Biology - Neural signalling

The nervous system

The nervous system

• two nervous systems: the central nervous system (CNS) of brain and spinal chord and peripheral nervous system of all the nerves in the body

• coordinates and regulates body functions

• information is sent through electrical impulses

• bundle of neurones = nerve

• CNS acts as a control coordinating centre

neurones

• main long fibre called the axon

• insulated by Schwann cells which from the myelin sheath and prevents loss of nerve impulse

• cell body and axon terminals contain extensions called dendrites

• dendrites allow them to connect to many other neurones and recieve impulses, network for easy communication

types of neurones

sensory: carry impulses from receptors to CNS

relay: connect sensory and motor neurones

motor: carry impulses from CNS to effectors

structure of neurones

motor: large cell body at one end within CNS and many dendrites

relay: short but highly branches

sensory: cell body in middle of cell and a single long dendron and single long axon

resting potential

• am impulse is a momentary reversal in electrical potential difference across neurone cell surface membrane

• an action that is not transmitting an impulse has a negative electrical potential inside called “resting potential”(around -70mv)

maintaining resting potential

• active transport of sodium and potassium ions, sodium-potassium pumps pump 3 Na out and 2 K in, creating a concentration gradient for both

• because of concentration gradient they both diffuse across membrane through sodium and potassium channels

• membrane is less permeable to Na ions so K ions diffuse out at a faster rate

• more positive ions on the outside, therefore negative charge inside

nerve impulses

• once resting potential is reached, the neurone membrane is said to be polarised

• nerve impulse is the depolarisation of this

• occurs when an action potential is generated

• an action potential is the rapid movement of sodium and potassium ions across the membrane

correlation to speed of transmission

• myelination - myelinated axons conduct faster due to insulation allowing faster saltatory conduction

• diameter - wider diameter means less resistance and thereby faster conduction

saltatory conduction

• schwann cells qrap themselves around axon forming myelin sheath

• myelin contains the phospholipids of the schwann cell membranes

• lipid content gives it high electrical resistance

• sheath has small uninsulated sections called nodes of ranvier

• electrical impulses can jump from one ranvier to the next (known as saltatory conduction) and speeds up rate of transmission

synapse

ends of two neurones + synaptic cleft. they are junctions between any cells in nervous system

synaptic transmission

1. electrical impulse arrives at the end of axon on presynaptic neurone and that membrane becomes depolarised, triggering influx of calcium ions into presynaptic cell via ion channels

2. calcium ions cause vesicles in presynaptic neurone to move towards presynaptic membrane where they fuse and release neurotransmitters into synaptic cleft (such as acetylcholine)

3. diffuse across synaptic cleft and bind with receptor molecules causing sodium channels to open and sodium ions to diffuse in

4. if there are enough neurotransmitters there is an action potential which travels down axon

5. neurotransmitters then break down by acetylcholinesterase

unidirectionality

• synapses ensure one-way transmission

• impulses can only pass one direction

depolarisation

• some ion channels are voltage gated meaning the open and close in response to electrical potential

• originally small number of ion channels open

• sodium ions move into axon down concentration gradient and reduces potential difference as inside becomes less negative

• if potential difference is reduced enough, voltage gated sodium ion channels open

• charge reverses to around 40 mb and an action potential is said to have been generated

how is an action potential propogated

• depolarisation causes sodium ions to diffuse along cytoplasm into the next section, depolarising the membrane in the new section and causing voltage gated ions to open

• triggers another action potential

• process repeats

repolarisation

• after an action potential is generated all voltage gated ion channels close

• voltage gated potassium channels open allowing diffusion of potassium ions out of axon

• axon becomes negatively charged again

• there is a short period of hyperpolarisation known as the refractory period where membrane is unresponsive

• makes sure action potentials are discrete and in one direction

• voltage gated potassium channels then close and pumps restore resting potential

local current inside

• the propagation of nerve impulses occurs due to local currents causing each section to reach threshold potential

• inside depolarised section there is a high concentration of sodium ions which diffuse within axon to neighbouring section which reduced negative membrane potential and begins action potential

local currents outside

• high sodium ion concentration outside of the section which is yet to be depolarised

• so they diffuse towards section which was just depolarised

• movements are known as local currents causing wave of depolarisations and repolarisations and propagation of nerve impulse

oscilloscopes

• membrane potentialls can be measured with electrodes on either side of the membrane (the difference)

• can be visually represented using an oscilloscope which graphically displays signal voltages

• x-axis = time ms

• y- axis = voltage/potential difference mV

nerve conduction velocity

• section of axon covered by myelin sheath means depolarisation cannot occur due to myelin sheath stopping diffusion

• therefore action potential jumps from one node to the next

• this saltatory conduction is up to 50 times faster than on myelinated

neonicotinoids

• synthetic compounds similar to nicotine and commonly found in pesticides

• can block synaptic transmission at cholinergic synapses in insects by binging to acetylcholine receptors

• stops impulses from being transmitted and leads to paralysis and deah

cocaine

• blocks reuptake of neurotransmitters into the presynaptic knob

• primarily affects reuptake of dopamine as it binds to dopamine transporter protiens

• so dopamine builds up in synapses creating a feeling of pleasure

• does the same for seratonin and norepirephrine which enhances confidence and anger

• brain thereby increases number of dopamine receptors so when levels return to normal can feel depression and sensitivity

inhibitory post-synaptic potentials

• neurotransmitters can inhibit impulse by opening gated potassium on channels

• post synaptic neurone becomes hyperpolarised and threshold will not be reached

• no action potential

excitatory and inhibitory at same time

• sodium ions enter

• potassium ions diffuse out

• cancels out effect and no action potential is reached

roles of inhibitory synapses

• prevents random impulses from being sent around the body

• specific pathways can be stimulated

• inhibitory pathways can develop over time

summation benefits

• effect of stimulus is magnified

• combination if stimuli can trigger a response

• avoids overwhelming nervous system by impulses

temporal summation

if multiple impulses arrive in quick succession their effects can be added together

spatial summation

multiple impulses arrive simultaneously at different synaptic knobs and effects are added together

perception of pain

• pain receptors are sensory receptors with free unencapsulated nerve endings

• exposed dendrites on these nerves have transient receptor potential channels which open in response to damaged tissue

• entry of positively charged ions causes threshold to be reached and stimulates action potential

• action potential travels to CNS and to the brain where pain is percieved

the cerebrum

• largest part of brain, carries out a variety of functions involved in conscious activity

• divided in two hemispheres with thin outer layer known as cerebral cortex/ grey matter

• cerebral cortex consists of cell bodies of neurones

• highly folded which increases SA, which means more neurones and more connections

• that is important as more connections means more complex behaviour

• interactions between neurones leads to consciousness (qualitative perception of feelings and complex awareness of the environment)