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Biology - Nerves

Nerve cells are called neurons. A neuron is like any other cell. It has a nucleus, cytoplasm, and the usual array of organelles. The middle of the neuron is called the cell body and that is where the nucleus and all of the usual cellular organelles are located. The neuron has two things that other cells do not have. It has dendrites and it has an axon. The neuron also has a cell membrane. The cell membrane surrounds the entire neuron including the axon and dendrites. The neuron is something that receives signals and sends signals. Signals are received at the dendrites. Signals are sent out via the axon. 

Simple Reflex Arc 

Let us consider a knee strike with a hammer. When we strike the knee, the dendrites sense the blow. They have received the signal, so the neuron fires. When the neuron fires, it sends an impulse through it cell body and out into its axon. The impulse is transmitted from the end of the axon to the dendrites of a second neuron. Now, the second neuron has received the signal from the first neuron. The second neuron now fires and sends an impulse through its cell body out into its axon. When the impulse reaches the end of the second axon, it is transmitted to the muscle on which the second neuron is sitting, and the muscle contracts. In short, when one neuron picks up a signal from the environment and it then sends an impulse to a second neuron and the second neuron then causes a bodily response, the whole setup is called a simple reflex arc. 

The neuron that picks up the signal is called the sensory neuron. The neuron that produces the response is called the effector neuron. Therefore, in our knee jerk example, the first neuron is the sensory neuron and the second neuron is the effector neuron. Remember, some sensory neurons are sensitive to being hit, others might be sensitive to light, and others might be sensitive to sound. Different sensory neurons are sensitive to different things. 

There are other simple reflex arcs. For example, if you tap the Achilles tendon with a hammer, the ankle will jerk. That is called the Achilles reflex. Simple reflex arc involves only one sensory neuron and one effector neuron. More complicated pathways involve a third kind of neuron called an interneuron. The interneuron acts between a sensory and effector neuron as a link. 

When a neuron is at rest, when it is not picking up and sending signals, there are two concentration gradients across its cell membrane. To begin with, the inside of the neuron has a much higher potassium ion concentration than does the outside, and it has a much lower concentration of sodium ions than does the outside. So when it comes to the resting neuron, there are two concentration gradients across the cell membrane, a potassium ion gradient and a sodium ion gradient. Remember that the inside of the neuron is high in potassium ions and low in sodium ions. 

The concentration gradient across the neuron cell membrane is created by the process of active transport. At rest or equilibrium, there are pumps that are busy pushing sodium out of the cell and pumping potassium into the cell. This sodium-potassium pump mechanism is what creates and maintains the concentration gradients. Remember that neurons use an energy-dependent pump that maintains a high concentration of sodium outside of the cell and a high concentration of potassium inside the cell. Keep in mind that the resting neurons are, by the process of active transport, constantly pushing sodium ions out of the cell and pulling potassium ions into the cell. 

The membrane of the resting neuron has a very low permeability to sodium and potassium, so the concentration gradients are maintained. The sodium gradient is of greater magnitude than the potassium gradient. The imbalance of sodium across the neuron’s membrane is greater than the imbalance of potassium. This means the inside of the membrane is negatively charged. The negative charge is -70 millivolts. Because the resting neuron is negatively charged relative to the outside, the resting neuron is said to be polarized. 

Every neuron is sensitive to some kind of stimulus rather. In order to cause a neuron to respond, the stimulus has to be of sufficient intensity to excite the neuron. That level of intensity is called the threshold. Therefore, a neuron’s threshold is the intensity of stimulus that is necessary to cause its response. 

When you reach a neuron’s threshold by stimulating its dendrites with sufficient intensity, a small area of the cell membrane suddenly becomes very permeable to sodium ions. Sodium then rushes into the cell along the concentration gradient. Now the inside of the cell has a lot of potassium ion and a lot of sodium ion. As soon as sodium rushes into the cell, the inside of the cell becomes positively charged relative to the outside of the cell. The charge increases from -70 millivolts to +50 millivolts. The increase to +50 millivolts is called the action potential. 

When a neuron is stimulated to its threshold, it becomes permeable to sodium which rushes into the cell and creates a positive charge of +50 millivolts relative to the outside of the cell. We can also say that the cell has become depolarized. 

Now at one small area, the neuron is depolarized. There is a charge of +50 millivolts inside the neuron. Next, the same area of membrane becomes impermeable to sodium and highly permeable to potassium. Potassium rushes out of the cell along its concentration gradient. This causes the inside of the cell to become negatively charged again, and it is called repolarized. 

Therefore, the sequence of events in exciting a neuron is, in the beginning, the cell is polarized, it has a relative charge inside of -70 millivolts. When the neuron is excited to its threshold, it becomes highly permeable to sodium. Sodium rushes inward and the cell is depolarized, its relative charge inside is +50 millivolts. That charge is called the action potential. The cell becomes permeable to potassium. Potassium rushes outward, and the cell is repolarized. It has a negative charge inside of -70 millivolts. The cell cannot respond to another stimulus for a while after an action potential. This period is called the refractory period. 

There are special voltage-gated channels along the axon. These channels open or close based on voltage changes in their area. The whole action potential takes place, thanks to voltage-gated sodium channels and voltage-gated potassium channels. When the membrane becomes a little bit depolarized from some stimulation; that triggers a few voltage-gated sodium channels to open and sodium enters the cell. The sodium ion coming into the cell depolarizes the cell a little more causing more sodium ion channels to open, more sodium ions to come in, and more depolarization. This will go on until the membrane potential of the cell goes all the way from -70 millivolts to +50 millivolts and initiate an action potential. 

At +50 millivolts, the electrochemical force driving sodium ions into the cell is zero, and the sodium channel becomes inactivated. They gradually start to close and they stay closed until the membrane recovers its resting potential. Meanwhile, voltage-gated potassium ion channels had also opened when the membrane was depolarized, but these potassium ion channels open slowly. Potassium permeability increases right about the time that sodium ion channels are inactivated. The potassium ions entering the cell help bring the cell back to its resting state, which also happens to be the equilibrium potential for potassium ions. Now, the potassium ion channel is closed and the sodium ion channels are ready for the next stimulus to come along and start the whole thing over again. 

Remember that stimulus must reach threshold for an action potential to happen. That means that the stimulus must be strong enough to open enough sodium ion channels to trigger an action potential. The sequence of depolarization followed by repolarization describes an action potential. As soon as one little area of a neuron undergoes an action potential, the action potential spreads like a chain reaction along the neuron’s entire length. Within milliseconds, every area of the neuron, in sequence, becomes depolarized and then repolarized meaning every area of the neuron in sequence becomes permeable to sodium and then permeable to potassium. When one small area of a neuron undergoes an action potential, every other area, in sequence, undergoes it too. 

The spreading of action potential is called a nerve impulse. In order for an impulse to travel, it synapses with other neurons. When the nerve impulse from a neuron reaches the end of the axon, it causes the axon to release a chemical into the synapse. When the chemical comes in contact with the dendrites of the next neuron, it brings on an action potential in that neuron. This chemical is called a neurotransmitter. 

A neurotransmitter is the means by which a nerve impulse crosses the synapse. When the neurotransmitter reaches the neuron, it binds to a receptor on the surface of the neuron. This binding opens ligand-gated channels for a particular ion, when the ligand gated channels open, the permeability of the neuron to a particular ion changes. These ions rush in and bring on an action potential in the neuron. That is how a neurotransmitter transmits a nerve impulse. 

Remember that one important neurotransmitter is a chemical called acetylcholine. Acetylcholine triggers muscle contractions. Remember also that excess acetylcholine is broken down by cholinesterase. Another neurotransmitter is norepinephrine. It is released by sympathetic neurons and some neurons in the brain and spinal cord. 

There is a kind of cell that tends to be around neurons. This cell is called a Schwann cell. Sometimes Schwann cells wrap themselves around axons. These cells are now called myelin sheath. The space between myelin sheaths is called the node of Ranvier. Axons that have myelin sheaths conduct nerve impulses more quickly than those that do not. 

Myelin sheaths wrapped around an axon do not let any current along the axon leak out. The only way a current can get anywhere is by jumping from point to point at the nodes of Ranvier. This kind of conduction is called saltatory. 

The Nervous System 

The whole nervous system is divided into two parts: the central nervous system and the peripheral nervous system. 

The central nervous system consists of the brain and spinal cord. Remember that the brain consists of the cerebral cortex, the largest part of the brain which integrates all sensory input and voluntary motor activity; the cerebellum, which is concerned with muscle coordination; the hypothalamus, which regulates homeostasis via hormones; and the medulla, which regulates involuntary actions such as coughing and breathing. 

As for the spinal cord, remember that its inner region is called the gray matter and its outer region is called the white matter. The gray matter is dark because it contains cell bodies. The white matter is white because it contains mostly myelinated axons. Spinal nerves, which contain both sensory and motor neurons, are associated with the spinal cord. Sensory neurons enter through the dorsal root and motor neurons exit through the ventral root. 

The peripheral nervous system is divided into two parts. One part is the somatic nervous system. The other part is the autonomic nervous system. The autonomic nervous system is divided itself into two components: the sympathetic and the parasympathetic components. Those parts of the body that are controlled by the autonomic nervous system have neurons from both the sympathetic and parasympathetic parts of the autonomic nervous system. 

The parasympathetic and sympathetic neurons have opposite effects on the body parts they control. The parasympathetic neurons have an effect that is opposite to that of the sympathetic neurons. The heart, for example, is under the control of the autonomic nervous system, so it is supplied with sympathetic and parasympathetic neurons. When the sympathetic neurons fire, the heart beats faster. When the parasympathetic neurons fire, the opposite happens. The heart beats slower. 

Receptors are nerve cells. The way to remember where these receptors are is to know that we have five senses: sight, touch, smell, taste, and hearing. 

The skin receptors sense touch as well as pressure, heat, cold and pain. Other receptors in the body are sensitive to the changing tensions in the muscles and tendons. 

Olfaction is another word for smell. Sensory receptors lining the nasal cavity detect many odors. 

Taste receptors have hair-like projections that are found in the taste buds and detect chemicals in the mouth. There are four different types of taste registered: sour, bitter, salty, and sweet. 

You should know a little about the ear. The best way to remember the important structures in the ear is to follow the path of sound through the ear. As sound enters the ear, it comes in contact with these structures: pinna, the auditory canal, tympanic membrane, the malleus, incus, and stapes; cochlea, and finally the auditory nerve also called the cochlear nerve. The cochlea is important because it contains hair cells, which send impulses to the auditory nerve when they are activated by a sound wave. Another name for the hair cells and related structures in the cochlea is the organ of Corti. 

Remember that the ear has three parts: external, which consists of the auditory canal; middle, which consists of malleus, incus, and stapes; and the inner, which consists of cochlea and vestibular system. The vestibular system helps us maintain our balance. It has three semicircular canals in the inner ear that contain hair cells, which detect certain types of movement. When you move your head, the fluid in the canals puts pressure on the hair cells, which then send impulses to the vestibular nerve and the brain. 

Now just as we did for the ear, we are going to follow the structures in the path of light as it goes through the eye: cornea, aqueous humor, pupil, lens, vitreous humor, light receptors and finally the optic nerve. The iris regulates the size of the pupil while the ciliary muscle regulates the shape of the lens. The light receptors which contain pigments are called rods and cones and are located in the retina. Cones detect color while rods are designed to work under poor lighting conditions. 

Now remember this, if you are nearsighted, light focuses in front of the retina when the object is far away. If you are farsighted, light focuses behind the retina when the object is near.