Chapter 7: The Nervous System
Schwann Cells
A type of Neuroglia, that produce myelin in the PNS. -forms regeneration tubes
electrical synapses
a mechanical and electrically conductive link between two neighboring neurons that is formed at a narrow gap between the pre- and postsynaptic neurons known as a gap junction.
serves as an electrical insulator that speeds nerve impulses to muscles and other effectors -fatty white substance that surrounds the axon of some nerve cells -produced by the Schwann Cells in PNS and oligodendrocytes in the CNS
describe the effects of myelination on a neuron
oligodendrocytes
glia cells that surround and insulate certain axons in the vertebrate brain and spinal cord.
chemical synapses
specialized junctions through which cells of the nervous system signal to one another and to non-neuronal cells such as muscles or glands
overshoot
the membrane potential continues to depolarize until the peak of the action potential is reached at about +40 millivolts
excitatory postsynaptic potentials
Depolarization of Postsynaptic Membrane. Cell is moved towards firing an Action Potential. Occurs when ion channels allow negatively charged ions to move out, and positively charged ions to move in.
inhibitory postsynaptic potentials
Depolarization of Postsynaptic Membrane. Cell is moved towards firing an Action Potential. Occurs when ion channels allow negatively charged ions to move out, and positively charged ions to move in.
microglia
a type of neuroglia (glial cell) located throughout the brain and spinal cord -As the resident macrophage cells, they act as the first and main form of active immune defence in the central nervous system (CNS).
ALL excitatory neurotransmitters cause an opening of ligand-gated sodium ion channels. As a result, sodium ions flow in and the cell becomes less negative on the inside. When we talk about acetylcholine, it activates ACh receptor sites and ligand gated sodium ion channels open. These excitatory neurotransmitters create a local increase of permeability of sodium ion channels (ligand gated sodium channels open) which leads to a local depolarization that's known as an excitatory postsynaptic potential **postsynaptic cell membrane depolarizes
actions of excitatory neurotransmitter on the postsynaptic neuron
When an inhibitory NT activates the receptor site, it causes additional potassium channels to open which may cause potassium ions to flow out of the cell and if additional positively charged potassium ions flow out of the cell, the inside of the cell will become more negative. In other words, inhibitory neurotransmitters cause an opening of ligand-gated potassium ion channels which leads to a local hyperpolarization (more negative than normal). This is known as a Inhibitory Postsynaptic Potential (IPSP) because it's going to be LESS likely to throw off an action potential. **postsynaptic cell membrane hyper polarizes
actions of inhibitory neurotransmitter on the postsynaptic neuron
interneuron
afferent and efferent neurons use a neuron which forms a connection between two or more neurons -most abundant type of neuron in your body -also called association neurons
action potential
brief, rapid, large (100 mV) changes in membrane potential -all or none -must reach threshold potential -not decremental -begins at axon hillock
Incoming signals from other neurons are (typically) received through its dendrites. The outgoing signal to other neurons flows along its axon. A neuron may have many thousands of dendrites, but it will have only one axon. The fourth distinct part of a neuron lies at the end of the axon, the axon terminals. These are the structures that contain neurotransmitters. Neurotransmitters are the chemical medium through which signals flow from one neuron to the next at chemical synapses.
describe the flow of information in a neuron
spatial summation
occurs when multiple presynaptic neurones together release enough neurotransmitter (e.g. acetylcholine) to exceed the threshold of the postsynaptic neurone. For example, neurone A and neurone B may individually release insufficient neurotransmitter but when these quantities are combined, threshold may be exceeded and an action potential generated.
temporal summation
occurs when one presynaptic neurone releases neurotransmitter many times over a period of time. The total amount of neurotransmitter released may exceed the threshold value of the postsynaptic neurone.
astrocytes
star-shaped cells surrounding neurons in the brain and spinal cord. -support of the cells that comprise the blood-brain barrier as well as maintaining the extracellular ion balance, supplying nutrients to nerve tissue and aiding in post-traumatic repair and scarring processes
-dendrites: Dendrites are treelike extensions at the beginning of a neuron that help increase the surface area of the cell body. These tiny protrusions receive information from other neurons and transmit electrical stimulation to the soma. Dendrites are also covered with synapses. -axon: the elongated fiber that extends from the cell body to the terminal endings and transmits the neural signal. The larger the diameter of the axon, the faster it transmits information. Some axons are covered with a fatty substance called myelin that acts as an insulator. These myelinated axons transmit information much faster than other neurons. -cell body: where the signals from the dendrites are joined and passed on
what are the different parts of the neuron?
During the depolarization phase of the action potential, open Na+ channels allow Na+ ions to diffuse into the cell. This inward movement of positive charge makes the membrane potential more positive (less negative). The depolarization phase is a positive feedback cycle where open Na+ channels cause depolarization, which in turn causes more voltage-gated Na+ channels to open.
what is Na+ and K+ permeability during an action potential?
saltatory conduction
-allows you to jump between nodes -action potential jumps from one node to the next -increase in AP speed (50x)
neural regeneration in PNS
-axons readily regenerate after nerve injury in this nervous system -the distal portion of the axon is disconnected from the cell body and undergoes degeneration -debris is removed by glial cell, mainly macrophages -proximal axons can then regenerate and reinnervate their targets allowing regeneration of their function
Central Nervous System
-brain and spinal chord -function in control and integration
neurons
-conduct electrical signals -basic cell of nervous system -structure consists of dendrites, cells body, axonal hillock, axon
Peripheral Nervous System
-cranial nerves and spinal nerves -communiction -connects CNS to receptors, glands, etc.
neuroglial
-majority of all nerve tissue cells -support neurons -nonconducting cells -in PNS and CNS -5 types
neural regeneration in CNS
-nerve cells in this system so not spontaneously regenerate after injury -This is the main reason why paralysis and loss of sensation is permanent in conditions such as spinal cord injury
acetylcholinesterase
-neurotransmitter found in synaptic cleft -functions to break down ACh into component parts, which are taken back into the presynaptic cell for reuse
efferent neuron
-somatic -autonomic nervous system -The nerves that carry signals away from the central nervous system in order to initiate an action
depolarization
-voltage-sensitive Na+ channels open as the membrane depolarizes -reach threshold--induces more Na+ channels to open -makes the cell less polar (membrane potential gets smaller as ions quickly begin to equalize the concentration gradients)
afferent neuron
The nerves responsible for sensing a stimulus and sending information about the stimulus to your central nervous system
all or none action potential
There are no big or small action potentials in one nerve cell - all action potentials are the same size. Therefore, the neuron either does not reach the threshold or a full action potential is fired
ependymal cells
These cells line the CSF-filled ventricles in the brain and the central canal of the spinal cord. These are nervous tissue cells with a ciliated simple columnar form much like that of some mucosal epithelial cells.
repolarization
brings the cell back to resting potential. The inactivation gates of the sodium channels close, stopping the inward rush of positive ions. At the same time, the potassium channels open. There is much more potassium inside the cell than out, so when these channels open, more potassium exits than comes in. This means the cell loses positively charged ions, and returns back toward its resting state.
graded potential
changes in membrane potential that vary in size, as opposed to being all-or-none. They arise from the summation of the individual actions of ligand-gated ion channel proteins, and decrease over time and space. -the magnitude is determined by the strength of the stimulus
Na+: ONLY voltage gated channels that are closed at rest and open when a particular membrane potential is reached (~55 mV) K+: 2 types of channels --leaky are not always open, but they are not gated --voltage gated channels: open when a particular membrane potential is reached, closed at resting potential
compare Na+ and K+ gating
When an action potential, or nerve impulse, arrives at the axon terminal, it activates voltage-gated calcium channels in the cell membrane. Ca2+ which is present at a much higher concentration outside the neuron than inside, rushes into the cell. The Ca2+ allows synaptic vesicles to fuse with the axon terminal membrane, releasing neurotransmitter into the synaptic cleft. The molecules of neurotransmitter diffuse across the synaptic cleft and bind to receptor proteins on the postsynaptic cell. Activation of postsynaptic receptors leads to the opening or closing of ion channels in the cell membrane. This may be depolarizing—make the inside of the cell more positive—or hyperpolarizing—make the inside of the cell more negative—depending on the ions involved.
describe the sequence of event in neural communication across and synapse
refractory period
during this time it is absolutely impossible to send another action potential. The inactivation (h) gates of the sodium channels lock shut for a time, and make it so no sodium will pass through. No sodium means no depolarization, which means no action potential -absolute: neuron will not send another message no matter what the stimulus -relative: during this time, it is really hard to send an action potential. This is the period after the absolute refractory period, when the h gates are open again. However, the cell is still hyperpolarized after sending an action potential.
-the neurotransmitter receptor is separate from the protein that serves as the ion channel -takes message from outside and bring it inside the cell -use cAMP as second messenger
explain how g-protein coupled signal pathways produce synaptic potentials
-action potential opens voltage gated Na+ channels -stimulus as stimulus for next depolarization of next adjacent region triggering an A{ at the segment -action potentials are produced continuously along the plasma membrane of unmyelinated axons
how are action potentials conducted?
-they are decremental: they decrease over time and die out -magnitude varies with signal strength -produced by some specific change in environment acting on specialized region -depolarizing or hyper polarizing **dont use graded potential for long distance communication because they'll die out
how are graded potentials conducted?
the more permeable the membrane is for an ion, the more the equilibrium potential of that ion will influence the membrane potential -the resting membrane is mainly permeable to K+, and thus the resting membrane potential should be close to the K+ equilibrium potential. However, the resting membrane potential is not exactly at the equilibrium potential for K+, but a bit higher at -70 mV. This is because the resting membrane is slightly permeable to Na+.
how do changes in permeability affect membrane potential?
changing the cell membrane potential -an electrical signal moves down the neuron -must reach threshold potentials • Briefly opening more membrane gates for Na+ and then for K+ • Thus making the neuron cell membrane o More permeable first to Na+ o Then to K+ • This causes a change in the numbers of plus charges inside and outside the cell membrane o Changing the cell membrane potential
how is an action potential produced?
exocytosis, which a cell uses to exude secretory vesicles out of the cell membrane. These membrane-bound vesicles contain soluble proteins to be secreted to the extracellular environment, as well as membrane proteins and lipids that are sent to become components of the cell membrane. Exocytosis in neuronal chemical synapses is Ca2+ triggered and serves interneuronal signalling
how is neurotransmitter released from axon terminal?
ligand gated channel
integral membrane proteins that contain a pore which allows the regulated flow of selected ions across the plasma membrane. Ion flux is passive and driven by the electrochemical gradient for the permeant ions. The channels are opened, or gated, by the binding of a neurotransmitter to an orthosteric site(s) that triggers a conformational change that results in the conducting state.
hyperpolization
makes the cell more negative than its typical resting membrane potential. As the action potential passes through, potassium channels stay open a little bit longer, and continue to let positive ions exit the neuron. This means that the cell gets even more negative than its resting state.
