A good starting point when preparing for the MCAT is to familiarize yourself with the syllabus and identify topics that you’ll need to study. Nerve cells, which are included in the Biological and Biochemical Foundations of Living Systems section of the exam, are just one of many MCAT subjects which you’ll need to know well. To keep things simple we’ve created a handy nerve cell study guide for the MCAT. This will put all the information in one place that you need to score highly on the nerve cell questions of the test.
Nerve cells, or neurons, are the building blocks of the human nervous system, allowing for all movements within the body, from waving an arm around to regulating the heartbeat. They are complex cells, all specialized and subspecialized to optimize their function depending on their location and role. Nerve cells are one of the few cell types within the body called ‘permanent cells’. This means that although pathways between these cells can grow, develop, or reduce over time, the cell population stays relatively stable through human life. As a result, the lifespan of a neuron in a human brain can last over 100 years. The basic structure of a nerve cell consists of a cell body, with a nucleus, that is connected to a long extension called an axon. The axon is often covered by a myelin sheath. One end of the nerve cell consists of the dendrite, which is initially depolarized through chemical synapses from other neurons. This impulse will then travel as an electrical impulse down the axon and cell body, to the telodendrion, the terminal part of the axon. Finally, the electrical impulse is converted to a chemical impulse and transmitted across the synaptic cleft to the dendrite of the postsynaptic nerve cell.
Four main types of neurons could be examined. As shown below, the structure of each of these nerve cells differ slightly.
Image source: https://commons.wikimedia.org/wiki/File:Neurons_uni_bi_multi_pseudouni.svg
Unipolar neurons (1) are a form of sensory (afferent) nerve cell that is found only in invertebrates. They are the only examinable nerve cell that does not exist in the human nervous system. They consist of a soma containing the nucleus on one end of the nerve cell, with one protoplasmic process. There is one central axon, with dendrites extending sideways from that axon.
Bipolar nerve cells (2) are another form of afferent nerve cell, but they are much more specialized than unipolar and pseudounipolar nerve cells. They exist within the retina of the eye (role in vision), the roof of the nasal cavity (role in olfaction) and within the inner ear (role in both hearing and balance). Structurally, these neurons consist of one central cell soma and nucleus, with two processes coming out of it, one of which contains the axon and the other containing the dendrites.
Multipolar neurons (3) are the main motor neurons in the nervous system, and most soma (cell bodies) within the CNS belong to these neurons. They have the longest axon of all the nerve cell types, and they have several dendrites extending from a large cell body containing the nucleus, allowing for them to integrate lots of information and connect with several other nerve cells. Interneurons are a type of multipolar neuron.
Pseudounipolar nerve cells (4), unlike unipolar nerve cells, are present in both invertebrates and vertebrates. In humans, their purpose is to interpret touch, pressure, and pain. They consist of a cell soma with a nucleus, similar to unipolar neurons, with one protoplasmic process. However, the difference is that with pseudounipolar neurons, this process splits in two, allowing for the cell nucleus to be away from the central axon. These are the most common sensory neurons in humans and exist within the dorsal root ganglion of the spinal cord.
Two of the most important organelles within the cell body are the vesicles and the mitochondria. However, organelles typical of normal eukaryotic cells are present in neurons, for instance, Nissl substance, which is vital for the formation of proteins within the neuron. Vesicles, in nerve cells, carry neurotransmitters across the synaptic bouton to allow for chemical synapses to occur. The mitochondria, similarly, are vital for the movement of these vesicles, as well as aiding in the reuptake of neurotransmitters from the synaptic cleft.
As previously mentioned, dendrites are vital for the function of neurons, by transmitting impulses towards the cell body. They tend to extend from the cell body (with the exception of unipolar neurons), and if a nerve cell contains more dendrites, it means that it holds a lot more connections with other nerve cells. Furthermore, an increased number of dendrites can hold and integrate much more information, allowing them to be more efficient nerve cells. These dendrites, as mentioned above, form part of the terminal part of the neuron.
Axons are long, filamentous extensions that are primarily responsible for the transmission of electrical impulses away from the cell body. Axons end at either the synapsis between two neurons or close to an effector organ, thereby triggering an effect (E.G. contraction of a muscle). Axons connect to the cell body at the axon hillock, which is important for determining whether an action potential is excitatory or inhibitory. An axon can split at the end to form several connections, producing arms called ‘telodendrions’. These widen at the end, forming a structure named the synaptic bouton, which functions to release neurotransmitters into the synaptic cleft to either further trigger an impulse down another neuron or trigger a reaction from a target organ.
Electrical signals must be transmitted very quickly, especially for reflex reactions. To increase this speed, the axons of several neurons are ‘myelinated’. This is the process of mechanically insulating the axon by wrapping several layers of fatty tissue, called myelin, around it, forming what is known as the ‘myelin sheath’. As a result, the strength of the electrical impulse is maintained throughout the axon, decreasing the likelihood that signals will dissipate as they travel across the nervous system. The action of myelination is completed by glial (supporting cells), which are either oligodendrocytes in the central nervous system or Schwann cells in the peripheral nervous system. Each Schwann cell is responsible for one segment of the axon and forms up to 20 rotations of myelin around the axon.
The speed of transmission of electrical impulses is further increased by nodes of Ranvier. These are gaps in the myelin sheath that occur every <1.5mm along the axon and are approximately 1-2μm in length. The axon is then exposed, and they are the only part along a myelinated axon which is permeable to the movement of ions. This is advantageous to the speed of the impulse, as it allows for a phenomenon called ‘saltatory conduction’, in which impulses ‘jump’ from each node of Ranvier to the next. Through this, the speed of the impulse is increased by 10-20 times.
A synapse is a chemical point of communication between nerve cells. It allows for impulses to be sent from one neuron to the next to continue the impulse and is the point at which the signal is converted from an electrical signal to a chemical signal, and back again. It is formed by the synaptic bouton, at the end of the axon on the presynaptic neuron (the neuron the signal is traveling from), the synaptic cleft (a 20-50nm gap between the two neurons) and the dendrites of the postsynaptic neuron (the neuron that is receiving the impulse).
Image source: https://commons.wikimedia.org/wiki/File:SynapseSchematic_unlabeled.svg
The process of transmitting the signal across the synaptic terminal consists of 7 steps:
Neurotransmitters are chemicals that act as messengers, helping to either initiate/increase (excitatory neurotransmitters) or reduce/prevent (inhibitory neurotransmitters) a specific response. This response can either affect the propagation of a signal between nerve cells, or from nerve cells to muscles. This regulation is important, because if there are no excitatory neurotransmitters, then muscle movement would not be able to occur, whereas if there are no inhibitory neurotransmitters, then involuntary movements would occur. The most important neurotransmitters to be aware of are acetylcholine, glutamate, GABA, and glycine.
Acetylcholine (ACh) is the main neurotransmitter of the parasympathetic nervous system, but also has a role in the sympathetic nervous system. Depending on the location, it can either be inhibitory (E.G. smooth muscles and cardiac muscle) or excitatory (neuromuscular junction, certain glands). It is released by nearly all presynaptic autonomic neurons.
Glutamate is one of the main excitatory neurotransmitters that is especially important within the central nervous system, aiding in neuroplasticity. Therefore, its function supports learning, as well as initiating movements. It is implicated in several movement disorders, notably Parkinson’s Disease.
GABA and glycine are both inhibitory neurotransmitters. GABA is a major inhibitory neurotransmitter throughout the body, but glycine is found in much higher concentrations within the spinal cord.
Alongside neurotransmitters, neuromodulators are other chemicals that can be found within the synaptic cleft. The main difference between these is that neurotransmitters transmit the signal, whereas neuromodulators have a role in the efficiency of this transmission. Examples of neuromodulators are dopamine and serotonin, both of which are implicated in mood disorders.
The resting potential refers to the general electrical charge of the neuron, when it is not being fired. In neurons, this is -70mV, which means that the inside of the nerve cell is negatively charged in relation to the external environment. This charge is maintained predominantly by the movement of two ions: sodium, which enters the nerve cell, and potassium, which leaves the nerve cell. The slow movement of these two ions is due to leak channels, which allow these ions to move along their concentration gradient. For potassium, this movement means that more potassium is drawn back into the cell through the ensuing electrical gradient. This phenomenon is known as the potassium equilibrium potential, and balances at approximately -90mV. Sodium enters the cell via sodium leak channels as the concentration of sodium is much higher outside of the cell than it is outside. Similar to potassium, the movement into the cell of sodium then alters the chemical charge, so sodium also leaks out of the cell and causes an equilibrium potential at approximately 60mV. The resting potential of the cell is therefore a balance of both ions. Although this potential would be assumed to be -15mV, the halfway point between -90mV and 60mV, this is not the case. In reality, within the nerve cell there are more potassium leak channels than there are sodium leak channels, therefore more potassium moves out of the nerve cell, so the net charge within the neuron itself decreases to -70mV.
In order for an impulse to be generated, the action potential threshold must be reached. This is the electrical charge at which the nerve cell is considered ‘depolarized’ and usually occurs at around -40mV. Nerve cells exhibit the ‘all or nothing principle’. This means that regardless of the level of depolarization, if it does not reach this threshold, no action potential will be generated and there will be no firing. Similarly, regardless of how far over this threshold the depolarization is, the strength of the impulse will not be affected. Instead, the strength of a nerve impulse is affected by either conduction velocity from myelination, or from repeated impulses causing constant depolarization. An example in which the latter applies is in tetanus, where the tetanospasmin toxin causes constant depolarization of nerve cells, and therefore ‘lockjaw’.
The mechanism behind which this depolarization occurs is originally through the electrical charge, then emphasized by the movement of sodium and potassium against their concentration gradients. This is completed by several pumps over a series of steps:
The sodium potassium ATPase pumps 2 potassium ions into the cell, and 3 sodium ions out. The net effect of this is one positive charge out of the cell per pump. This, alongside the highly permeable potassium channel within the membrane, allows for an influx of sodium and efflux of potassium until the overall potential and ion concentrations return to normal.
Although this mechanism refers to one part of the axon, the rest of the axon is depolarized in a unidirectional way to allow the signal to travel through the nerve cell. When the cell is originally depolarized and the influx of sodium occurs, the sodium diffuses along the axon to areas of lower sodium concentrations, allowing the potential of the next segment to reach the threshold and therefore allowing for the voltage-gated sodium channels to open in the sequential part of the axon.
Excitatory fibers are ones that increase firing by depolarizing the postsynaptic nerve cell, whereas inhibitory fibers are ones that decrease firing by hyperpolarizing the postsynaptic cells. The cumulative effect of this is known as summation. It is important to understand the basics of how these fibers work together:
There are two main types of summation that occur in the human nervous system.
The first is temporal summation, temporal meaning ‘time’. This phenomenon explains that when there are a few fibers that are fired at one neuron at the same time, they are more likely to be able to create a summative effect. This means that fewer excitatory signals are needed to depolarize the nerve cell, as long as these signals occur at the same time, than if these signals were to occur in a more spaced-out time period.
The second phenomenon is spatial summation, spatial meaning ‘location’. Signals sent directly to the cell soma are likely to have a greater effect than those that pass through the dendrites first. Using the same example as before, this means that fewer excitatory signals are needed to depolarize the nerve cell if they fire at the soma in comparison to at the dendrites. Together, both of these types of summation mean that depolarization of a nerve cell, in several circumstances, relies on the additive effects of several nerve fibers as opposed to the level of depolarization produced by one individual signal.
The absolute refractory period is the period of the cell cycle after the nerve cell has become hyperpolarized due to the great efflux of potassium ions. In this period, regardless of the frequency of firing, or the level of potential depolarization, the nerve cell cannot be further depolarized. It lasts approximately 4ms.
Support cells within the nervous system are referred to as glial cells. They work to protect the nerve cells and their connections to maintain the human nervous system. There are 4 main types of neuroglia that are important to know: Schwann cells/oligodendrocytes, astrocytes, ependymal cells, and microglia.
Schwann cells and oligodendrocytes are the cells that are responsible for myelination of axons. Schwann cells work in the peripheral nervous system and oligodendrocytes work in the central nervous system. Due to these, the firing of the nerve cell is much faster, and there is a much higher chance of depolarization continuing across the axon due to the impermeable nature of myelin.
The function of astrocytes is not well understood, however there are two theories that are generally accepted by scientists. The first is their role in synapses, both in the formation, maintenance, and breakdown of them. In studies, they are shown to be implicated in the process of synaptic pruning: a process in which synaptic connections that are largely unused are broken down, whilst those that are used often are further strengthened to prioritize information coding and retrieval. The second purpose is to do with the regulation of neuronal circulation. It is believed that astrocytes have vasodilative and vasoconstrictive effects that affect the microcirculation of the CNS, allowing for adaptation depending on changes in the microenvironment.
Ependymal cells are glial cells in the CNS that line the ventricles of the brain. The ventricles are responsible for storage and flow of cerebrospinal fluid and are implicated in disorders such as hydrocephalus (increase in cerebrospinal fluid in the ventricles). The production of cerebrospinal fluid, however, is due to ependymal cells, which create it from the blood that circulates the brain.
The final important glial cells to be aware of are microglia. These, in essence, are the phagocytes of the CNS. They assist in the breakdown and disposal of synapses during synaptic pruning, as well as the breakdown of waste products around the CNS.
All these cells and their subtypes, together, help to maintain and regulate the structure and function of the CNS and the nerve cells within it.
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