The Nervous System

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### Objectives 1. Compare nervous system complexity, from nerve nets in invertebrates to a centralized brain in vertebrates. 2. Discuss the relevance to neurological disorders and venomous species. ### Irritability and Coordination - Ability to respond to environmental stimuli - Adjustment in response and complex linking of various processes ### Organization of the Nervous System - **PNS (Peripheral Nervous System):** Sensory receptors, nerves, ganglia, and plexuses. Sensory division conducts action potentials from the periphery. - **CNS (Central Nervous System):** Brain and spinal cord. Processing and integrating information, initiates responses, mental activity. - **Somatic Nervous System:** Controls skeletal muscle. - **Autonomic Nervous System:** Controls cardiac muscle, smooth muscle, and glands. Motor division conducts action potentials to the periphery. ### Basic Plan - Receive information from external and internal environments - Encode and transmit information and to process it for appropriate action ### Basic Functions a. **Sensory function:** To sense changes (known as stimuli) both outside and within the body. b. **Integrative function:** Processing the information received from the sense organs. c. **Motor function:** Response to the stimuli that causes muscles to contract or glands to secrete. ### Subdivisions - **Central Nervous System (CNS):** Brain and spinal cord. - **Peripheral Nervous System (PNS):** All nervous tissue outside the CNS. ### The Neuron: Functional units of the nervous system - Transmit nerve impulses - **Parts:** a. Cell body (soma) b. Dendrites c. Axon ### Types of Neuron: structural - **Multipolar neurons:** Usually have several dendrites and one axon. - **Bipolar neurons:** Have one main dendrite and one axon. - **Unipolar neurons:** Have a single process extending from the cell body. ### Types of Neuron: direction of nerve impulse - **Afferent or sensory neuron:** Convey action potential into the CNS through cranial or spinal nerves. - **Efferent or motor neuron:** Convey action potentials away from the CNS to effectors (muscles and glands). - **Interneurons or associated neurons:** Connect sensory and motor neurons, integrate signals. ### Neuroglia /glial cells - Extremely numerous in the vertebrate brain. - Major supporting cells in the CNS. - **Functions:** - Forms a permeability barrier. - Phagocytize foreign substances. - Produce cerebrospinal fluid. - Form myelin sheaths around axons. ### Schwann cells - Special glial cells. - Forms the myelin sheath around axons in the PNS. - Participate in axon regeneration. ### Oligodendrocytes - Responsible for forming and maintaining the myelin sheath around CNS axons. ### Astrocytes - Star-shaped cells that have many processes. - Largest and most numerous of the neuroglia. - Multifunctional glial cells essential for maintaining CNS homeostasis. ### Microglia - Small cells with slender processes. - Serve as the primary immune defense cells within the CNS. ### Nature of a Nerve Action Potential - **Action potential:** - Nerve signal. - Electrochemical message of neuron. ### Resting membrane potential - Electrical potential difference across the cell membrane of a neuron when it is not actively sending a signal. - **Ionic composition:** - An active sodium-potassium exchange pump located in the plasma membrane drives sodium to the outside, keeping its concentration low inside and potassium concentration high inside. The membrane is permeable to potassium, but this ion is held inside by the repelling positive charge outside the membrane, by its attraction to large negatively charged anions, such as proteins, which cannot leave the cell, and by a constant influx of potassium due to the sodium-potassium exchange pump. ### Sodium pump - Complex of protein subunits embedded in the plasma membrane of the axon. - Actively transport sodium ions ($\text{Na}^+$) out of the cell and potassium ions ($\text{K}^+$) into the cell using ATP. ### Action potential - A rapid, temporary change in the electrical charge across the membrane of a neuron. - Allows transmission of electrical signals over long distances. - **Phases:** - **Depolarizing phase:** The negative membrane potential becomes less negative, reaches zero, and then becomes positive. - **Repolarizing phase:** The membrane potential is restored to the resting state of -70 mV. ### High-speed conduction - Ionic and electrical events associated with action potentials are much the same throughout the animal kingdom. - Conduction velocities vary enormously from neuron to neuron and from animal to animal. - From as slow as 0.1 m/sec in sea anemones to as fast as 120 m/sec in some mammalian motor axons. - Highly correlated with diameter of the axon. - **In invertebrates:** - Fast conduction velocities are important for quick response. - Some have giant axons. - **In vertebrates:** - Cooperative relationship between axons and the investing layers of myelin laid down by the Schwann cells or oligodendrocytes. ### Synapses: Junctions between nerves - Specialized junction where a neuron communicates with another neuron or with a target cell. - **Components:** - **Presynaptic neuron:** The neuron sending the signal. - **Postsynaptic neuron:** The neuron receiving the message. - **Types:** - **Electrical synapses:** Points at which ionic currents flow through narrow gap junctions from one neuron to another. - **Chemical synapses:** Use neurotransmitters to transmit signals across the synaptic cleft. ### Evolution of Nervous System - **Invertebrates:** Development of centralized nervous systems. - **Vertebrates:** Fruition of encephalization. - **Encephalization:** Evolutionary process or condition in which an animal develops an increase in size, configuration, and functional capacity of the brain.