Neurons and Synapses PDF
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Hamilton College
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This document provides an overview of neurons and synapses, explaining the structure and function of neurons and how they communicate. It covers the divisions of the nervous system, including the somatic and autonomic nervous systems, and describes the process of neurotransmission.
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eurons and Synapses N Nervous system is a network of neurons that run throughout your brain and body. nerves—collections of neurons—carry signals to and from your brain, relating perceptions, thoughts, and feelings into actions. Spina...
eurons and Synapses N Nervous system is a network of neurons that run throughout your brain and body. nerves—collections of neurons—carry signals to and from your brain, relating perceptions, thoughts, and feelings into actions. Spinal cord: major bundle of nerves that connects your body to your brain The nervous system also allows us to have some important behaviors such as reflexes without requiring the brain at all Neuron Fundamentals Neuron = Nerve cell in the brain & nervous system ○ Sensory neuron: Carry messages from sensory organs to CNS. Carry information from within your body and the outside world to your brain ○ Interneuron: Carry messages from one set of neurons to another. They interpret, store, and retrieve information about the world, allowing you to make informed decision before you act ○ Motor neuron: Carry messages from CNS to muscles and glands Divisions of the Nervous System The central nervous system consists of the brain and spinal cord. The peripheral nervous system has two divisions: the somatic nervous system and the autonomic nervous system. Somatic nervous system is related to voluntary commands-or commands that we choose to do ○ not only senses the body, but also controls your conscious body movements ○ includes your skeletal muscles. Autonomic nervous system is related to involuntary commands, those largely not in your control ○ maintains the operations of the inside of your body-for example, your heart-and is mostly outside of your conscious control. ○ i ncludes your organs, blood vessels, and glands, which are hormone-secreting organs ○ branches into the sympathetic and parasympathetic nervous systems sympathetic branch ensures that your body provides essential resources needed for the fight-or-flight response, To make fight-or-flight possible, your body redirects energy from processes that are not essential in the moment, such as digestion and sexual reproduction. acts on blood vessels, organs, and glands in ways that prepare the body for action, especially in life-threatening situations The parasympathetic nervous system acts on blood vessels, organs, and glands in a way that returns the body to a resting state by both counteracting and complementing the actions of the sympathetic system. The restorative function of the rest-and-digest response allows you to regenerate the energy that your body needs when it is safe to do so. Eating is one important restorative activity. "Emotional eating" is their attempt to regulate stress by engaging the parasympathetic nervous system's anti-stress response. Although often working in opposition to each other, the sympathetic and parasympathetic nervous systems work together to prepare the body for the challenges that the brain sees lying ahead. The Structure of Neurons Cell Body: The cell body collects neural impulses, contains the nucleus, and provides life-sustaining functions for the cell Dendrites : receive chemical messages from other neurons. The axon: transports electrical impulses called action potentials to the terminal branches, where they are converted into chemical messages that are sent to other neurons. Myelin: is a layer of fatty tissue that covers and insulates the axon to ensure electrical messages are kept intact and travel quickly. ○ Degradation of myelin, called demyelination, is a central characteristic of neurodegenerative diseases ○ Shrinks with aging ○ Glia: cells that make up the myelin. Insulate, support, and nourish the neuron. Serve as cellular glue contribute to information processing during childhood development and into adulthood Are essential for brain development, providing a scaffold along which axons grow and guiding them to their correct location in the nervous system Aid in the formation of neural networks: cluster of cells that work together as a functional unit Terminal branches: convert electrical signals into chemical messages that they then send to other neurons. Action Potential and How Nerves Fire Them Neurons are bathed in extracellular fluid composed of positively and negatively charged particles or ions (sodium (NA+), chloride (Cl-), potassium (k+), and calcium (CA2+)) The membrane that separates the intracellular (inside the cell) and extracellular (outside the cell) fluids is selective, which means that only certain ions can pass through the membrane to the inside of a neuron Normally, at resting potential, positively charged ions are outside the cell so the intracellular fluid is relatively positive compared to the negative intracellular environment ○ A neuron cannot fire an action potential as long as this resting potential is strongly negatively polarized If a neuron is stimulated sufficiently to pass its voltage threshold, an action potential fires ○ When other neurons sufficiently stimulate a neuron’s dendrites ion changes open allowing positively charged sodium (Na + ) ions into the neuron. As these positively charged ions flood into the neuron, they set off a chain reaction as they spread down the axon, causing more channels to open. 1) Depolarization: occurs when the voltage of a neuron becomes less negatively polarized and moves toward and past a critical voltage threshold to fire an action potential. a) positive ions flowing into the axon. 2) Voltage threshold: the critical voltage(around -50 millivolts) that the neuron must reach to fire an action potential. The neuron's voltage then surges rapidly and becomes positive as it passes zero. 3) Repolarization: occurs as the neuron returns to its resting state voltage a ) positive ions flowing out of the axon. 4) Refractory period: is the time required before a neuron is able to fire its next action potential a) during which it is difficult or impossible to get a neuron to fire an action potential again Neuron Signals One of two kinds: Excitatory or Inhibitory excitatory messages: move the voltage of the neuron closer to its threshold. inhibitory messages: move it farther away from its voltage threshold. ○ If the excitatory (positive/+) messages outweigh the inhibitory (negative/-) messages enough to reach the voltage threshold, then the neuron fires an action potential ○ If the neuron receives many inhibitory (negative/-) signals, it is much less likely to reach the threshold necessary for firing. Neurotransmission: How Neurons Communicate Synapse: The gap where a sending neuron communicates with the dendrites or the cell body of the receiving neuron The process of neurotransmission allows the electrical message to bridge the synaptic gap by converting the electrical signal into a chemical one, thus allowing neurons to transmit their signals to one another. Electrical-to-chemical translation is critical for communication between neurons-from the sending presynaptic neuron's terminals to the receiving postsynaptic target neuron's dendrites Without making anatomical contact, the terminal branches release chemical messengers called neurotransmitters , across the synaptic gap toward the target, receiving neuron On the surface of target neurons are receptors that recognize and bind with specific neurotransmitters. Each neurotransmitter has specific receptors that selectively recognize it. 'Thus, each receptor is like a lock with a key-a neurotransmitter-that will open it. Once the neurotransmitter binds to its receptor ion channels open hereby inducing changes in on flow across the target neurons cell membrane. As a result an electronic signal is generated in the target neuron Receptor Response to Neurotransmitters R eceptors targeted by neurotransmitters produce excitatory or inhibitory electrical signals in the target neuron Ions enter the receptor, moving the target neuron closer to or farther from its action potential threshold The receptor's response, not the neurotransmitter itself, determines whether the signal is excitatory or inhibitory Neurotransmitter Inactivation Inactivation of neurotransmitters in the synapse is crucial after signal generation Prevents constant stimulation and maintains neuronal balance Mechanisms for neurotransmitter removal: ○ a) Diffusion: Neurotransmitters drift out of the synapse into extracellular space ○ b) Degradation: Chemical reactions break down neurotransmitters in the synapse ○ c) Reuptake: Presynaptic terminals reabsorb neurotransmitters Antidepressants and Neurotransmitter Function Some antidepressants prevent neurotransmitter reuptake Selective Serotonin Reuptake Inhibitors (SSRIs) keep neurotransmitters in the synapse longer Can be helpful for depression but may cause side effects like upset stomach or insomnia Class of neurotransmitters Amino acids: such as glutamate and gamma-aminobutyric acid (GABA), are the brain’s most abundant neurotransmitters M onoamines: are important for fight-or-flight response activation Acetylcholine: can behave as both an inhibitory and an excitatory signal. It supports heart and skeletal muscle, and cognitive function.
