Overview of the Nervous System PDF

Overview of the Nervous System Functions of the Nervous System ​ The nervous system and endocrine system work together to control body actions. ​ The nervous system transmits rapid electrical signals for quick responses. ​ In contrast, the endocrine system uses slower chemical signals for pro...

Overview of the Nervous System Functions of the Nervous System ​ The nervous system and endocrine system work together to control body actions. ​ The nervous system transmits rapid electrical signals for quick responses. ​ In contrast, the endocrine system uses slower chemical signals for prolonged responses. ​ The nervous system is crucial for immediate reactions to stimuli, while the endocrine system regulates longer-term processes. ​ Both systems are essential for maintaining homeostasis and responding to environmental changes. Organization of the Nervous System ​ The nervous system is divided into two main parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). ​ The CNS consists of the brain and spinal cord, primarily made up of interneurons that coordinate information. ​ The PNS includes all other nerves, which are categorized into sensory (afferent) and motor (efferent) neurons. ​ Sensory neurons carry information from receptors to the CNS, while motor neurons transmit signals from the CNS to effectors. ​ The PNS is essential for connecting the CNS to the rest of the body, facilitating communication and response. Components of Neurons Structure of a Neuron ​ Neurons are specialized cells that transmit nerve impulses and cannot survive independently; they rely on glial cells for support. ​ Glial cells perform essential functions such as nourishing neurons, removing waste, and defending against infections. ​ Key parts of a neuron include the cell body, axon, dendrites, myelin sheath, axon hillock, axon terminals, and nodes of Ranvier. ​ The myelin sheath, produced by Schwann cells, insulates the axon and increases the speed of signal transmission. ​ Dendrites receive signals from other neurons or sensory receptors, while axons conduct impulses away from the cell body.​  Classes of Neurons ​ Sensory (afferent) neurons transmit information from sensory receptors to the CNS, providing data about the internal and external environment. ​ Motor (efferent) neurons convey impulses from the CNS to effector cells, such as muscles and glands, facilitating movement and response. ​ Interneurons, located within the CNS, connect sensory and motor neurons, playing a critical role in reflexes and neural circuits. ​ Each class of neuron has a distinct function, contributing to the overall operation of the nervous system. ​ The interaction between these neuron types is essential for processing information and executing responses. Neural Circuits and Reflex Arcs Understanding Neural Circuits ​ Neural circuits are pathways through which signals travel, allowing for communication between different parts of the nervous system. ​ The simplest neural circuit involves a direct pathway that does not require brain involvement, enabling quick reflex actions. ​ Essential components of a simple neural circuit include receptors, sensory neurons, interneurons, motor neurons, and effectors. ​ This organization allows for rapid responses to stimuli, crucial for survival in many situations. ​ Reflex arcs exemplify how the nervous system can respond to stimuli without conscious thought, demonstrating efficiency in neural processing.​ ![Here's a concise alt text/caption for the provided image: The Reflex Arc  ​ The reflex arc is a fundamental neural pathway that mediates reflex actions, providing a rapid response to stimuli. ​ It consists of five essential components: receptor, sensory neuron, interneuron, motor neuron, and effector. ​ The receptor detects a stimulus and activates the sensory neuron, which transmits the signal to the interneuron in the CNS. ​ The interneuron processes the information and relays it to the motor neuron, which then activates the effector (muscle or gland). ​ Reflex arcs are critical for protective responses, such as withdrawing a hand from a hot surface, showcasing the nervous system's role in immediate reaction. 1. Electrochemical Impulse and Membrane Potential Electrochemical Impulse ​ Neurons transmit signals through electrochemical currents, primarily involving the movement of ions across the membrane. ​ The process of depolarization is crucial for the conduction of impulses, where the inside of the neuron becomes less negative relative to the outside. Membrane Potential ​ All animal cells exhibit a voltage difference across their plasma membranes due to the separation of charges: positive outside and negative inside. ​ This voltage difference is termed the membrane potential, which is essential for neuron function. Resting Membrane Potential ​ The resting membrane potential is approximately -70 mV, indicating that the inside of the neuron is negatively charged compared to the outside. ​ The sodium-potassium pump actively transports Na+ ions out and K+ ions into the neuron, maintaining the negative charge inside. ​ The membrane is more permeable to K+ ions, allowing them to diffuse out, which contributes to the resting potential. Factors Maintaining Resting Membrane Potential ​ Large negatively charged proteins (anions) are present inside the cell, contributing to the negative charge.  ​ Ion-specific channels in the plasma membrane facilitate the passive movement of ions, primarily K+ during resting potential. ​ The sodium-potassium pump's active transport ensures a higher concentration of Na+ outside and K+ inside, maintaining polarization. 2. Action Potential and Its Phases Overview of Action Potential ​ Action potential is the electrical impulse that travels along the axon, characterized by rapid changes in membrane potential. ​ The process involves five stages: stimulus and initiation, depolarization, repolarization, hyperpolarization, and recovery of resting membrane potential. Stages of Action Potential 1.​ Stimulus and Initiation: A stimulus causes depolarization, raising the membrane potential to the threshold (-50 to -55 mV). 2.​ Depolarization: Na+ channels open, allowing Na+ to flow in, causing a sharp increase in membrane potential to +40 mV. 3.​ Repolarization: K+ channels open, K+ exits the neuron, and the membrane potential decreases back towards resting levels. 4.​ Hyperpolarization: Slow closing of K+ channels causes a brief undershoot of resting potential. 5.​ Recovery: The membrane stabilizes back to resting potential, ready for another action potential. Refractory Period ​ A brief refractory period occurs after an action potential, lasting 1 to 10 ms, during which a new action potential cannot be initiated. ​ This ensures that impulses travel in one direction along the axon and prevents backflow. 3. Propagation of Impulse Mechanism of Propagation ​ Action potentials propagate along the axon as a wave of depolarization, requiring no further stimulus once initiated. ​ Neighboring membrane areas depolarize due to local electrical currents, creating a chain reaction. Saltatory Conduction ​ Myelinated axons have Schwann cells that insulate the axon, with Nodes of Ranvier allowing for rapid depolarization at these points. ​ The action potential 'jumps' from node to node, significantly increasing the speed of transmission (up to 120 m/s). Comparison of Myelinated vs Unmyelinated Axons > Type of Speed of Mechanism Axon Transmission of Propagation Unmyelinated Slower Continuous depolarization Myelinated Faster Saltatory conduction 4. Synaptic Transmission Electrical vs Chemical Synapses ​ Electrical Synapses: Direct contact between presynaptic and postsynaptic cells via gap junctions, allowing rapid transmission. ​ Chemical Synapses: Separated by a synaptic cleft where neurotransmitters diffuse to transmit signals. Role of Neurotransmitters ​ Neurotransmitters are stored in synaptic vesicles and released upon action potential arrival, facilitating communication between neurons. ​ They bind to receptors on the postsynaptic membrane, causing ion channels to open and influencing the postsynaptic cell's potential. Inhibitory vs Excitatory Responses  ​ Neurotransmitters can have excitatory (depolarizing) or inhibitory (hyperpolarizing) effects depending on the receptor type. ​ Example: Acetylcholine can cause muscle contraction (excitatory) or inhibit heart rate (inhibitory). 5. Neurotransmitter Examples and Functions Key Neurotransmitters > Neurotransmitter Function Associate d Conditions Dopamine Movement control, Schizophrenia, pleasure Parkinson's Serotonin Mood regulation, Depression sensory perception Endorphins Natural painkillers Emotional effects Norepinephrine Autonomic High blood functions, stress pressure, response anxiety Mechanisms of Neurotransmitter Action ​ Neurotransmitters are released through exocytosis and bind to specific receptors, leading to changes in the postsynaptic cell's membrane potential. ​ Enzymatic degradation and reuptake mechanisms ensure neurotransmitters do not overstimulate the postsynaptic cell.