Structure Function and Information Flow

The human nervous system consists of three major components. The brain and spinal cord together constitute the central nervous system (CNS). Information is transmitted to and from the CNS by means of an enormous network of nerves that make up the peripheral nervous system (PNS). The PNS reaches every tissue of the body. It connects to the CNS via spinal nerves and cranial nerves.

A nerve is a bundle of axons that carries information about many things simultaneously. It is important to distinguish between the axon of a single neuron and a nerve. Some axons in a nerve may be carrying information to the CNS, while other axons in the same nerve are carrying information from the CNS to the organs of the body.

A conceptual diagram of the nervous system traces information flow

The major avenues of information flow through the nervous system are illustrated in Figure 46.1. The afferent portion of the peripheral nervous system carries information to the CNS. We are consciously aware of much of the information that moves through these afferent pathways (for example, vision, hearing, temperature, pain, the position of limbs). We are not consciously aware of other afferent information that is important for physiological regulation (for example, blood pressure, deep body temperature, blood oxygen supply).

The efferent portion of the peripheral nervous system carries information from the CNS to the muscles and glands of the body. Efferent pathways can be divided into a voluntary division, which executes our conscious movements, and an involuntary, or autonomic, division, which controls physiological functions.

Phylogeny Mammals
46.1 Organization of the Nervous System The peripheral nervous system (indicated by pink and blue) carries information both to and from the central nervous system.The CNS also receives hormonal inputs and produces hormonal outputs.

In addition to neuronal information, the CNS receives chemical information in the form of hormones circulating in the blood. Neurohormones released by neurons into the extracellular fluids of the brain can send chemical information to other neurons in the brain or can leave the brain and enter the circulation. In Chapter 42 we learned of the important role of neurohormones (such as GnRH) in the control of the anterior pituitary, and we saw that other neurohormones (such as oxytocin) are released from the posterior pituitary into the circulation.

The vertebrate CNS develops from the embryonic neural tube

Early in the development of all vertebrate embryos, a hollow tube of neural tissue forms. This neural tube runs the length of the embryo on its dorsal side. At the anterior end of the embryo, the neural tube forms three swellings that become the basic divisions of the brain: the hindbrain, the midbrain, and the forebrain. The rest of the neural tube becomes the spinal cord (Figure 46.2). The cranial and spinal nerves, which make up the peripheral nervous system, sprout from the neural tube and grow throughout the embryo.

Each of the three regions of the embryonic brain develops into several structures in the adult brain. From the hindbrain come the medulla, the pons, and the cerebellum. The medulla is continuous with the spinal cord. The pons is in front of the medulla, and the cerebellum is a dorsal outgrowth of the pons. The medulla and pons contain distinct groups of neurons that are involved in the control of physi-

The streched-out neural tube, viewed from above, shows three swellings that will form the adult brain.

Lateral views

Neural tube

Lateral views

Neural tube

25 days

25 days

35 days

Dorsal views

Forebrain

Midbrain

Hindbrain

Spinal cord

Dorsal views

Forebrain

Midbrain

Hindbrain

Spinal cord

Plasma Membrane Function
25 days

35 days

The forebrain develops into two major divisions, the telencephalon and diencephalon.

Forebrain Midbrain

Hindbrain

Telencephalon

Diencephalon-

Midbrain

40 days

Forebrain Midbrain

40 days

Hindbrain

Cerebral hemisphere

40 days

Cerebral hemisphere

Hindbrain

Cerebral hemisphere

Cerebral hemisphere

Forebrain Structure And Function
46.2 Development of the Human Nervous System Three swellings at the anterior end of the hollow neural tube in the early vertebrate embryo develop into the parts of the adult brain.The final view is an adult human brain cut in half through the midline.

ological functions such as breathing and circulation or basic motor patterns such as swallowing and vomiting. All information traveling between the spinal cord and higher brain areas must pass through the pons and the medulla.

The cerebellum is like the conductor of an orchestra; it receives "copies" of the commands going to the muscles from higher brain areas, and it receives information coming up the spinal cord from the joints and muscles. Thus it can compare the motor "score" with the actual behavior of the muscles and refine the motor commands.

From the embryonic midbrain come structures that process aspects of visual and auditory information. In addition, all information traveling between higher brain areas and the spinal cord must pass through the midbrain. The structures that develop from the hindbrain and the midbrain are collectively known as the brain stem.

The embryonic forebrain develops a central region called the diencephalon and a surrounding structure called the te-lencephalon. The diencephalon is the core of the forebrain and consists of an upper structure called the thalamus and a lower structure called the hypothalamus. The thalamus is the final relay station for sensory information going to the telen-cephalon, and the hypothalamus is responsible for the regulation of many physiological functions and biological drives.

The telencephalon consists of two cerebral hemispheres, left and right (and is also referred to as the cerebrum). In humans, the telencephalon is by far the largest part of the brain and plays major roles in sensory perception, learning, memory, and conscious behavior.

As we go up the vertebrate phylogenetic scale from fishes to mammals, the telencephalon increases in size, complexity, and importance. The forebrain dominates the nervous systems of mammals, and damage to this region results in severe impairment of sensory, motor, or cognitive functions, and even coma. In contrast, a shark with its telencephalon removed can swim almost normally.

Peripheral Neuropathy Natural Treatment Options

Peripheral Neuropathy Natural Treatment Options

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