nervous system cells and communication Flashcards
describe the general functions of the nervous system
sensory, integration and motor
sensory function
receives information about the external and internal environments and communicates this information to the integrative system
integration function
process and distribute input signals and then distributes output signals to the motor region to initiate body responses
motor function
activates the effector organs/ cells to cause an appropriate action in response to the sensory information
what are the 2 divisions of the nervous system
CNS central nervous system (the brain and spinal cord) and PNS peripheral nervous system (nerves).
what are neurons
cells which communicate by receiving, transmitting and sending signals between each other
what is a dendrite (cell body and multipolar neurons) on a neuron
the receptive region. It receives and responds to incoming chemical signals
what is the axon on a neuron
the conducting region. it transmits electrical impulses (action potentials)
what is the axon terminals on a neuron
the secretory region. They secrete neurotransmitters that stimulate or inhibit other neurons or body cells
what is a multipolar neuron
They have 2 or more dendrites and a single axon extending from the cell body. Most common neuron type in the nervous system
what are unipolar neurons
dendrites are continuous with the axon and the cell body lies off to one side of the axon. most sensory neurons are unipolar neurons
what are anaxonic neurons
all cell processes extending from the cell body look alike (no distinct axon). located in brain and some special sense organs, but functions are poorly understood
bipolar neurons
have 2 distinct processes- a single dendrite with distal branches and a single axon- extending from the cell body. Rare, only found in some special sense organs
what is the neuroglia
cells which support and protect neurons in the CNS and PNS
astrocytes CNS
the most abundant in the CNS.
- regulate ECF concentrations and maintain the blood- brain barrier
ependymal cells CNS
form an epithelium called the ependyma which lines fluid- filled spaces in the brain and spinal cord. these cells produce, monitor and circulate cerebrospinal fluid
microglia CNS
move through nervous tissue and remove cellular debris, waste products and pathogens by phagocytosis
oligodendrocytes CNS
- provide structural framework by stabilising the position of neuronal axons
- produce myelin sheaths for neurons in the CNS
Schwann cells PNS
- form multi layered myelin sheaths around a segment of the axon in the PNS, just like oligodendrocytes in the CNS
satellite cells PNS
They surround neuron cell bodies within ganglia and regulate the environment around the neurons in the PNS
what is white matter
regions of the nervous system that are dominated by myelinated axons
what is grey matter
areas that contain unmyelinated axons, cell bodies and dendrites
what is resting membrane potential
when a neuron is neither sending nor recieving signals it is considered to be resting. In this state the cell is charged, but there is no change in voltage. it is 70mV
what is the general value of the resting membrane potential
-70mV
what is depolarization
the voltage change is positive. the RMP moves towards 0mV. it is caused by excitatory stimuli and can lead to the generation of action potentials
what is hyperpolarization
the voltage change is negative. the RMP moves further away from 0mV. it is caused by inhibitory stimuli as it cannot lead to an action potential
what does the change of RMP depend on
- the location of the membrane voltage change
- the direction (positive or negative) of the voltage change
- the size of the voltage change.
what are some factors of graded (local) potential
- includes small depolarizations
- occur in the receptive regions of neurons (dendrites)
- includes small hyperpolarizations
- can excite or inhibit the neuron
what are some factors of action potentials
- voltage change from about -59mV to +30mV
- will cause the release of neurotransmitters
- is a large depolarization
- conducted along the length of the axon
what causes a graded potential
stimuli received within the receptive region of the neuron. the stimulus causes the opening of ion membrane channels, allowing ions in and/or out of the cell. the movement of these ions changes the RMP at the location of the stimulus reception. they can cause depolarization or hyperpolarization
what are excitatory post synaptic potentials
they are graded potentials caused by excitatory stimuli which cause depolarization
what are inhibitory post- synaptic potentials
they are graded potentials caused by inhibitory stimuli which cause hyperpolarization
what is summation
the total change of voltage within the receptive region of the neuron. summation determines whether or not a neuron will fire an action potential, as a particular voltage value (threshold value) must be met for an action potential to fire
what does the conduction of an action potential depend on
the conduction of an action potential depends on the state of the ion channels, the size of the axon and the presence of myelin.
ion channel inactivation (how does the action potential move down the axon)
an action potential is conducted down the length of the axon towards the axon terminals. this happens by the inactivation of the sodium channels after they close at the end of depolarization. inactive sodium channels cannot be stimulated to re open immediately. So while the particular segment of axon is depolarized it stimulates the voltage-gated ion channels in the adjacent segments, only the axonal segment towards the axon terminals can open their ion channels, therefore the action potential continues in the same direction.
what are refractory periods
the periods after an action potential has occurred that prevents or limits when the next action potential can take place
absolute refractory period
while sodium channels on an axon segment are open, that segment cannot respond to any stimuli
relative refractory period
when the sodium channels on an axon segment become inactive it causes a brief increase in the threshold value. during this period only very strong stimuli can cause the generation of a new action potential
what is AP velocity
the speed of an action potential which depends on 2 factors (axon diameter and myelination)
what is axon diameter
the wider the axon, the faster the AP travels. this is because there is less resistance to the flow of the local currents due to the increased space
what is myelination and how does it help APs
the presence and amount of myelin on axons increases the speed of the APs. Because myelin sheaths wrap only segments of axon, they leave gaps of exposed axon called nodes of ranvier. It is at these nodes that the movement of ions can take place. therefore the AP skips from node to node, getting to the axon terminals more quickly
what are the components of a chemical synapse
the presynaptic element, the synaptic cleft and the postsynaptic element
how does synaptic communication take place
it is an area where 2 neurons come close enough together that they are able to pass chemical signals from one cell to the other. the neurons are separated by a synaptic cleft. the neuron where the signal is initiated is called the pre synaptic neuron and the neuron that recieves the signal is called the post synaptic neuron. in the pre synaptic neuron there are neurotransmitters. when the presynaptic neuron is excited by an action potential, calcium ions cause the release of vesicles that hold neurotransmitters which fuse with the presynaptic membrane and release their content into the synaptic cleft. once they are there the neurotransmitters interact with the receptors on the post synaptic membrane. they bind to the receptor and can cause an action to occur in the postsynaptic cell.
what is the synapse
a functional junction between 2 neurons
how is an action potential conducted along the length of the axon
if graded potential lead to a depolarisation that reaches the neurons threshold value at the axon hillock, voltage-gated na+ channels open at the first axonal segment allowing an influx of na+ and a rapid depolarisation of the axonal membrane at that location (which will be followed by repolarization due to K+ channels opening). this depolarisation stimulates voltage-gated Na+ channels in the next segment of the axon to open and the Na+ to rush in and depolarises that segment, which stimulates voltage-gated Na+ channels in the next segment to open.. and so forth all the way down the length of the axon until the depolarisation reaches the axon terminal
what are some neurotransmitters common in the body
histamine, endorphins, serotonin, GABA, norepinephrine, nitric oxide, acetylcholine, epinephrine/ adrenaline and dopamine
what are neurotransmitters
chemicals produced and released by neurons to stimulate or inhibit particular actions in the postsynaptic cell
what is saltatory conduction
occurs in myelinated axons and allows the APs to generate only at the nodes of Ranvier
the process of an action potential moving along the axon in bullet points
- neurons threshold value is met
- sodium channels open
- sodium rushes into the cell
- membrane voltage reaches +30mV
- potassium channels open, sodium channels close
- potassium rushes out of the cell
- neuronal membrane repolarizes and hyperpolarises
- potassium channels close
- RMP reset
