Intro to Nervous System Review (ANAT-1049)
Introduction to the Nervous System and Its Primary Functions
The nervous system performs three essential functions to maintain the body's internal stability and interact with the environment. First, it monitors changes occurring both inside and outside the body through sensory input, which is perceived as a stimulus. Second, the system processes these inputs through integration, a process where information is interpreted and prioritized to make decisions. Finally, it generates a motor output or response, which activates specific mechanisms such as the movement of muscles, the functional adjustments of organs, or the release of hormones to mediate the necessary physiological responses.
Divisions and Anatomical Organization of the Nervous System
The nervous system is organized into two primary anatomical divisions: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS serves as the command center and consists of the brain, located within the cranium, and the spinal cord, situated within the vertebral canal. The PNS serves as a relay system, consisting of cranial nerves that emerge from the brain and spinal nerves that emerge from the spinal cord. The role of the PNS is to transmit information to and from the CNS. Once the brain receives this information, it integrates the data and initiates the appropriate response.
Functional Classifications: Somatic and Autonomic Systems
Beyond anatomical divisions, the nervous system is categorized by function into the Somatic Nervous System (SNS) and the Autonomic Nervous System (ANS). The Somatic Nervous System includes a motor division that innervates somatic effectors, specifically the voluntary skeletal muscles, and a sensory division that transmits information from these effectors to the somatic centers of the CNS for integration. Conversely, the Autonomic Nervous System (ANS) innervates visceral effectors, which include smooth muscle, cardiac muscle, and glandular tissue. The ANS manages involuntary responses initiated by visceral sensory information, although it remains influenced by the conscious mind through voluntary regulation.
Cellular Composition: Neurons and Neuroglia
The nervous tissue is composed of two major classifications of cells. Neurons are referred to as the excitable cells and are responsible for the transmission of nerve impulses throughout the body. Neuroglia, once thought to be simple supporting "glue," are now recognized as vital for the proper function of nervous tissue. There may be as many as neuroglia per nervous system across various cell types. Unlike neurons, neuroglia retain the ability to reproduce, which has significant clinical implications. The primary types of neuroglia include astrocytes, Schwann cells, oligodendrocytes, ependymal cells, and microglia.
Neuronal Structure and Classification
A typical neuron consists of several specialized regions. Dendrites serve as the receptive regions, often featuring dendritic spines to increase surface area. The cell body acts as the biosynthetic center and receptive region, containing the nucleus, nucleolus, and Nissl bodies. The axon hillock connects the cell body to the axon, which is the impulse-generating and conducting region. The axon is often covered by a myelin sheath composed of Schwann cells in the PNS, with the outer layer called the neurilemma. Gaps between Schwann cells are known as Nodes of Ranvier. The axon terminates in terminal branches, also called telodendria, which end in axonal terminals that serve as the secretory component of the cell.
Neurons are classified by the direction in which they transmit impulses. Sensory (afferent) neurons carry impulses from sensors or receptors toward the CNS, with their cell bodies typically located outside the CNS. Motor (efferent) neurons carry impulses away from the CNS to effectors like muscles or glands; somatic motor neurons link to skeletal muscle and have cell bodies located inside the CNS. Interneurons (association neurons) are found entirely within the CNS and serve as an intermediate form designed to integrate sensory and motor functions. These three types work in concert to facilitate characteristic nervous responses.
Physiology of Nerve Impulse Transmission: Axonal and Synaptic
Nerve impulses originate in the neuronal cell body and travel along the axon. Axonal transmission is a result of ion movement across the neuronal membrane. Initially, the membrane is polarized, maintaining a positive charge outside and a negative charge inside. When a stimulus occurs, the membrane's permeability to increases, leading to depolarization. This depolarization spreads to adjacent areas, creating an action potential that moves toward the end of the axon. Immediately following this, permeability to increases, and ions rush out of the cell to cause repolarization. These changes in permeability are regulated by the activation of specific and channels.
Synaptic transmission occurs because neurons do not make direct contact. The gap between the axon of the presynaptic neuron and the dendrite or effector of the postsynaptic cell is called the synaptic cleft. When an action potential reaches the axon terminal, a neurotransmitter is released into the synapse. This chemical diffuses across the cleft and binds to specific receptors on the postsynaptic membrane, initiating a new action potential in the receiving cell. To ensure the signal is discrete, the neurotransmitter is eventually deactivated by a chemical inactivator.
Neurotransmitters and the Organization of Neural Tissue
Acetylcholine is a prominent neurotransmitter found at the neuromuscular junction, where it is inactivated by the enzyme acetylcholinesterase. Other neurotransmitters include dopamine, Gamma-Aminobutyric Acid (GABA), norepinephrine, and serotonin. Due to their chemical nature and the specificity of their receptors and inactivators, synapses are highly susceptible to pharmacological and chemical agents, such as antihypertensives, antidepressants, and nerve gas.
Neural tissue is organized into white and gray matter. White matter consists of groups of myelinated axons and forms nerve tracts within the CNS. Gray matter consists of nerve cell bodies, dendrites, and unmyelinated axon bundles; it forms the cerebral cortex on the brain's surface and the horns in the spinal cord. In the PNS, a bundle of fibers is called a nerve, and a collection of cell bodies is a ganglion. In the CNS, a bundle of fibers is a tract, and a collection of cell bodies and dendrites is called a nucleus.
The Central Nervous System: Brain Structure and Protective Coverings
The CNS is protected externally by the cranium and the vertebral column. Internally, it is shielded by three layers of membranes called meninges. The Dura Mater is the strong, white, fibrous two-layered outer membrane that also acts as the inner periosteum of the cranial bones. The Arachnoid Mater is a thin, mesh-like middle layer. The Pia Mater is a thin, transparent membrane adhering directly to the surface of the brain and spinal cord, containing fine blood vessels. Clinical risks include severe epidural hematomas resulting from head trauma. The brain itself is divided into the Cerebrum, the Diencephalon, the Brain Stem (comprising the midbrain or mesencephalon, pons, and medulla oblongata), and the Cerebellum.
Specific Brain Regions: Diencephalon, Brain Stem, and Cerebellum
The Diencephalon contains the Thalamus and the Hypothalamus. The Thalamus consists of two lobes separated by the third ventricle and acting as a relay for sensory impulses to the cortex, facilitating awareness and cognition. The Hypothalamus forms the floor of the diencephalon and regulates the ANS, emotions, body temperature, sleep-wake cycles, thirst, appetite, sex drives, and the endocrine system. The brain stem includes the superior Midbrain, the Pons (which links the cerebellum to other CNS parts and controls breathing rhythm), and the inferior Medulla Oblongata. The medulla acts as a vital link for autonomic control of visceral activities, such as regulating heart rate, force of contraction, and setting the pace for ventilation.
The Cerebellum provides precise timing for skeletal muscle activity and maintains balance and equilibrium, ensuring movements are smooth and coordinated. It receives sensory inputs from the inner ear, the eye, and skeletal muscle proprioceptors. Within the brain, four ventricles (two lateral, a third, and a fourth) facilitate the circulation of Cerebrospinal Fluid (CSF). CSF is produced by the Choroid Plexus in the ventricles, leaves through the fourth ventricle, and enters the subarachnoid space.
The Cerebrum: Cerebral Cortex and Hemispheres
The Cerebrum is the "seat of intelligence," responsible for communication, memory, logic, and emotional response, comprising over of total brain mass. It is divided into right and left hemispheres by the longitudinal fissure, which are connected by the Corpus Callosum. Each hemisphere acts on the opposite side of the body. There are five lobes: Frontal, Parietal, Occipital, Temporal, and the Insula. Functional areas include motor areas (primary motor area in the frontal lobe), sensory areas (somatosensory, auditory, taste, olfactory, visual), and association areas linked by tracts. Notable association areas include the Premotor area (movement patterns), Prefrontal area (intellect), Wernicke's Area (language comprehension), Broca’s Area (speech), and the Parieto-occipital Area (spatial awareness).
Spinal Cord and the Peripheral Nervous System
The spinal cord is continuous with the medulla oblongata, exiting the skull via the Foramen magnum and extending to the vertebral level. It features cervical and lumbar enlargements and serves as a center for spinal reflexes. It is protected by the meninges, and CSF circulates in the subarachnoid space. The PNS consists of nerves arising from the CNS, organized into pairs of cranial nerves and pairs of spinal nerves. Cranial nerves are identified by name and number (I-XII): I Olfactory (smell), II Optic (vision), III Oculomotor (eye muscle), IV Trochlear (eye muscle), V Trigeminal (face sensory/chewing), VI Abducens (eye), VII Facial (taste/expression), VIII Vestibulocochlear (hearing/balance), IX Glossopharyngeal (pharyngeal muscle), X Vagus (thoracic/abdominopelvic), XI Accessory (neck), and XII Hypoglossal (tongue).
Spinal Nerves, Plexuses, and Reflexes
Spinal nerves are numbered by their emergence from the spinal cavity: cervical (C1-C8), thoracic (T1-T12), lumbar (L1-L5), sacral (S1-S5), and coccygeal. Because the cord ends at , lower nerve pairs descend before emerging. Nerve plexuses are complex "braids" formed by the ventral rami (except T2-T12) for regional supply, including the Cervical, Brachial, Lumbar, Sacral, and Coccygeal plexuses. Reflexes, such as the patellar, gag, withdrawal, and stretch reflexes, serve as protective mechanisms to maintain homeostasis.
Sympathetic and Parasympathetic Divisions of the ANS
The ANS regulates visceral functions like heart rate and digestion through two divisions. The Sympathetic Nervous System is the "fight or flight" system, mobilizing the body during stress by increasing heart rate and breathing, widening pupils, releasing liver sugar, and inhibiting digestion. The Parasympathetic Nervous System is the "rest and digest" system, active at rest to conserve energy and promote normal digestion and waste elimination. Homeostasis is maintained by the balance between these two systems.