Normal Physiology

Immune System

Moreover, our bodies are protected against microbes and parasites via the immune system. Specific organs are used to fight off and filter out the bacteria and virus from entering our body system and send out immune cells that circulate in the blood flow to respond quickly to the attack. The innate immune system is the first to respond to the infection. This will direct to acute inflammation, but lacks the specificity for microbes and produces no antibodies. However, the adaptive immune system would take longer to develop in which it is specific and responds rapidly to the same microbes that previously encountered. [4]

For a healthy person, different lymphocytes are sensitive to specific antigens, or also called foreign substances. Lymphocytes with specific receptors will search for a specific antigen that is present in the bloodstream and bind to it. Then, it will activate proliferation and differentiation that results in mass production of clonal selection of cells that are specific to that antigen. There are two types of lymphocytes: T cells and B cells. T cells mature in the thymus, are stimulated by the antigens, and are responsible for cellular immune activity. B cells mature in the bone marrow and are responsible for humoral immunity, producing antibodies (immunoglobulin) soluble molecules. [4]

There are two immune pathways of the adaptive immune system: humoral and cell-mediated immunity. The humoral immunity pathway occurs when B lymphocytes are introduced to exogenous antigens in which causes a mass production of antibodies. Monocytes are recruited to the infected inflamed tissue and further differentiate into macrophages. Macrophages engulf exogenous antigens. These antigens will be broken down by lysosomes and then excrete the antigen particles. As a result, major histocompatibility complex (MHC) molecules class II receptors will present these foreign particles on the surface of macrophages. In this case, macrophages are considered as antigen-presenting cells which also comprise B cells and dendritic cells. These antigens will then be presented to T helper cells where they release cytokines that trigger and activate specific B lymphocytes. This specific B lymphocyte will proliferate and differentiate (clonal selection), creating antibodies and eventually plasma cells. These plasma cells (memory cells) produce antibodies once activated when exogenous antigens are present. [4]

Cell-mediated immunity only targets endogenous antigens without producing new antibodies. Typically, cancer cells and cells infected by viruses are not recognized by the immune system since they are considered non-foreign antigens. These endogenous antigens will be presented by a special MHC class I on the surface of the cell. T helper cells have T helper cells receptors on the surface which will bind to the antigens and recognize these infected or cancerous cells. This will trigger cytotoxic T cells to be activated in which highly specific to antigens and release perforating enzymes. These cells will later be degraded by enzymes. Eventually, it will stop further spreading of the virus-infected cells [4]

Pathway of the Neuron

To begin, a stimulus initiates a feedback loop to produce a bodily response. The stimulus signal is received by the body's receptors (the eyes, on the skin, etc.), sent to the relay neurons and then toward the neurons in the central nervous system. The central nervous system includes the brain and the spinal cord. At a cellular level, the dendrites of the neuron receive the signal, pass it through the nucleus and cell body, and down to the axon hillock. At the axon hillock, also referred to as the neuron's integrating center, a decision is made about whether or not an action potential can be sent down the length of the axon. The action potential is a digital, or an "all-or-nothing", signal. If the graded potential, or sum of graded potential, does not exceed the threshold, nothing further happens. Once the threshold is exceeded, an action potential is conducted down the length of the axon. Going down the axon, action potentials will jump from one node of Ranvier to the next because of the presence of voltage-gated sodium ion channels. Eventually the signal reaches the synapse, the region where the terminal of the axon communicates with a neuron nearby. In the synapse the signal travels from the presynaptic axon terminal to the synaptic cleft and finally the postsynaptic dendrites. Then the process begins again with this new neuron. The signal will be passed on to the motor neurons and eventually an effector (like a muscle) that will produce a bodily response. In terms of its interactions with the immune system, infection is one of the primary stimuli for modulation of the brain. The brain modulates the immune system in response to environmental stress through the hypothalamic-pituitary axis, orchestrating immune responses with corticotropin releasing factor, or CRF. [5]

Myelin is an insulating layer formed around the axon of a neuron. In the peripheral nervous system, living Schwann cells will wrap around the axon to form this insulating layer. For the central nervous system, this layer is formed by oligodendrocytes. The purpose of myelin is to help maintain proper saltatory conduction in the axon. Normally, the signal propagates down the axon, into the terminals and synaptic cleft, and eventually passes the signal to a neighboring neuron. In closer detail, a depolarizing action causes the voltage-gated sodium ion channels to open and allow Na+ to rapidly enter the axon. The positive charges from the trigger zone spreads to the neighboring parts of the axon. When the axon's active region reaches the action potential peak, the sodium ion channels close. Then the potassium ion channels will open. This allows K+ to exit the cytoplasm. After a depolarizing stimulus, the closing of K+ channels and decrease in K+ causes the axon segment to return to its resting membrane potential. The absolute refractory period prevents any backward conduction, propagating the action potential signal towards the terminals. However, not all neurons are myelinated. The additional layer of myelin results in quicker action potential conduction, a highly resistant membrane, and prevents current leaks. Demyelinating diseases like multiple sclerosis reduce or block saltatory conduction when current leaks out of the axon nodes, which were previously insulated. If there's a substantial amount of current that leaks out from the nodes, the depolarization level may no longer exceed the threshold. This will cause action potential failure. [5]