Nervous System

Unit Overview: In this section you will learn about how neurons are the functional part of the nervous system. You will also learn that neurons are located in the PNS and CNS. Continue with learning about ACh receptors and the two different kinds: nicotinic and muscarinic. You will also learn the four different stages of the release of a neurotransmitter.

1. Neurons and Supporting Cells
Well first of all just a little reminder that our nervous system is split into two different parts the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS contains the brain and spinal cord, while the PNS contains everything outside of the CNS. The nervous system as a whole has only two different types of cells, the supporting cells and the neurons. Neurons are the functional part of the nervous system. "They are specialized to respond to physical and chemical stimuli, conduct impulses, and release chemical regulators," Fox, Stuart. By having the ability to do all of these different functions they allow the control of glands, muscles, and learning memory. Every neuron is different in size but they all contain three main parts. These include: dendrites, cell body, and an axon.


The cell body is the larger portion of the neuron. It is the place were the nucleus is located and macromolecules are made. The cell bodies of neurons like to form a type of cluster or group called nuclei. In the PNS they are known as ganglia when they form a group or cluster. Dendrites are the tree like branches that come off of the cell body. Dendrites are afferent and take impulses to the cell body. The axon is the longer process coming off of the cell body at a place called the axon hillock. Every axon can be different in length. Axons are efferent and take impulses away from the cell body. Axon collaterals are what come off of the axon at their ends. Sensory also known as afferent neurons bring information into the CNS. While on the other hand, Motor also known as efferent neurons bring information out of the CNS to what ever needs to be targeted whether its a muscle or gland. Located inside the CNS are the association neurons that merge together functions of the nervous system. Somatic and autonomic neurons are the two different types of efferent neurons. The somatic is what controls are reflexive and voluntary actions over are skeleton muscles. The autonomic sends out information to smooth muscles, cardiac muscles, and glands. 8547neurons.jpg

"The structural classification of neurons is based on the number of processes that extend from the cell body of the neuron," Fox, Stuart. One type known as the pseudounipolar are composed of a short single process that branches out into a T to go into two larger process. Out of these two larger processes, each one has a different job. One is a sensory, which receives and produces nerve impulses. The other one is takes the impulses either to the brain or spinal cord. Another type of classification is known by bipolar neurons which are found in the retina of the eye. Another type is called the multipolar neurons. These are the most common type and contain many dendrites and only one axon coming from the cell body.

Nerves are a cluster of axons that are outside of the CNS. Most are made up of both the motor and sensory fibers which make them mixed nerves. You will find the ones only containing sensory fiber in the cranial nerves, mostly found in the hearing, tasting, smelling, and sight senses. When you have a cluster of axons in the CNS they are called a tract.

Supporting cells come from embryonic tissue layer. In the PNS there are two different types of supporting cells: Schwann and satellite (ganglionic gliocytes) cells. The schwann cells form the myelin sheath that covers the axon and the satellite supports the cell bodies. There are four different types of supporting cells in the CNS. First there is oligodendrocytes which also forms the myelin sheath around axons (this takes place after birth), then there is the microglia which gets rid of cellular debris, then astrocytes that help with regulating the external environment of neurons, lastly you have the ependymal cells which lines the central part in the spinal cord and the cavities in the brain. Myelin sheath makes the axons myelinated which helps with the ability to transmit impulses faster than those that are unmyelinated. These schwann cells wrap around the axon as if you where wrapping it up in a blanket. So in other words they over lap and have many layers. Each of these cells only take up about one millimeter of the axon, leaving a gap before another schwann cell wraps around the axon. In the middle of each schwann cell were the space is, that is called the nodes of Ranvier which produce nerve impulses. Back to the oligodendrocytes, they have extension which allow it to form myelin over the many axons. The myelin over axons in the CNS makes a white color while the ones that are not myelinated are the gray color. If by chance an axon in a peripheral nerve is cut off, it will be cleaned up by the schwann cells but then the schwann cells make a regeneration tube which starts the growing of a new axon tip. Astrocytes make up most of the glial cells in the CNS. The functions of astrocyes are: To take up K+ form the extracellular fluid, neurotransmitters that are released from the axon terminals and glucose from the blood. They are also needed for the formation of synapses in the CNS, of blood-brain barrier, to regulate neurogenesis in adults brain, and to release transmitter chemicals that can stimulate or inhibit neurons.

2. Acetylcholine as a neurotransmitter, Monoamines as neurotransmitter, Other neurotransmitters

Acetylcholine as a Neurotransmitter

There are two different subtypes of ACh receptors: nicotinic and muscarinic.

  • Nicotinic receptors enclose membrane channels and open when ACh bonds to the receptor. This causes a depolarization called an excitatory postsynaptic potential (EPSP) in skeletal muscle cells.



  • The binding of ACh to muscarinic receptors opens ion channels indirectly, through the action of G-proteins. This can cause a hyperpolarization called an inhibitory postsynaptic potential (IPSP).


- After ACh acts at the synapse it is inactivated by the enzyme acetylcholinesterase (AChE).


Chemically Regulated Channels

Ligand-Gated Channels,
G-Protein-Coupled Channels

EPSPs are graded and capable of summation. They decrease in amplitude with distance as they are conducted.



ACh is used in the PNS as the neurotransmitter of somatic motor neurons, which stimulate skeletal muscles to contract, and by some autonomic neurons.

ACh in the CNS produces EPSPs at synapses in the dendrites or cell body. These EPSPs travel to the axon hillock, stimulate opening of voltage-regulated gates, and generate action potentials in the axon.



Monoamines as Neurotransmitters

Monoamines include serotonin, dopamine, norepinephrine, and epinephrine. The last three are also included in the subcategory known as catecholamines.

  • These neurotransmitters are inactivated after being released, primarily by reuptake into the presynaptic nerve endings.
  • Catecholamines may activate adenylate cyclase in the postsynaptic cell, which catalyzes the formation of cyclic AMP.

click picture scroll down to figure. 8 at end of the page

Dopaminergic neurons (those that use dopamine as a neurotransmitter) are implicated in the development of Parkinson’s disease and schizophrenia. Norepinephrine is used as a neurotransmitter by sympathetic neurons in the PNS and by some neurons in the CNS.

Other Neurotransmitters

The amino acids glutamate and aspartate are excitatory in the CNS.

  • The subclass of glutamate receptor designated as NMDA receptors are implicated in learning and memory.

  • The amino acids glycine and GABA are inhibitory. They produce hyperpolarizations, causing IPSPs, by opening Cl- channels.


There are a large number of polypeptides that function as neurotransmitters, including the endogenous opioids.


Nitric oxide functions as both a local tissue regulator and a neurotransmitter in the PNS and CNS. It promotes smooth muscle relaxation and is implicated in memory.

3. The Synapse & Synaptic Integration

A synapse is the functional connection between a neuron (presynaptic) and a second cell (postsynaptic). This other cell is also a neuron in the CNS. In the PNS, the other cell may be either a neuron or an effector cell within a muscle or gland. the synapse occurs most commonly between the axon of the presynaptic neuron and the dendrites or cell body of the postsynaptic neuron.

There are chemical and electrical synapses.
Electrical synapses: for two cells to be electrically coupled, they must be close to equal in size and they must be joined by areas of contact with low electrical resistance. Depolarization flows from presynaptic into postsynaptic cells through channels called gap junctions. Each gap junction is composed of 12 proteins known as connexins. Gap junctions are found in cardiac muscle. They allow action potentials to spread from cell to cell so that myocardium can contract as one unit. Gap junctions are also found in smooth muscles to allow many cells to be stimulated and contract together, which produces a stronger contractions. These junctions can also be found between neuroglial cells.

Chemical synapses: Transmission across the majority of synapses in the nervous system is one-way and occurs through the release of chemical neurotransmitters from presynaptic axon endings. These endings are terminal boutons. The synaptic cleft separates terminal bouton of presynaptic from postsynaptic cells.


Release of a Neurotransmitter


Steps 1-4 summarize how action potentials stimulate the exocytosis of synaptic vesicles. Action potentials open channels for Ca2+. Docked vesicles are held to the plasma membrane of the axon terminals by a complex of SNARE proteins. Ca2+ sensor protein complex alters the SNARE complex to allow the complete fusion of the synaptic vesicles with the plasma membrane.

The ligand (chemically) gated channels are located in the dendrites and cell body. This allows these regions to respond to neurotransmitter chemicals. The depolarization produced by those channels must spread to the axon hillock. The axon hillock is where the first action potentials are produced. After depolarization stimulus causes the opening of voltage gated channels, the action potentials can be conducted without decrement along the axon.

Synaptic Integration

Divergence of neural pathways can occur because axons can have collateral branches. One neuron can make synapses with other neurons. A number of axons can synapse on a single neuron which allows convergence of neural pathways.
Spatial summation is a result of the convergence of presynaptic axon terminals on dendrites and cell body of postsynaptic neuron. It takes place when EPSPs from different synapses occur in a postsynaptic cell at the same time.
Temporal Summation occurs because EPSPs that occur closely in time can sum before they fade.

Spatial Summation

Postsynaptic inhibition is inhibition of a postsynaptic neuron by axon endings that release a neurotransmitter that induces hyperpolarization.
Presynaptic inhibition is a neural inhibition in which axoaxonic synapses inhibit the release of neurotransmitter chemicals from presynaptic axon. It occurs when one neuron synapses onto an axon or bouton of another neuron. This inhibits the release of its NT.

Postsynaptic Inhibition

Essential Questions:
-Describe how the dendrite or cell body of the postsynaptic neuron is stimulated (excited) to send an impulse from the axon hillock to the rest of the neuron. -Include these key words in your description: neurotransmitter, ligand gated channels, Na+, depolarization, EPSP, axon hillock, voltage-gated channels (Na+ and K+), action potential.
When nerotransmitters are released they travel past the axon terminals and diffuse quickly past the synaptic cleft going into the postsyamptic cell. It binds with a specific part of the receptor proteins that is part of the postsynaptic membrane. This special part where the neurotransmitter contexts to is called the ligand, causing the ligand-regulated gates to open (chemically-regulated gates). When these voltage channels open, particularly sodium and calcium, they enter the cell causing depolarization which is called EPSP, because the membrane action potential moves towards the threshold needed. The action potentials are first produced at the axon hillock where the highest density of voltage gated potassium and sodium channels are first produced. The depolarization produced from these channels by the stimulation of EPSP will determine if the axon potential will fire and how frequently it will fire. When the action potential is fired it sends a chain reaction down the rest of axon.

-Describe the sequence of events that occur to get an action potential (neuron impulses) to stimulate the release of neurotransmitters from the presynapic axon. What happens when the neuron is inhibited? -Include these key words in your description: resting membrane potential (-70 mv), depolarization, threshold (-55 mv), action potential, hyperpolarization (<-70 mv), voltage-gated channel (Na+, K+, Ca2+), repolarization, sodium/potassium pump, Ca2+, vesicles, neurotransmitters, exocytosis.
When the neuron is at a resting potential it will hit a certain level of depolarization going from -70mV to -55 mV causing the sodium channels to open at the threshold level. As sodium enters the cell it causes more sodium gates to open causing a negative feedback loop. Once the sodium goes from a -70 mV to +30mV the sodium channels close creating a potassium voltage gate channel to open. Before potassium enters the neuron hyperpolarization occurs: as sodium (which is negative) leaves the cell, potassium enters (which is positive) creating hyperpolarization. Potassium diffuses quickly out of the cell because it is positively charged. It will restore the cell back to its resting membrane potential. This is called repolarization and creates a negative feedback loop. The change between sodium and potassium diffusion in the neuron creates the action potential or nerve impulse. When sodium channels are inactive, potassium channels open and the membrane potential moves in to a potassium equilibrium. This then causes potassium to repolarize the membrane. This usually causes too much potassium to be created and causes potassium channels to close, causing the resting membrane potential to be reestablished. Sodium and potassium pumps constantly work in the plasma membrane it pumps out the sodium which enter the axon during action potential which pumps in the potassium. active transport by the pumps help move the sodium out of the axon and to move potassium into the axon after an action potential. At the end of the neuron contained in small enclosed membranes called vesicles neurotransmitters are releases into the synaptic cleft where exocytois occurs. during exocytosis the neurotransmitters cause an action potential that stimulates Ca2+ to enter the axon terminal through the calcium voltage gate channels.

-How this applies to PTA:

Knowing how the nervous system works is very important in our field as PTA's. Because everything we do to try and help people has to do with how well the body functions. The only way that can happen is by the nervous system working properly. By testing the cervical nerves we can figure out what is causing numbness or weakness in the arms when trying to figure out how to help a patient if they come in with an upper body problem or condition.Such as if there is a nerve pinch somewhere. It may be shooting pain down an arm or numbness in the area because of the impingement. We as PTA's can understand what is happening with the nerve and nerve impulses because of the symptoms our patient is feeling and where the nerve might be being pinched. Understanding how the nervous system works is also important when we are working with patients on medication; how that will affect their body and sense of pain when receiving therapy. It will help us know if what they are feeling or doing is because if their condition or because of their medication affecting the nerve impulses to different areas of the body.

1. Fox, Stuart I. "Human Physiology." New York: McGraw-Hill, 2011. Print.