The Sensory Nervous System

A Brief Description of the Peripheral Nervous System
       The nervous system consists of two main interconnected subsystems, the central nervous system (CNS) the brain and the spinal cord and the peripheral nervous system (PNS). The PNS is responsible for connecting the CNS to the rest of the body. It includes two subsystems, the autonomic nervous system (ANS) which, in turn, includes the sympathetic, the parasympathetic, and the enteric nervous systems and the sensory nervous system (SNS). The ANS is responsible for managing involuntary body functions (e.g., digestion), while the SNS is responsible for managing voluntary functions (e.g., moving a finger) and providing the CNS with information about the inside and the outside of the body (Hoyle and Arthur, 2005, p. 1-5).

        The peripheral nervous system is composed of functional units called neurons (also known as nerve cells), and they are classified into two main types 1. Sensory (afferent) neurons They receive information about the internal and external environments (i.e., stimuli) through sensory receptors, and then send this information to the central nervous system (Farabee, 2007, p. 1). 2. Motor (efferent) neurons The central nervous system issues commands and sends them to effectors (i.e., glands, muscles) through these neurons (Farabee, 2007, p. 1). The Structure of the NeuronDendrites

       A root-like structure that surrounds the cell body. Dendrites are specialized for receiving nervous impulses from another cell and transmitting them to the cell body. The dendrites branched structure ensures a strong ability to receive impulses (Weis, 1996, p. 1).Cell Body
       The part of the neuron that contains the nucleus. It receives nervous impulses from the dendrites and transmits them to the axon, which in turn, transmits them to another neurons dendrites (Weis, 1996, p. 1).Axon (Nerve Fiber)

       A tube that connects between two neurons, its main function is receiving impulses from the cell body and transmitting them to the axon terminals, and thus, to another neurons dendrites. The axon is enveloped in a protective sheath comprised of a fatty compound called myelin, and hence, this sheath is called the myelin sheath. Myelin is produced by the Schwann cells, which are the cells that surround the axon. The Schwann cells build multiple protective structures along the axon separated by small nodes, which are known as the Nodes of Ranvier. These structures are very important for neurons performance because they isolate axons from each other and help them increase the speed of impulse transmission (Farabee, 2007, p. 1 Rose, 2001, p. 1 Weis, 1996, p. 1).

               Where and How Are Sensory Stimuli Perceived in the Central Nervous System       The functional units in the peripheral nervous system are the neurons, and they are found in the form of groups of clusters known as ganglia (plural of ganglion, which is a cluster) in certain parts of the body (Ferr, 2002, p. 1 Finlay, 2010, p. 1). There are 12 cranial and 31 spinal pairs of ganglia groups, and they are responsible for managing different types of processes including perceiving sensory stimuli 1. 12 cranial pairs They control different types of processes some of them are sensory some manage voluntary functions others manage more than one type of functions including involuntary functions. The main functions of these ganglia are Receiving information from sensory receptors and transmitting it to the brain, and receiving commands from the brain and transmitting them to effectors (Farabee, 2007, p. 1 Hoyle and Arthur, 2005, p. 1 Rose, 2001, p. 1) 2. 31 spinal pairs Each pair consists of two ganglia groups, dorsal (sensory) and ventral (motor). The main functions of these ganglia are Receiving information from sensory receptors and transmitting it to the spinal cord (which is connected to the brain), and transmitting commands from the spinal cord to effectors. There are some processes that the spinal cord and spinal ganglia handle without the brains assistance, the reflex arcs, which are instant and involuntary reactions to certain stimuli. An example of a reflex arc is Removing the hand quickly after touching a very hot object.

                 How Do Nervous Impulses Move in the Sensory Nervous System       Nervous impulses move through neurons in the form of electrical impulses, which are also known as action potentials. Sensory stimuli work as stimulations for neurons to generate action potentials. Each neuron has a membrane, and by default, each of its two sides has a different electrical charge, and the difference between these charges generates an electrical voltage known as the resting potential. The axons membrane, by default, contains potassium inside (negative ions), while its outer side is rich of sodium (positive ions). The resting potential in this case is about -70 millivolts (Farabee, 2007, p. 1 Finley, 2010, p. 1 Weis, 1996, p. 1).

       When a nervous impulse is generated, the sodium and potassium ion gates get opened and become more permeable, and that enables ions to move into and out of the membrane easily. At the same time, the membrane protein channel gates get opened in order to ease the flow of ions inward and outward. Positive sodium ions move first, they rapidly rush into the membrane, and as a result, the inside becomes more positive than the outside. Negative potassium ions, in turn, move outside the membrane in order to create a negative charge on the outside equal to the positive charge in the inside. This process increases the electrical voltage in the membrane to 30 millivolts (action potential). Ions cant stay in their new positions because the protein that comprises the membrane is actively moving ions against their concentration gradients (i.e., returning them to their original positions), this process is known as the sodium-potassium pump. These interconnected processes continue and include the rest of the membrane, and as a result, the action potential moves to the axon terminals (Farabee, 2007, p. 1 Finley 2010, p. 1 Weis, 1996, p. 1).

            The point at which one neurons axon transmits an impulse to another neurons dendrites is known as the synapse. In the synapse, there is a tiny space that separates between the two neurons called the synaptic cleft. A neuron can transmit impulses through this cleft by releasing neurotransmitters, which are stored in its axon terminals in the form of synaptic vesicles. When an impulse reaches the axon terminals, it stimulates syntactic vesicles to move to the cleft where they release their contents. The neurotransmitters move to the new neurons dendrites, and then stimulate that neuron to initiate a new action potential by causing ion gates to open (Farabee, 2007, p. 1).

       After transmitting the action potential, there is a small period in which its impossible for the membrane to transmit any more impulses, this period is known as the refractory period (0.0004 of a second). The membrane spends this period returning to the default state. The neurons behavior during this period plays an important role in its performance because this behavior prevents impulses from being transmitted to the opposite direction (Farabee, 2007, p. 1 Finlay, 2010, p. 1).


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