Overview of the Nervous System

Now we will examine some of the fundamentals of the nervous system by looking at the nervous system and neural tissue. First let's look at the basic structure and function of the nervous system. The divisions of the nervous system include the central nervous system, which is composed of the brain and spinal cord. This is also considered to be the seat of all mental activity. It interprets sensory input and dictates motor responses based on past experiences, reflexes, and current conditions. The peripheral nervous system is composed of the cranial and spinal nerves. These are the communication lines between the central nervous system and the rest of the body. There are two divisions of the peripheral nervous system. The sensory division, which is the afferent, and conducts impulses from sensory receptors to the central nervous system. Sensory receptors, as you will see, are within somatic, your skin, muscle, and joints, and visceral, which are the organs. The sensory division is considered the input region. The motor division, which is efferent, consists of the motor neurons, which conduct impulses from the central nervous system. to the effectors, which are the muscles and glands. The motor division is considered the output, and the motor division is further subdivided into the somatic nervous system and the autonomic nervous system. Now the functions of the nervous system are listed here. The nervous system uses sensory receptors to monitor changes both inside and outside of the body. That's the input. Nervous system tissue processes and interprets sensory input. That's the integration. And affects a response appropriate to the stimulus, which is the motor output. This allows for homeostasis to be maintained. Here is the nervous system organization that I mentioned. And within the autonomic, we also have the sympathetic nervous system, which mobilizes the body during emergency situations like fight or flight. and the parasympathetic, which conserves energy and promotes non-emergency functions like during resting and digesting. Here is an additional slide showing you the organization of the nervous system that we just talked about. It gives you some additional detail that I mentioned previously. Now, let's examine... nerve tissue by first looking at neurons. Nervous tissue is composed of densely packed intertwined cells of two specific types. Neurons are the functional cells of nervous tissue responsible for receiving, interpreting, and sending stimuli. Neurons exhibit several unique characteristics like being excitable. They possess a polarized membrane which allows them to conduct messages in the form of a nerve impulse from one part of the body to another. Neurons also have longevity. They can function for nearly a hundred years. They have a high metabolic rate. Neurons cannot survive for more than a few minutes without oxygen, glucose, or ATP. Neurons can be very large. They're some of the largest cells in the body and they're amyototic. Most central nervous system neurons lose their ability to divide after they assume the role as communication lines. You can also see here some of the parts of a typical neuron. We have the cell body, which is also called the soma. This is the enlarged metabolic region of the cell where the nucleus is located. Neurons possess large amounts of rough endoplasmic reticulum, which is clustered within the cell body to form nissle bodies. Clusters of cell bodies in the central nervous system are often called nuclei, and clusters of cell bodies within the peripheral nervous system are called ganglia. You can also see several processes. Cellular processes are either called tracts in the central nervous system or nerves in the peripheral nervous system. Dendrites are neuron processes which possess large surface area because of numerous branches. They receive chemical stimuli as well as electrical signals. towards the cell body and these electrical signals are not nerve impulses. They're called graded potentials which we will talk about later. You can also see the axon. The axon is capable of generating action potentials and transmitting nerve impulses away from the cell body during axoplasmic transport. Whenever the signal travels from the axon terminals Back toward the cell body, it's called retrograde flow. The plasma membrane surrounding the axon is called the axilemma, while the cytoplasm is called the axoplasm. The axon forms at a tapered area called the axon hillock. This is the trigger zone because graded potentials much... must reach this area of the neuron before they can be converted into action potentials. As the axon approaches the next cell, many fine extensions branch from its end forming telodendria each ending in an axon terminal. The synaptic terminals possess many synaptic vesicles that contain neurotransmitters which are used across the synapse or synaptic cleft found between the presynaptic cell and the postsynaptic cell. The myelin sheath some axons are covered with a white fatty protein lipid and which is known as the myelin sheath. Myelin protects electrically insulates fibers from one another. Myelinated fibers conduct impulses rapidly and are white in appearance. Unmyelinated fibers conduct impulses quite slowly and generally have a gray appearance. And we'll talk about what makes the mylon sheath later on. This slide shows you how neurons can synapse with other neurons for cell-to-cell communication. You can have a synapse with another neuron. You can have a synapse at a neuromuscular junction or a neuroglandular synapse. All of these will allow for cell-to-cell communication between neurons with a neuron, a muscle, or a gland. Now there is different types of neuron classification. Neurons are classified by their structure or the number of cell processes. Bipolar neurons have two cell processes, one dendrite and one axon. Bipolar neurons are rare but occur in the retina of the eye and within the nasal mucosa. Unipolar neurons, the dendrites and axon, are continuous and basically fused with the cell body located off to the side. Most sensory neurons of the peripheral nervous system are unipolar. Multipolar neurons have one axon and two or more dendrites. These are the most common neurons of the central nervous system, and all motor neurons that control skeletal muscles are multipolar neurons. Now let's examine some of the neuroglial cells, which are simply called glial cells. Neuroglial cells feed, protect, and insulate neurons. It is estimated that there are between 700 to 900 neuroglial cells per neuron. Neuroglial cells differ within the central nervous system and the peripheral nervous system. Neural glial cells of the central nervous system consist of astrocytes, which make up about half of all neural volume, and they're generally star-shaped. They possess numerous projections with a bulb at the end that cling to neurons and capillaries, and therefore they serve as connections between neurons and the blood nutrient supply. This is called the blood-brain barrier. Astrocytes control the chemical environment around neurons by regulating ions, nutrients, dissolved gas concentrations, and hormones. They also absorb and recycle neurotransmitters that cannot be broken down and they form scar tissue after injury. Another type of neuroglial cell in the central nervous system is microglia. Microglia are ovoid cells with highly branched processes. These act as macrophages that engulf microbes and dead neural cells as well as remove cellular debris, waste products, and pathogens. Another type is the oligodendrocytes. These possess few branches and these cells line up along thicker neuron fibers in the central nervous system and wrap their extensions around nerve fibers forming the myelin sheath to insulate the neurons from each other. The last type of neuroglial cell within the central nervous system is the epidermal cells. These cells line the central cavities of the brain and spinal cord creating a barrier between the central nervous system cavities and the tissues surrounding the cavities. Epidymal cells assist in producing, monitoring, and circulating cerebral spinal fluid. They use their cilia to circulate the CSF within the cavities of the central nervous system. Neuroglial cells of the peripheral nervous system are the Schwann cells and satellite cells. Schwann cells form the myelin sheath around large nerve fibers in the peripheral nervous system. Schwann cells can also act as phagocytic cells that engulf damaged or dying nerve cells and are important in directing the process of regeneration. The myelin sheath forms by the Schwann cells wrapping around and around the axon. Satellite cells surround the nerve cell body and may aid in controlling the chemical environment around the neuron, much like the astrocytes of the central nervous system. And here you can see the myelin sheath and the neurolemma. The intercompressed layers of the myelin, while you have the outermost metabolically active layer, which is known as the neurolemma. And gaps between, for example, Schwann cells are called the nodes of Ranvier. This is where the action potential will jump from node to node during conduction. Now the sensory motor response is shown here and we will talk about the sensory motor response. in a little more detail. A sensory neuron, for example, we discussed how you have sensory neurons throughout the body. One in the skin is shown here that might be sensitive to temperature changes. And the graded potential from the sensory ending, if it is strong enough, can initiate an action potential. The axon of the sensory neuron then enters the spinal cord and may contact another neuron in the gray matter. The action potential is initiated at the initial segment of the neuron and travels up the sensory pathway to a region of the brain called the thalamus. Almost all sensory neurons must go through the thalamus on their way up to higher brain centers with the exception of olfaction or your sense of smell. Another synapse passes the information. along to the next neuron. The sensory pathway then ends when the signal reaches the appropriate area of the cerebral cortex. After integration with neurons in other parts of the cortex, a motor command is then initiated. The motor neuron will send an action potential down the spinal cord to another motor neuron in the gray matter of the spinal cord. The axon of the motor neuron emerges from the spinal cord in a nerve and connects to a muscle through the neuromuscular junction to cause contraction of the target muscle. So this shows you the pathway from the sensory afferent going towards the central nervous system back out through the motor efferent division to the appropriate muscle or gland. Okay, so now we're going to talk about neurophysiology. In the body, electrical currents correspond to the flow of ions across cellular membranes. The transmission of a nerve impulse is very similar to the process of a muscle contraction that you learned about previously and occurs in the following steps. We have a resting membrane potential where the cell membrane is considered to be polarized. We have a depolarization. and the production of a graded potential. Then there is a conversion of the graded potential to an action potential and then the propagation of an action potential down the length of the axon to the synaptic terminals. Next, repolarization or re-establishing the resting membrane conditions occurs. Ion channels play a crucial role in establishing ion concentrations on either side of the neuron cell membrane. And they can be classified in the following ways. Passive, or leaky, channels are protein channels that are always open, allowing certain ions to pass through. These channels are responsible for maintaining the resting membrane potential and are located all over the surface of a neuron. Active, or leaky, channels are the ones that are most likely to pass through the membrane. or gated channels are protein channels that open and close in response to various signals. Chemically gated ion channels open when the appropriate neurotransmitter or chemical binds to the receptor site on the protein. These are important for depolarization and the production of the graded potential and are located only on the dendrites and cell body of the neuron. Voltage gated ion channels open in response to changes in the membrane potential. These are important in the generation and propagation of an action potential and are located only on the axon. Mechanically gated ion channels open in response to some physical deformation of the membrane surface caused by exposure to touch, pressure, or vibration. And remember ions are moving down an electrical chemical gradient. Here you can see some examples of the cell membrane and transmembrane proteins, which make up the different types of channels that we just discussed. Here is an example of a ligand-gated channel or chemically-gated channel, where a neurotransmitter or chemical binds to its appropriate receptor site on the protein, which opens the channel. Mechanically gated channels open in response to some physical deformation of the membrane surface, which again can be caused by... touch, pressure, or vibration, and voltage-gated channels, which open in response to changes in the membrane's potential. Leaky channels, remember, are always open, and they allow for only certain ions to pass through. And again, certain channels are located in the... Axon of the nerve and other channels are located on the dendrites and cell body like the chemically gated ion channels versus the voltage gated ion channels which are located on the axon. The resting membrane potential is referring to when the cell membrane is polarized. The resting membrane potential exists only across the membrane. That is, there is a bulk of solutions inside and outside the cell that are electrically neutral. The resting membrane potential in neurons is approximately minus 70 millivolts. The inside of the neuron's membrane is negatively charged, while the outside of the neuron's membrane is positively charged. Sodium is in its highest concentration outside the cell, while potassium is in its highest concentration inside the cell. All voltage-gated sodium and potassium ion channels are closed so that the neuron cell membrane is relatively impermeable to the two ions. Passive gates for both ions remain open, but movement is minimal. And here we could measure the resting membrane potential using a voltmeter to see the distribution of charge across the cell membrane. Now, graded potentials are processed and can sum to produce an action potential. When we have a neurotransmitter, within the synaptic cleft and it opens sodium chemically gated channels and sodium begins to rush into the neuron down its concentration gradient this process can begin the formation of a graded potential and hence depolarization of the membrane. In that local area of the membrane the interior side of the membrane begins to change from a negative charge to a more positive charge while the exterior changes from a positive to a negative charge. Any change from that resting membrane potential of minus 70 millivolts is called depolarization. As depolarization occurs, the membrane potential becomes less negative, moving from minus 70 millivolts towards, for example, minus 60 millivolts. This switch in charge begins to spread across the dendrites and cell body and is now called a graded potential. If the threshold is great enough, an action potential may ensue. And here you can see some of the steps we just talked about. The chemical stimulus opens the sodium channels, and as sodium channels are opened, we go through a depolarization phase. During the repolarization phase, sodium channels are closing and potassium channels are opening. Graded potentials do show some differences between action potentials. Graded potentials have what we call a decremental conduction, which means You can see here in this bottom figure the strength of the stimulus decays as it gets farther and farther away from the origin of the stimulus. Graded potentials also can go in both directions, whereas action potentials only go in one direction. Now, action potentials can occur as a result of graded potentials. If the graded potential reaches the axon hillock, the voltage gated channels within the axon hillock open, which cause the sodium ions to flow into the axon, switching the charge across the axilemma. This causes the voltage channels to start opening all the way down the axon, and the action potential now moves down the length of the axon. This is called the propagation of the action potential and can generate a change in the charge from that minus 60 millivolts to positive 30 millivolts. Once generated, an action potential cannot be stopped. This is referred to as the all-or-none principle. Myelin on the myelinated nerves causes the local depolarization to jump to the next node of rainbow and then from node to node. This type of propagation that occurs in myelinated axons is called saltatory conduction and is a very rapid form of signaling. On unmyelinated nerves, local depolarization... must spread to sites immediately adjacent to each other, creating a continuous conduction pattern. This type of propagation is relatively slow. And here you can see the phases of an action potential. Hyperpolarization can sometimes occur because potassium channels are slowed to close. So the membrane potential can actually go below minus 70 millivolts. Now during the repolarization phase of an action potential, the removal of the neurotransmitter from the synaptic cleft causes the sodium channels to close so that no additional sodium enters the cell. And that's what's occurring in step four here of the figure. The rapid outflow reduces the total number of positive charges within the cell. causing the charge to switch back across the membrane from positive plus 30 millivolts to minus 70 millivolts. The membrane now goes back to positive outside and negative inside. And again, because the potassium channels can stay open longer, the membrane may become hyperpolarized, which is shown here in step 5. And that would be... let's say approximately minus 90 millivolts. At this point, the sodium-potassium pump is signaled on and pumps three sodium ions to the outside for every two potassium ions pumped to the inside. This reestablishes the resting location of ions while also reestablishing the resting membrane potential of minus 70 millivolts. And that's at step 6 here in the figure, where the membrane potential has returned to its resting state. Now, graded potentials are different from action potentials. Graded potentials can produce temporary changes in the membrane voltage. And the characteristic does depend on the size of the stimulus. Some types of stimuli will cause depolarization of the membrane, but other types can cause hyperpolarization. And it depends on the ion channels that are activated in the cell membrane. So as you can see here, a small stimulus causes a small depolarization in the cell membrane. A larger stimulus causes more depolarization. A stimulus that lasts longer causes a longer depolarization and then a larger stimulus that goes beyond the threshold level may trigger an action potential and this might result from sodium ion channels opening because sodium channel ion channels generally depolarize the membrane and are excitatory. Some stimuli may result in hyperpolarization. And this might be an example of potassium channels opening. And again, the stimuli may produce different types of hyperpolarization depending on the size of the stimulus and how long it lasts. Inhibitory postsynaptic potentials, or IPSPs. And these potential summations can sum. If the summation together, as shown at point B, where a mix of excitatory and inhibitory postsynaptic potentials can result in an action potential at the axon hillock. Now if the graded potential reaches the axon hillock, voltage-gated channels within the axon hillock open and cause sodium ions to flow into the axon, which can change the charge across the axilemma. Now the propagation of the action potential can vary based on the level of myelination. Once generated an action potential cannot be stopped and that's referred to as the all-or-none principle. In a bare plasma membrane the voltage would decay as it goes across along the axon because there's current links across the membrane. In an unmyelinated axon, the voltage-gated sodium and potassium channels can regenerate the action potential at each point along the axon, so the voltage doesn't decay. In a myelinated axon, the myelon current doesn't decay as much. The action potential is generated only in the nodes of Ranvir. And so the action potential jumps from node to node, and this type of propagation is called saltatory conduction and is very rapid. So myelin on the myelinated nerves causes the local depolarization to jump to the next node and then from node to node. So we have gone through some of the steps of the neurophysiology shown here, where we have examined the resting potential, what the stimulus can produce in a neuron on the soma or cell body in the form of graded potentials, which may sum to produce an action potential, and then there is the transmission effects in the postsynaptic cell where the information is processed via the neurotransmitter traveling across the synaptic cleft. So let's look at some of that postsynaptic processing. The net effect from the summation of all the neurotransmitter effects can be an excitatory postsynaptic potential which is known as an EPSP with depolarization. Again this would result from possibly sodium channels opening. which are excitatory and depolarizing, or inhibitory postsynaptic potential, IPSPs, with graded hyperpolarization from potassium channels opening, which generally are inhibitory. Now, there's different classes of neurotransmitters that you should be familiar with. There are... Cholinergic transmitters such as acetylcholine which is excitatory and skeletal muscle. Biogenic amines and some classes of those are dopamine, norepinephrine and epinephrine. Serotonin which alters your mood, sleep, appetite, anger and these are generally inhibitory. histamine. Two classes of amino acid neurotransmitters that you should be familiar with are GABA, gamma-aminobutyric acid, which is inhibitory, and glutamate, which is excitatory. And then peptides, classes of peptides to be familiar with are the endorphins and enkephalins, which are inhibitory, and these are your opioids. Glutamate is important in memory and learning. GABA opens chloride channels and can have an indirect effect on potassium, thereby demonstrating its inhibitory effects. Now there are several different receptor types, two of which are noted here. An ionotropic receptor is a channel that opens the neurotransmitter when it's bound to it. A metabotropic receptor causes metabolic changes in the cell when the neurotransmitter is bound to it. So in this case, after binding, a G protein, which is associated with the receptor, hydrolyzes GTP, guanosine triphosphate, and moves to the effector protein. Then the G protein contacts the effector protein, and a second messenger is generated. sometimes cyclic AMP. Second messengers can often go on to do many biological things within the cell. It can cause changes in the neuron like opening or closing ion channels. It can have an impact on metabolic changes or changes in gene transcription. So there can be a cascade of events anytime a second messenger is activated. This concludes our general look at the nervous system and neural tissue.

This is also considered to be the seat of all mental activity. It interprets sensory input and dictates motor responses based on past experiences, reflexes, and current conditions. The peripheral nervous system is composed of the cranial and spinal nerves. These are the communication lines between the central nervous system and the rest of the body. There are two divisions of the peripheral nervous system.

The sensory division, which is the afferent, and conducts impulses from sensory receptors to the central nervous system. Sensory receptors, as you will see, are within somatic, your skin, muscle, and joints, and visceral, which are the organs. The sensory division is considered the input region.

The motor division, which is efferent, consists of the motor neurons, which conduct impulses from the central nervous system. to the effectors, which are the muscles and glands. The motor division is considered the output, and the motor division is further subdivided into the somatic nervous system and the autonomic nervous system.

Now the functions of the nervous system are listed here. The nervous system uses sensory receptors to monitor changes both inside and outside of the body. That's the input.

Nervous system tissue processes and interprets sensory input. That's the integration. And affects a response appropriate to the stimulus, which is the motor output.

This allows for homeostasis to be maintained. Here is the nervous system organization that I mentioned. And within the autonomic, we also have the sympathetic nervous system, which mobilizes the body during emergency situations like fight or flight.

and the parasympathetic, which conserves energy and promotes non-emergency functions like during resting and digesting. Here is an additional slide showing you the organization of the nervous system that we just talked about. It gives you some additional detail that I mentioned previously. Now, let's examine...

nerve tissue by first looking at neurons. Nervous tissue is composed of densely packed intertwined cells of two specific types. Neurons are the functional cells of nervous tissue responsible for receiving, interpreting, and sending stimuli.

Neurons exhibit several unique characteristics like being excitable. They possess a polarized membrane which allows them to conduct messages in the form of a nerve impulse from one part of the body to another. Neurons also have longevity. They can function for nearly a hundred years.

They have a high metabolic rate. Neurons cannot survive for more than a few minutes without oxygen, glucose, or ATP. Neurons can be very large.

They're some of the largest cells in the body and they're amyototic. Most central nervous system neurons lose their ability to divide after they assume the role as communication lines. You can also see here some of the parts of a typical neuron. We have the cell body, which is also called the soma. This is the enlarged metabolic region of the cell where the nucleus is located.

Neurons possess large amounts of rough endoplasmic reticulum, which is clustered within the cell body to form nissle bodies. Clusters of cell bodies in the central nervous system are often called nuclei, and clusters of cell bodies within the peripheral nervous system are called ganglia. You can also see several processes.

Cellular processes are either called tracts in the central nervous system or nerves in the peripheral nervous system. Dendrites are neuron processes which possess large surface area because of numerous branches. They receive chemical stimuli as well as electrical signals. towards the cell body and these electrical signals are not nerve impulses.

They're called graded potentials which we will talk about later. You can also see the axon. The axon is capable of generating action potentials and transmitting nerve impulses away from the cell body during axoplasmic transport.

Whenever the signal travels from the axon terminals Back toward the cell body, it's called retrograde flow. The plasma membrane surrounding the axon is called the axilemma, while the cytoplasm is called the axoplasm. The axon forms at a tapered area called the axon hillock. This is the trigger zone because graded potentials much...

must reach this area of the neuron before they can be converted into action potentials. As the axon approaches the next cell, many fine extensions branch from its end forming telodendria each ending in an axon terminal. The synaptic terminals possess many synaptic vesicles that contain neurotransmitters which are used across the synapse or synaptic cleft found between the presynaptic cell and the postsynaptic cell. The myelin sheath some axons are covered with a white fatty protein lipid and which is known as the myelin sheath.

Myelin protects electrically insulates fibers from one another. Myelinated fibers conduct impulses rapidly and are white in appearance. Unmyelinated fibers conduct impulses quite slowly and generally have a gray appearance. And we'll talk about what makes the mylon sheath later on.

This slide shows you how neurons can synapse with other neurons for cell-to-cell communication. You can have a synapse with another neuron. You can have a synapse at a neuromuscular junction or a neuroglandular synapse. All of these will allow for cell-to-cell communication between neurons with a neuron, a muscle, or a gland. Now there is different types of neuron classification.

Neurons are classified by their structure or the number of cell processes. Bipolar neurons have two cell processes, one dendrite and one axon. Bipolar neurons are rare but occur in the retina of the eye and within the nasal mucosa. Unipolar neurons, the dendrites and axon, are continuous and basically fused with the cell body located off to the side. Most sensory neurons of the peripheral nervous system are unipolar.

Multipolar neurons have one axon and two or more dendrites. These are the most common neurons of the central nervous system, and all motor neurons that control skeletal muscles are multipolar neurons. Now let's examine some of the neuroglial cells, which are simply called glial cells. Neuroglial cells feed, protect, and insulate neurons. It is estimated that there are between 700 to 900 neuroglial cells per neuron.

Neuroglial cells differ within the central nervous system and the peripheral nervous system. Neural glial cells of the central nervous system consist of astrocytes, which make up about half of all neural volume, and they're generally star-shaped. They possess numerous projections with a bulb at the end that cling to neurons and capillaries, and therefore they serve as connections between neurons and the blood nutrient supply. This is called the blood-brain barrier.

Astrocytes control the chemical environment around neurons by regulating ions, nutrients, dissolved gas concentrations, and hormones. They also absorb and recycle neurotransmitters that cannot be broken down and they form scar tissue after injury. Another type of neuroglial cell in the central nervous system is microglia.

Microglia are ovoid cells with highly branched processes. These act as macrophages that engulf microbes and dead neural cells as well as remove cellular debris, waste products, and pathogens. Another type is the oligodendrocytes. These possess few branches and these cells line up along thicker neuron fibers in the central nervous system and wrap their extensions around nerve fibers forming the myelin sheath to insulate the neurons from each other. The last type of neuroglial cell within the central nervous system is the epidermal cells.

These cells line the central cavities of the brain and spinal cord creating a barrier between the central nervous system cavities and the tissues surrounding the cavities. Epidymal cells assist in producing, monitoring, and circulating cerebral spinal fluid. They use their cilia to circulate the CSF within the cavities of the central nervous system.

Neuroglial cells of the peripheral nervous system are the Schwann cells and satellite cells. Schwann cells form the myelin sheath around large nerve fibers in the peripheral nervous system. Schwann cells can also act as phagocytic cells that engulf damaged or dying nerve cells and are important in directing the process of regeneration. The myelin sheath forms by the Schwann cells wrapping around and around the axon. Satellite cells surround the nerve cell body and may aid in controlling the chemical environment around the neuron, much like the astrocytes of the central nervous system.

And here you can see the myelin sheath and the neurolemma. The intercompressed layers of the myelin, while you have the outermost metabolically active layer, which is known as the neurolemma. And gaps between, for example, Schwann cells are called the nodes of Ranvier.

This is where the action potential will jump from node to node during conduction. Now the sensory motor response is shown here and we will talk about the sensory motor response. in a little more detail. A sensory neuron, for example, we discussed how you have sensory neurons throughout the body.

One in the skin is shown here that might be sensitive to temperature changes. And the graded potential from the sensory ending, if it is strong enough, can initiate an action potential. The axon of the sensory neuron then enters the spinal cord and may contact another neuron in the gray matter. The action potential is initiated at the initial segment of the neuron and travels up the sensory pathway to a region of the brain called the thalamus.

Almost all sensory neurons must go through the thalamus on their way up to higher brain centers with the exception of olfaction or your sense of smell. Another synapse passes the information. along to the next neuron.

The sensory pathway then ends when the signal reaches the appropriate area of the cerebral cortex. After integration with neurons in other parts of the cortex, a motor command is then initiated. The motor neuron will send an action potential down the spinal cord to another motor neuron in the gray matter of the spinal cord.

The axon of the motor neuron emerges from the spinal cord in a nerve and connects to a muscle through the neuromuscular junction to cause contraction of the target muscle. So this shows you the pathway from the sensory afferent going towards the central nervous system back out through the motor efferent division to the appropriate muscle or gland. Okay, so now we're going to talk about neurophysiology. In the body, electrical currents correspond to the flow of ions across cellular membranes.

The transmission of a nerve impulse is very similar to the process of a muscle contraction that you learned about previously and occurs in the following steps. We have a resting membrane potential where the cell membrane is considered to be polarized. We have a depolarization.

and the production of a graded potential. Then there is a conversion of the graded potential to an action potential and then the propagation of an action potential down the length of the axon to the synaptic terminals. Next, repolarization or re-establishing the resting membrane conditions occurs.

Ion channels play a crucial role in establishing ion concentrations on either side of the neuron cell membrane. And they can be classified in the following ways. Passive, or leaky, channels are protein channels that are always open, allowing certain ions to pass through. These channels are responsible for maintaining the resting membrane potential and are located all over the surface of a neuron.

Active, or leaky, channels are the ones that are most likely to pass through the membrane. or gated channels are protein channels that open and close in response to various signals. Chemically gated ion channels open when the appropriate neurotransmitter or chemical binds to the receptor site on the protein. These are important for depolarization and the production of the graded potential and are located only on the dendrites and cell body of the neuron. Voltage gated ion channels open in response to changes in the membrane potential.

These are important in the generation and propagation of an action potential and are located only on the axon. Mechanically gated ion channels open in response to some physical deformation of the membrane surface caused by exposure to touch, pressure, or vibration. And remember ions are moving down an electrical chemical gradient. Here you can see some examples of the cell membrane and transmembrane proteins, which make up the different types of channels that we just discussed. Here is an example of a ligand-gated channel or chemically-gated channel, where a neurotransmitter or chemical binds to its appropriate receptor site on the protein, which opens the channel.

Mechanically gated channels open in response to some physical deformation of the membrane surface, which again can be caused by... touch, pressure, or vibration, and voltage-gated channels, which open in response to changes in the membrane's potential. Leaky channels, remember, are always open, and they allow for only certain ions to pass through. And again, certain channels are located in the...

Axon of the nerve and other channels are located on the dendrites and cell body like the chemically gated ion channels versus the voltage gated ion channels which are located on the axon. The resting membrane potential is referring to when the cell membrane is polarized. The resting membrane potential exists only across the membrane. That is, there is a bulk of solutions inside and outside the cell that are electrically neutral.

The resting membrane potential in neurons is approximately minus 70 millivolts. The inside of the neuron's membrane is negatively charged, while the outside of the neuron's membrane is positively charged. Sodium is in its highest concentration outside the cell, while potassium is in its highest concentration inside the cell. All voltage-gated sodium and potassium ion channels are closed so that the neuron cell membrane is relatively impermeable to the two ions.

Passive gates for both ions remain open, but movement is minimal. And here we could measure the resting membrane potential using a voltmeter to see the distribution of charge across the cell membrane. Now, graded potentials are processed and can sum to produce an action potential.

When we have a neurotransmitter, within the synaptic cleft and it opens sodium chemically gated channels and sodium begins to rush into the neuron down its concentration gradient this process can begin the formation of a graded potential and hence depolarization of the membrane. In that local area of the membrane the interior side of the membrane begins to change from a negative charge to a more positive charge while the exterior changes from a positive to a negative charge. Any change from that resting membrane potential of minus 70 millivolts is called depolarization. As depolarization occurs, the membrane potential becomes less negative, moving from minus 70 millivolts towards, for example, minus 60 millivolts.

This switch in charge begins to spread across the dendrites and cell body and is now called a graded potential. If the threshold is great enough, an action potential may ensue. And here you can see some of the steps we just talked about.

The chemical stimulus opens the sodium channels, and as sodium channels are opened, we go through a depolarization phase. During the repolarization phase, sodium channels are closing and potassium channels are opening. Graded potentials do show some differences between action potentials.

Graded potentials have what we call a decremental conduction, which means You can see here in this bottom figure the strength of the stimulus decays as it gets farther and farther away from the origin of the stimulus. Graded potentials also can go in both directions, whereas action potentials only go in one direction. Now, action potentials can occur as a result of graded potentials.

If the graded potential reaches the axon hillock, the voltage gated channels within the axon hillock open, which cause the sodium ions to flow into the axon, switching the charge across the axilemma. This causes the voltage channels to start opening all the way down the axon, and the action potential now moves down the length of the axon. This is called the propagation of the action potential and can generate a change in the charge from that minus 60 millivolts to positive 30 millivolts. Once generated, an action potential cannot be stopped.

This is referred to as the all-or-none principle. Myelin on the myelinated nerves causes the local depolarization to jump to the next node of rainbow and then from node to node. This type of propagation that occurs in myelinated axons is called saltatory conduction and is a very rapid form of signaling. On unmyelinated nerves, local depolarization... must spread to sites immediately adjacent to each other, creating a continuous conduction pattern.

This type of propagation is relatively slow. And here you can see the phases of an action potential. Hyperpolarization can sometimes occur because potassium channels are slowed to close. So the membrane potential can actually go below minus 70 millivolts. Now during the repolarization phase of an action potential, the removal of the neurotransmitter from the synaptic cleft causes the sodium channels to close so that no additional sodium enters the cell.

And that's what's occurring in step four here of the figure. The rapid outflow reduces the total number of positive charges within the cell. causing the charge to switch back across the membrane from positive plus 30 millivolts to minus 70 millivolts.

The membrane now goes back to positive outside and negative inside. And again, because the potassium channels can stay open longer, the membrane may become hyperpolarized, which is shown here in step 5. And that would be... let's say approximately minus 90 millivolts.

At this point, the sodium-potassium pump is signaled on and pumps three sodium ions to the outside for every two potassium ions pumped to the inside. This reestablishes the resting location of ions while also reestablishing the resting membrane potential of minus 70 millivolts. And that's at step 6 here in the figure, where the membrane potential has returned to its resting state.

Now, graded potentials are different from action potentials. Graded potentials can produce temporary changes in the membrane voltage. And the characteristic does depend on the size of the stimulus.

Some types of stimuli will cause depolarization of the membrane, but other types can cause hyperpolarization. And it depends on the ion channels that are activated in the cell membrane. So as you can see here, a small stimulus causes a small depolarization in the cell membrane.

A larger stimulus causes more depolarization. A stimulus that lasts longer causes a longer depolarization and then a larger stimulus that goes beyond the threshold level may trigger an action potential and this might result from sodium ion channels opening because sodium channel ion channels generally depolarize the membrane and are excitatory. Some stimuli may result in hyperpolarization. And this might be an example of potassium channels opening. And again, the stimuli may produce different types of hyperpolarization depending on the size of the stimulus and how long it lasts.

Inhibitory postsynaptic potentials, or IPSPs. And these potential summations can sum. If the summation together, as shown at point B, where a mix of excitatory and inhibitory postsynaptic potentials can result in an action potential at the axon hillock. Now if the graded potential reaches the axon hillock, voltage-gated channels within the axon hillock open and cause sodium ions to flow into the axon, which can change the charge across the axilemma.

Now the propagation of the action potential can vary based on the level of myelination. Once generated an action potential cannot be stopped and that's referred to as the all-or-none principle. In a bare plasma membrane the voltage would decay as it goes across along the axon because there's current links across the membrane. In an unmyelinated axon, the voltage-gated sodium and potassium channels can regenerate the action potential at each point along the axon, so the voltage doesn't decay. In a myelinated axon, the myelon current doesn't decay as much.

The action potential is generated only in the nodes of Ranvir. And so the action potential jumps from node to node, and this type of propagation is called saltatory conduction and is very rapid. So myelin on the myelinated nerves causes the local depolarization to jump to the next node and then from node to node.

So we have gone through some of the steps of the neurophysiology shown here, where we have examined the resting potential, what the stimulus can produce in a neuron on the soma or cell body in the form of graded potentials, which may sum to produce an action potential, and then there is the transmission effects in the postsynaptic cell where the information is processed via the neurotransmitter traveling across the synaptic cleft. So let's look at some of that postsynaptic processing. The net effect from the summation of all the neurotransmitter effects can be an excitatory postsynaptic potential which is known as an EPSP with depolarization.

Again this would result from possibly sodium channels opening. which are excitatory and depolarizing, or inhibitory postsynaptic potential, IPSPs, with graded hyperpolarization from potassium channels opening, which generally are inhibitory. Now, there's different classes of neurotransmitters that you should be familiar with.

There are... Cholinergic transmitters such as acetylcholine which is excitatory and skeletal muscle. Biogenic amines and some classes of those are dopamine, norepinephrine and epinephrine. Serotonin which alters your mood, sleep, appetite, anger and these are generally inhibitory.

histamine. Two classes of amino acid neurotransmitters that you should be familiar with are GABA, gamma-aminobutyric acid, which is inhibitory, and glutamate, which is excitatory. And then peptides, classes of peptides to be familiar with are the endorphins and enkephalins, which are inhibitory, and these are your opioids.

Glutamate is important in memory and learning. GABA opens chloride channels and can have an indirect effect on potassium, thereby demonstrating its inhibitory effects. Now there are several different receptor types, two of which are noted here.

An ionotropic receptor is a channel that opens the neurotransmitter when it's bound to it. A metabotropic receptor causes metabolic changes in the cell when the neurotransmitter is bound to it. So in this case, after binding, a G protein, which is associated with the receptor, hydrolyzes GTP, guanosine triphosphate, and moves to the effector protein. Then the G protein contacts the effector protein, and a second messenger is generated. sometimes cyclic AMP.

Second messengers can often go on to do many biological things within the cell. It can cause changes in the neuron like opening or closing ion channels. It can have an impact on metabolic changes or changes in gene transcription. So there can be a cascade of events anytime a second messenger is activated.

This concludes our general look at the nervous system and neural tissue.

Transcript for:Overview of the Nervous System

Transcript for:
Overview of the Nervous System