Introduction to nervous system
⦁ With a mass of only 2 kg (4.5 lb), about 3% of total body weight, the nervous system is one of the smallest and yet the most complex of the 11 body systems.
⦁ The nervous system is an intricate, highly organized network of billions of neurons and even more neuroglia.
⦁ The structures that make up the nervous system include the brain, cranial nerves and their branches, the spinal cord, spinal nerves and their branches, ganglia, enteric plexuses, and sensory receptors.
⦁ The skull encloses the brain, which contains about 100 billion neurons. Twelve pairs (right and left) of cranial nerves emerge from the base of the brain.
⦁ The spinal cord connects to the brain through the foramen magnum of the skull and is encircled by the bones of the vertebral column. It contains about 100 million neurons. Thirty-one pairs of spinal nerves emerge from the spinal cord, each serving a specific region on the right or left side of the body.
⦁ Ganglia (singular is ganglion) are small masses of nervous tissue, consisting primarily of neuron cell bodies, that are located outside the brain and spinal cord. Ganglia are closely associated with cranial and spinal nerves.
⦁ In the walls of organs of the gastrointestinal tract, extensive networks of neurons, called enteric plexuses, help regulate the digestive system.
⦁ The term sensory receptor is used to refer to the dendrites of sensory neurons as well as separate, specialized cells that monitor changes in the internal or external environment, such as photoreceptors in the retina of the eye.
⦁ With a mass of only 2 kg (4.5 lb), about 3% of total body weight, the nervous system is one of the smallest and yet the most complex of the 11 body systems.
⦁ The nervous system is an intricate, highly organized network of billions of neurons and even more neuroglia.
⦁ The structures that make up the nervous system include the brain, cranial nerves and their branches, the spinal cord, spinal nerves and their branches, ganglia, enteric plexuses, and sensory receptors.
⦁ The skull encloses the brain, which contains about 100 billion neurons. Twelve pairs (right and left) of cranial nerves emerge from the base of the brain.
⦁ The spinal cord connects to the brain through the foramen magnum of the skull and is encircled by the bones of the vertebral column. It contains about 100 million neurons. Thirty-one pairs of spinal nerves emerge from the spinal cord, each serving a specific region on the right or left side of the body.
⦁ Ganglia (singular is ganglion) are small masses of nervous tissue, consisting primarily of neuron cell bodies, that are located outside the brain and spinal cord. Ganglia are closely associated with cranial and spinal nerves.
⦁ In the walls of organs of the gastrointestinal tract, extensive networks of neurons, called enteric plexuses, help regulate the digestive system.
⦁ The term sensory receptor is used to refer to the dendrites of sensory neurons as well as separate, specialized cells that monitor changes in the internal or external environment, such as photoreceptors in the retina of the eye.
Functions of the Nervous System
The nervous system carries out a complex array of tasks. It allows us to sense various smells, produce speech, and remember past events; in addition, it provides signals that control body movements, and regulates the operation of internal organs. These diverse activities can be grouped into three basic functions: sensory, integrative, and motor.
1. Sensory function. Sensory receptors detect internal stimuli, such as an increase in blood acidity, and external stimuli, such as a raindrop landing on your arm. This sensory information is then carried into the brain and spinal cord through cranial and spinal nerves.
2. Integrative function. The nervous system integrates (processes) sensory information by analyzing and storing some of it and by making decisions for appropriate responses. An important integrative function is perception, the conscious awareness of sensory stimuli. Perception occurs in the brain.
3. Motor function. Once sensory information is integrated, the nervous system may elicit an appropriate motor response by activating effectors (muscles and glands) through cranial and spinal nerves. Stimulation of the effectors causes muscles to contract and glands to secrete.
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