5.3 - Neuronal communication Flashcards
What are the functions of sensory receptors?
- Can detect changes in environment
- Can convert one form of energy to another
What is a pacinian corpuscles?
A pressure sensor that detects changes in pressure to the skin
What is the pacinian corpuscle made up of?
Consists of a series of concentric rings of connective tissues wrapped around the end of a nerve cell
How is an impulse transmitted to other parts of the body?
The impulse is transmitted along the neurones as an action potential. The action potential is then carried as a rapid depolarisation of the membrane caused by the influx of sodium ions
What happens once a stimulus is detected?
The energy is converted to a depolarisation of the receptor cell membrane
What are motor neurones?
Neurones that carry an action potential from the central nervous system to an effector such as a muscle or a gland (short dendron and long axon)
What are sensory neurones?
Neurones that carry the action potential from a sensory receptor to the CNS (long dendron and short axon)
What are relay neurones?
Neurones that connect sensory and motor neurones (short dendrites, no dendron and a short axon)
What are the structural features of neurones?
- Very long, so that they can transmit the action potential over a long distance
- Cell body contains nucleus, ribosomes and many mitochondria
- Surrounded by a fatty layer, this insulates the cell
- Ion channels on the cell surface, to control entry and exit of Na, K and Ca ions
What carries the impulses towards and away from the body?
- Numerous dendrites connect the neurones, they carry the impulse towards the body
- An axon carries the impulses away form the cell body
What is a stimulus?
A change in the energy levels in the environment
What is the myelin sheath made up of?
Very tightly wrapped Schwann cells
What are the gaps in the myelin sheath called?
Nodes of Ranvier
What is the purpose of the Nodes of Ranvier?
The Nodes of Ranvier allow the ions to cross the membrane as there is a gap in the myelin sheath. This allows the conduction to be much more rapid (100-120ms-1)
Why is the action potential not as fast in non-myelinated neurones? (2-20ms-1)
As several neurones would be loosely wrapped in the Schwann cells it means that the action potential moves along as a wave and cannot jump from node to node
Where would myelinated neurones be used in the body?
Myelinated neurones can be longer and faster meaning they can carry action potentials from sensory receptors to the CNS and from the CNS to effectors e.g for quick reflexes
Where would non-myelinated neurones be used in the body?
Non-myelinated neurones tend to be shorter and are often used when coordinating slow responses in the body such as breathing and for the digestive system
What are the differences between sensory and motor neurones?
Sensory -Cell body outside CNS -Short axon -Long dendron -Carry AP to CNS Motor -Cell body in the CNS -Long axon -Short dendrite -Carry AP away from the CNS
Why do neurones need to contain many mitochondria?
Because transmitting action potentials is an active process as they need to pump ions to create concentration gradients
Why are receptors called transducers?
Because they transfer energy into electrical energy
How is the resting potential maintained?
The sodium potassium pump actively moves 3 Na+ ions out
What are neurones and what do they do?
-Neurones are nerve cells that work by transmitting electrical impulses
(This is done by creating an action potential)
How are neurones suited to start an action potential?
-Neurones start out in a polarised state
(They have an electrical polarisation of their
membrane)
-This allows the action potential to transmit an electrical impulse along their length
What are the 3 types of neurones?
Motor, Sensory & Relay
What are the features of a motor neurone?
- Cell body at the top
- Dendrites coming off of the cell body
- Axons going along/coming off the cell body
- Wrapped in a myelin sheath with nodes of Ranvier
- Transmission is away from the cell body and down the axon
What is the role of the dendrons and axons in neurones?
Dendrons - carries impulses towards the cell body
Axons - carries impulses away from the cell body
Which types of neurones are myelinated (in Schwann cells)?
- Motor and sensory neurones are myelinated
- This means that they have an increased speed of transmission due to saltatory conduction, the jumping of the action potential from node to node faster
What are the features of a sensory neurone?
- Cell body at the bottom
- Leading to the synaptic endings
- Dendron at the start with receptors before
- Wrapped in a myelin sheath with nodes of Ranvier
- Transmission is towards the CNS from receptors
What is the full direction of transmission?
SENSORY NEURONES -From receptors on the sensory neurone down the dendron to the cell body of the sensory neurone
-From the cell body to the synaptic endings to the CNS
RELAY NEURONE -Connects sensory to motor
MOTOR NEURONES -Dendrites on the motor neurone
-Down the axon towards a muscle, subsequent motor neurone or a gland to produce a response
What are the features of a relay neurone?
- Big cell body
- Dendrites coming off the cell body with synaptic endings
- Connects motor to sensory
- Present in the brain
What is the process of the action potential?
1) -At the start the neurone is polarised (-60mv)
- This is because of the movement of 3 sodium ions out of the cell and the movement of 2 potassium ions into the cell (Na/K pump) [The outside is positively charged and the inside is negatively charged]
- This is also because K+ channels are open so the potassium ions can also leak out {There are more sodium ions outside the cell than inside and more potassium ions inside the cell}
2) -Some sodium ion channels open, the na+ ions move down the gradient into the cell (depolarisation so the potential increases from -60 to -50 when the action potential occurs)
3) -Threshold potential is reached (-50mv) this then triggers the opening of voltage gated sodium ion channels
4) -More and more sodium ions flood into the cell causing the membrane to become more depolarised
5) -The membrane reaches +40mv as Na+ ions flood in
6) -Sodium ion channel closes and the potassium ion channels open The potassium ions then flood out the cell causing the potential to decrease
7) -Repolarisation as the K+ ions exit the cell down the concentration gradient
8) -Hyperpolarisation (To around -70mv)
9) -AP is fired and the potential returns to the resting state, there is potentially a brief refractory period where the voltage gated channels don’t allow another AP to be fired
What is the simplified process of the action potential?
RESTING POTENTIAL
-There is a difference in charge (POLARISED -60mv)
-The inside is negative (more potassium ions)
-The outside is positive (more sodium ions)
ACTION POTENTIAL STARTS - (DEPOLARISATION)
-Voltage gated channels are used
-Sodium ions move into the cell (passively)
-Inside of the cell becomes more positive (depolarised)
-Reaches threshold potential causing positive feedback
[As the inside of the cell becomes more positive, the sodium ions diffuse across, propagating down the axon, the sodium ions inside the cell cause more channels to open and more Na+ ions to enter making the cell more positive]
REPOLARISATION
-Sodium channels close
-Potential returns to the resting
HYPERPOLARISATION
-Potassium channels open,
-Potassium moves out of the axon causing a brief hyperpolarisation
-Potential returns to the resting due to the NA+/K+ pump
Why is the action potential an all or nothing response?
- It can only move one direction down the axon
- This means that during depolarisation of the membrane, once sodium ions enter the cell (Making it more positive) the action potential must move down the cell causing a wave of depolarisation (Making the rest of the cell more positive)
What do sensory receptors do?
- Transduce one stimulus and turn it into an electrical signal for example: (Cells in eyes turn light into an electrical signal)
- Contain gated ion channels
- Channels open so sodium ions can enter
- An example is the Pacinian Corpuscle which detects pressure
What the process occurring in the synapse?
1) The AP moves down to the synapse
2) Voltage gated calcium channels open, calcium ions move into the presynaptic bulb
3) Vesicles containing ACh move to the membrane to fuse and then release via exocytosis
4) Na+ channels open, Na+ moves in causing depolarisation
5) ACh is then broken down by acetylcholinesterase to be reabsorbed back into the presynaptic bulb, they then re-joined back, requiring ATP
How is the synapse fit for function?
- Many mitochondria in the presynaptic bulb
- Many endoplasmic reticulum in the presynaptic bulb
- Voltage gated calcium ion channels
- Pre-existing concentration gradient as there are more calcium ions outside than inside the presynaptic bulb
- ACh gated Na+ channels on the synapse
What is temporal summation? (Time, three in quick succession build as one)
-Where the neurone after, decides whether it wants to fire the AP based off of the building of three small depolarisations that join to reach the threshold value
(Using EPSP, Excitatory post synaptic potential)
What is spatial summation? (Space, three build as one in steps of one AP)
-When three separate axons arrive at one synapse at the same time that release ACh and trigger a depolarisation that is able to reach the threshold potential and then trigger the AP
What is a neuromuscular junction and how does it cause the AP to bring about muscle contraction?
When an AP connects to a muscle:
- It is myelinated meaning the speed of transmission is about ten times faster
- The AP arrives the calcium moves in via calcium channels that open
- Vesicles fuse to the membrane and release the neurotransmitter via exocytosis
- ACh binds to the receptors on the surface of the sarcolemma (surface membrane of the muscle cell)
- The AP spreads along the muscle fibre down to the T tubule which carries the AP into the cisternae of the muscle fibre
- Channels open and release Ca+ ions into the muscle fibre where it binds to the troponin which causes the contraction
What is the structure of the muscle fibre?
- Surrounded by the sarcolemma
- T-tubule (transverse) connects to the terminal cisternae of the sarcoplasmic reticulum
- Multinucleated as it is more than one fibre fused together
- Many mitochondria to produce ATP needed for muscle contraction
