Medical brain notes 41

  Organization of the Nervous System The nervous system includes all neural tissue present in the body and accounts for just 3% of total body weight. The functional unit of the nervous system is the  neuron.  The neurons are supported and pro-tected by specialized tissue known as  neuroglia.  The organs of the nervous system are formed by the neu-rons, neuroglia, connective tissue, and blood vessels. The nervous system consists of the  brain  and  spinal cord,  enclosed in the skull and vertebrae, re-spectively. The  sensors  sense the changes in the in- ternal and external environment, and the  nerves  con-nect the sensors to the brain and spinal cord and take commands to tissue from the spinal cord to produce a response. Classically, the brain and spinal cord are known as the  central nervous system  (CNS); the rest of the nervous system is the peripheral nervous system (PNS). The CNS (see Figure 5.1) helps int...

  STRUCTURE OF THE NEURON The structure of the neuron (see Figure 5.2), the func-tional unit of the nervous system, varies from site to site. Typically, a neuron has a  cell body / soma,  or  perikaryon,  with a nucleus and cytoplasm, alongwith the organelles normally found in a cell. The prominent rough endoplasmic reticulum is known as  Nissl bodies.  However, most neurons do not have acentriole and lose the ability to multiply. Nuclei  are clusters of cell bodies of neurons in the CNS (excep-tion, basal ganglia). These clusters in the PNS are known as  ganglia. Many processes lead off from the soma. The  axon  is long and helps conduct impulses  away  from the cell body. The axon may have many branches, known as  collaterals.  The collaterals help the cell commu-nicate with more than one neuron. The  dendrites  are highly branched processes from the cell body that take impulses  to  the som...

  THE SYNAPSE A  synapse  (see Figure 5.3) is the region where neurons communicate with each other. The axon of the neuron, which brings impulses to the synapse, is the  presy-naptic neuron.  The neuron, which receives the im pulse, is the  postsynaptic neuron.   The end of the axon of the presynaptic neuron is enlarged into a bulb, the  synaptic knob,  or  terminal.  The synaptic knob usually has neurotransmitters packaged in small structures called  synaptic vesicles . When the presynaptic neuron is stimulated, it re-leases neurotransmitters into the gap between the two neurons. These then become attached to receptors on the cell membrane of the postsynaptic neuron, pro-ducing electrical changes. A synapse may be at a dendrite ( axodendritic),  on the soma ( axosomatic ), or along the length of the axon ( axoaxonic).  Rarely, a synapse may exist be-tween two dendrites ( dendrodendritic ). The neuro-transmitters are ...

  CLASSIFICATION OF NEURONS Neurons are classified in many ways, according to their anatomic structure and function. Anatomic Classification Bipolar Neurons These neurons have two processes extending from ei-ther end of the cell body, the dendrite and the axon (see Figure 5.4). This type of neuron is rare and is found in the retina of the eye. Unipolar Neurons The cell body in this type lies to one side, with a sin-gle process leading off from one side of the body. This process divides at once into two processes: the axon and the dendrite. Sensory neurons are of this type. Multipolar Neurons This is the most common neuron, with the cell body having several dendrites and one axon. All the neu-rons motor to the skeletal muscles are of this type. Functional Classification The neurons may be also classified according to func-tion. These are the  sensory, motor,  and  interneu-rons,  or  association neurons.  Those that take im-pulses  to  the CNS...

  NEUROGLIA Neuroglias are the supporting cells. They are five times more abundant than neurons. There are four types of glial cells in the CNS: the  ependymal cells, astro-cytes, microglia,  and  oligodendrocytes  and twotypes in the PNS:  Schwann cells,  or  neurolemmo-cytes, and  satellite cells,  or  ganglionic gliocytes. The ependymal cells line the cavities in the brain and spinal cord and are responsible for producing, circulating, and monitoring the cerebrospinal fluid—the fluid inside and around the CNS that cushions and protects the brain. The astrocytes, as the name suggests, are star-shaped. They are present between the blood capillaries and the brain and spinal cord, monitoring the sub-stances that enter and leave the brain and preventing sudden changes in the environment around the CNS. This is the  blood-brain barrier,  and the astrocytes are responsible for its creation. The astrocytes also help at the time...

  PRODUCTION AND PROPAGATION OF IMPULSES Impulse formation is a complex process and is related to the properties of the cell membrane (review the section on cell membrane, if necessary). The neurons communicate with each other by chang-ing the electrical potential inside the cell. This is achieved by movement of ions in and out of the cell and is determined by the permeability of the nerve cell membrane. The changes in the neuron (at rest and when stimulated) have been studied using minute electrodes that penetrate inside the neuron. Resting Membrane Potential If two electrodes are placed on the surface of the cell membrane of a neuron and connected to a measuring device (see Figure 5.6), no electrical changes are de-tected. However, if one of the electrodes is pushed into the cell and the other placed on the surface, the recording device will show that the inside of the cell is negative to that of the outside. This is known as the  resting membrane potential,  or  t...

  DIFFERENCES IN PROPAGATION OFACTION POTENTIAL IN MYELINATEDAND UNMYELINATED AXONS The action potential in an unmyelinated neuron trav-els slowly along the axon because every region of the axon has sodium and potassium channels. In a myelinated cell, the myelin sheath serves as insulators, preventing movement of ions through the membrane. Ions move only through the numerous channels lo-cated in the nodes and the action potential is propa- gated from one node of Ranvier to another, literally jumping from node to node across the myelin. Hence, propagation is rapid. This is known as  saltatory con-duction.  It should be noted that jumping is only ametaphor. Actually, the action potential in one node depolarizes the membrane at the next node to thresh-old and a new action potential is produced there. Ac-tion potential is also faster in thicker axons. The rate of conduction ranges from 1.0 m/sec in thin, un-myelinated fibers to 100 m/sec (225 miles per hour) in thick, myelina...

  SYNAPTIC TRANSMISSION For the neurotransmitters to have an effect on the postsynaptic neuron, sufficient amounts of neuro-transmitters must be released. The number of synap-tic vesicles that fuse with the cell membrane of the axon terminal, to be released into the synaptic cleft by exocytosis, depends on the frequency of action poten-tials. With greater frequency, more vesicles release the neurotransmitters contained within them. The neuro-transmitters become attached to receptors on the post-synaptic membrane that open chemical-gated sodium  channels (see Figure 5.9A). If sufficient channels open, they depolarize the neuron to reach threshold poten-tial and produce an action potential. This is an exam-ple of a  stimulatory neurotransmitter.  The potential changes that occur at the nerve junction are known as  excitatory postsynaptic potential  (EPSP). Certain neurotransmitters become attached to re-ceptors that open chemical-gated  potassium  c...

  EXAMPLES OF NEUROTRANSMITTERS There are many neurotransmitters in the nervous system. Some examples of common neurotransmitters are norepinephrine, dopamine, serotonin, -aminobutyric acid (GABA), glutamate, glycine, enkephalins, endorphins, substance P, nitric oxide (yes, a gas!), among many others. Some neurotransmitters are predomi-nant in certain areas of the nervous system. If production of these neurotransmitters is affected, the func-tioning of this region of the nervous system is affected. Many drugs affect the nervous system at the synapse level. For example, symptoms of strychnine poisoning (spasm of skeletal muscles) is a result of the blocking of glycine receptors. Glycine is the neurotransmitter in neurons that inhibits motor neurons to muscle. If these neurons don’t function, the motor neurons fire continuously, causing muscles to spasm. Similarly, cocaine causes euphoria by blocking dopamine removal from certain areas of the brain. Increased levels of dopamine in th...

  ELECTRICAL SYNAPSES Most synapses are chemical synapses; however, more recently, electrical synapses (Figure 5.9B) have been  discovered in the brain.   Such synapses also exist be-tween smooth muscle cells, between cardiac cells, and between glial cells. In the region of such a synapse, there are gap junctions  present between adjacent cells. These junctions allow ions to move in both directions and are a route of communication of impulses from one cell to another. As a result of the presence of gap junctions, transmission of impulses is faster than in chemical synapses.

  SUMMATION As already mentioned, each neuron can have many synapses. Therefore, the potential changes that occur in it depend on the net effect of all synapses. Certain synapses may produce inhibitory effects and others may produce stimulatory effects. What happens in the postsynaptic neuron depends on which effect is predominant. For example, if action potentials arrive rapidly in a synapse that has a stimulatory effect, the potential in the postsynaptic neuron may reach threshold quickly and produce an action potential. At the same time, if action potentials arrive in a synapse that produces an inhibitory effect, the postsynaptic membrane will be-come hyperpolarized, making it difficult for action potential to be generated. This mechanism of inte-grating the effects of two or more neurons by the postsynaptic neuron is known as  summation.

  FACTORS THAT AFFECT NEURAL FUNCTION Other factors that affect neuronal functioning are the changes in the extracellular environment and the metabolic demands of the neuron. Neurons are very sensitive to pH. If the pH becomes too high (more al-kaline), they start discharging action potentials spon-taneously. If the pH becomes too low, the opposite happens and the nervous system shuts down. As can be expected, fluctuating levels of ions, espe-cially sodium, potassium, and calcium, have a marked effect on impulse production. Similarly, an increase in body temperature makes neurons more excitable. Neurons require energy for manufacturing neuro-transmitters and for maintaining ionic composition. They can be easily injured if metabolic demands are not met.

  FUNCTIONAL ORGANIZATION OF NEURONS The body has about 10 million sensory neurons, 20 billion interneurons, and one-half million motor neurons. These neurons are arranged in so many waysthat impulses generated can converge on one neuron or diverge to many neurons or even have a feedback on the neuron that originally generated the impulse. All these possibilities help the body better coordinate its activities. For some different ways that impulses can be modified, see Figure 5.10. A  diverging arrangement  allows for a wide distri-bution of a specific input. For example, sensory input is distributed to other neurons in the spinal cord and the brain.  Parallel processing  allows the informa-tion to be processed by different neurons at the same time and produce a response in different regions of the body. For example, if you encounter a grizzly bear face-to-face on a hike, I do not know what you would do, but I would scream and run at the same time (but, please do...

  STANDARD TERMS AND GROUPING Anatomically, the neurons are arranged in a system-atic and logical manner in the brain and spinal cord, with neurons having the same or similar functions invariably grouped together. Many standard terms describe these areas and groupings: Ganglia. A collection of cell bodies of neurons(e.g., Preganglionic nerves of the sympathetic and parasympathetic nerves synapse with postganglionic neurons in a region located outside the spinal cord and brain). The collection of cell bodies of the post-ganglionic neurons of one region is known as gan-glia. Another example is the  dorsal root ganglion,  a collection of the cell bodies of the unipolar sensory neurons that lies just outside the spinal cord. Centers,  located in the CNS, are collections of cellbodies of neurons having the same function. For ex-ample, the vasomotor center in the brain has cell bodies of neurons involved in regulating the activities of the smooth muscles in the walls of bl...

  REGENERATION AND DEGENERATIONOF NEURONS Effect of Pressure on Neurons Neurons generally have a limited capacity to regenerate. For most neurons, cell division stops at birth. Although the whole neuron cannot be replaced if dam-aged, it is possible for the dendrites and axons to regenerate if the cell body is intact. If pressure is applied to the axon of a neuron, the lack of oxygen and blood supply reduces its ability to conduct. If the pressure is released after a few hours, the neurons recover in a few weeks. Cut Injury More severe pressure will present with the same symptoms as a cut to the nerve. If the axon of a neu-ron is cut, the part of the axon distal to the cut de-generates and is phagocytized by the Schwann cells that surround it. This process is known as  walleriandegeneration.  Macrophages come to the area and re-move the debris. The Schwann cells, however, do not degenerate. Instead, they multiply along the path of the original axon. The axon stump connect...

  SENSE ORGANS AND INITIATION OF IMPULSES The neurons that convey information about the in-ternal and external environment—the sensory or afferent neurons—detect the actual changes in the environment by means of  sensory receptors,  which are located at that end of unipolar neurons. Sensory receptors are transducers that convert different forms of energy into action potentials. The endings of sensory nerves alone may have transducer function or they may be surrounded by other non-neural cells that produce action potentials in the neuron. In the latter case, it is known as a  sense organ. Some different forms of energy that receptors con-vert into action potentials are mechanical (touch, pres-sure), thermal (degrees of warmth and cold), electro-magnetic (light), and chemical energy (taste, smell, oxygen content in blood, and carbon dioxide content). Each receptor responds maximally and is sensitive to one type of energy. The particular form of energy to which the rece...