Medical brain notes 27

  The Nervous System M ost of us can probably remember being told,   when we were children, not to touch the stove or some other source of potential harm. Because children are curious, such warnings often go unheeded. The result? Touching a hot stove brings about an immedi-ate response of pulling away and a vivid memory of painful fingers. This simple and familiar experience illustrates the functions of the nervous system :   1.             To detect changes and feel sensations   2.             To initiate appropriate responses to changes   3.             To organize information for immediate use and store it for future use   The nervous system is one of the regulating systems (the endocrine system is the other). Electrochemical impulses of the nervous system make it possible to obtain infor-mat...

  NERVOUS SYSTEM DIVISIONS   The nervous system has two divisions. The  central   nervous system  ( CNS ) consists of the brain and   spinal cord. The  peripheral nervous system  ( PNS ) consists of cranial nerves and spinal nerves. The PNS includes the autonomic nervous system (ANS).   The peripheral nervous system relays information to and from the central nervous system, and the brain is the center of activity that integrates this informa-tion, initiates responses, and makes us the individuals we are.

  NERVE TISSUE   Nerve tissue was briefly described, so we will begin by reviewing what you already know and then add to it.   Nerve cells are called  neurons , or  nerve fibers . Whatever their specific functions, all neurons have the same physical parts. The  cell body  contains the nucleus (Fig. 8–1) and is essential for the continued life of the neuron. As you will see, neuron cell bodies are found in the central nervous system or close to it in the trunk of the body. In these locations, cell bodies are protected by bone. There are no cell bodies in the arms and legs, which are much more subject to injury.   Figure 8–1.  Neuron structure. ( A ) A typical sensory neu-ron. ( B ) A typical motor neuron. The arrows indicate the direc-tion of impulse transmission. ( C ) Details of the myelin sheath and neurolemma formed by Schwann cells.   QUESTION:  The axon terminal of the motor neuron would be   Cell body  found at ...

  TYPES OF NEURONS   Neurons may be classified into three groups: sensory neurons, motor neurons, and interneurons (Fig. 8–3).   Sensory neurons  (or  afferent neurons ) carry im-pulses from receptors to the central nervous system.  Receptors  detect external or internal changes and   send the information to the CNS in the form of impulses by way of the afferent neurons. The central nervous system interprets these impulses as a sense tion. Sensory neurons from receptors in skin, skeletal muscles, and joints are called somatic ; those from receptors in internal organs are called  visceral  sensory neurons. Motor neurons  (or  efferent neurons ) carry   impulses from the central nervous system to  effectors . The two types of effectors are muscles and glands. In response to impulses, muscles contract or relax and glands secrete. Motor neurons linked to skeletal mus-cle are called somatic; those to smooth muscle, cardiac...

  NERVES AND NERVE TRACTS   A  nerve  is a group of axons and/or dendrites of many neurons, with blood vessels and connective tissue.  Sensory nerves  are made only of sensory neurons.   The optic nerves for vision and olfactory nerves for smell are examples of nerves with a purely sensory function.  Motor nerves  are made only of motor neu-rons; autonomic nerves are motor nerves. A  mixed   nerve  contains both sensory and motor neurons. Most   of our peripheral nerves, such as the sciatic nerves in the legs, are mixed nerves.   The term  nerve tract  refers to groups of neurons within the central nervous system. All the neurons in a nerve tract are concerned with either sensory or motor activity. These tracts are often referred to as  white matter ; the myelin sheaths of the neurons give   them a white color.

  THE NERVE IMPULSE   The events of an electrical nerve impulse are the same as those of the electrical impulse generated in muscle fibers. Stated simply, a neuron not carrying an impulse is in a state of  polar-ization , with Na +   ions more abundant outside the   cell, and K +  ions and negative ions more abundant inside the cell. The neuron has a positive charge on the outside of the cell membrane and a relative nega-tive charge inside. A stimulus (such as a neurotrans- mitter) makes the membrane very permeable to Na +  ions, which rush into the cell. This brings about  depolarization , a reversal of charges on the mem-brane. The outside now has a negative charge, and the inside has a positive charge.   As soon as depolarization takes place, the neuron membrane becomes very permeable to K +  ions, which rush out of the cell. This restores the positive charge outside and the negative charge inside, and is called  repolarization ....

  THE SPINAL CORD   The  spinal cord  transmits impulses to and from the brain and is the integrating center for the spinal cord reflexes. Although this statement of functions is very brief and sounds very simple, the spinal cord is of great importance to the nervous system and to the body as a whole.   Enclosed within the vertebral canal and the meninges, the spinal cord is well protected from mechanical  injury. In length, the spinal cord extends from the foramen magnum of the occipital bone to the disc between the first and second lumbar vertebrae.   A cross-section of the spinal cord is shown in Fig. 8–3; refer to it as you read the following. The internal  gray matter  is shaped like the letter H; gray matter   consists of the cell bodies of motor neurons and interneurons. The external  white matter  is made of myelinated axons and dendrites of interneurons. These nerve fibers are grouped into nerve tracts based on their f...

  SPINAL NERVES   There are 31 pairs of  spinal nerves , those that emerge from the spinal cord. The nerves are named according to their respective vertebrae: 8 cervical pairs, 12 tho-racic pairs, 5 lumbar pairs, 5 sacral pairs, and 1 very small coccygeal pair. These are shown in Fig. 8–4; notice that each nerve is designated by a letter and a number. The 8th cervical nerve is C8, the 1st thoracic nerve is T1, and so on.   In general, the cervical nerves supply the back of the head, neck, shoulders, arms, and diaphragm (the phrenic nerves). The first thoracic nerve also con-tributes to nerves in the arms. The remaining thoracic nerves supply the trunk of the body. The lumbar and sacral nerves supply the hips, pelvic cavity, and legs. Notice that the lumbar and sacral nerves hang below the end of the spinal cord (in order to reach their proper openings to exit from the vertebral canal); this is called the  cauda equina , literally, the “horse’s tail.” Some of the...