SN 1 PDF.pdf

# Classification of the nervous system The nervous system can be classified based on its location and its function. ## 1. Classification of the nervous system The nervous system is broadly classified into two main categories: topographical (based on location) and functional (based on role) [3](#page=3). ### 1.1 Topographical classification This classification divides the nervous system into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS) [3](#page=3). #### 1.1.1 Central nervous system (CNS) - Nevrax The CNS, also known as the nevrax, includes the encephalon (brain) and the spinal cord [3](#page=3). #### 1.1.2 Peripheral nervous system (PNS) The PNS comprises all nervous structures outside the CNS. It consists of [3](#page=3): * **Peripheral nerves:** These include cranial nerves, which are typically categorized into sensory (or sensory/afferent), motor (or efferent), and mixed types, with 12 pairs originating from the brain. Spinal nerves are mixed nerves and there are 31 pairs connecting to the spinal cord [3](#page=3). * **Nervous ganglia:** These are aggregations of neuronal cell bodies. They can be spinal, cranial, or autonomic (vegetative) ganglia [3](#page=3). ### 1.2 Functional classification This classification divides the nervous system based on its role in controlling different bodily functions [3](#page=3). #### 1.2.1 Somatic nervous system (SN Somatic) The somatic nervous system is responsible for controlling skeletal muscles [3](#page=3). #### 1.2.2 Autonomic nervous system (SN Vegetative) The autonomic nervous system, also referred to as the vegetative nervous system, innervates smooth muscles, the cardiac muscle, and glands. It further divides into two subdivisions [3](#page=3): * **Sympathetic system:** Generally associated with the "fight or flight" response. * **Parasympathetic system:** Generally associated with "rest and digest" functions. ### 1.3 Functional compartments of the nervous system Nervous centers regulate bodily functions by processing incoming information and generating commands transmitted to effectors. These functional compartments are [4](#page=4): 1. **Sensory compartment:** This is where information collected by receptors arrives [4](#page=4). 2. **Motor compartment:** This transmits commands to effectors [4](#page=4). > **Tip:** Every nervous organ (e.g., brain, spinal cord) has both sensory and motor functions. At the level of the cerebral hemispheres, a third function, the psychic function, also emerges [4](#page=4). > **Note:** The separation of these functions is schematic. In reality, there is no purely sensory activity without motor manifestations, and vice versa. Psychic states arise from the integration of sensory and motor activities [4](#page=4). --- # The neuron: structure and components The neuron is the fundamental morpho-functional unit of the nervous system, composed of a cell body and one or more extensions [5](#page=5) [6](#page=6). ### 2.1 Neuron structure Neurons are highly diverse in their shape and dimensions. They consist of three primary components [5](#page=5): * **Cell body (perikaryon)**: The central part of the neuron containing the nucleus and most organelles [6](#page=6). * **Dendrites**: Typically multiple, shorter extensions that conduct nerve impulses towards the cell body (celulipetal conduction) [10](#page=10) [6](#page=6). * **Axon**: A single, long extension that conducts nerve impulses away from the cell body (celulifugal conduction) [11](#page=11) [6](#page=6). ### 2.2 The cell body (perikaryon) The cell body of a neuron is enclosed by a thin, lipoproteic membrane called the neurilemma. Its cytoplasm, or neuroplasma, contains common organelles such as mitochondria and ribosomes, but it lacks a centrosome because mature neurons do not divide. Specific to the cell body are [8](#page=8): #### 2.2.1 Common organelles and inclusions * **Mitochondria**: Provide energy for neuronal functions [8](#page=8). * **Ribosomes**: Involved in protein synthesis [8](#page=8). * **Endoplasmic reticulum (RE)**: Participates in cellular metabolism [8](#page=8). * **Pigment inclusions**: Varying deposits found within the cytoplasm [8](#page=8). #### 2.2.2 Specific organelles * **Nissl bodies (tigroid bodies, chromophilic substance)**: These are equivalent to ergastoplasm and are found in the cell body and at the base of dendrites. They play a crucial role in neuronal metabolism [8](#page=8). * **Neurofibrils**: A network of filaments present throughout the cell body, dendrites, axon, and terminal buttons. They provide mechanical support and are involved in impulse conduction [8](#page=8). #### 2.2.3 Nucleus Most motor, sensory, and association neurons have a single nucleus with one or two nucleoli. Vegetative neurons, whether central or peripheral, often exhibit an eccentric nucleus and may possess double or multiple nuclei [8](#page=8). ### 2.3 Dendrites Dendrites are cellular extensions that receive nerve impulses and transmit them toward the neuron's cell body. They are thicker at their origin and gradually taper. Dendrites contain neurofibrils and Nissl bodies, particularly at their base. Their primary function is the reception and centripetal conduction of nerve impulses [10](#page=10) [6](#page=6). ### 2.4 Axon The axon is a single, typically long and thicker, cellular extension that carries nerve impulses away from the cell body. It can extend up to one meter in length. Along its length, the axon may emit perpendicular branches called collaterals, which can be myelinated and participate in synapses. The terminal part of the axon branches extensively into terminal buttons [11](#page=11) [6](#page=6). #### 2.4.1 Terminal buttons These specialized structures at the end of axon branches contain small vesicles filled with neurotransmitters, facilitating the transmission of nerve impulses to other neurons or target cells at synapses. Terminal buttons also house neurofibrils and mitochondria [11](#page=11). #### 2.4.2 Axon membranes and cytoplasm The axon is covered by a membrane called the **axolemma** and contains cytoplasm known as **axoplasma**. The axolemma is critical for the propagation of nerve impulses [11](#page=11) [12](#page=12). ##### 2.4.2.1 Axolemma and surrounding sheaths From the inside out, the axolemma is surrounded by three sheaths: 1. **Myelin sheath**: * Synthesized by Schwann cells in the peripheral nervous system (PNS), with each cell myelinating a single axon [13](#page=13). * Synthesized by oligodendrocytes in the central nervous system (CNS), with each cell myelinating multiple axons [13](#page=13). * In the PNS, the myelin sheath has periodic constrictions called nodes of Ranvier, which are gaps between successive Schwann cells [13](#page=13). * The myelin sheath is absent in axons with a diameter less than 2 micrometers and in postganglionic fibers [13](#page=13). * It acts as an electrical insulator, accelerating nerve impulse conduction [13](#page=13). 2. **Schwann sheath**: * Formed by Schwann cells arranged around the myelin sheath. Each segment between two nodes of Ranvier corresponds to a single Schwann cell [13](#page=13). * These are cylindrical, uninucleated cells with an eccentric nucleus [13](#page=13). * This sheath is absent in CNS axons [13](#page=13). 3. **Henle sheath**: * Separates the Schwann cell's plasma membrane from the surrounding connective tissue [13](#page=13). * It contributes to permeability and resistance [13](#page=13). * This sheath is absent in CNS axons [13](#page=13). > **Tip:** The presence and structure of the myelin sheath are crucial for understanding the speed of nerve impulse transmission, particularly the concept of saltatory conduction at the nodes of Ranvier. ##### 2.4.2.2 Axoplasma The cytoplasm of the axon, axoplasma, contains mitochondria, vesicles of the endoplasmic reticulum, and neurofibrils [14](#page=14). --- # Neuron classification and function This section explores the multifaceted classification of neurons based on their morphology and functional roles, and introduces the supportive neuroglia. ### 3.1 Neuron classification Neurons can be categorized using three primary criteria: their shape, the number of extensions they possess, and their functional roles within the nervous system [15](#page=15). #### 3.1.1 Classification by shape Classification by shape identifies several distinct neuronal morphologies [16](#page=16): * **Stellate neurons:** These neurons have a star-like appearance and are found, for instance, in the anterior horns of the spinal cord, in the association neurons of the posterior horns, and are a type of multipolar neuron [16](#page=16). * **Spherical or oval neurons:** Neurons with a rounded or oval cell body include those found in spinal ganglia, as well as unipolar, pseudounipolar, and bipolar neurons [16](#page=16). * **Pyramidal neurons:** Characterized by a pyramid-shaped soma, these neurons are located in the motor areas of the cerebral cortex and are also a type of multipolar neuron [16](#page=16). * **Fusiform neurons:** These neurons have a spindle or elongated shape and are found in the deep layer of the cerebral cortex, and are also a type of bipolar neuron [16](#page=16). #### 3.1.2 Classification by the number of extensions The number and arrangement of a neuron's extensions (dendrites and axons) provide another basis for classification [17](#page=17): * **Unipolar neurons:** These neurons have a globular cell body with a single extension, which functions as the axon. Examples include the cone and rod cells in the retina [17](#page=17). * **Pseudounipolar neurons:** These are spherical or oval neurons where a single extension emerges from the cell body and then bifurcates in a "T" shape. The dendrite of this extension extends to the periphery, while the axon enters the central nervous system (CNS). These are characteristic of neurons in the spinal ganglia and cranial nerve ganglia [17](#page=17). * **Bipolar neurons:** These neurons are round, oval, or fusiform and have two extensions originating from opposite poles of the cell. They are found in the spiral ganglion of Corti, the vestibular ganglion of Scarpa, the retina, and the olfactory mucosa [17](#page=17). * **Multipolar neurons:** Exhibiting a diverse range of shapes including stellate, pyramidal, or piriform, these neurons possess numerous dendritic extensions and a single axon. They are prevalent in the cerebral and cerebellar cortices, the anterior horns of the spinal cord, the posterior horns (as association neurons), the retina, and include mitral cells in the olfactory bulb [17](#page=17). #### 3.1.3 Classification by function Neurons are functionally classified into three main types based on their role in information processing [20](#page=20): * **Sensory (or afferent) neurons:** These neurons receive stimuli from the external environment (somatosensory), the musculoskeletal system (proprioceptive), or the internal organs (viscerosensory) via their dendrites. Sensory neurons are typically pseudounipolar, with both their dendrite and axon being myelinated [20](#page=20). * **Motor (or efferent) neurons:** The axons of motor neurons connect to effector organs. They can be somatomotor, innervating skeletal muscles, or visceromotor, innervating internal organs, blood vessels, or glands. Motor neurons are usually multipolar, and only their axon is myelinated [20](#page=20). * **Interneurons (or association neurons):** These neurons act as intermediaries, forming connections between sensory and motor neurons. Interneurons in the spinal cord are located in the posterior horns and are characterized as multipolar, stellate neurons [20](#page=20). ### 3.2 Neuroglia (glial cells) Neuroglia, also known as glial cells, are supportive cells within the nervous system. In higher mammals, glial cells significantly outnumber neurons, by more than ten times. They vary in shape and size, with extensions that are also variable in number [21](#page=21). #### 3.2.1 Types of neuroglia Key types of neuroglia include: * Schwann cells * Astrocytes * Oligodendroglia * Microglia * Ependymal cells * Satellite cells [21](#page=21). #### 3.2.2 Characteristics and roles of neuroglia Unlike neurons, glial cells are capable of intense division, making them the source of tumors in the CNS. They do not contain neurofibrils or Nissl bodies. Their diverse roles include [21](#page=21): * Providing structural support for neurons [21](#page=21). * Offering protection [21](#page=21). * Performing trophic functions, supplying nutrients and growth factors [21](#page=21). * Phagocytosis (carried out by microglia) [21](#page=21). * Synthesizing the myelin sheath [21](#page=21). * Synthesizing RNA and other substances that are transferred to neurons [21](#page=21). --- # Properties and transmission of the nerve impulse The transmission of a nerve impulse involves the fundamental properties of nerve cells, excitability and conductivity, and detailed mechanisms across different axon types and synapses [23](#page=23). ### 4.1 Properties of nerve cells Nerve cells possess two key properties related to impulse transmission: * **Excitability:** The capacity of a neuron to generate an action potential (AP). The occurrence of an AP in one region of the neuronal membrane triggers a new AP in the adjacent region due to depolarization from a preceding AP [23](#page=23) [24](#page=24). * **Conductivity:** The ability to propagate an AP that has arisen in one part of the neuronal membrane to other parts. This conduction is a regenerative process, with each new AP being a completely new event that repeats along the axon [23](#page=23) [25](#page=25). ### 4.2 Conduction along axons #### 4.2.1 Conduction in unmyelinated axons In unmyelinated axons, an action potential can arise anywhere along the membrane. The electrical properties of the membrane allow adjacent regions to depolarize. However, the AP propagates in only one direction because the area where the previous AP occurred is in an absolute refractory state. The speed of propagation in unmyelinated fibers is approximately 10 meters per second [25](#page=25). #### 4.2.2 Conduction in myelinated axons Myelin acts as an electrical insulator, causing action potentials to appear only at the nodes of Ranvier. The impulse "jumps" from one node to the next, a process known as saltatory conduction. This results in a significantly higher conduction velocity, approximately 100 meters per second. This faster conduction speed explains variations in reflex response times [25](#page=25). ### 4.3 The synapse A synapse is a functional connection between a neuron and another cell. In the central nervous system (CNS), the second cell is typically another neuron. In the peripheral nervous system (PNS), it can be an effector cell, such as a muscle or secretory cell. A neuromuscular synapse is specifically termed a motor plate or neuromuscular junction. Synaptic transmission is unidirectional [28](#page=28). #### 4.3.1 Classification of synapses Synapses can be classified based on: **A. Type of cells forming the synapse:** 1. **Neuro-neuronal synapses:** Occur between two neurons. * Axosomatic: Axon of the first neuron synapses with the cell body of the second [29](#page=29). * Axodendritic: Axon of the first neuron synapses with the dendrite of the second [29](#page=29). * Axoaxonic: Between the axons of two neurons [29](#page=29). * Dendrodendritic: Between the dendrites of two neurons [29](#page=29). 2. **Neuro-muscular synapses:** Between a neuron and a muscle cell [29](#page=29). 3. **Neuro-secretory synapses:** Between a neuron and a glandular cell [29](#page=29). **B. Type of transmission:** 1. **Chemical synapses:** Operate via chemical messengers (neurotransmitters) [29](#page=29). 2. **Electrical synapses:** Often dendrodendritic in nature [29](#page=29). #### 4.3.2 Chemical synapses A chemical synapse comprises three main components: 1. **Presynaptic terminal:** Contains vesicles filled with chemical mediators, of which acetylcholine is the most common [31](#page=31). 2. **Synaptic cleft:** The space between the presynaptic terminal and the postsynaptic cell [31](#page=31). 3. **Postsynaptic cell:** Features receptors for the chemical mediator [31](#page=31). Upon arrival of a nerve impulse, quanta of chemical mediators are released into the synaptic cleft via exocytosis. These mediators then interact with receptors on the postsynaptic membrane, increasing its conductance to sodium ions ($Na^+$). This influx of $Na^+$ leads to depolarization of the postsynaptic membrane. Transmission is unidirectional, from the presynaptic to the postsynaptic terminal. Examples include most synapses in the CNS, motor plates, and the autonomic nervous system [31](#page=31). > **Tip:** The depolarization of the postsynaptic membrane is known as an excitatory postsynaptic potential (EPSP) if the postsynaptic cell is a neuron, or a terminal plate potential if it's a skeletal muscle fiber. This is distinct from an action potential [33](#page=33). #### 4.3.3 Transmission at the chemical synapse The excitatory postsynaptic potential (EPSP) has two key properties: 1. **Temporal summation:** Two potentials generated by rapid sequential releases of neurotransmitter from the same presynaptic neuron can summate to produce a larger potential [33](#page=33). 2. **Spatial summation:** EPSPs generated by two nearby presynaptic terminals on the same postsynaptic membrane can accumulate [33](#page=33). An EPSP will only trigger an "all-or-nothing" action potential in the postsynaptic cell if a critical threshold is reached [33](#page=33). > **Example:** If a single presynaptic neuron fires rapidly, it can cause a stronger depolarization in the postsynaptic neuron than if it fired slowly, due to temporal summation. Similarly, if two adjacent presynaptic neurons fire simultaneously, their combined effect on the postsynaptic neuron can be greater than either one alone, illustrating spatial summation. Repeated and rapid stimulation of excitatory synapses can lead to a significant increase in the number of action potentials in the postsynaptic neuron initially. However, this is followed by a sharp decrease, a protective mechanism against overstimulation. This occurs through the depletion of chemical mediator (neurotransmitter) stores at the presynaptic terminal, leading to synaptic transmission fatigue [34](#page=34). Certain drugs can increase synaptic excitability (e.g., caffeine), while others decrease it (e.g., some anesthetics) [34](#page=34). #### 4.3.4 Electrical synapses Electrical synapses consist of two cells of similar size that are closely apposed at areas of minimal electrical resistance. They function through the direct passage of ions and molecules between cells at their junctional sites. Conduction in electrical synapses appears to be bidirectional. Examples include the myocardium, smooth muscle, and certain brain regions [35](#page=35). --- ## Common mistakes to avoid - Review all topics thoroughly before exams - Pay attention to formulas and key definitions - Practice with examples provided in each section - Don't memorize without understanding the underlying concepts

| Term | Definition | |------|------------| | Nervous System (SN) | The nervous system, along with the endocrine system, regulates most bodily functions, with a close interdependence between the two. The nervous system particularly regulates muscle and gland activity, while the endocrine system primarily regulates metabolic functions. | | Central Nervous System (CNS) | Also known as the neuraxis, it comprises the brain (encefal) and the spinal cord (maduva spinarii). | | Peripheral Nervous System (SNP) | This system includes peripheral nerves, such as cranial nerves (12 pairs: sensory/sensorial, motor, mixed) and spinal nerves (31 pairs: mixed), as well as nerve ganglia (clusters of neuronal cell bodies). | | Somatic Nervous System | This part of the nervous system innervates skeletal muscles. | | Autonomic Nervous System | This part of the nervous system innervates smooth muscles, the cardiac muscle, and glands. It is further divided into the sympathetic and parasympathetic divisions. | | Sensory Compartment | This functional compartment of the nervous system receives information collected by receptors. | | Motor Compartment | This functional compartment of the nervous system transmits commands to effectors. | | Neuron | The morpho-functional unit of the nervous system, composed of a cell body, dendrites, and an axon. | | Cell Body (Perikaryon) | The main part of the neuron, containing the nucleus and organelles. | | Dendrites | Branch-like extensions of a neuron that receive neural impulses and conduct them towards the cell body (centripetal conduction). | | Axon | A single, long extension of a neuron that conducts neural impulses away from the cell body (centrifugal conduction). It can branch and terminates in synaptic boutons. | | Neurolemma | The thin membrane that encloses the neuron. | | Neuroplasma | The cytoplasm of a neuron, containing organelles and specific structures like Nissl bodies and neurofilaments. | | Nissl Bodies (Tigroid Bodies) | Structures within the neuron's cell body and dendrites, equivalent to the rough endoplasmic reticulum, involved in neuronal metabolism. | | Neurofilaments | Filamentous structures found throughout the neuron, providing mechanical support and aiding in nerve impulse conduction. | | Axolemma | The membrane of the axon, crucial for the propagation of the nerve impulse. | | Myelin Sheath | An insulating layer around some axons, formed by Schwann cells in the PNS and oligodendrocytes in the CNS, which speeds up nerve impulse conduction. | | Schwann Cells | Cells in the peripheral nervous system that form the myelin sheath around axons. | | Nodes of Ranvier | Gaps in the myelin sheath along an axon where the nerve impulse is regenerated. | | Axoplasm | The cytoplasm within the axon, containing mitochondria, ER vesicles, and neurofilaments. | | Intercalary Neurons (Association Neurons) | Neurons that form connections between sensory and motor neurons. | | Neuroglia (Glial Cells) | Supportive cells of the nervous tissue, which are more numerous than neurons in mammals. They perform functions such as support, protection, phagocytosis, and myelination. | | Excitability | The capacity of a nerve cell to generate an action potential in response to a stimulus. | | Conductivity | The capacity of a nerve cell to conduct an action potential generated in one part of its membrane to another part. | | Action Potential (PA) | A rapid, transient change in the electrical potential across a neuron's membrane, representing the nerve impulse. | | Unmyelinated Axons | Axons that lack a myelin sheath, where nerve impulses propagate continuously along the membrane. | | Myelinated Axons | Axons covered by a myelin sheath, where nerve impulses propagate in a saltatory manner, jumping between the Nodes of Ranvier. | | Saltatory Conduction | The mode of nerve impulse propagation in myelinated axons, where the action potential "jumps" from one Node of Ranvier to the next, leading to faster conduction. | | Synapse | A functional connection between a neuron and another cell, allowing for the transmission of neural signals. | | Presynaptic Terminal | The end of an axon terminal that contains vesicles filled with neurotransmitters, which are released into the synaptic cleft. | | Synaptic Cleft | The small gap between the presynaptic terminal and the postsynaptic cell. | | Postsynaptic Cell | The cell that receives the signal from the presynaptic neuron at a synapse, containing receptors for neurotransmitters. | | Chemical Synapse | A synapse that transmits information through the release of chemical neurotransmitters. | | Electrical Synapse | A synapse where electrical signals are transmitted directly from one cell to another through gap junctions. | | Neurotransmitter | A chemical messenger released from a presynaptic neuron that binds to receptors on a postsynaptic cell to transmit a signal. | | Exocytosis | The process by which vesicles within the presynaptic terminal fuse with the presynaptic membrane to release neurotransmitters into the synaptic cleft. | | Postsynaptic Potential (PSP) | A change in the membrane potential of the postsynaptic cell caused by the binding of neurotransmitters. It can be excitatory (EPSP) or inhibitory (IPSP). | | Excitatory Postsynaptic Potential (EPSP) | A depolarization of the postsynaptic membrane that brings the neuron closer to firing an action potential. | | Temporal Summation | The process where multiple excitatory postsynaptic potentials arriving from the same presynaptic neuron in rapid succession combine to reach the threshold for firing an action potential. | | Spatial Summation | The process where excitatory postsynaptic potentials generated by multiple presynaptic neurons converging on the same postsynaptic neuron combine to reach the threshold for firing an action potential. | | Synaptic Fatigue | A phenomenon where repeated stimulation of an excitatory synapse leads to a decrease in the number of postsynaptic neuron discharges, often due to the depletion of neurotransmitter stores. |