neuronal communications Flashcards
define pacinian corpuscle
a pressure sensor found in the skin
define sensory receptors
cells/ sensory nerve endings that respond to a stimulus in the internal/ external environment of an organism and can create action potentials
define transducer
a cell that converts one form of energy into another , in this case to an electrical impulse
define motor neurone
neurones that carry an action potential from the CNS to the effector
define myelinated neurones
has an individual layer of myelin around it
define non myelinated neurones
has no individual layers of myelin
what are the different types of neurones
motor
sensory
relay
define sensory neurones
carry the action potential from a sensory receptor to the CNS
define relay neurones
connect sensory and motor neurones
each type of transducer is adapted to detect ?
changes in a particular form of energy , this may be a change in light levels , change in pressure on the skin or one of many other energy changes , other receptors detect the presence of chemicals
name the sensory receptor and energy change involved for each stimulus
- change in light sensitivity
- change in temperature
- change in skin pressure
- change in sound
- movement
- change in length of muscle
- chemicals in the air
- chemicals in food
- light sensitive cells (cones and rods) in the retina - light to electrical
- temperature receptors in the skin and the hypothalamus - heat to electrical
- pacinian corpuscles in the skin- movement to electrical
- vibration receptors in the cochlea of the year- movement to electrical
- hairs in the inner ear- movement to electrical
- muscle spindles in skeletal - movement to electrical
- olfactory cells in epithelium lining the nose- these receptors detect the presence of a chemical and create an electrical nerve impulse
- chemical receptors in taste buds on tongue - same as above
describe the structure and function of the pacinian corpuscle
is an oval shaped structure that consists of a series of concentric rings of connective tissue wrapped around the end of a nerve cell. when pressure on the skin changes this deforms the rings of connective tissue , which push against the nerve ending
the corpuscle is sensitive only to changes in pressure that deform the rings of connective tissue . therefore when the pressure is constant they stop responding
describe the structure of neurons
- many are very long so that they can transmit the action potential over a long distance
- the cell surface membrane has many gated ion channels that control the entry or exit of sodium , potassium or calcium ions
- sodium/ potassium pumps use ATP to actively transport sodium ions out of the cell and potassium ions into the cell
- neurones maintain a potential difference across their cell surface membrane
- a cell body contains the nucleus , many mitochondria and ribosomes
- numerous dendrites connect to other neurones. the dendrites carry impulses towards the cell body
- an axon carries impulses away from the cell body
- neurones are surrounded by a fatty layer that insulates the cell from electrical activity in other nerve cells nearby, this fatty layer is composed of schwann cells closely associated with the neurone
what are the differences between the different types of neurones
- motor neurones have their cell body in the CNS and have a long axon that carries the action potential out of the effector
- sensory neurones have a long dndron carrying the action potential from the sensory receptor to the cell body , which is positioned just outside the CNS . they have a short axon carrying the acyion potential into the CNS
- relay neurones connect the sensory and motor neurones together. they have many short dendrites and a short axon . the number of dendrites and the number of divisions of the axon is variable . relay neurones are an essential part of the nervous system , which conducts impulses in coordinated pathways
describe myelinated neurones
- has an individual layer of myelin around it
most sensory and motor neurones are associated with many schwann cells , which make up a fatty sheath called the myelin sheath . these schwann cells are wrapped tightly around the neurone so the sheath actually consists of several layers of membrane and thin cytoplasm from the schwann cells
at intervals of 1-3 mm along the neurone there are gaps in the myelin sheath. these are called the nodes of ranvier. each node is very short.
because the myelin sheath Is tightly wrapped around the neurone it prevents the movement of ions across the neurone membranes. therefore movement of ions across the membrane can only occur at the node of ranvier , this means that the impulse or action potential , jumps from one node to the next . this makes conduction much more rapid
describe non myelinated cells
has no individual layer of myelin
are also associated with schwann cells, but several neurones may be enshrouded in one loosely wrapped schwann cells . this means that the action potential moves along the neurones in a wave rather than jumping from node to node as seen in myelinated neurones
advantages of myelination
- yelinated neurones can transmit an action potential more quickly than non myelinated neurones
- myelinated neurones carry action potentials from sensory receptors to the CNS and from the CNS to effectors. they carry action potentials over long distances , this increases speed of transmission means that the action potential reaches the end of the neurones more quickly , this enables a more rapid response to a stimulus
- non myelinated neurones tend to be shorter and carry action potentials only over a short distance. they are often used in coordinating body functions such as breathing and the action of the digestive system. therefore the increased speed of transmission is not so important
define resting potential
the potential difference across the membrane while the neurone is at rest
describe resting potential
it is actively pumping ions across its cell surface membrane . sodium / potassium ion pumps use ATP to pump three sodium ions out of the cell for every two potassium ions that are pumped in . the gated sodium ion channels are kept closed . however some of the the potassium ion channels are open , and therefore the plasma membrane is more permeable to potassium ions than to sodium ions. potassium ions tends to diffuse out of the cell. the cell cytoplasm also contains large organic anions . hence the interior of the cell is maintained at a negative potential compared with the outside . the cell membrane is said to be polarised. the potential difference across the cell membrane is about -60mv . this is called the resting potential
describe the stages of an action potential
1- the membrane starts in its resting potential - polarised with the inside of the cell being -60mv compared to the outside . there is a high concentration of sodium ions outside than inside and higher concentration of potassium ions inside than outside
2- sodium ions channels open and some sodium ions diffuse into the cell
3- the membrane depolarises , it becomes less negative with respect to the outside and reaches the threshold value of -50mv
4- positive feedback causes nearby voltage sodium gated sodium ions to flood in . as more sodium ions enter the cell becomes more positively charged inside compared with outside
5- the potential difference across the plasma membrane reaches +40 mv. the inside of the cell of the cell is positive compared with the outside
6- the sodium ion channels close and potassium channels open
7- potassium ions diffuse out of the cell bringing the potential difference back to negative inside compared with the outside , this is called polarisation
8- the potential difference overshoots slightly making the cell hyperpolarised
9- the original potential difference is restored so that the cell returns to its resting state
describe the refractory period
after an action potential the sodium and potassium ions are in the wrong places. the concentrations of these ions inside and outside the cell must be restored by the action of the sodium/ potassium ion pumps. for a short time after each action potential it is impossible to stimulate the cell membrane to reach another action potential. this is known as the refractory period and allows the cell to recover after an action potential . it also ensures that action potentials are transmitted in only one direction.
describe the formation of local currents
- when an action potential occurs the sodium ion channels open at that point in the neurone
- the open sodium ion channels allow sodium ions to diffuse across the membrane from the region of higher concentration outside of the neurone into the neuron . the concentration of the sodium ions inside the neurone rises at the point where the sodium ion channels open.
- sodium ions continue to diffuse sideways along the neuron , away from the region of increased concentration.. this movement of charged particles is a current called a local current
- the local current causes a slight depolarisation further along the neurone which effects the voltage gated sodium ions channels , causing them to open . the open channels allow rapid influx of sodium ions causing a full depolarisation further along the neuron the action potential has therefore moved along the neurone
the action potential will continue to move in the same direction until it reaches the end of the neurone
describe saltatory conduction
the myelin sheath is an insulating layer of fatty material , composed of schwann cells wrapped tightly around the neurone . sodium and potassium ions cannot diffuse through this fatty layer . in between the schwann cells are small gaps called the nodes of ranvier . therefore the , the ionic movements that create an action potential occur over much of the length of the neurone, they can only occur at the node of ranvier . in myelinated neurones the local currents are therefore elongated and sodium ions diffuse along the neurone from one node of ranvier to the next . this means that the potentials jump from one node to another
advantages of saltatory conduction
the myelin sheath means that action potentials can only occur at the gaps between the schwann cells that make up the myelin sheath .effectively the action potentials jump from one node of ranvier to the next . this speeds up the transmission of the action potential along a neurone .
explain the frequency of transmission
the impulse carried by a neurone is an action potential . all action potentials are the same intensity / size, each one produces a depolarisation of + 40 mv . this is the all or nothing rule
although the size of the action potential is unelated to the intensity of the stimulus that caused the action potential , we can still detect stimuli of different intensities , such as loud or quiet sounds . our brain determines the intensity of the stimulus from the frequency of action potentials arriving in the sensory region of the brain. a higher frequency of action potentials means a more intense stimulus
when a stimulus is a high intensity more sodium channels are opened in the sensory receptor. this produces more generator potentials. as a result there are more frequent action potentials in the sensory neurones. therefore there are more frequent action potentials entering the central nervous system
describe cholinergic synapses
a synapse that uses acetycholine as its neurotransmitter
define neurotransmitter
a chemical used as a signalling molecule between two neurones in a synapse
define synapse
is a junction between two or more neurones , where one neurone can communicate with , or signal to , another neurone between the two neurones is a small gap called the synaptic cleft
the pre synaptic bulb contains a number of features including
- many mitochondria , indicating that an active process needing ATP is involved
a large amount of smooth endoplasmic reticulum which packages the neurotransmitters into vesicles
large numbers of vesicles , containing molecules of a chemical called acetycholine , the transmitter that will diffuse across the synaptic cleft
a number of voltage gated calcium ion channels on the cell surface membrane
describe the post synaptic membrane
the post synaptic membrane contains specialised sodium ion channels that can respond to the neurotransmitter . these channels consist of five polypeptide molecules . two of these polypeptides have a special receptor site that is specific to acetycholine . the receptor sites have a shape that is complementary to the shape of the acetycholine molecule . when acetycholine is present in the synaptic cleft it binds to the two receptor sites and causes the sodium ion channel to open.
describe the transmission across the synapse
- an action potential arrives at the synaptic bulb
- the voltage gated calcium ion channels open
- calcium ions diffuse into the synaptic bulb
- the calcium ions cause the synaptic vesicles to move to and fuse with the pre synaptic membrane
- acetycholine is released by exocytosis
- acetycholine molecules diffuse across the cleft
- acetycholine molecules bind to the receptors sites on the sodium ions channels in the post synaptic membranes
- the sodium ion channels open
- sodium ions diffuse across the post synaptic membrane into the posy synaptic neurone
- a generator potential or excitatory post synaptic potential is created
- if sufficient generator potentials combine then the potential across the post synaptic membrane reaches the threshold potential
- a new neurone potential is created in the post synaptic neurone
role of acetylcholinesterase
if acetycholine is left in the synaptic cleft it will continue to open the sodium ion channels in the post synaptic membrane and will continue to cause action potentials . acetylcholinesterase is an enzyme found in the synaptic cleft . it hydrolyses the acetycholine to ethanoic acid and choline . this stops the transmission of signals , so that the synapse does not continue to produce action potentials In the post synaptic neurone
the ethanoic acid and choline are recycled . they re enter the synaptic bulb by diffusion and are recombined to acetycholine using ATP from respiration in the mitochondria . the recycled acetychloine is stored in the synaptic vesicles for future use
describe summation
-occurs when the effects of several excitatory post synaptic potentials (epsp) are added together
describe inhibitory post synaptic potentials
these can reduce the effect of summation and prevent an action potential in the post synaptic neurone
describe neuron junctions
involve several neurones, this could be several , this could be several neurones from different places converging on one neurone , or it could be one neurone sending signals out to several neurones that diverge to different effectors
describe excitatory post synaptic potentials (EPSP)
when one action potential passes down the axon to the synapse it will cause a few vesicles to move , and fuse with the pre synaptic membrane .the relatively small number of acetycholine molecules diffusing across the cleft producers a small depolarisation- this on its own will not be enough to cause an action potential in the post synaptic neurone
describe temporal summation
summation can result from several action potentials in the same pre synaptic neurone
describe spatial summation
summation can result from action potentials arriving from several different pre synaptic neurones
describe habituated
after repeated stimulation a synapse may run out of a vesicles containing the neurotransmitter. the synapse is said to be fatigued . this means the nervous system no longer responds to the stimulus , we have become habituated. , it explains why we soon get used to a smell or a background noise , it may help over stimulation of an effector, which could cause damage
