The Nervous System (Part 1)
March 25th 2008 13:13
THE NERVOUS SYSTEM
The human nervous system consists of around a hundred billion specialized cells (Kahl, 2003), known as neurons. The general function of these neurons is to receive sensory stimuli and to transmit them to effector organs, muscular or glandular.
However aside from the neurons the nervous system also includes cells whose main functions are to support the neurons and keep their fibers intact and well-organized. These cells are collectively known as the neuroglia (Snell, 1997).
Though these two groups of specialized cells differ from each other, they work hand in hand to compliment the endocrine functions in carrying out the bodyÂ’s responses to the ever-changing environment (www.biologymad.com, 2004).
The Two Major Groups of Cells Comprising the Nervous System
Two differentiated cell types comprise the neural tissues. They are the nerve cells (neurons) and the neuroglial (glia) cells (www.sci.uidaho.edu, 2007). The neurons (Figure 1) can be considered as the primary role players, since they are the cells which generally handle the reception and interpretation of sensory stimuli, as well as the organization of the bodyÂ’s responses to such stimuli. But neurons cannot function as they have been destined to function without the enhancements provided by the neuroglia. And so the neuroglial cells are also important in a sense that they provide structural integrity to the nervous system and functional support that enables neurons to perform.
Neurons
A typical neuron varies considerably in size and shape, but each possesses a soma (cell body) from whose surface project one or more processes known as neurites (nerve fibers). Those neurites responsible for receiving information and conducting it towards the cell body are called dendrites. Those neurites that conduct impulses away from the cell body are called axons (Snell, 1997).
Figure 1. A typical neuron
The soma of neurons appears to be gray in complexion and the neurites usually appear to be white because of the myelin sheath covering these fibers.
We can keep the above facts in mind if we are going to classify the neurons according to their structural characteristics. And so if a neuron contains only one projection, that neuron can be classified as a unipolar neuron. But if two fibers project from the soma, that neuron can be considered as a bipolar neuron (Figure 2). In addition, there are instances that a neuron contains more than two neurites. This neuron is a multipolar type of neuron (Figure 3).
Figure 2. Upper portion shows an example of a bipolar neuron, while lower portion shows an example of a unipolar neuron (formerly known as pseudounipolar).
Figure 3. Multipolar Neuron
In the illustrations below, we can observe an example of how the nervous system works. And from these pictures, we can infer that neurons can also be grouped according to their functions (Figure 4).
Figure 4. The knee jerk
The knee jerk is a reflex elicited once the patellar tendon is tapped (by a neurohammer). The force from the neurohammer is being detected by the sensory receptors in the knee region especially in the patellar tendon. The sensory receptors then relay the signal to the sensory nerves which in turn pass the information (that the tendon is being tapped) to the spinal cord. Within the spinal cord there are neurons that connect the sensory neurons to the motor neurons. These are the interneurons. These interneurons integrate the sensory information to the central nervous system and then to the motor neurons, to produce a meaningful response. And after such integration, the central nervous system relays the ideal response to the motor neurons: that is to move the leg towards extension. The motor neurons then pass the information about the appropriate response to the effectors (in this case the quadriceps femoris muscle). The quadriceps, being the effector, thus executes the response and contracts to move the knee towards extension.
In the above scenario, we can observe three groups of neurons: the sensory or afferent neurons, the interneurons, and the motor or efferent neurons.
The afferent or sensory neurons have long axons and transmit nerve impulses from sensory receptors all over the body to the central nervous system. The peripheral sensory neurons form ganglia at the dorsl aspect of the spinal cord and enter the central nervous system via aspect of the spinal cord. Interneurons are also called association neurons exclusively within the spinal cord and the brain. The efferent or motor neurons transmit impulses from the central nervous system to the muscles and glands that carry out the response. They exit the central nervous system via the anterior gray horn cells of the spinal cord.
On the other hand, the nerve fibers can also be classified physiologically. Generally nerve fibers can be classified either as type A or C (Guyton and Hall, 2000). Type A fibers are myelinated fibers, capable of fast conduction of signals and impulses. Type C are unmyelinated fibers, that conduct impulses at low velocities. Type A fibers usually relay sensory impulses for stretch, pressure, vibration and discrimination, as well as motor signals to move the muscles of the body. Type C fibers are capable of relaying painful stimuli from the body to the central nervous system, and can also send reflex responses to effectors.
Neuroglia
There are four groups of neuroglial cells (Snell, 1997). These are the astrocytes, myelin sheath- producing cells, microglia, and the ependymal cells.
Astrocytes
There are two types of astrocytes: fibrous and protoplasmic.
Fibrous astrocytes (Figure 5) are found mainly in the white matter, where their processes pass between the nerve fibers. Each process is long, slender, smooth, and not much branched. The cell bodies and processes contain many filaments in their cytoplasm.
Figure 5. Fibrous Astrocytes
On the other hand protoplasmic astrocytes (Figure 6) are found mainly in gray matter, where processes pass between the nerve cell bodies. The processes are shorter, thicker, and more branched. Their cytoplasms contain fewer filaments.
Figure 6. Protoplasmic Astrocytes
The astrocytes with their branching processes form a supporting framework for the nerve cells and nerve fibers. In the embryo, they serve as scaffolding for the migration of immature neurons. Astrocytes can also be neurotransmitter regulators, limiting the effect of GABA and glutamic acid secreted by nerve terminals. They also serve as storage for glycogen which can be broken down to become energy source for neurons in response to norepinephrine release. They can also become phagocytes cleaning up debris and wastes surrounding neurons.
Figure 7. Nerve Fiber wrapped with myelin sheath
Myelin Sheath-Producing Cells
Myelin sheath is a structure that speeds up the transmission of signals along the nerve fibers (Figure 7). The increased velocity of the transmission is due to the saltatory conduction, when the action potential only occurs from one node of Ranvier to another.
In the central nervous system, the cells that produce the myelin sheath is known as the oligodendrocytes (Figure 8). And in the peripheral nervous system, they are known as Schwann cells (Figure 9)
.Figure 8. Oligodendrocytes
Figure 9. Scwann cells
Microglia
In the normal brain and spinal cord microglia appear to be inactive. However in inflammatory and degenerative lesions of the central nervous system, they proliferate to the sie of lesions and become active phagocytes. Joined by the monocytes of the blood, they work to remove dead cells and debris on sites of lesion (Figure 10)
Figure 10. Microglia
Ependymal Cells
These cells ensure that the cerebrospinal fluid are constantly circulating around the central nervous system in order to facilitate the transport of nutrients and essential substances to the system. Ependymal cells (Figure 11) come in three forms: ependymocytes, tanycytes, and choroidal epithelial cells.
Figure 11. Ependymal cells
The Structural and Functional Organization of the Nervous System
Based on anatomical features the nervous system can be divided into two parts: the central and the peripheral nervous system. While the brain and the spinal cord constitute the central nervous system, the so-called cranial nerves and the spinal nerves form parts of the peripheral. The peripheral nerves connect the central nervous system with the sense organs, i.e. the organs for vision, hearing, smell, taste, perceptional touch, and other effector organs like muscles and glands.
Another way of dividing the nervous system is to look at it from a functional point of view: autonomic and somatic.
The autonomic nervous system is in connection with glands, internal organs, and involuntary muscles. The functions of the autonomic nervous system stand above our will and awareness, in the sense that the autonomic nervous system automatically takes care of the body.
In contrast, the somatic nervous system handles the skeletal muscles of the body, in other words functions of which are governed by our will (Kahl, 2003).
Below are charts that feature the subdivisions of the nervous system.
ANATOMICAL ORGANIZATION
PHYSIOLOGICAL ORGANIZATION
TO BE CONTINUED...
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