Neurons and Communication in the Brain Flashcards
how many neurons does the human brain contain?
100 billion or more
up to how many neurons can one neuron be connected to?
10,000 other ones
where does neurogenesis occur?
not born with full set of neurons
neurogenesis occurs in some parts of the brain: olfactory bulb, hippocampus and amygdala
how many cortical neurons do we lose each second?
1 each second
neurons have a high metabolic rate and require a continuous source of fuel
what is the nervous system?
one of the body’s major command and control systems
complex system with 100 billion cells in the brain which is interconnected and interacting in complex ways
how is the nervous system divided?
into the central and peripheral nervous system
CNS: brain and spinal cord
PNS: everyone outside the brain and spinal cord
how is the peripheral nervous system divided?
into the somatic and autonomic nervous system
somatic: controls movement and muscles and registers touch
autonomic: controls automatic processes like heart rate, organs, stress
how is the autonomic nervous system divided?
into sympathetic and parasympathetic nervous system
sympathetic: gets body ready for fight or flight
parasympathetic: calms body for rest and digest
what does the volume of the CNS consist of?
neurones consist of half the volume of the CNS
supporting cells consist of the other half
what are supporting cells?
supporting cells provide neurons with nutrients from the capillaries and keep neurons from getting damaged
why are the supporting cells important?
neurons have a very high metabolic rate but have no means of storing nutrients so they must constantly be supplied with nutrients and oxygen or they will quickly die
types of supporting cells (glia cells)
astrocytes
oligodendrocytes
schwann cells
function of astrocytes
serve as the matrix that holds neurons in place to insulate them
delivers nutrients from blood supply to neurons
surrounds and isolates synapses to limit the dispersion of neurotransmitters that are released by the terminal buttons
structure of astrocytes
located in the centre and connect to many neurons
many thin tendrils go out and attach to the soma of neurons and capillaries
these thin tendrils have U-shaped buttons that attach
function of oligodendrocytes
provide support for neurons in the CNS by wrapping around axons to form the myelin sheath which speeds up the communication in the brain
structure of oligodendrocytes
has a soma similar to the structure of a neuron
many tendrils go out of it which merge with the myelin sheaths of the surrounding axons
function of schwann cells
provide support for sensory and motor neurons in the PNS by wrapping around axons to form the myelin sheath which speeds up communication between neurons
structure of schwann cells
large circular structure that engulfs axon by wrapping itself round in many layers that form the myelin sheath
what is the blood brain barrier?
a semipermeable barrier between the blood and the brain produced by the cells in the walls of the brain’s capillaries
properties of the blood brain barrier
selectively permeable so some substances e.g. water molecules can pass through the cells of the capillaries passively whereas other molecules enter via active transport which requires ATP to move between the tightly packed cells of the capillaries
what are sensory neurons?
a neuron that detects changes in the external or internal environment from receptors in skin, tongue etc and sends information about these changes to the CNS
what are motor neurons?
a neuron that controls the contraction of a muscle or the secretion of a gland, cell body located in the CNS but axons go to PNS
what are interneurons?
a neuron that connects sensory and motor neurons; located entirely within the CNS
doesn’t have a long axon
which processes to do with neurons require the brain and which do not?
reflex arcs do not require the brain
inhibition requires the brain
how does a reflex arc work?
The axon of the sensory neuron carries the pain signal from the hand holding the hot object to the interneuron in the spinal cord
The interneuron in the spinal cord excites the motor neuron, causing a withdrawal reflex of the hand
how does inhibition of a reflex arc work?
The axon of the sensory neuron carries the pain signal from the hand holding the hot object to the interneuron in the spinal cord
The interneuron has inhibitory signals from the brain which inhibits the motor neuron from withdrawing, ergo preventing the hand from dropping the object
what are synapses?
where two neurones join together without touching
what are dendrites?
similar structure to trees
carry signals from a receptor/synapse to the cell body
what are axons?
has myelin sheath wrapped around
carry signals away from the cell body to the terminal buttons
what are terminal buttons?
where synapses occur
found at the end of an axon
what is the soma?
cell body
contains the cytoplasm and nuclei with DNA
what is the function of the neuron?
to transmit signals from one part of the body to another
what are multipolar neurones?
most common type of neuron
consist of a single axon and multiple dendrites with the cell body near the dendrites
what are bipolar neurons?
consist of one dendrite and one axon with cell body in the middle
often sensory neurons
what are unipolar neurons?
single axon with the cell body located on one side
what structures can be found into the soma?
membrane
cytoplasm
mitochondria
nucleus
chromosome
microtubule
ribosomes
define membrane
a structure consisting principally of lipid molecules that defines the outer boundaries of a cell and also constitutes many of the cell organelles
define cytoplasm
the viscous, semiliquid substance contained in the interior of a cell
define mitochondria
an organelle that is responsible for extracting energy from nutrients
define ribosome
protein synthesis site
define nucleus
a structure in the central region of a cell, containing the chromosomes
define chromosome
a strand of DNA, with associated proteins, found in the nucleus; carries genetic information
define microtubule
part of the cytoskeleton; transports substances like neurotransmitters and proteins
how does the microtubules transport material?
anterograde axoplasmic transport: transports large neurotransmitters and proteins from cell body to terminal buttons
retrograde transport: transports material like recycled neurotransmitters from the terminal buttons to the soma
how are large neurotransmitters made and transported?
made in the soma
transported via anterograde axoplasmic transport from cell body to terminal buttons
how are small neurotransmitters made and transported?
made in the terminal buttons and do not need to be transported
what is an electrode?
a conductive medium that can be used to apply electrical stimulation or to record electrical potentials
what is a microelectrode?
a very fine electrode, generally used to record activity of individual neurons
what is membrane potential?
the electrical change across a cell membrane; the difference in electrical potential inside and outside of the cell
how was neuron activity initially measured?
measured charge of axon using giant squid as their axons are much thicker than humans
did this by putting axon in saltwater and measuring the charge of the saltwater and the charge inside the squid axon
how does communication along axon occur?
occurs due to changes in charge within the axon compared to outside the axon
what makes up the charge outside the axon at resting potential?
lots of sodium ions outside (Na+)
inside is relatively negative compared to outside
what makes up the charge inside the axon at resting potential?
lots of potassium ions inside (K+)
lots of anions inside (-)
makes overall charge inside relatively negative compared to outside
why is there are build up of anions inside?
anions cannot leave due to the impermeability of the cell membrane
what is resting potential?
-70mv
what are the 3 pressures that act on the ions to maintain resting potential?
diffusion
electrostatic forces
sodium-potassium pump
how does diffusion maintain resting potential?
movement of substances from an area of high concentration to an area of low concentration along concentration gradient
pressure for sodium ions to move into the axon as there is less inside than outside and pressure for potassium ions to move out of the axon as there is less outside than inside
how do electrostatic forces maintain resting potential?
negative and positive charges attract
positive sodium and potassium ions move into the axon as it is negative
how does the sodium-potassium pump maintain resting potential?
active transport
pumps 3 sodium ions out for every 2 potassium ions in which maintains the resting potential
what happens to chloride ions?
negatively charged
remain at equilibrium
how was the first action potential found?
by Darwin in a venus fly trap
what is an action potential?
the brief electrical impulse that provides the basis for conduction of information along an axon
what is threshold of excitation?
the value of the membrane potential that must be reached to produce an action
how does an action potential work?
- stimulus triggers a change in charge called the action potential which travels along the axon
- this stimulus must reach a certain threshold of -60mv for an action potential to be fired
- once this threshold is reached, sodium ion channels open and sodium ions flood in which increases the membrane potential to +40mv inside the axon
- at the same time potassium ion channels open and potassium ions leave
- due to the influx of ions in and out of the axon, once the membrane potential reaches +40mv, the sodium ion channels become refractory so diffusion no longer occurs and less sodium ions enter
- this causes the membrane potential to decrease as potassium ions continue to leave
- membrane potential returns to resting potential as potassium ion channels close
- too many potassium ions leave so hyperpolarisation occurs which requires the sodium-potassium pump to bring the membrane potential back to resting potential
how is an action potential triggered?
Activation by a previous neuron after a synapse or activation by previous action potential
explain the process of depolarisaiton
- at the start of an action potential, sodium ions enter the axon and diffuse along the axon via diffusion as it is negative - this increases the membrane potential
- depolarisation decreases as the action potential spreads down the axon but it is still enough to trigger a new action potential at the next node of Ranvier as the myelin sheath prevents an action potential from firing
- this process repeats at each node of Ranvier
what is saltatory conduction?
Saltatory conduction is when the action potential jumps from node to node which speeds up the velocity at which an axon can conduct an action potential as it does not have to depolarise every bit of the axon
if decremental conduction of message is faster than action potentials, why are action potential important?
Depolarisation decreases as it spreads down the axon but it is still enough to trigger a new action potential at the next node but would peter out before getting to the end of the axon
what is the all or none law?
the principle that once an action potential is triggered in an axon, it is propagated, without decrement, to the end of the fiber
action potentials are always the same size
what is the rate law?
the principle that variations in the intensity of a stimulus or other information being transmitted in an axon are represented by variations in the rate at which that axon fires.
intensity of the action potential is detected by the rate of action potential
when are myelin sheaths required and not required?
Myelin sheaths are not required when travelling short distances e.g. interneurons and some brain neurons travel small distances so do not require a myelin sheath
Myelin sheaths are required for travelling long distances e.g. along the spinal cord and in the peripheral NS
what is the presynaptic membrane?
the membrane of a terminal button that lies adjacent to the postsynaptic membrane and through which the neurotransmitter is released
what is the postsynaptic membrane?
the cell membrane opposite the terminal button in a synapse; the membrane of the cell that receives the message
what is the synaptic cleft?
the space between the presynaptic membrane and the postsynaptic membrane
common neurotransmitters
Acetylcholine
Dopamine
Noradrenaline (norepinephrine)
Serotonin (5-HT)
Glutamate
GABA
what are the 4 stages of the lifecycle of neurotransmitters?
synthesis
release
binding
inactivation
explain how neurotransmitters are synthesised
- Small neurotransmitter molecules are synthesised in the terminal buttons and packaged into synaptic vesicles
- Large neurotransmitter molecules are assembled in the cell body, packaged in vesicles and then transported to the axon terminal via microtubules
explain how neurotransmitters are released
When an action potential arrives, the synaptic vesicles fuse with the presynaptic membrane and neurotransmitters are released into synaptic cleft.
explain how neurotransmitters are binded
The neurotransmitters move across and bind to receptors on the postsynaptic neuron
what are the two different types of receptors on the postsynaptic membrane?
ionotropic receptors and metabotropic receptors
what are ionotropic receptors and how do they work?
Receptors that control ion channels
Ligand gated ion channels respond to the presence of neurotransmitters and cause postsynaptic potentials which can be inhibitory or excitatory depending on the ion
what happens to an excitatory potential at the postsynaptic membrane?
Excitatory potential means Na+ channels are opened which leads to an action potential firing again as the inside of the axon becomes positive due to sodium ions flowing in and the process of depolarisation repeats
what happens to an inhibitory potential at the postsynaptic membrane?
Inhibitory potential means K+ channels are opened which leads to an action potential not firing as the inside of the axon is too negative due to potassium ions flowing out
what are metabotropic receptors and how do they work?
Slower and less direct mode of action
An autoreceptor is a type of metabotropic receptor located on the presynaptic membrane and is sensitive to the neurotransmitter released.
The neurotransmitter and the metabotropic receptor travels through a channel and has an effect on the G-protein which changes the shape of a protein which effects the second messenger which affects the ion channel
explain how neurotransmitters are inactivated
The two mechanisms for terminating neurotransmitter action in the synapse: reuptake and enzymatic degradation
reuptake: e.g. dopamine is recycled to be used again
enzymatic degradation: e.g. Acetylcholine is broken down by AcHE, the products are recycled by astrocytes and go back to the soma
what effect does drugs have on inactivation of neurotransmitters?
Some drugs e.g. opiates may block the autoreceptors so dopamine cannot be recycled by reuptake and stays in the synapse - this creates a high-like effect due to the build up of dopamine in the synapse
how can you inhibit excitatory reflexes?
neural integration
- If several excitatory synapses are active at the same time, the EPSPs they produce summate as they travel toward the axon and the axon fires
- If several inhibitory synapses are active at the same time, the IPSPs they produce diminish the size of the EPSPs and prevent the axon from firing
