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01 Introduction to Human Physiology
02 Interactions Between Cells and Their Environment
03 From Cells to Systems
04 Nervous System - Neurons and Synapses
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04 Nervous System - Neurons and Synapses
Individuals with neurological disorders can suffer significant problems in a large variety of functional categories. Physical therapy can supply an aggressive rehab technique to reduce the impairment an affected person will deal with. A wide range of neurological disorders can be dealt with, as neurological physical therapy covers disorders, diseases, or injury that affects parts of the CNS. A neurologic clinical specialist (NCS) is a specially trained physical therapist who provides hope and relief to persons who deal with the effects of neurological diseases and disorders. Some of the disorders that can be treated include ALS, cerebral palsey, multiple sclerosis, Parkinson's, to go along with many other diseases and disorders. Some conditions suffered by individuals with these disorders are paralysis, sense impairment, decreased range of motion, and decreased independence. Some of the interventions that can be performed on people with neurelogical disorders include Gait retraining, hydrotherapy, range of motion activities, strength training, and many others. All of this is according to an article found on the internet.
This unit was one of the most interesting and most confusing units so far, there is a lot of complicated concepts to try to grasp. The unit starts out describing the anatomical make up of the nervous system. It goes through what is incorporated in the CNS, or central nervous system, (brain and spinal cord) and the PNS, or peripheral nervous system, (cranial nerves and spinal nerves). Introduced next are neurons which are the functional unit of the nervous system. Neurons conduct electrical signals to send messages from one point to the next. The structure of a neuron consists of a cell body, which contains the nucleus and organelles, dendrites, which receive stimulation, and an axon, which sends the message to the destination. Along with the neurons are the nueroglia, which is the most numerous structure in the nervous system. There are two ways a neuron can be broken down, the first is by functional types. The first functional type is sensory, or afferent, neurons. These neurons are apart of the PNS and transmit electrical signals from tissues to the brain and the spinal cord. Their main function is to detect changes in the environment (hot or cold, pain, etc) and relay that information to the CNS. The next functional type of neuron is the motor, or efferent, neuron. It also is apart of the PNS but the difference is it transmits signals from the CNS to the tissues or effectors. The final type of neuron is called the association, or interneuron. These are located within the CNS and analyze and send out the signal from the brain. Part two of this chapter is where the confusion starts. This first covers resting potential of a membrane. Resting potential is due to difference in permeability of a membrane to be charged with particles. At resting potential neither Potassium or sodium are in an equal state. Electrical signals are charges in the membrane potential away from the resting potential. An impulse, also known as depolarization, is due to a decrease in the difference in charge between the inside of the cell and the outside of the cell. Depolarization happens when there is more of a positive charge inside of the cell. Suppression, or hyperpolarization, is due to the increase difference in charge between the inside and outside of the cell. Basically, when there is more of a negative charge inside the cell than there is outside the cell. To make the neurons fire different proteins need to connect to channels on the membrane to open these gates so positive and negative ions can flow either in or out of the cell. One type of gate that is used is called a leakage gate. These channels are always open but still only let specific ions through. There are two types of gated channels ligand-gated and voltage-gated. Ligand gated need a binding of a chemical signal to open and these are found on cell bodies and dendrites. The voltage gated open when there is a change in the membrane potential. The Sodium/Potassium pump is another way ions can get in and out of the neuron. This is an active transport, meaning it needs energy to work. The Na/K pump acts to maintain resting potential and restore the membrane potential. When these gates open and membrane potential reaches its threshold the neuron fires. Depolarization (neuron goes from a negative charge to a positive charge) is the triggering event that causes the neuron to fire. The opposite of that is repolarization, when the neuron reaches its peak and the Na channels are deactived. At this point the K voltage channels open and potassium flows into the cell giving the neuron a more negative charge. When the neuron is repolarizing this period is known as the refractory period. This is the time that must pass before the neuron can fire again. An absolute refractory period the sodium channels are either fully open or in an inactive state. Relative refractory period refers to the period when the sodium channels can be activated again, but potassium is still flowing out of the cell at a high rate. This electric charge must travel down the axon to a synapse. A synapse is a communication junction between another neuron or different kind of effector. The synapse sends neurotransmitters out into another cell body or into the effector which makes us react in many different ways.
- A synapse is a communication junction between a neuron and another neuron or an effector. Chemical signals are released from an axon and bind to different receptors on another cell. The presynaptic neuron is the neuron that fires upon the next cell. They contain synaptic vessicles filled with neurotransmitters that stimulate the cell body of the next neuron, or postsynaptic neuron. Chemicals in the synaptic vessicle have many voltage-gated calcium channels in their membrane at the axon terminals. Action potential triggers an opening of the calcium channels and the ions rush into the cell. Transmitters in the presynaptic neuron diffuse across the synaptic cleft by exocytosis and bind to receptors on the postsynaptic cell membrane. There are also ligand-gated ion channels that aid in the transfer of neurotransmitters. The picture below depicts what a synapse looks like, also it shows how the neurotransmitters flow across the synaptic cleft into the receptors on the dendrites
- Neurotransmitters are the chemical signals that are sent from an axon terminal to the receptors on the postsynaptic dendrite. The most common neurotransmitter is acetycholine (ACh) and is used by many diverse neurons. Nicotinic ACh receptors are found in certain brain regions, and cell bodies of autonomic motor neurons. This receptor is a ligand-gated ion channel which is turned on by the binding of ACh. The ACh is quickly broken down by an enzyme called acetylcholinesterase embedded in the post-synaptic neuron. Another important neurotransmitter in the body is called gamma-aminobutyric acid (GABA) and is an important centeral nervous system transmitter. The video below does a good job of summing up the whole chapter in about a minute, toward the end it shows how the neurotransmitters flow from the pre-synaptic neuron to the post-synaptic neuron through the ligand-gated channels on the post-synaptic receptors.
Myelin is an insulating layer that forms around nerves, including those in the brain and spinal cord. It is made up of protein and fatty substances.
The myelin sheath allows faster and more energetically efficient conduction of impulses. The sheath is formed by the cell membranes of glial cells (Schwann cells in the peripheral and oligodendroglia in the central nervous system). The Schwann cell wraps itself around the axon and when cut into a cross section can look like rings in a tree. The spaces in between each Schwann cell is called a Node of Ranvier, which if I understand correctly is where most of the ion exchange from the outside to the inside takes place. If the myelin sheath is damaged it can slow down the electrical impulse being transferred by the axon. One disorder that affects the myelin sheath is called multiple sclerosis. This disease makes it so the neuron cannot transfer the signal down the axon.
1. The pre-synaptic neuron is transmitting an electrical impulse down the axon to the post-synaptic neuron. The neurotransmitters in the pre-synaptic neuron diffuse across the synaptic cleft and binds to receptors on the axon hillock of the postsynaptic cell membrane. These transmitters stimulate proteins to activate specific ligand-gated ion channels in the postsynaptic membrane. The cell body then sends the transmission down the axon. Electrical signals provoke changes in the membrane potential away from the resting membrane potential. The impulse sent down the axon is also known as depolarization. Depolarization is regulated by voltage-gated channels. The changes in the membrane potential cause the channel to open. This requires the membrane to be depolarized to -55 mV. Once the membrane hits this threshold the axon can go up to its action potential which causes the neuron to fire. The Na/K pump is used to maintain the balance of sodium and potassium (the two ions that cause the neuron to fire). This could also be explained by Excititory postsynaptic potential (EPSP). If an EPSP is a strong enough depolarization to reach threshold then action potential forms in the postsynaptic cell, according to the narrated notes.
2. When the membrane is at resting potential (which is -70 mV) it takes a charge big enough to stimulate the axon. The charge needed to stimulate the axon is called the threshold which is -50 mV. Once the threshold is met then the membrane is to its action potential. The charge travels down the axon by depolarization and after the charge passes through the cell voltage-gated channels are open, after that the axon suppresses by repolarization. When the membrane potential reaches +30 mV the Na channels are
and K channels are opened so potassium can flow out of the cell (this is regulated by the sodium/potassium pump). As the charge flows down the presynaptic neuron it comes to the synapse where the charge must jump across to the next axon hillock. The voltage-gated Ca+ channels are triggered and the calcium rushes into the axon terminal. The calcium induces exocytosis of the synaptic vesicles and the neurotransmitters diffuse across the synaptic cleft to the next neuron.
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