The Analysis of the Sensory and Motor Neurons - Coursework Example

NERVOUS SYSTEM Department Structure and Functions of sensory and Motor Neurons A neuron is cell that function to relay electrochemical impulses known as nerve impulses or action potentials. The diagram below shows the structure and parts of the sensory neuron. The cell bodies of the sensory neurons that connect to the spinal cord are often located in clusters, DRG (dorsal root ganglia) just next to the spinal cord (Brodal, 2010 p.70). The axon of the sensory neuron also extends from both directions: to receptors known as peripheral axon and to spinal cord known as the central axon (Swanson, 2012 p.124). Figure 1: Structure of Sensory neuron Source: Brodal, 2010 p.71 Figure 2 shows the structure of the motor neuron. Motor neuron functions by taking response or information from the brain to the organs or muscles which are referred to as the effectors (Starr et al, 2012 p.555). The information is taken by the motor neuron through the dendrites which then relays the same information to the cell body through the synaptic endings. Within the cell body of a motor neuron, there is axon which then transmits the information to the axon terminals which finally connect with the muscles (Chiras, 2014 p.228). These are referred to as the motor end plates and play an important role in relaying the information to the muscles for the necessary action. Figure 2 the Structure of motor neuron Source: Brodal, 2010 p.70 All neurons within and outside the central nervous system contain cytoplasmic extensions known as axons upon which impulses are conducted. The axons are often very long but thin, for instance axons connecting the foot and spinal cord can be up to one meter (Palastanga, 2006 p.15). The dendrites are branched fibres upon which nerve impulses are generated and transmitted through the axon which brunch into many times as it approaches its end. Myelin sheath is a glistening fatty sheath that covers the axon and is normally the expanded plasma membrane of an accessory cell known as Schwann cell. The node of ranvier is the point at which the sheath of one Schwann cell meets the other and it plays an important role in the propagation of nerve impulses (Brown, 2001 p.2). A Reflex Arc Involving Three Neurones A reflex action is a spontaneous, fast and predictable motor response to stimulus which is repeated in the same manner whenever the same stimulus is presented (Clark, 2005 p.217). A reflex arc is completed by five components including the stimulus receptor, which is a group of nerve endings or a specialised cell which respond to the cell stimuli and receive the stimulus from internal or external environment. The second is the sensory neuron known as afferent neuron and is found in peripheral ganglion like dorsal root ganglion. Sensory neurones transmit impulses with the information about stimuli to the central nervous system. The central nervous system is the control centre as well as the coordinator. Figure 3: The Functions of sensory and motor neurons in Reflex arc Sensory neuron Motor Neuron Source: Martini, 2005 p.331 The function of the control centre is to process the information from sensory neurones and generate motor commands to the effectors organs so as to respond to the stimuli. Inter-neurones are found within the central nervous system and their function as a link between sensory and motor neurone and transmit information within different parts of CNS (Martini, 2005 p.332). The motor neurones transmit efferent impulses away from the CNS and last but not least the effectors which respond to the information relayed through by the specific motor neurons (Brodal, 2010 p.289). This anatomical route is referred to as a reflex arc. Figure 3 below shows reflex arc involving Figure 4: Withdrawal Reflex Arc Source: Clark, 2005 p.218 How an Action Potential Passes Along the Axon An action potential refers to a short-lasting spike like change within the membrane potential that is normally passed along an axon as a result of stimulus or trigger of the receptors cell and organs. The entire process of action potential takes approximately 2 seconds (Simmons & Young, 2010 p.26). This takes place when the neurotransmitters on the ligand-gated channels or mechanically-gated channels respond to mechanical stress resulting from the trigger thereby causing a small change in charge distribution within the membrane. This result into application of a positive charge (Na+) within the axon makes it even more positive and this is referred to as depolarization. Figure 5 how an action potential passes along the axon Source: Cambridge International AS and A Level Biology Coursebook When more charges are applied inside the axon, a threshold of excitation is often reached and the neuron produces an action potential (Coon, 2010 p.56). During this time, the membrane potential is quickly reversed resulting into a very strong positive charge of up to +40mV with regard to the exterior (Williams, 2000 p.329). The membrane potential then rapidly returns to normal but first overshoots its resting potential and drops to around -75mv a condition refers to hyperpolarisation. Figure 4 illustrates the whole process of action potential through the axon. Sections 1, 2, 3 represents rising phase, falling phase and resting phase respectively. Similarities between the Synapse and neuromuscular junction Figure 6: Neuromuscular Junction and Synapse Source: Favero et al (2009 p.2327) A synapse is a site through which information is relayed from one cell to the other either through electrical means or chemicals means thus we have electrical synapse and chemical synapse respectively (Favero et al 2009 p.2327). It is the functional or structural contact area between the target cell and neurone. It is the junctional area where the synaptic knob of a neurone comes into contact with a dendrite s shown in figure 1 and 2. However, when the second cell is a muscle fibre, the synapse is referred to as a neuromuscular junction. Synapse and neuromuscular junction are similar and yet difference at the same time. They are both separated by a synaptic cleft though excitable and the cleft plays a key important role in preventing direct transmission of electrical activity between them (Matthews, 2003 p.111). The major difference is that synapse is a junction exiting between neurons while neuromuscular junction exists between skeletal muscle and a motor neuron. While direct transmission of action potential occurs at neuromuscular junction an action potential in within postsynaptic neuron takes place only when the membrane is at threshold. It is also important to note that neuromuscular junction is an excitatory (EPP) and synapse can either be inhibitory (IPSP) or excitatory (EPSP) (Johnson & Byrne, 2003 p.114). Multiple Sclerosis (MS) Multiple sclerosis (MS) is a neurological disease that affects the structure of the neurones and is commonly exhibited by ataxia, blindness, fatigue, depression, pain, and cognitive impairment. Pathology of MS includes the loss of myelin sheath, proliferation of the astrocytes and the disappearance of oligodenrocytes (Gold & Wolinsky, 2011 p.76). These changes in the neurons lead to the formation of scattered plagues throughout the Central nervous system. At the initial stages, the myelin sheaths of the spinal cord and brain neurons are attacked sparing the nerve fibre. During this time, myelin is capable of regenerating and patients may disappear thus leading to remission (Solaro et al p.2). The glial scar tissue can also replace myelin; however, this slows down the nerve impulses. However, the destruction of axons, leads to the complete blocking of nerve impulses which result into the permanent loss of nerve functions. This is characterized by chronic as well as progressive deterioration of some, remissions and exacerbations in other patients.References Brodal P., 2010, The Central Nervous System: Structure and Function: Fourth Edition, Oxford: Oxford University Press. Brodal, P 2010, The Central Nervous System: Structure and Function. Oxford: Oxford University Press. Brown, AG 2001, Nerve Cells and Nervous Systems: An Introduction To Neuroscience, New York: Springer. 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