Nervous systems of vertebrates

Animals must be able to coordinate their behaviors and to maintain a relatively constant internal environment, despite fluctuations in the external environment, to survive and reproduce. To do so, animals must monitor their external and internal environments, integrate this sensory information, and then generate appropriate responses. The evolution of the vertebrate nervous system has provided for the efficient performance of these tasks.

The Pattern of the Vertebrate Nervous System

Although the various vertebrates show differences in the organization of their respective nervous systems, they all follow a similar anatomical pattern. The nervous system can be partitioned into two major divisions: the peripheral nervous system (PNS) and the central nervous system (CNS). These divisions are determined by their location and function. The CNS consists of the spinal cord and the brain. The PNS, that part of the nervous system outside the CNS, connects the CNS with the various sense organs, glands, and muscles of the body. In humans, the PNS consists of thirty-one pairs of spinal nerves, twelve cranial nerves, and the autonomic nervous system. However, the composition of the nervous system varies between species.

The PNS joins the CNS in the form of nerves, which are cordlike bundles of hundreds to thousands of individual, parallel nerve-cell (neuron) axons (long tubular extensions of the neurons) extending from the brain and spinal cord. The nerves extending from the spine are called spinal nerves, while those from the brain are called cranial nerves. The elements of the PNS include sensory neurons (for example, those in the eyes and in the tongue) and motor neurons (which activate muscles and glands, thereby causing some sort of action or change to occur). Most nerves contain both sensory and motor axons.

Thus, the PNS can be divided into two major subdivisions—sensory (or afferent) neurons and motor (or efferent) neurons. Little information processing occurs in the PNS. Instead, it relays environmental information to the CNS (sensory function) and the CNS responses to the body’s muscles and glands (motor function). Sensory neurons of the PNS are classified as somatic afferents if they carry signals from the body's skin, skeletal muscles, or joints. Sensory neurons from the visceral organs (internal body organs) are called visceral afferents.

The PNS motor subdivision also has two parts. One is the somatic efferent nervous system, which carries neuron impulses from the CNS to skeletal muscles. The other is the autonomic nervous system (ANS), which carries signals from the CNS to regulate the body’s internal environment by controlling the smooth muscles, the glands, and the heart. The ANS itself is subdivided into the sympathetic and parasympathetic nervous systems. These are generally both connected to any given target and cause approximately opposite effects to each other on that target (for example, slowing or increasing the heart rate).

The CNS, where essentially all information processing occurs, has two major subdivisions: the spinal cord and the brain. Virtually all vertebrates have similarly organized spinal cords, with two distinct regions of nervous tissue: gray and white matter. Gray matter is centrally located and consists of neuron cell bodies and unmyelinated axons (bare axons without the glistening sheaths called myelin, created by supporting cells wrapping around the axons). White matter contains mostly bundles of myelinated axons (white because they have glistening myelin sheaths around them). Bundles of axons in the CNS are called nerve tracts. Within the spinal cord, these are either sensory tracts carrying impulses toward the brain or they are motor tracts transmitting information in the opposite direction.

Interneurons are neurons positioned between two or more other neurons. They accept and integrate signals from some of the cells and then influence others in turn. Interneurons are particularly numerous within the gray matter. In the spinal cord, they permit communication up, down, and laterally. Most axons in the cord’s tracts belong to interneurons.

The Vertebrate Brain

The brain of vertebrates is a continuation of the spinal cord, which undergoes regional expansions during embryonic development. The subdivisions of the brain show more variety among vertebrate species than does the spinal cord. The brain has three regions: the hindbrain, the midbrain, and the forebrain. Their structures are complex, and various systems of subdividing them exist. The major components forming the brain regions are the hindbrain’s medulla oblongata, pons, and cerebellum; the midbrain’s inferior and superior colliculi, tegmentum, and substantia nigra; and the forebrain’s hypothalamus, thalamus, limbic system, basal ganglia, and cerebral cortex.

The hindbrain begins as a continuation of the spinal cord called the medulla oblongata. Most sensory fiber tracts of the spinal cord continue into the medulla, but it also contains clusters of neurons called nuclei. The posterior cranial nerves extend from the medulla, with most of their nuclei located there.

Also in the medulla, and extending beyond it through the pons and midbrain, is the complexly organized reticular formation. This mixture of gray and white matter is found in the central part of the brain stem but has indistinct boundaries. Essentially all sensory systems and parts of the body send impulses into the reticular formation. There are also various nuclei within its structure. Impulses from the reticular formation go to widely distributed areas of the CNS. This activity is important for maintaining a conscious state and for regulating muscle tone.

Prominent on the anterior (front) surface of the mammalian medulla oblongata are the pyramids: tracts of motor fibers originating in the forebrain and passing without interruption into the spinal cord to control muscle contraction. These tracts cross to the opposite side of the medulla before entering the spinal cord, which results in each side of the forebrain controlling muscle contraction in the opposite side of the body.

Many sensory fibers from the spinal cord terminate in two paired nuclei at the lower end of the medulla, the gracile and cuneate nuclei. Axons leaving these nuclei cross to the opposite side of the medulla and then continue as large tracts (the medial lemniscus) into the forebrain. Thus, each side of the brain gets sensory stimuli mostly from the opposite side of the body.

Immediately above the medulla is the pons. It contains major fiber pathways carrying signals through the brain stem and many nuclei, including several for cranial nerves. Some pontine nuclei get impulses from the forebrain and send axons into the cerebellum, again with a majority crossing to the opposite side of the brain stem before entering the cerebellum.

On the dorsal (back) side of the medulla and pons is the cerebellum, an ancient part of the brain that varies in size among vertebrate species. The cerebellum forms a very important part of the control system for body movements, but it is not the source of motor signals. Its gray matter forms a thin layer near its surface called the cerebellar cortex and surrounds central white matter.

Vertebrates with well-developed muscular systems (for example, birds and mammals) have a large cerebellum, with several lobes and convex folding of its cortex. It is attached to the brain stem by three pairs of fiber tracts called cerebellar peduncles, which transmit signals between the left and right sides of the cerebellum and between the cerebellum and motor areas of the spinal cord, brain stem, midbrain, and forebrain. The cerebellum times the order of muscle contractions to coordinate rapid body movements.

The Midbrain and the Forebrain

The midbrain is the second major region of the brain. The midbrain’s dorsal aspect, called the tectum, is a target for some of the auditory and visual information that an animal receives. The paired inferior colliculi form the lower half of the tectum. They help to coordinate auditory reflexes to relay acoustic signals to the cerebrum. The two superior colliculi, the other half of the tectum, assist the localization in the space of visual stimuli by causing appropriate eye and trunk movements. In lower vertebrates, the superior colliculi form the major brain target for visual signals. Connecting fiber pathways (commissures) link the individual lobes of each pair of colliculi.

The midbrain’s tegmentum contains several fiber tracts carrying sensory information to the forebrain and carrying impulses among various brain-stem nuclei and the forebrain. Two cranial nerve nuclei concerned with the control of eye movements are also in the tegmentum. The reticular formation extends through the tegmentum and regulates the level of arousal. It also helps to control various stereotyped body movements, especially those involving the trunk and neck muscles. Finally, the tegmentum contains the red nucleus, which, in conjunction with the cerebellum and basal ganglia, serves to coordinate body movements. The substantia nigra functions as part of the basal ganglia to permit subconscious muscle control.

The forebrain, the final major area of the brain, differs from the lower areas in the more highly evolved functions it controls. It has a small but extremely important collection of about a dozen pairs of nuclei called the hypothalamus. These control many of the body’s internal functions (such as temperature, blood pressure, water balance, and appetite) and drives (such as sexual behavior and emotions). Immediately above the hypothalamus lies the thalamus, another collection of more than thirty paired nuclei. The two thalami are the largest anterior brain-stem structures. Their ventral (front) parts relay motor signals to lower parts of the brain. The dorsal (back) parts transmit impulses from every sensory system (except olfaction, the sense of smell) to the cerebrum.

The limbic system is organized from a number of forebrain structures, mostly surrounding the hypothalamus and thalamus. It determines arousal levels, emotional and sexual behavior, feeding behavior, memory formation, learning, and motivation. In general, the limbic system exchanges information with the hypothalamus and thalamus and receives impulses from auditory, visual, and olfactory areas of the brain.

The basal ganglia function with the midbrain’s tegmentum and substantia nigra, the cerebral cortex, the thalamus, and the cerebellum. These paired structures’ functions are unclear, but it is known that they are important for adjusting the body’s background motor activities, such as gross positioning of the trunk and limbs before the cerebral cortex superimposes the precise final movements.

The cerebral cortex, like the cerebellum, is an ancient brain structure; however, it shows even more variation among vertebrate species than the cerebellum. It is formed into two hemispheres, which have olfactory bulbs projecting from their anterior (front) ends. The olfactory bulbs receive impulses from olfactory nerves for the sense of smell. The gray matter of the cerebral cortex is at the surface, enclosing the white matter (fiber tracts) beneath. The white matter connects various parts of the gray matter of one hemisphere with others within the same hemisphere and with corresponding parts in the opposite hemisphere. It also connects the cortical gray matter with lower brain structures. The ultimate control of voluntary motor activity resides in the motor areas of the cortex, although this control is heavily influenced by all the previously mentioned motor-control areas of the CNS.

Corresponding to each of the major senses (touch, vision, audition), there are primary sensory areas. These areas get the most direct input from their sensory organs by way of the corresponding sensory thalamic nuclei. Surrounding each primary area are association areas that receive a less direct sensory input but also more inputs from other sensory cortical areas. In general, the more intelligent an animal is, the larger are its association areas.

Studying the Nervous System

Many methods are used in studying vertebrate nervous systems. The level of description desired often determines the methods employed. For example, the gross structure visible to the unaided eye is usually investigated using the entire brain or spinal cord of the animal under study. It will then be photographed or drawn sliced at various points either parallel to its long axis or across its long axis, and again photographed or drawn, until a complete series of such “sections” has been assembled.

To see finer structural details requires microscopes and very thin slices of nervous tissue (less than a millimeter in thickness). Preparation of such thin slices of this soft tissue requires that it first be either frozen or embedded in a block of paraffin wax. A special slicing device called a microtome is then used to produce the thin sections. For easy observation of different structural details (such as nuclei and fiber tracts), various chemical stains can be applied to the slices of tissue. These stains specifically color particular structural features green, blue, or some other color, thereby making them more visible.

Nervous tissue can be selectively and painlessly destroyed in an anesthetized animal by cutting a nerve trunk, inserting a fine wire electrode into the CNS and destroying tissue with electricity, or inserting a fine needle and then either injecting a chemical agent that kills nervous tissue or using a suction device to remove areas of tissue. The precision and reproducibility of wire or needle placement within the CNS is possible with a device called a stereotaxic frame. This instrument positions the animal’s head and brain in an exact standard position. Then, wires or needles are inserted a certain distance away from (for example, behind, below, or to one side of) common landmarks on the skull. Stereotaxic atlases are books published by investigators for specific animals, with the exact coordinates in three-dimensional space for most CNS structures.

Following such procedures, the animal may be immediately and painlessly sacrificed, its brain or spinal cord removed, and the previously described thin slices prepared and stained. It may be necessary to allow the animal to recover from its surgical treatment since several days or weeks must sometimes pass for the severed fiber tracts to degenerate. Then, following a painless lethal injection, the nervous tissue is prepared as above using special staining techniques, which reveal the pathways of degenerating nerve fibers in the tissue sections studied later under the microscope.

Although new techniques are constantly being developed, the preceding methods have revealed that the hundred billion neurons of the vertebrate nervous system form the most complexly organized structure known. Through this knowledge of the structure of the nervous system, it has become possible to study its functions, to diagnose its diseases, and to devise methods of treatment when it becomes damaged or diseased.

Advanced twenty-first-century technology has supported increased scientific exploration of the human brain and nervous system. While magnetic resonance imaging (MRI), functional magnetic resonance imaging (fMRI), and positron emission tomography (PET) continue to improve and remain reliable instruments, further technology allows the manipulation and restoration of specific brain functions. UNSLICE, a three-dimensional imaging system, creates a subcellular resolution, layered model of the entire brain, expanding the research potential of neuropsychological and neurodegenerative studies aiming to understand, diagnose, and reverse diseases like Alzheimer's disease. Another device, Nano-MIND, allows the manipulation of brain regions using magnetic fields, and Airy beam technology uses sound waves to create beam patterns that have successfully altered behaviors in mice.

Scientists aiming to understand the origins of the vertebrate brain created a cell-type atlas of the brain of a sea lamprey using single-cell RNA sequencing and in situ sequencing. The sea lamprey is a parasitic jawless fish that has a nervous system believed to resemble that of ancient vertebrate ancestors. There were several similarities when comparing the sea lamprey atlas to mice and other jawed animals. However, the data indicated that, most likely, ancestral brains had no cerebellar or myelinating cells. This data provides insight into the evolutionary progression of vertebrates' brains and offers a revolutionary platform on which to build research.

A Complex Structure

The vertebrate nervous system is the most complex structure known to humankind. The human nervous system, for example, has more than a hundred billion cellular elements and perhaps a hundred trillion points of information exchange between these elements. It is impossible to understand the details of such a structure in the same way that one can understand the structure of a radio; however, the general organizational plan can be discovered through the application of modern neuroanatomical techniques.

It is a widely accepted tenet of physiology that, to comprehend the functioning of an organ or an organ system, it is necessary to have a critical understanding of its structure. In fact, physiology and anatomy are inseparable because function (physiology) always reflects structure (anatomy): It is impossible for an organ to perform in any other way than its architecture permits.

The nervous system is the ultimate control and communication system in the vertebrate body. Its complexity allows the vast range of vertebrate behaviors, as well as the rapid and precise regulation of the body’s internal environment. The most complex vertebrate nervous systems display self-consciousness, reasoning, and language capabilities.

There are many reasons for studying the organization of vertebrate nervous systems, ranging from purely theoretical (such as determining the mechanisms of memory recall or clarifying the evolutionary relationships among vertebrate species) to very practical (such as treatments for mental illnesses, precisely defining brain death, or designing better computers).

For many, the goal is to obtain a better understanding of the relationship between the brain and the mind. It has been proposed that the mind and mental processes are emergent properties that appear when a certain degree of organizational complexity within the nervous system has been reached. The individual elements of the nervous system (the neurons) are not the constituents that think or possess consciousness. These unique capabilities are achieved as a result of the specific connections between neurons and sensory organs and among neurons themselves.

It does not matter whether one performs an analysis of a machine or a nervous system; it can never be expected to reveal its soul or its consciousness—if they exist. All that can be done is to admire the intelligence of its designer or the wisdom of nature.

Principal Terms

Axon: An extension of a neuron’s cell membrane that conducts nerve impulses from the neuron to the point or points of axon termination

Gray Matter: The part of the central nervous system primarily containing neuron cell bodies and unmyelinated axons

Interneuron: A central nervous system neuron that does not extend into the peripheral nervous system and is interposed between other neurons

Myelinated Axon: An axon surrounded by a glistening sheath formed when a supporting cell has grown around the axon

Neural Integration: Continuous summation of the incoming signals acting on a neuron

Neurons: Complete nerve cells that respond to specific internal or external environmental stimuli, integrate incoming signals, and sometimes send signals to other cells

Nucleus (pl. nuclei): Cluster of neuron cell bodies within the central nervous system

Tract: A cordlike bundle of parallel axons within the central nervous system

White Matter: The part of the central nervous system primarily containing myelinated axon tracts

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