous system, allowing for the transmission of signals that regulate various bodily functions and behaviors. Proper neurotransmission is essential for brain function, impacting everything from mood to motor control.
Neurotransmitter release: Neurotransmitter release refers to the process by which chemical messengers called neurotransmitters are released from the synaptic vesicles of a neuron into the synaptic cleft, allowing for communication between neurons. This process is crucial for transmitting signals across synapses, influencing various functions such as muscle contraction, mood regulation, and cognitive processes. The precise timing and amount of neurotransmitter release are essential for maintaining effective neuronal communication and ensuring proper functioning of the nervous system.
Node of Ranvier: A Node of Ranvier is a small gap in the myelin sheath surrounding a neuron, where the axonal membrane is exposed. These nodes play a crucial role in the propagation of action potentials along myelinated axons, allowing electrical signals to jump from one node to the next, which significantly increases the speed of neural transmission. This process is known as saltatory conduction, and it is essential for efficient communication between neurons.
Pyramidal Neurons: Pyramidal neurons are a type of excitatory neuron found in the cerebral cortex and other parts of the brain, characterized by their pyramid-shaped cell body and long dendrites. These neurons play a crucial role in cognitive functions, such as decision-making, memory, and learning due to their ability to integrate and transmit information across long distances within the brain.
Refractory Period: The refractory period is the time following an action potential during which a neuron is less excitable and cannot generate another action potential. This period ensures that action potentials are unidirectional and allows the neuron to recover its resting state, contributing to the regulation of neuronal firing rates and overall neural communication.
Santiago Ramón y Cajal: Santiago Ramón y Cajal was a Spanish neuroscientist and pathologist known as the father of modern neuroscience for his groundbreaking work on the structure of the nervous system. He pioneered the use of histological staining techniques to visualize neurons, which allowed him to formulate the neuron doctrine, proposing that neurons are the fundamental units of the nervous system and communicate via synapses.
Sensory Neurons: Sensory neurons are specialized nerve cells responsible for converting external stimuli from the environment into electrical signals that can be interpreted by the brain. They play a crucial role in our ability to perceive sensations such as touch, temperature, pain, and taste, making them essential for interacting with the world around us.
Synapse: A synapse is a junction between two neurons that allows them to communicate with each other. It plays a crucial role in transmitting signals through the nervous system, facilitating the flow of information by connecting the axon terminal of one neuron to the dendrites or cell body of another. This connection is vital for the functioning of neural circuits, influencing everything from reflexes to complex behaviors.
Synaptic Plasticity: Synaptic plasticity refers to the ability of synapses, the connections between neurons, to strengthen or weaken over time in response to increases or decreases in their activity. This dynamic process is crucial for learning and memory, as it allows the nervous system to adapt based on experiences. The changes can happen through various mechanisms, including long-term potentiation (LTP) and long-term depression (LTD), which are essential for neural communication and overall brain function.