MedicoPlexus
Human Anatomy & Histology
157. nervous system.
The nervous system consists of the brain, spinal cord, sensory organs, and all of the nerves that connect these organs with the rest of the body. Together, these organs are responsible for the control of the body and communication among its parts. The brain and spinal cord form the control center known as the central nervous system (CNS), where information is evaluated, and decisions made. The sensory nerves and sense organs of the peripheral nervous system (PNS) monitor conditions inside and outside of the body and send this information to the CNS. Efferent nerves in the PNS carry signals from the control center to the muscles, glands, and organs to regulate their functions. Nervous Tissue
The majority of the nervous system is tissue made up of two classes of cells: neurons and neuroglia.
Neurons. Neurons, also known as nerve cells, communicate within the body by transmitting electrochemical signals. Neurons look quite different from other cells in the body due to the many long cellular processes that extend from their central cell body. The cell body is the roughly round part of a neuron that contains the nucleus, mitochondria, and most of the cellular organelles. Small tree-like structures called dendrites extend from the cell body to pick up stimuli from the environment, other neurons, or sensory receptor cells. Long transmitting processes called axons extend from the cell body to send signals onward to other neurons or effector cells in the body.
There are 3 basic classes of neurons: afferent neurons, efferent neurons, and interneurons.
Afferent neurons. Also known as sensory neurons, afferent neurons transmit sensory signals to the central nervous system from receptors in the body.
Efferent neurons. Also known as motor neurons, efferent neurons transmit signals from the central nervous system to effectors in the body such as muscles and glands.
Interneurons. Interneurons form complex networks within the central nervous system to integrate the information received from afferent neurons and to direct the function of the body through efferent neurons.
Neuroglia. Neuroglia, also known as glial cells, act as the “helper” cells of the nervous system. Each neuron in the body is surrounded by anywhere from 6 to 60 neuroglia that protect, feed, and insulate the neuron. Because neurons are extremely specialized cells that are essential to body function and almost never reproduce, neuroglia are vital to maintaining a functional nervous system.
Brain
The brain, a soft, wrinkled organ that weighs about 3 pounds, is located inside the cranial cavity, where the bones of the skull surround and protect it. The approximately 100 billion neurons of the brain form the main control center of the body. The brain and spinal cord together form the central nervous system (CNS), where information is processed, and responses originate. The brain, the seat of higher mental functions such as consciousness, memory, planning, and voluntary actions, also controls lower body functions such as the maintenance of respiration, heart rate, blood pressure, and digestion.
Spinal Cord
The spinal cord is a long, thin mass of bundled neurons that carries information through the vertebral cavity of the spine beginning at the medulla oblongata of the brain on its superior end and continuing inferiorly to the lumbar region of the spine. In the lumbar region, the spinal cord separates into a bundle of individual nerves called the cauda equina (due to its resemblance to a horse’s tail) that continues inferiorly to the sacrum and coccyx. The white matter of the spinal cord functions as the main conduit of nerve signals to the body from the brain. The grey matter of the spinal cord integrates reflexes to stimuli.
Nerves
Nerves are bundles of axons in the peripheral nervous system (PNS) that act as information highways to carry signals between the brain and spinal cord and the rest of the body. Each axon is wrapped in a connective tissue sheath called the endoneurium. Individual axons of the nerve are bundled into groups of axons called fascicles, wrapped in a sheath of connective tissue called the perineurium. Finally, many fascicles are wrapped together in another layer of connective tissue called the epineurium to form a whole nerve. The wrapping of nerves with connective tissue helps to protect the axons and to increase the speed of their communication within the body.
Afferent, Efferent, and Mixed Nerves. Some of the nerves in the body are specialized for carrying information in only one direction, similar to a one-way street. Nerves that carry information from sensory receptors to the central nervous system only are called afferent nerves. Other neurons, known as efferent nerves, carry signals only from the central nervous system to effectors such as muscles and glands. Finally, some nerves are mixed nerves that contain both afferent and efferent axons. Mixed nerves function like 2-way streets where afferent axons act as lanes heading toward the central nervous system and efferent axons act as lanes heading away from the central nervous system.
Cranial Nerves. Extending from the inferior side of the brain are 12 pairs of cranial nerves. Each cranial nerve pair is identified by a Roman numeral 1 to 12 based upon its location along the anterior-posterior axis of the brain. Each nerve also has a descriptive name (e.g., olfactory, optic, etc.) that identifies its function or location. The cranial nerves provide a direct connection to the brain for the special sense organs, muscles of the head, neck, and shoulders, the heart, and the GI tract.
Spinal Nerves. Extending from the left and right sides of the spinal cord are 31 pairs of spinal nerves. The spinal nerves are mixed nerves that carry both sensory and motor signals between the spinal cord and specific regions of the body. The 31 spinal nerves are split into 5 groups named for the 5 regions of the vertebral column. Thus, there are 8 pairs of cervical nerves, 12 pairs of thoracic nerves, 5 pairs of lumbar nerves, 5 pairs of sacral nerves, and 1 pair of coccygeal nerves. Each spinal nerve exits from the spinal cord through the intervertebral foramen between a pair of vertebrae or between the C1 vertebra and the occipital bone of the skull.
Meninges
The meninges are the protective coverings of the central nervous system (CNS). They consist of three layers: the dura mater, arachnoid mater, and pia mater.
Dura mater. The dura mater, which means “tough mother,” is the thickest, toughest, and most superficial layer of meninges. Made of dense irregular connective tissue, it contains many tough collagen fibers and blood vessels. Dura mater protects the CNS from external damage, contains the cerebrospinal fluid that surrounds the CNS, and provides blood to the nervous tissue of the CNS.
Arachnoid mater. The arachnoid mater, which means “spider-like mother,” is much thinner and more delicate than the dura mater. It lines the inside of the dura mater and contains many thin fibers that connect it to the underlying pia mater. These fibers cross a fluid-filled space called the subarachnoid space between the arachnoid mater and the pia mater.
Pia mater. The pia mater, which means “tender mother,” is a thin and delicate layer of tissue that rests on the outside of the brain and spinal cord. Containing many blood vessels that feed the nervous tissue of the CNS, the pia mater penetrates into the valleys of the sulci and fissures of the brain as it covers the entire surface of the
CNS.
Cerebrospinal Fluid
The space surrounding the organs of the CNS is filled with a clear fluid known as cerebrospinal fluid (CSF). CSF is formed from blood plasma by special structures called choroid plexuses. The choroid plexuses contain many capillaries lined with epithelial tissue that filters blood plasma and allows the filtered fluid to enter the space around the brain.
Newly created CSF flows through the inside of the brain in hollow spaces called ventricles and through a small cavity in the middle of the spinal cord called the central canal. CSF also flows through the subarachnoid space around the outside of the brain and spinal cord. CSF is constantly produced at the choroid plexuses and is reabsorbed into the bloodstream at structures called arachnoid villi.
Cerebrospinal fluid provides several vital functions to the central nervous system:
CSF absorbs shocks between the brain and skull and between the spinal cord and vertebrae. This shock absorption protects the CNS from blows or sudden changes in velocity, such as during a car accident.
The brain and spinal cord float within the CSF, reducing their apparent weight through buoyancy. The brain is a very large but soft organ that requires a high volume of blood to function effectively. The reduced weight in cerebrospinal fluid allows the blood vessels of the brain to remain open and helps protect the nervous tissue from becoming crushed under its own weight.
CSF helps to maintain chemical homeostasis within the central nervous system. It contains ions, nutrients, oxygen, and albumins that support the chemical and osmotic balance of nervous tissue. CSF also removes waste products that form as byproducts of cellular metabolism within nervous tissue.
Sense Organs
All of the bodies’ many sense organs are components of the nervous system. What are known as the special senses—vision, taste, smell, hearing, and balance—are all detected by specialized organs such as the eyes, taste buds, and olfactory epithelium. Sensory receptors for the general senses like touch, temperature, and pain are found throughout most of the body. All of the sensory receptors of the body are connected to afferent neurons that carry their sensory information to the CNS to be processed and integrated.
Functions of the Nervous System
The nervous system has 3 main functions: sensory, integration, and motor.
Sensory. The sensory function of the nervous system involves collecting information from sensory receptors that monitor the body’s internal and external conditions. These signals are then passed on to the central nervous system (CNS) for further processing by afferent neurons (and nerves).
Integration. The process of integration is the processing of the many sensory signals that are passed into the CNS at any given time. These signals are evaluated, compared, used for decision making, discarded or committed to memory as deemed appropriate. Integration takes place in the gray matter of the brain and spinal cord and is performed by interneurons. Many interneurons work together to form complex networks that provide this processing power.
Motor. Once the networks of interneurons in the CNS evaluate sensory information and decide on an action, they stimulate efferent neurons. Efferent neurons (also called motor neurons) carry signals from the gray matter of the CNS through the nerves of the peripheral nervous system to effector cells. The effector may be smooth, cardiac, or skeletal muscle tissue or glandular tissue. The effector then releases a hormone or moves a part of the body to respond to the stimulus.
Divisions of the Nervous System
Central Nervous System
The brain and spinal cord together form the central nervous system, or CNS. The CNS acts as the control center of the body by providing its processing, memory, and regulation systems. The CNS takes in all of the conscious and subconscious sensory information from the body’s sensory receptors to stay aware of the body’s internal and external conditions. Using this sensory information, it makes decisions about both conscious and subconscious actions to take to maintain the body’s homeostasis and ensure its survival. The CNS is also responsible for the higher functions of the nervous system such as language, creativity, expression, emotions, and personality. The brain is the seat of consciousness and determines who we are as individuals.
Peripheral Nervous System
The peripheral nervous system (PNS) includes all of the parts of the nervous system outside of the brain and spinal cord. These parts include all of the cranial and spinal nerves, ganglia, and sensory receptors.
Somatic Nervous System
The somatic nervous system (SNS) is a division of the PNS that includes all of the voluntary efferent neurons. The SNS is the only consciously controlled part of the PNS and is responsible for stimulating skeletal muscles in the body.
Autonomic Nervous System
The autonomic nervous system (ANS) is a division of the PNS that includes all of the involuntary efferent neurons. The ANS controls subconscious effectors such as visceral muscle tissue, cardiac muscle tissue, and glandular tissue.
There are 2 divisions of the autonomic nervous system in the body: the sympathetic and parasympathetic divisions.
Sympathetic. The sympathetic division forms the body’s “fight or flight” response to stress, danger, excitement, exercise, emotions, and embarrassment. The sympathetic division increases respiration and heart rate, releases adrenaline and other stress hormones, and decreases digestion to cope with these situations.
Parasympathetic. The parasympathetic division forms the body’s “rest and digest” response when the body is relaxed, resting, or feeding. The parasympathetic works to undo the work of the sympathetic division after a stressful situation. Among other functions, the parasympathetic division works to decrease respiration and heart rate, increase digestion, and permit the elimination of wastes.
Enteric Nervous System
The enteric nervous system (ENS) is the division of the ANS that is responsible for regulating digestion and the function of the digestive organs. The ENS receives signals from the central nervous system through both the sympathetic and parasympathetic divisions of the autonomic nervous system to help regulate its functions.
However, the ENS mostly works independently of the CNS and continues to function without any outside input. For this reason, the ENS is often called the “brain of the gut” or the body’s “second brain.” The ENS is an immense system—almost as many neurons exist in the ENS as in the spinal cord.
158. histology of nerve system
Central Nervous System (CNS)
The CNS consists of the brain (encephalon), which is enclosed in the skull, and the spinal cord, which is contained within the vertebral canal. Nervous tissue of the CNS does not contain connective tissue other than that in the three meninges (dura mater, arachnoid membrane and pia mater) and in the walls of large blood vessels. Collagenous fibers or fibrocytes/blasts are consequently not observed, which is quite unlike other tissues. Because of the absence of connective tissue, fresh CNS tissue has a very soft, somewhat jelly-like consistency. The two major classes of cells that make up the nervous tissue are nerve cells, neurons, and supporting cells, glia.
Neurons
The vast majority of neurons is generated before birth. Persisting stem cells give rise to a small number of new neurons throughout the lifetime of mammals, including humans. The permanent addition of neurons may be important for the maintenance and plasticity of some parts of the CNS, but it is insufficient to replace neurons that die because of disease or acute damage to the CNS. Neurons should last a lifetime. Mature neurons are not mitotically active, i.e., they do not divide.
Neurons are generally large cells. Neural activity and its control require the expression of many genes, which is reflected in the large and light nuclei of most neurons. The keys to the understanding of the function of a neuron lies in (1) the shape of the neuron and, in particular, its processes, (2) the chemicals the neuron uses to communicate with other neurons (neurotransmitters) and (3) the ways in which the neuron may react to the neurotransmitters released by other neurons. the shape of the neuron and its processes
Neurons have long processes, which extend from the part of the cell body around the nucleus, the perikaryon or soma. The processes can be divided into two functionally and morphologically different groups, dendrites and axons.
Dendrites are part of the receptive surface of the neuron. As a rule, neurons have one to several primary dendrites, which emerge from the perikaryon. Primary dendrites may divide into secondary, tertiary etc.
dendrites. Dendrites can be smooth, or they can be studded with small, mushroom-shaped appendages, which are called spines.
Each neuron has as a rule one axon, and never more than one axon which emerges from the perikaryon or close to the trunks of one of the primary dendrites. The point of origin of the axon from the perikaryon is the axon hillock. The axon may, like the dendrites, branch as it travels through the nervous tissue to its destination(s). The axon is the “transmitting” process of the neuron.
The axon forms small, bulb-shaped swellings called boutons at the ends (terminal boutons) or along the course (boutons en passant) of its branches. Synapses are morphologically specialized contacts between a bouton formed by one neuron, the presynaptic neuron, and the cell surface of another neuron, the postsynaptic neuron. Synaptic vesicles contain the neurotransmitters. Synaptic vesicles typically accumulate close to the site of contact between the bouton and the postsynaptic neuron. The release of the neurotransmitter from the synaptic vesicles into the synaptic cleft, i.e., the space between the bouton and the postsynaptic neuron, mediates the transfer of information from the pre- to the postsynaptic neuron. Both the release of the synaptic vesicles and the mediation of the response to the transmitter require membrane-associated specializations – the pre- and postsynaptic densities.
The shape and orientation of the dendritic tree of the neuron determines the amount and type of information that may reach the neuron. The course of its axon determines to which neurons this information may be passed on. The location of the neuron within the CNS determines to which major system the neuron belongs.
There are several hundred functionally different areas, i.e., groups of neurons, in the CNS. Based on their location, the shape of their dendritic tree and the course of their axon, several thousand types of neurons can be distinguished in the CNS.
Transmitters
Neurotransmitters either excite or inhibit the postsynaptic neuron. The most prominent excitatory transmitter in the CNS is L-glutamate. The most prominent inhibitory transmitter in the CNS is GABA (gamma-amino butyric acid). Other “main” neurotransmitters are e.g., dopamine, serotonin, acetylcholine, noradrenaline and glycine. Each neuron uses only one of the main transmitters, and this transmitter is used at all synaptic boutons that originate from the neuron.
One or more of the “minor” transmitters (there are several dozens of them – such as cholecystokinin, endogenous opioids, somatostatin, substance P) may be used together with a main transmitter.
The molecular machinery that is needed to mediate the events occurring at excitatory synapses differs from that at inhibitory synapses. Differences in the morphological appearances of the synapses accompany the functional differences. The pre-and postsynaptic densities are typically of equal width, or symmetric, at inhibitory synapses. The postsynaptic density is thicker than the presynaptic density at asymmetric synapses, which are typically excitatory.
Receptors
Usually there exists a multitude of receptors which are all sensitive to one particular neurotransmitter. Different receptors have different response properties, i.e., they allow the flux of different ions over the plasma membrane of the neuron or they may address different second messenger systems in the postsynaptic neurons. The precise reaction of the neuron to the various neurotransmitters released onto its plasma membrane at the synapses is determined by the types of receptors expressed by the neuron. Suitable Slides sections of spinal cord – H&E, luxol fast blue/cresyl violet (LFB/CV), toluidine blue, Giemsa
Thoracic Spinal Cord, sheep – LFB/CV
Most neurons have a light, large nucleus with a distinct nucleolus. The cytoplasm of many neurons contains fairly large amounts of rough endoplasmic reticulum, which may aggregate within the cytoplasm of the neuron to form Nissl-bodies. Nissl-bodies are prominent in motor neurons located in the ventral horn of the grey matter of the spinal cord. The neurites are difficult to identify in most types of stained sections. Only the most proximal segments of the primary dendrites are seen clearly. The size of the perikaryon depends on the level of activity of the neuron and the length of the processes which the neuron has to support. A usable range for the size of the perikaryon would be 15 – 50 µm, although much smaller and much larger neuronal perikarya exist.
Draw the spinal cord at low magnification and indicate the distribution of grey matter and white matter. Find a nice group of neurons in the grey matter and draw them at a high magnification. Finally, have a look at the white matter and identify the nuclei of glial cells. You will find similar nuclei also in the grey matter.
Glia
CNS tissue contains several types of non-neuronal, supporting cells, neuroglia.
Astrocytes (or astroglia) are star-shaped cells. Their processes are often in contact with a blood vessel
(perivascular foot processes). Astrocytes provide mechanical and metabolic support to the neurons of the CNS. They participate in the maintenance of the composition of the extracellular fluid. Although not themselves directly involved in the process of communication between neurons, they may be involved in the removal of transmitters from synapses and the metabolism of transmitters. Astrocytes are also the scar-forming cells of the CNS.
Oligodendrocytes (or oligoglia) have fewer and shorter processes. Oligodendrocytes form myelin sheath (see below) around axons in the CNS and are the functional homologue of peripheral Schwann cells.
Oligodendrocytes may, in contrast to Schwann cells in the periphery, form parts of the myelin sheath around several axons.
Microglia are small cells with complex shapes. Microglia are, in contrast to neurons and the other types of glial cells, of mesodermal origin. They are derived from the cell line which also gives rise to monocytes, i.e., macrophage precursors which circulate in the blood stream. In the case of tissue damage, microglia can proliferate and differentiate into phagocytotic cells.
The ventricles of the brain and the central canal of the spinal cord are lined with ependymal cells. The cells are often ciliated and form a simple cuboidal or low columnar epithelium. The lack of tight junctions between ependymal cells allows a free exchange between cerebrospinal fluid and nervous tissue.
Ependymal cells can specialize into tanycytes, which are rarely ciliated and have long basal processes.
Tanycytes form the ventricular lining over the few CNS regions in which the blood-brain barrier is incomplete.
They do form tight junctions and control the exchange of substances between these regions and surrounding nervous tissue or cerebrospinal fluid.
Peripheral Nervous System (PNS)
The PNS comprises all nervous tissue outside the brain and spinal cord. It consists of groups of neurons (ganglion cells), called ganglia, feltworks of nerve fibres, called plexuses, and bundles of parallel nerve fibres that form the nerves and nerve roots. Nerve fibres, which originate from neurons within the CNS and pass out of the CNS in cranial and spinal nerves, are called efferent or motor fibers. Nerve fibres which originate from nerve cells outside the CNS but enter the CNS by way of the cranial or spinal nerves are called afferent or sensory nerve fibres.
The principal neurotransmitters in the PNS are acetylcholine and noradrenalin
Peripheral Nerves
Afferent, sensory fibres enter the spinal cord via the dorsal roots, while efferent, motor fibres leave the spinal cord via the ventral roots. Dorsal and ventral roots merge to form the spinal nerves, which consequently contain both sensory and motor fibres. As the spinal nerves travel into the periphery they split into branches and the exact composition of the nerve in terms of motor and sensory fibres is, of course, determined by the structures the nerve will innervate.
One nerve fibre consists of an axon and its nerve sheath. Each axon in the peripheral nervous system is surrounded by a sheath of Schwann cells. An individual Schwann cell may surround the axon for several hundred micrometers, and it may, in the case of unmyelinated nerve fibers, surround up to 30 separate axons. The axons are housed within infoldings of the Schwann cell cytoplasm and cell membrane, the mesaxon .
In the case of myelinated nerve fibres, Schwann cells form a sheath around one axon and surround this axon with several double layers (up to hundreds) of cell membrane. The myelin sheath formed by the Schwann cell insulates the axon, improves its ability to conduct and, thus, provides the basis for the fast saltatory transmission of impulses. Each Schwann cell forms a myelin segment, in which the cell nucleus is located approximately in the middle of the segment. The node of Ranvier is the place along the course of the axon where two myelin segments abut.
Fibre types in peripheral nerves:
Type A fibres (myelinated) are 4 – 20 µm in diameter and conduct impulses at high velocities (15 – 120 m per second). Examples: motor fibers, which innervate skeletal muscles, and sensory fibres.
Type B fibres (myelinated) are 1 – 4 µm in diameter and conduct impulses with a velocity of 3 – 14 m per second. Example: preganglionic autonomic fibres.
Type C fibres (unmyelinated) are 0.2 – 1 µm thick and conduct impulses at velocities ranging from 0.2 to 2 m per second. Examples: autonomic and sensory fibres.
Peripheral nerves contain a considerable amount of connective tissue. The entire nerve is surrounded by a thick layer of dense connective tissue, the epineurium. Nerve fibres are frequently grouped into distinct bundles, fascicles, within the nerve. The layer of connective tissue surrounding the individual bundles is called perineurium. The perineurium is formed by several layers of flattened cells, which maintain the appropriate microenvironment for the nerve fibres surrounded by them. The space between individual nerve fibres is filled by loose connective tissue, the endoneurium.
Fibrocytes, macrophages and mast cells are present in the endoneurium.
Nerves are richly supplied by intraneural blood vessels, which form numerous anastomoses. Arteries pass into the epineurium, form arteriolar networks beneath the perineurium and give off capillaries to the endoneurium.
Ganglia
Ganglia are aggregations of nerve cells (ganglion cells) outside the CNS. Cranial nerve and dorsal root ganglia are surrounded by a connective tissue capsule, which is continuous with the dorsal root epi- and perineurium. Individual ganglion cells are surrounded by a layer of flattened satellite cells. Neurons in cranial nerve and dorsal root ganglia are pseudo unipolar. They have a T-shaped process. The arms of the T represent branches of the neurite connecting the ganglion cell with the CNS (central branch) and the periphery (peripheral branch). Both branches function as one actively conducting axon, which transmits information from the periphery to the CNS. The stem is connected to the perikaryon of the ganglion cell and is the only process originating from it. Ganglion cells in dorsal root ganglia do not receive synapses. Their function is the trophic support of their neurites.
Early in development two processes emerge from the perikaryon of dorsal root ganglion cells, which merge in the course of development. These ganglion cells are therefore also called pseudo unipolar neurons. Two processes emerge from the perikaryon of bipolar neurons. The majority of CNS neurons are multipolar, i.e., more than two processes (but only one axon) emerge from their perikaryon.
Autonomic ganglia do contain synapses, and the ganglion cells within them do have dendrites. They receive synapses from the first neuron of the two-neuron chain, which characterizes most of the efferent connections of the autonomic nervous system. The second neuron is the ganglion cell itself. Some autonomic ganglia are embedded within the walls of the organs which they innervate (intramural ganglia – e.g., GIT and bladder).
159. ontogenesis of NS
The central nervous system (CNS) consists of the brain and the spinal cord. It is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except radially symmetric animals such as sponges and jellyfish.
Some classifications of the CNS also include the retina and the cranial nerves. Together with the peripheral nervous system, it has a fundamental role in the control of behavior. The CNS is contained within the dorsal cavity, with the brain in the cranial cavity and the spinal cord in the spinal cavity. In vertebrates, the brain is protected by the skull, while the spinal cord is protected by the vertebrae, and both are enclosed in the meninges .
During early development of the vertebrate embryo, a longitudinal groove on the neural plate gradually deepens and the ridges on either side of it (the neural folds) become elevated and ultimately meet, transforming the groove into a closed tube, the ectodermal wall of which forms the rudiment of the nervous system. This tube initially differentiates into three vesicles (pockets): the prosencephalon at the front, the mesencephalon, and between the mesencephalon and the spinal cord, the rhombencephalon. At six weeks in the human embryo’s development, the prosencephalon divides further into the telencephalon and diencephalon; and the rhombencephalon divides into the metencephalon and myelencephalon.
As the vertebrate grows, these vesicles differentiate further still. The telencephalon differentiates into, among other things, the striatum, the hippocampus, and the neocortex, and its cavity becomes the first and second ventricles. Diencephalon elaborations include the subthalamus, hypothalamus, thalamus, and epithalamus, and its cavity forms the third ventricle. The tectum, pretectum, cerebral peduncle, and other structures develop out of the mesencephalon, and its cavity grows into the mesencephalic duct (cerebral aqueduct). The metencephalon becomes, among other things, the pons and the cerebellum; the myelencephalon forms the medulla oblongata; and their cavities develop into the fourth ventricle.
G OBJECTIVE
Describe peripheral nervous system development
KEY POINTS
The first sign of the nervous system is the appearance of a thin strip of cells along the center of the back, called the neural plate. The inner portion of the neural plate is destined to become the central nervous system, while the outer portion will become the peripheral nervous system.
Neurulation (neural development) progresses with the formation of the neural groove which closes to form the neural tube and neural crest.
Neural crest cells from the roof plate of the neural tube migrate through the periphery where they differentiate into varied cell types, including pigment cells and the cells of the peripheral nervous system.
TERMS
neurulation
The process by which the beginnings of the vertebrate nervous system is formed in embryos.
neural crest
A strip of ectodermal material in the early vertebrate embryo inserted between the prospective neural plate and the epidermis. neural plate
A thick, flat bundle of ectoderm formed in vertebrate embryos after induction by the notochord.
In vertebrates, the first sign of the nervous system is the appearance of a thin strip of cells along the center of the back, called the neural plate. The inner portion of the neural plate (along the midline) is destined to become the central nervous system (CNS), the outer portion the peripheral nervous system (PNS). As development proceeds, a fold called the neural groove appears along the midline. This fold deepens and then closes up at the top. At this point the future CNS appears as a cylindrical structure called the neural tube, whereas the future PNS appears as two strips of tissue called the neural crest, running lengthwise above the neural tube. The sequence of stages from neural plate to neural tube and neural crest is known as neurulation .
Formation of the Fetal Nervous System
The neural tube will give rise to the central nervous system, while the neural crest will give rise to the peripheral nervous system.
Neural crest cells are a transient, multipotent, migratory cell population unique to vertebrates that gives rise to a diverse cell lineage including melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons and glia. After gastrulation, neural crest cells are specified at the border of the neural plate and the non-neural ectoderm. During neurulation, the borders of the neural plate, also known as the neural folds, converge at the dorsal midline to form the neural tube. Subsequently, neural crest cells from the roof plate of the neural tube undergo an epithelial to mesenchymal transition, delaminating from the neuroepithelium and migrating through the periphery where they differentiate into varied cell types, including pigment cells and the cells of the peripheral nervous system. The emergence of the neural crest was important in vertebrate evolution because many of its structural derivatives are defining features of the vertebrate clade.
- external morphology of spinal cord
The spinal cord extends caudally from the brainstem, running from the medullary-spinal junction at about the level of the first cervical vertebra to about the level of the twelfth thoracic vertebra. The vertebral column (and the spinal cord within it) is divided into cervical, thoracic, lumbar, sacral, and coccygeal regions. The peripheral nerves (called the spinal or segmental nerves) that innervate much of the body arise from the spinal cords 31 segmental pairs. The cervical region of the cord gives rise to eight cervical nerves (C1—C8), the thoracic to twelve thoracic nerves (T1—T12), the lumbar to five lumbar nerves (L1—L5), the sacral to five sacral nerves (S1 —S5), and the coccygeal to one coccygeal nerve. The segmental spinal nerves leave the vertebral column through the intervertebral foramina that lie adjacent to the respectively numbered vertebral body. Sensory information carried by the afferent axons of the spinal nerves enters the cord via the dorsal roots, and motor commands carried by the efferent axons leave the cord via the ventral roots . Once the dorsal and ventral roots join, sensory and motor axons (with some exceptions) travel together in the segmental spinal nerves.
Two regions of the spinal cord are enlarged to accommodate the greater number of nerve cells and connections needed to process information related to the upper and lower limbs. The spinal cord expansion that corresponds to the arms is called the cervical enlargement and includes spinal segments C5—T1; the expansion that corresponds to the legs is called the lumbar enlargement and includes spinal segments L2—S3. Because the spinal cord is considerably shorter than the vertebral column, lumbar and sacral nerves run for some distance in the vertebral canal before emerging, thus forming a collection of nerve roots known as the cauda equina. This region is the target for an important clinical procedure called a “lumbar puncture” that allows for the collection of cerebrospinal fluid by placing a needle into the space surrounding these nerves to withdraw fluid for analysis. In addition, local anesthetics can be safely introduced to produce spinal anesthesia; at this level, the risk of damage to the spinal cord from a poorly placed needle is minimized.
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