Lecture Notes in Human Anatomy - Hryhorieva O.A., Svitlytskyi A.O. 2020

Splanchnology (The Study of Viscera)
Anatomy of the Nervous System
Nervous System

Lecture Outline:

1. General Overview of the Anatomy of The Nervous system.

2. Central nervous system: Anatomy of the Spinal Cord and brain.

3. Peripheral Nervous System.

4. Somatic nervous system.

5. Autonomic nervous system.

The nervous system is built of Nervous Tissue, which consists of Nerve Cells—Neurons—and neuroglia. Neuroglia is a collection of cellular elements of nervous tissue that perform trophic, protective, supportive, and secretory Functions.

The Structural and functional unit of the nervous system is the nerve Cell, known as a neuron or neurocyte. Neurons determine the specific function of nervous tissue: they receive stimuli, generate impulses, and transmit them. A neuron consists of a cell body and processes. Extending from the body in one direction is a single, unbranched long process called an axon (or neurite), while short, branching processes called dendrites extend in the other direction. The transmission of nervous excitation within a neuron proceeds in the direction from the dendrites to The Cell body (soma) of the neuron, and from there to the axon; the axon conducts the excitation away from the cell body. Axons and dendrites terminate in specialized apparatuses called nerve endings. According to their functional purpose, nerve endings are subdivided into sensory endings (or receptors), motor endings (or effectors), and synaptic endings (or synapses).

Receptors are the nerve endings of dendrites that perceive stimuli from the Skin, Muscles, Blood Vessels, etc. Depending on whether stimuli are received from the external or internal environment, they are divided into exteroceptors and interoceptors. Exteroceptors include receptors that perceive stimuli from the external environment, such as skin and sensory system receptors. Interoceptors perceive stimuli from the internal environment; these include receptors that pick up signals from muscles and joints (proprioceptors) and from Internal Organs and blood vessels (visceroreceptors).

Effectors are the motor endings of axons from motor Cells of the somatic and autonomic nervous systems; they transmit impulses to muscles.

The transmission of a Nerve Impulse from one neuron to another is carried out with the help of specially structured terminal apparatuses, or synapses. A synaptic ending—a synapse—is the site of contact between two neurons (or a neuron and another cell) where the transmission of excitation from one cell to another occurs chemically (via a neurotransmitter). A synapse consists of the presynaptic membrane (a region of The cell membrane near which neurotransmitter vesicles are located within the cell), the synaptic cleft, and the postsynaptic membrane (a region of the cell membrane). Because vesicles containing the neurotransmitter are located exclusively near the presynaptic membrane, excitation is transmitted in only one direction. There are axosomatic connections between neurons, where the axon of one neuron approaches the cell body of another, and phylogenetically younger axodendritic connections (contact between an axon and a dendrite).

Thus, the entire nervous system is a complex of neurons that form synaptic connections with one another without fusing together. In this way, a nerve excitation arising in any given Location is transmitted along the processes of nerve cells through synapses from one neuron to another. An example of such a connection between organs via neurons is the reflex arc, which forms The basis of a reflex.

A simple reflex arc must consist of at least two neurons: one of them is connected to a sensory surface (such as the skin), while the other terminates with its neurite in a Muscle (or gland). Upon stimulation of the sensory surface, the excitation travels along the associated sensory neuron in an afferent direction toward the reflex center, where the synapse is located. Here, the excitation passes to the second neuron and travels in an efferent direction to the muscle (or gland), resulting in either Muscle contraction or A change in glandular secretion, and so on. Often, a simple (two-neuron) reflex arc includes a third, interneuron, which serves as a Relay from the sensory chain to the motor chain. However, in humans, the majority of reflex arcs are multineuronal, passing through various levels of the CNS.

Nerve fibers are the processes of neurons covered by a sheath. The sheath consists either solely of Schwann cells or additionally includes a myelin sheath, which acts as a unique insulator. Depending on this, nerve fibers are divided into myelinated and unmyelinated fibers. Myelinated fibers are thicker and feature unmyelinated gaps known as nodes of Ranvier. Bundles of nerve fibers enclosed within a common Connective Tissue sheath form nerves or nerve trunks. In most cases, a nerve combines both sensory and motor fibers, constituting a mixed nerve. Nerves consisting exclusively of sensory fibers are called sensory nerves, while those consisting solely of motor fibers are called motor nerves.

The nervous system is unified, yet conventionally it is divided into parts.

According to the topographical principle, the nervous system is divided into the central and peripheral nervous systems. The central nervous system comprises the BRAIN AND SPINAL cord, whereas the peripheral system includes the nerves emerging from the brain (12 pairs of Cranial Nerves) and spinal cord (31 pairs of Spinal Nerves), as well as nerve knots (ganglia) located peripherally.

The spinal cord (medulla spinalis) lies within the vertebral canal and, in adults, appears as a long (41–45 cm) cylindrical cord slightly flattened from front to back, which transitions superiorly directly into the Medulla Oblongata and terminates inferiorly in a conical taper at the level of the 2nd lumbar vertebra. The spinal cord consists of segments; a segment is a section of the spinal cord corresponding to the exit of a single pair of spinal nerves. There are 31 spinal cord segments. The spinal cord comprises Gray matter and White matter: the gray matter represents the cell bodies of nerve cells, while the white matter consists of nerve processes.

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The gray matter is represented by anterior, posterior, and lateral horns (columns) and is surrounded by white matter. In cross-section, the gray matter resembles a butterfly or the letter H, distinguishing the anterior horn, posterior horn, and lateral horn (present only in the 8th cervical segment, the 1st–12th thoracic segments, and the 1st–3rd lumbar segments). The anterior horn consists of 5 motor nuclei formed by a collection of motor neurons; the posterior horn is represented by 6 sensory nuclei, which are subdivided into nuclei that transmit information within the spinal cord and those that relay information to the brain. The lateral horn is formed by a collection of interneurons.

The white matter is represented by pathways: associative pathways (connecting spinal cord segments within the same half), Projection Pathways (connecting the spinal cord to the brain via ascending tracts and the brain to the spinal cord via descending tracts), and Commissural pathways—the white commissure—which interconnect the two halves of the spinal cord.

There are three Meninges of the spinal cord:

- outer—dura mater;

- middle—arachnoid mater;

- inner—pia mater.

The brain (encephalon) is located within the cranial cavity and consists of five divisions: the medulla oblongata, Hindbrain, Midbrain, Diencephalon, and Telencephalon.

The medulla oblongata is bulb-shaped, about 2.5 cm long, and lies in the cranial cavity on the clivus. On its anterior surface, lateral to the anterior median fissure,

there are longitudinal thickenings known as pyramids, which consist of pyramidal motor tracts connecting the brain and the spinal cord. Flanking the pyramids are oval olives, whose function is related to maintaining posture. The medulla oblongata consists of gray and white matter. The gray matter forms clusters called nuclei, or centers. The medulla houses the autonomously functioning respiratory center, centers regulating cardiac and vascular activity, digestive gland secretion, and Reflexes such as sneezing, coughing, and swallowing, as well as the nuclei of the IX-XII cranial nerve pairs. The white matter is composed of ascending (sensory) and descending (motor) pathways.

The hindbrain lies within the cranial fossa and includes the Pons and the Cerebellum.

The pons is situated anterior to the medulla oblongata and consists of both gray and white matter. The gray matter forms the pontine nuclei—the nuclei of the V-VIII cranial nerve pairs—through which the Cerebral Cortex connects to the cerebellum. The white matter is made up of longitudinal and transverse fibers; the longitudinal fibers comprise ascending and descending pathways, while the transverse fibers connect the pons to the cerebellum.

The cerebellum lies posterior to the pons and the medulla oblongata, featuring two hemispheres and a central region (the vermis). It has three pairs of cerebellar peduncles: superior, middle, and inferior. The superior peduncles connect the cerebellum to the roof of the midbrain and contain proprioceptive spinal tracts. The middle peduncles connect it to the pons and consist of fibers linking the pontine nuclei to the cerebellar cortex. The inferior peduncles connect it to the medulla oblongata, transmitting proprioceptive pathways from the spinal cord to the cerebellum. The cerebellar white matter consists of fibers connecting the cerebellum to the spinal cord and other PARTS OF THE brain, while the gray matter forms the cerebellar cortex and its nuclei.

The midbrain consists of the tectum and cerebral peduncles; its cavity is the cerebral aqueduct, which connects the IV ventricle to the III ventricle.

The tectum of the midbrain comprises four colliculi: the two superior colliculi serve as subcortical reflex visual startle centers, and the two inferior colliculi act as subcortical reflex auditory startle centers. The cerebral peduncles are divided into the crus cerebri and the tegmentum, separated by the substantia nigra. The crus cerebri consists of white matter containing important descending pathways. The tegmentum contains both gray and white matter, with the gray matter represented by nuclei and the white matter by pathways. Of particular importance are the four decussations (crossings) of these pathways.

The diencephalon includes the thalamencephalon and the Hypothalamus, enclosing the III ventricle. The thalamencephalon comprises the thalamus, epithalamus, and metathalamus. The thalamus is a paired cluster of gray matter that makes up the bulk of the diencephalon; its nuclei relay afferent pathways for cutaneous, neuromuscular, olfactory, and visual sensations. The hypothalamus lies inferior to the thalamus, its most prominent feature being the infundibulum, which Supports the Pituitary Gland—the master endocrine gland; the tuber cinereum and mamillary bodies are also part of the hypothalamus. Acting as the vegetative brain, the hypothalamus contains subcortical autonomic centers, with its gray matter organized into nuclei.

The telencephalon consists of two hemispheres connected by the corpus callosum. The hemispheres include the rhinencephalon (olfactory brain), basal nuclei (or central nuclei), and the Lateral ventricles. The corpus callosum lies deep within the longitudinal fissure and consists of nerve fibers connecting the cerebral hemispheres. Beneath the corpus callosum lies the fornix, appearing as two arching white bands. The cortex of each hemisphere is composed of a 1.3–4.5 mm thick layer of gray matter, forming folds, sulci, and gyri that divide each hemisphere into five lobes: frontal, occipital, parietal, temporal, and the insula (a lobe covered by the temporal, parietal, and frontal lobes). The frontal lobe is located anterior to the central sulcus, the parietal lobe lies between the central and parieto-occipital sulci, the occipital lobe is posterior to the parieto-occipital sulcus, the temporal lobe sits inferior to the lateral sulcus, and the insular lobe is located at the floor of the lateral sulcus.

The total cortical surface area is 220–250 thousand mm². The cerebral cortex is the most crucial substrate for Higher Nervous Activity in humans. The basal nuclei of the hemispheres are clusters of gray matter that form the so-called subcortex.

The rhinencephalon is the oldest part of the Forebrain (telencephalon). Nerve fibers running to and from the cortex and lower Regions of the central nervous system form a layer of white matter. There are three systems of nerve fibers in the white matter: association, commissural, and projection fibers.

The cortex is divided into areas where information is received and processed. There are over 200 such areas, the eight most critical being those that receive and process sensory information:

- precentral gyrus: cortical end of the proprioceptive analyzer (center of neuromuscular sense);

- postcentral gyrus: cortical end of the cutaneous analyzer (center for general sensations of pain, Temperature, and Touch);

- lower part of the postcentral gyrus: cortical end of the gustatory analyzer (taste center);

- cuneus and calcarine sulcus in the occipital lobe: cortical end of the visual analyzer (visual center);

- superior temporal gyrus: cortical end of the Auditory Analyzer (auditory center);

- middle and inferior temporal gyri: cortical end of the vestibular analyzer (balance center);

- parahippocampal gyrus: cortical end of the olfactory analyzer (olfactory center).

Topography of speech centers:

1) sensory speech centers:

a) auditory speech center: posterior part of the superior temporal gyrus;

b) visual speech center: angular gyrus in the parietal lobe.

2) motor speech centers:

a) center for spoken language – inferior frontal gyrus;

b) center for written language – posterior part of the middle frontal gyrus.