Nervous System Cells (Bio 12.1) PDF

Automatic ZoomActual SizePage Width100%50%75%100%125%150%200%300%400% Chapter 12: Nervous System 418 Lesson 12.1 **Cells of the Nervous System** Introduction The **nervous system** includes both neurons, cells responsible for communicating information throughout the body via electrochemical si...

Automatic ZoomActual SizePage Width100%50%75%100%125%150%200%300%400% Chapter 12: Nervous System 418 Lesson 12.1 **Cells of the Nervous System** Introduction The **nervous system** includes both neurons, cells responsible for communicating information throughout the body via electrochemical signaling, and glial cells (sometimes called neuroglia or neuroglial cells), which provide various means of support to neurons. Some glial cells modify neurons by providing an electrically insulating layer called myelin. This lesson presents the basic characteristics of neurons and the ways that the various types of glial cells interact with neurons. 12.1.01 The Neuron **Neurons** (ie, nerve cells) are the cells of the nervous system specialized to receive, integrate, and transmit information via electrochemical signaling. The sites of information transmission, called **synapses**, are areas of neuron plasma membrane either in close proximity to or in actual contact with other cells, including other neurons. A typical neuron (Figure 12.1) consists of a **soma** (cell body), which contains the nucleus and participates in information reception and integration, and specialized branching extensions. **Dendrites** are neuron extensions generally specialized for signal *reception* and contain many receptors for molecules involved in information transfer. **Axons** are neuron extensions typically specialized for signal *transmission*. The **axon hillock** is the region where an axon originates from a neuron cell body, and the **axon terminal** is the end of the axon, where signal transmission occurs. **Figure 12.1** The neuron. A diagram of a nerve cell Description automatically generated Chapter 12: Nervous System 419 Although all neurons have at least one synapse, neuron morphology varies considerably. **Bipolar neurons** have a single dendrite and a single axon, whereas **multipolar neurons** have more than one dendrite and a single axon (Figure 12.1 illustrates a multipolar neuron). **Unipolar neurons** have cell bodies that lay to the side of a single extension formed from the fusion of a dendrite and an axon. Signals in unipolar neurons travel from the \"dendritic\" (ie, receptive) end to the \"axonal\" (ie, transmissive) end; however, the entire extension is called an axon. Neurons can also be classified according to their function and location. Neurons in the brain and spinal cord are part of the central nervous system (CNS), and neurons in the periphery are part of the peripheral nervous system (PNS). **Sensory** (**afferent**) neurons transmit information from the periphery to the CNS and are often linked to a specific type of sensory receptor. **Motor** (**efferent** or **somatic**) neurons transmit signals from the CNS to the periphery (eg, skeletal muscle fibers) to cause an action (eg, movement). Together, sensory and motor neurons account for roughly 1% of all neurons; the other 99% are **interneurons**, which are CNS neurons connecting two neurons. The flow of ions across the plasma membrane plays an essential role in neuron function, as discussed in Lesson 12.2, and, to regulate such flow, neurons invest a large amount of energy in pumping ions across the plasma membrane. In addition, the structure of neurons necessitates the transport of molecules to and from distant cellular locations (eg, from the cell body to the axon terminal). Neurons use cellular transport systems with ATP-dependent [motor proteins](javascript:void(0)) to move molecules throughout the cell. To help meet the cell\'s energy demands, neurons have an abundance of [mitochondria](javascript:void(0)). Concept Check **12**.**1** Identify each region in the diagram of a bipolar neuron as a region that primarily receives information at synapses or a region that primarily transmits information at synapses. [**Solution**](javascript:void(0))  A diagram of a nerve cell Description automatically generated Chapter 12: Nervous System 420 12.1.02 Myelin Sheath As part of their specialization for transmitting information via electrical impulses, certain neurons display different structural characteristics. **Myelinated neurons** have an electrically insulating sheath known as **myelin** around their axons, whereas **unmyelinated neurons** lack this layer. The myelin sheath consists of multiple layers of a glial cell [phospholipid](javascript:void(0)) membrane wrapped around the axon, as shown in Figure 12.2. Myelination alters ion and nutrient transport along axons, and myelinating cells provide nutrients to myelinated regions via gap junctions and transporters. Cell bodies and dendrites are not myelinated. The myelin layer is not continuous along the length of a myelinated axon; it is interrupted at small, regularly spaced sites called the **nodes of Ranvier**. These nodes are rich in ion channels (ie, membrane proteins through which ions can pass). By focusing ion movement to the nodes of Ranvier, myelination causes electrical impulses to \"jump\" along the axon. This form of electrical impulse transmission is called **saltatory conduction** and greatly augments the rate of signal transmission along myelinated axons (discussed in detail in Lesson 12.2). **Figure 12.2** A myelinated axon. Of the various types of glial cells (discussed in Concept 12.1.03), [two participate](javascript:void(0)) in myelination: **Oligodendrocytes** myelinate central nervous system (CNS) neurons and **Schwann cells** myelinate peripheral nervous system (PNS) neurons. In demyelinating diseases (eg, multiple sclerosis, Guillain-Barré syndrome) or when nerves are otherwise damaged (eg, through mechanical trauma such as cutting or compression), the myelin sheath is damaged or removed (Figure 12.3). Demyelination can profoundly slow or block signal transmission along neurons that are myelinated when healthy.  Chapter 12: Nervous System 421 **Figure 12.3** Axon myelination states. 12.1.03 Glial Cells Like a surgeon requires a supporting group of individuals carrying out specific tasks (eg, monitoring vital signs, administering anesthesia, assisting with surgical procedures) to safely perform surgery, neurons require a group of support cells for the protection and optimization of neuronal impulse transmission. **Glial cells** (sometimes called glia or neuroglia) are nervous system cells that help fuel, protect, and structurally support neurons. There are multiple types of glial cells. Some of the central nervous system (CNS) glial cells are shown in Figure 12.4: **Ependymal cells** line compartments in the CNS, thereby forming a selectively permeable barrier between the compartments, and produce cerebrospinal fluid, which helps cushion and support structures, thereby protecting neurons from mechanical trauma. **Oligodendrocytes** are myelinating cells, forming myelin sheaths around multiple axons to reduce ion leakage, decrease capacitance, and increase action potential propagation. **Microglia** are immune cells that can transform into specialized macrophages in response to neuronal damage or infection and can [phagocytose](javascript:void(0)) pathogens, damaged cells, and waste materials. Microglia functions are essential because other immune cells (eg, [B cells](javascript:void(0)), cells of the [innate immune system](javascript:void(0))) are largely prevented from entering the CNS. **Astrocytes** are a network of cells connected by [gap junctions](javascript:void(0)) and perform diverse roles. Some contact with blood vessels and regulate blood flow or form the [blood-brain barrier](javascript:void(0)); others are found Several images of different types of neurons Description automatically generated with medium confidence Chapter 12: Nervous System 422 near synapses, where they help regulate chemical composition by taking up K+ ions and signaling molecules that might interfere with signal transmission. Yet others associate with neurons to exchange metabolic substrates. Chapter 12: Nervous System Two of the glial cell types found in the peripheral nervous system (PNS) are illustrated in Figure 12.5: **Schwann cells** each associate with a single axon to form a myelin sheath that increases the speed of electrical impulse conduction. **Satellite cells** are nonmyelinating Schwann cells that provide structural support and supply nutrients to neuron cell bodies, similar to the role that some astrocytes play in the CNS