The Nervous System
There are some organs and tissues that we can live without. We could lose one or both kidneys and still survive with the help of a machine. We could
even live with a mechanical heart. But the brain is essential! No one can live without a brain. The brain controls almost everything that happens in your body. Because it is so crucial to our survival, it is not surprising, then, that the brain is protected inside a very hard skull. However, this protection does not completely guarantee its safety; the brain is a very fragile organ. The brain is just part of the nervous system: that delicate network of nerves that carries messages around the body, allowing us to interact successfully and safely with our environment.
There are some organs and tissues that we can live without. We could lose one or both kidneys and still survive with the help of a machine. We could
even live with a mechanical heart. But the brain is essential! No one can live without a brain. The brain controls almost everything that happens in your body. Because it is so crucial to our survival, it is not surprising, then, that the brain is protected inside a very hard skull. However, this protection does not completely guarantee its safety; the brain is a very fragile organ. The brain is just part of the nervous system: that delicate network of nerves that carries messages around the body, allowing us to interact successfully and safely with our environment.
Structural Features
The core of the nervous system the central nervous system consists of the brain and spinal cord. The nerves that carry signals between the central nervous system and the body make up the peripheral nervous system. The peripheral nervous system relays information about the internal and external environments to the brain. It also relays instructions from the brain to other parts of the body to control many of the body’s functions and responses. The peripheral nervous system can be further divided into three groups of nerves:
• nerves that control the voluntary muscles
• nerves that carry information from
the sensory organs, such as the eyes,
ears, taste buds, and touch receptors,
to the brain
• nerves that regulate involuntary
functions such as breathing, heartbeat,
and digestion.
To protect it from physical damage, the central nervous system is shielded by bones. The skull protects the brain, and the spine guards the spinal cord (Figure 2). The brain and the spinal cord are surrounded by cerebrospinal fluid. This fluid helps cushion the brain and spinal cord from injury, transports chemicals, and removes wastes that are produced in the brain.
The core of the nervous system the central nervous system consists of the brain and spinal cord. The nerves that carry signals between the central nervous system and the body make up the peripheral nervous system. The peripheral nervous system relays information about the internal and external environments to the brain. It also relays instructions from the brain to other parts of the body to control many of the body’s functions and responses. The peripheral nervous system can be further divided into three groups of nerves:
• nerves that control the voluntary muscles
• nerves that carry information from
the sensory organs, such as the eyes,
ears, taste buds, and touch receptors,
to the brain
• nerves that regulate involuntary
functions such as breathing, heartbeat,
and digestion.
To protect it from physical damage, the central nervous system is shielded by bones. The skull protects the brain, and the spine guards the spinal cord (Figure 2). The brain and the spinal cord are surrounded by cerebrospinal fluid. This fluid helps cushion the brain and spinal cord from injury, transports chemicals, and removes wastes that are produced in the brain.
Nerve Tissue
Nerve tissue is made up of special cells called neurons. Nerve tissue is found in the brain, spinal cord, and nerves. There are an estimated 100 billion neurons in the human brain. Neurons are communication specialists. Their structure enables them to send information around your body. They do this by conducting electrical signals—nerve impulses—from one area of the body to another (Figure 3). The axons of some neurons are covered by a fatty material called myelin. The myelin sheath acts like the insulation on an electrical wire, preventing electrical impulses from passing to the wrong neuron.
Nerve tissue is made up of special cells called neurons. Nerve tissue is found in the brain, spinal cord, and nerves. There are an estimated 100 billion neurons in the human brain. Neurons are communication specialists. Their structure enables them to send information around your body. They do this by conducting electrical signals—nerve impulses—from one area of the body to another (Figure 3). The axons of some neurons are covered by a fatty material called myelin. The myelin sheath acts like the insulation on an electrical wire, preventing electrical impulses from passing to the wrong neuron.
Nerves are bundles of neurons that are surrounded by connective tissue (Figure 4). Nerves allow a two-way flow of information even though each neuron transmits information only in one direction.Many cells in the body undergo cell division to repair an injury. Injured neurons in the central nervous system, however, do not easily regenerate. Some neurons in the peripheral nervous system can regrow to repair a small gap (a few millimetres) between the ends of severed nerves.
Sensory Receptors
Sensory receptors are special cells or tissues that receive input from our external environment and send signals along the peripheral nerves to our central nervous system. Our eyes have receptors that are sensitive to light. The ears, mouth, nose, muscles, and skin have other sensory receptors.
Sensory receptors are special cells or tissues that receive input from our external environment and send signals along the peripheral nerves to our central nervous system. Our eyes have receptors that are sensitive to light. The ears, mouth, nose, muscles, and skin have other sensory receptors.
In addition to the familiar senses (sight, hearing, taste, smell, and touch), we have receptors in our muscles and skin that are sensitive to pressure, temperature, and pain and receptors that make us aware of our balance, position, and motion. All of the sensory receptors around the body send information to the brain, where it is processed. Information from different sensory receptors goes to specific parts of the brain (Figure 6).
Communication, Coordination, and Perception
The overall function of the nervous system is to transmit signals in both directions between your brain and the rest of your body. This allows your body to respond both to the outside world and to the internal environment. For example, the nervous system tells the respiratory system when to increase the breathing rate and tells the circulatory system when the heart should beat faster. The nervous system lets us know when we should eat or drink and when we should stop. Another function of the brain is perception: interpreting or making sense of all the information we receive from our environment. The spinal cord has another important function: it acts as a short cut for reflexes. Reflexes are actions that do not require the involvement of the brain; they occur without conscious thought. Quickly moving your hand away from a hot surface is an example of a reflex action.
Communication, Coordination, and Perception
The overall function of the nervous system is to transmit signals in both directions between your brain and the rest of your body. This allows your body to respond both to the outside world and to the internal environment. For example, the nervous system tells the respiratory system when to increase the breathing rate and tells the circulatory system when the heart should beat faster. The nervous system lets us know when we should eat or drink and when we should stop. Another function of the brain is perception: interpreting or making sense of all the information we receive from our environment. The spinal cord has another important function: it acts as a short cut for reflexes. Reflexes are actions that do not require the involvement of the brain; they occur without conscious thought. Quickly moving your hand away from a hot surface is an example of a reflex action.
Diseases and Disorders of the Nervous System
Problems with the nervous system can be very serious. The brain can be permanently damaged by viruses or bacteria. Diseases can also be caused by
problems in other body systems. For example, multiple sclerosis is caused by a malfunction of the immune system. This disease destroys the myelin
sheaths of neurons in the central nervous system. Symptoms, which worsen as the disease progresses, include muscle weakness, slurred speech, and
difficulty walking.
Physical trauma, such as a fall or a blow, can cause severe damage to the spinal cord, often resulting in paralysis. One of the most serious—and possibly avoidable—sports injuries is damage to the brain. We hear about hockey players suffering from head traumas, ranging from concussions to death, when their heads hit the ice. Helmets can reduce the effect of head trauma. If someone is hit on the head and a doctor suspects brain injury, the person is likely to be sent for a CT or MRI scan. The CT scan in Figure 8 shows an area where blood has collected between the brain and its protective membrane, causing pressure on the brain tissue.