Nervous System Structure and Function PDF

440 Lesson 12.3 **Nervous System Structure and Function** Introduction The nervous system consists of multiple branches spread across several layers of organization. This complex organization reflects the efficient division of labor, as well as robust fine-tuning via antagonistic control. This l...

440 Lesson 12.3 **Nervous System Structure and Function** Introduction The nervous system consists of multiple branches spread across several layers of organization. This complex organization reflects the efficient division of labor, as well as robust fine-tuning via antagonistic control. This lesson first presents the general organization of the nervous system before considering the general pathways by which the central and peripheral nervous systems communicate, the antagonistic divisions of the autonomic nervous system, and the participation of the nervous system in feedback regulation. 12.3.01 Organization of the Nervous System The **nervous system**, which is responsible for the control and integration of all body systems, consists of the brain, spinal cord, and associated neurons and glial cells. The nervous system receives incoming information from external and internal environments, processes this information, and uses the processed information to coordinate purposeful responses. Through these actions, the nervous system coordinates movement, thought, and processes in the body that help maintain homeostasis. The components of the nervous system can be divided and subdivided along several levels of organization, as shown in Figure 12.21. Anatomically, the **central nervous system (CNS)** consists of the cells of the brain and spinal cord, whereas the **peripheral nervous system (PNS)** consists of the nervous system components outside of the brain and spinal cord. Functionally, the CNS is the part of the nervous system that receives and processes sensory information and coordinates responses to this information throughout the body. The PNS is the part of the nervous system that relays information to and from the CNS and carries out responses. Chapter 12: Nervous System **Figure 12.21** The nervous system is organized into central and peripheral components. Analogous structures in the CNS and PNS have different names. Bundles of cell bodies are called **nuclei** within the CNS and **ganglia** within the PNS. Likewise, bundles of axons within the CNS are called **tracts**, whereas, in the PNS, such bundles are called **nerves**. Notably, neurons extending from the CNS to a ganglion are called **preganglionic** fibers, and neurons extending from a ganglion to a target tissue are called **postganglionic** fibers. Within the CNS, neuron components are grouped to form **gray matter** (consisting of dendrites, cell bodies, and unmyelinated axons) and **white matter** (primarily myelinated axons). In the brain, gray matter is found on the surface of the cerebral cortex, with the many interconnecting axons of the white matter found on the interior. This arrangement is reversed in the spinal cord, with white matter on the surface and gray matter in the core. The PNS can be further divided into **motor** and **sensory divisions** (sometimes called efferent and afferent divisions, discussed further in Concept 12.3.03). The motor division includes both the **somatic nervous system**, which innervates [skeletal muscles](javascript:void(0)), and the **autonomic nervous system (ANS)**, which innervates cardiac and smooth muscles, as well as glands. The enteric nervous system (see Concept 15.3.02) is also partly controlled by the ANS. The ANS is further divided into **sympathetic** and **parasympathetic divisions**, which are discussed in Concept 12.3.02. Figure 12.22 summarizes the PNS divisions. A diagram of a human body Description automatically generated Chapter 12: Nervous System 442 **Figure 12.22** Divisions of the peripheral nervous system. 12.3.02 The Sympathetic and Parasympathetic Nervous Systems The autonomic nervous system (ANS), itself part of the motor (efferent) division of the peripheral nervous system (PNS), can be divided into two largely antagonistic divisions, both regulating involuntary bodily functions: The **sympathetic division** (sympathetic nervous system) promotes \"fight-or-flight\" responses that prepare the body for action under stressful conditions. For example, the sympathetic nervous system activates processes that lead to increased heart rate, increased liver glucose release, and dilation of airways. At the same time, sympathetic activation inhibits nonessential activities such as digestion so that energy can be redirected toward addressing immediate stressors. The **parasympathetic division** (parasympathetic nervous system) promotes \"rest-and-digest\" responses under low-stress conditions. These responses include *increased* digestive functions and energy storage, normal urination patterns, and *decreased* blood pressure (ie, via lower heart rate and vasodilation). Both sympathetic and parasympathetic responses are transmitted via preganglionic neurons arising from the central nervous system (CNS) and postganglionic fibers that synapse with the target tissue(s) (Figure 12.23). Preganglionic fibers are typically shorter than postganglionic fibers in sympathetic pathways, with the opposite being true in parasympathetic pathways. In both pathways, preganglionic neurons release  Chapter 12: Nervous System 443 acetylcholine, but the neurotransmitter released by the postsynaptic neuron varies, with norepinephrine typically released in sympathetic pathways and acetylcholine typically released in parasympathetic pathways. **Figure 12.23** Pre- and postganglionic neuron lengths and neurotransmitters released in the sympathetic and parasympathetic nervous systems. Figure 12.24 summarizes a number of the processes mediated by the parasympathetic and sympathetic nervous systems and highlights the differences in the locations from which parasympathetic and sympathetic preganglionic fibers arise from the CNS. A diagram of a brain Description automatically generated Chapter 12: Nervous System 444 **Figure 12.24** Parasympathetic and sympathetic divisions of the autonomic nervous system. 12.3.03 Afferent and Efferent Pathways The terms afferent and efferent denote the direction a neural signal travels (toward and away from a point of reference, respectively). The terms afferent and efferent are used here to refer to neurons carrying information to and from the central nervous system (CNS), respectively. The peripheral nervous system (PNS) communicates with the CNS via two groups of neurons (Figure 12.25): **Afferent (sensory) neurons** carry sensory information from the periphery (eg, pain, touch, pressure) to the CNS. The cell bodies of afferent neurons are located in ganglia on the dorsal (ie, posterior) part of the spinal cord, and afferent neurons synapse with neurons in the dorsal part of the spinal cord or in the medulla (ie, portion of brainstem). **Efferent (motor) neurons** carry motor commands from the brain to effector organs (eg, skeletal muscle, glands). Efferent neurons exit the ventral (ie, anterior) part of the spinal cord.  Chapter 12: Nervous System 445 The nerves carrying information to and from the spinal cord are called spinal nerves. These nerves split into two branches, sometimes called roots, shortly before entering the spinal cord. Therefore, on each side of the spinal cord a dorsal root carries sensory information from the periphery to the spinal cord, and a ventral root carries motor commands from the spinal cord to the periphery. Within the spinal cord, the components of the communicating neurons are grouped into white matter (primarily axons) and dark matter (primarily cell bodies and dendrites). **Figure 12.25** Afferent and efferent neurons. Afferent neurons typically either interact with a specialized receptor (eg, for temperature, pain) or have ends (dendrites or \"dendritic ends\" of unipolar neurons) that serve as receptors. In many cases, an afferent neuron can activate a reflex response, mediated by an efferent neuron, as discussed in Concept 12.3.04. 12.3.04 Reflexes **Reflexes** are involuntary responses to stimuli and may or may not require input from the brain. The specific neuronal pathway by which a stimulus directly causes the muscular or glandular effect associated with a particular reflex is called a **reflex arc**. Reflex arcs include a sensory (afferent) neuron, an effector (efferent) neuron, and, sometimes, an interneuron. A reflex arc begins with stimulation of a sensory receptor, which leads to an afferent electrical impulse that travels toward the spine or brain along a sensory nerve. This afferent impulse is then transmitted to an effector neuron in one of two ways: **Directly**, via a synapse between the afferent sensory neuron and the efferent effector neuron. This pathway is known as a **monosynaptic reflex arc**. **Indirectly,** through an interneuron between the sensory and the effector neuron. This pathway is known as a **polysynaptic reflex arc**. Subsequently, electrical impulses travel along efferent neuron axons to stimulate a muscle fiber or gland, either directly or after synapsing with a postganglionic effector neuron. Some somatic motor reflexes are A diagram of a human body Description automatically generated Chapter 12: Nervous System monosynaptic, whereas others are polysynaptic, as shown in Figure 12.26. Autonomic reflex arcs are always polysynaptic. **Figure 12.26** Monosynaptic and polysynaptic reflexes. Reflexes can be modulated (ie, dampened or enhanced) by input from the brain. In the case of polysynaptic reflexes, this modulation takes the form of descending signals from higher areas in the central nervous system that act on preganglionic neurons in the reflex arc. Reflexes involving input from the brain are called [supraspinal reflexes](javascript:void(0)), whereas those mediated entirely within the spinal cord are called **spinal reflexes**. 12.3.05 Feedback Control Nervous system responses play a vital role in the maintenance of homeostasis and can be viewed as operating as components in a **feedback loop**. In such a loop, afferent (sensory) nervous system pathways are activated in response to a deviation from a set point in a sensory pathway (eg, body temperature), providing a signal for the central nervous system, acting as a control center, to act on. The resulting efferent (motor) pathway then activates responses in effector organs that bring the regulated system back toward the baseline set point range. In this way, the feedback loop functions to maintain homeostasis, as depicted in Figure 12.27. Feedback loops that function to maintain homeostasis are examples of negative feedback loops. In a negative feedback loop, the neural response to the activation of a neural receptor *inhibits* the initial stimulus in the same pathway. Negative feedback loops act to reduce the stimuli that move a system away from its normal range and thus help maintain homeostasis. As a result of this homeostatic tendency, nervous system feedback typically operates as part of a negative feedback loop. Although negative feedback responses by the nervous system are much more common, in special circumstances, neural pathways can operate as part of a positive feedback loop in which activation of a sensory receptor triggers a response that moves a system *away* from homeostasis. For example, during childbirth (ie, parturition), uterine contractions increase pressure on the cervix, activating maternal pain receptors. Activation of these receptors leads to oxytocin release that, in turn, further stimulates painful uterine contractions, as discussed in Concept 10.4.03