Tuesday, July 17, 2007

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Chapter 12 Nervous Tissue

OVERVIEW OF THE NERVOUS SYSTEM

Structures of the Nervous System

With a mass of only 2 kg, about 3% of total body weight, the nervous system is one of the smallest andyet 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, ganglia, enteric plexuses, and sensory receptors.

The skull encloses the brain, which contains about 100 billion (1011) neurons. Twelve pairs (right and left) of cranial nerves, numbered I through XII, emerge from the base of the brain. A nerve is a bundle of hundreds to thousands of axons plus associated connective tissue and blood vessels that lies outside the brain and spinal cord. Each nerve follows a defined path and serves a specific region of the body.

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 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.

 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. Neurons called sensory or afferent neurons carry this sensory information into the brain and spinal cord through cranial and spinal nerves.
 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. Many of the neurons that participate in integration are interneurons, with axons that extend only for a short distance and contact nearby neurons in the brain or spinal cord. The vast majority of neurons in the body are interneurons.
 Motor function. Once sensory information is integrated, the nervous system may elicit an appropriate motor response such as muscle contraction or gland secretion. The neurons that serve this function are called motor or efferent neurons. Motor neurons carry information from the brain toward the spinal cord or out of the brain and spinal cord to effectors (muscles and glands) through cranial and spinal nerves. Stimulation of the effectors by motor neurons causes muscles to contract and glands to secrete.

Organization of the Nervous System

The two main subdivisions of the nervous system are the central nervous system (CNS), which consists of the brain and spinal cord, and the peripheral nervous system (PNS), which includes all nervous tissue outside the CNS. The CNS processes many different kinds of incoming sensory information. It is also the source of thoughts, emotions and memories. Most nerve impulses that stimulate muscles to contract and glands to secrete originate in the CNS. Components of the PNS include cranial nerves and their branches, spinal nerves and their branches, ganglia, and sensory receptors. The PNS may be subdivided further into a somatic nervous system (SNS), and autonomic nervous system (ANS) and an enteric nervous system (ENS).

The SNS consists of: 1) sensory neurons that convey information from somatic receptors in the head, body wall, and limbs and from receptors for the special senses of vision, hearing, taste and smell to the CNS, and 2) motor neurons that conduct impulses from the CNS to skeletal muscles only. Because these motor responses can be consciously controlled, the action of this part of the PNS is voluntary.

The ANS consists of: 1) sensory neurons that convey information from autonomic sensory receptors, located primary in visceral organs such as the stomach and the lungs, to the CNS, and 2) motor neurons that conduct nerve impulses from the CNS to smooth muscle, cardiac muscle, and glands. Because its motor responses are not normally under conscious control, the action of the ANS is involuntarily. The motor part of the ANS consists of two branches, the sympathetic division and the parasympathetic division. With a few exceptions, effectors receive nerves from both divisions, and usually the two divisions have opposing actions. For example, sympathetic neurons increase heart rate, and parasympathic neurons slow it down. In general, the sympathetic division helps support exercise or emergency actions, so-called "fight-or-flight" responses, and the parasympathetic division takes care of "rest-and-digest" activities.

The operation of the ENS, the "brain of the gut," is involuntary. Once considered part of the ANS, the ENS consists of approximately 100 million neurons in enteric plexuses that extend most of the length of the gastrointestinal (GI) tract. Many of the neurons of the enteric plexuses function independently of the ANS and CNS to some extent, although they also communicate with the CNS via sympathetic and parasympathetic neurons. Sensory neurons of the ENS monitor chemical changes within the GI tract as well as the stretching of its walls. Enteric motor neurons govern contraction of the GI tract smooth muscle to propel food through the GI tract, secretions of the GI tract organs such as acid from the stomach, and activity of GI tract endocrine cells, which secrete hormones.

HISTOLOGY OF NERVOUS TISSUE

Nervous tissue consists of two types of cells: neurons and neuroglia. Neurons provide most of the unique functions of the nervous sytem, such as sensing, thinking, remembering, controlling muscle activity, and regulating glandular secretions. Neuroglia support, nourish, and protect the neurons and maintain homeostasis in the interstitial fluid that bathes them.

Neurons

Like muscle cells, neurons (nerve cells) possess electrical excitability, the ability to respond to a stimulus and convert it into an action potential. A stimulus is any change in the environment that is strong enough to initiate an action potential. An action potential (nerve impulse) is an electrical signal that propagates (travels) along the surface of the membrane of a neuron. It begins and travels due to the movement of ions (such as sodium and potassium) between interstitial fluid and the inside of a neuron through specific ion channels in its plasma membrane. Once begun, a nerve impulse travels rapidly and at a constant strength.

Some neurons are tiny and propagate impulses over a short distance (less than 1 mm) within the CNS. Others are the longest cells in the body. The motor neurons that enable you to wiggle your toes, for example, extend from the lumbar region of your spinal cord (just above waist level) to the muscles in your foot. Some sensory neurons are even longer. Those that allow you to feel the position of your wiggling toes stretch all the way from your foot to the lower portion of your brain. Nerve impulses travel these great distances at speeds ranging from 0.5 to 1.30 metres per second.

Parts of a Neuron

Most neurons have three parts: 1) a cell body, 2) dendrites, and 3) an exon. The cell body (perikaryon) contains a nucleus surrounded by cytoplasm that includes typical cellular organelles such as lysosomes, mitochondria, and a Golgi complex. Neuronal cells bodies also contain free ribosomes and prominent clusters of rough endoplasmic reticulum, termed Nissl bodies. The ribosomes are the sites of protein synthesis. Newly synthesized proteins produced by Nissl bodies are used to replace cellular components as material for growth of neurons and to regenerate damaged axons in the PNS. The cytoskeleton includes both neurofibrils, composed of bundles of intermediate filaments that provide the cell shape and support, and microtubules, which assist in moving materials between the cell body and axon. Many neurons also contain lipofuscin, a pigment that occurs as clumps of yellowish brown granules in the cytoplasm. Lypofuscin is a product of neuronal lysosomes that accumulates as the neuron ages, but does not seem to harm the neuron.

A nerve fibre

Chapter 12 Nervous Tissue

OVERVIEW OF THE NERVOUS SYSTEM

Structures of the Nervous System

With a mass of only 2 kg, about 3% of total body weight, the nervous system is one of the smallest andyet 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, ganglia, enteric plexuses, and sensory receptors.

The skull encloses the brain, which contains about 100 billion (1011) neurons. Twelve pairs (right and left) of cranial nerves, numbered I through XII, emerge from the base of the brain. A nerve is a bundle of hundreds to thousands of axons plus associated connective tissue and blood vessels that lies outside the brain and spinal cord. Each nerve follows a defined path and serves a specific region of the body.

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 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.

 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. Neurons called sensory or afferent neurons carry this sensory information into the brain and spinal cord through cranial and spinal nerves.
 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. Many of the neurons that participate in integration are interneurons, with axons that extend only for a short distance and contact nearby neurons in the brain or spinal cord. The vast majority of neurons in the body are interneurons.
 Motor function. Once sensory information is integrated, the nervous system may elicit an appropriate motor response such as muscle contraction or gland secretion. The neurons that serve this function are called motor or efferent neurons. Motor neurons carry information from the brain toward the spinal cord or out of the brain and spinal cord to effectors (muscles and glands) through cranial and spinal nerves. Stimulation of the effectors by motor neurons causes muscles to contract and glands to secrete.

Organization of the Nervous System

The two main subdivisions of the nervous system are the central nervous system (CNS), which consists of the brain and spinal cord, and the peripheral nervous system (PNS), which includes all nervous tissue outside the CNS. The CNS processes many different kinds of incoming sensory information. It is also the source of thoughts, emotions and memories. Most nerve impulses that stimulate muscles to contract and glands to secrete originate in the CNS. Components of the PNS include cranial nerves and their branches, spinal nerves and their branches, ganglia, and sensory receptors. The PNS may be subdivided further into a somatic nervous system (SNS), and autonomic nervous system (ANS) and an enteric nervous system (ENS).

The SNS consists of: 1) sensory neurons that convey information from somatic receptors in the head, body wall, and limbs and from receptors for the special senses of vision, hearing, taste and smell to the CNS, and 2) motor neurons that conduct impulses from the CNS to skeletal muscles only. Because these motor responses can be consciously controlled, the action of this part of the PNS is voluntary.

The ANS consists of: 1) sensory neurons that convey information from autonomic sensory receptors, located primary in visceral organs such as the stomach and the lungs, to the CNS, and 2) motor neurons that conduct nerve impulses from the CNS to smooth muscle, cardiac muscle, and glands. Because its motor responses are not normally under conscious control, the action of the ANS is involuntarily. The motor part of the ANS consists of two branches, the sympathetic division and the parasympathetic division. With a few exceptions, effectors receive nerves from both divisions, and usually the two divisions have opposing actions. For example, sympathetic neurons increase heart rate, and parasympathic neurons slow it down. In general, the sympathetic division helps support exercise or emergency actions, so-called "fight-or-flight" responses, and the parasympathetic division takes care of "rest-and-digest" activities.

The operation of the ENS, the "brain of the gut," is involuntary. Once considered part of the ANS, the ENS consists of approximately 100 million neurons in enteric plexuses that extend most of the length of the gastrointestinal (GI) tract. Many of the neurons of the enteric plexuses function independently of the ANS and CNS to some extent, although they also communicate with the CNS via sympathetic and parasympathetic neurons. Sensory neurons of the ENS monitor chemical changes within the GI tract as well as the stretching of its walls. Enteric motor neurons govern contraction of the GI tract smooth muscle to propel food through the GI tract, secretions of the GI tract organs such as acid from the stomach, and activity of GI tract endocrine cells, which secrete hormones.

HISTOLOGY OF NERVOUS TISSUE

Nervous tissue consists of two types of cells: neurons and neuroglia. Neurons provide most of the unique functions of the nervous sytem, such as sensing, thinking, remembering, controlling muscle activity, and regulating glandular secretions. Neuroglia support, nourish, and protect the neurons and maintain homeostasis in the interstitial fluid that bathes them.

Neurons

Like muscle cells, neurons (nerve cells) possess electrical excitability, the ability to respond to a stimulus and convert it into an action potential. A stimulus is any change in the environment that is strong enough to initiate an action potential. An action potential (nerve impulse) is an electrical signal that propagates (travels) along the surface of the membrane of a neuron. It begins and travels due to the movement of ions (such as sodium and potassium) between interstitial fluid and the inside of a neuron through specific ion channels in its plasma membrane. Once begun, a nerve impulse travels rapidly and at a constant strength.

Some neurons are tiny and propagate impulses over a short distance (less than 1 mm) within the CNS. Others are the longest cells in the body. The motor neurons that enable you to wiggle your toes, for example, extend from the lumbar region of your spinal cord (just above waist level) to the muscles in your foot. Some sensory neurons are even longer. Those that allow you to feel the position of your wiggling toes stretch all the way from your foot to the lower portion of your brain. Nerve impulses travel these great distances at speeds ranging from 0.5 to 1.30 metres per second.

Parts of a Neuron

Most neurons have three parts: 1) a cell body, 2) dendrites, and 3) an exon. The cell body (perikaryon) contains a nucleus surrounded by cytoplasm that includes typical cellular organelles such as lysosomes, mitochondria, and a Golgi complex. Neuronal cells bodies also contain free ribosomes and prominent clusters of rough endoplasmic reticulum, termed Nissl bodies. The ribosomes are the sites of protein synthesis. Newly synthesized proteins produced by Nissl bodies are used to replace cellular components as material for growth of neurons and to regenerate damaged axons in the PNS. The cytoskeleton includes both neurofibrils, composed of bundles of intermediate filaments that provide the cell shape and support, and microtubules, which assist in moving materials between the cell body and axon. Many neurons also contain lipofuscin, a pigment that occurs as clumps of yellowish brown granules in the cytoplasm. Lypofuscin is a product of neuronal lysosomes that accumulates as the neuron ages, but does not seem to harm the neuron.

A nerve fibre

Monday, July 16, 2007

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