Why is coordination needed?

• Organisms have to coordinate all their different cells and organs to make sure they're operating effectively overall
• Have to be able to respond to changes in their internal and external environments

Neuronal communication

• Neurones have:
  • cell body --> contains the nucleus with a cytoplasm full of endoplasmic reticulum and mitochondria which are involved in the production of neurotransmitters
  • dendrons --> responsible for transmitting the impulse TOWARDS the cell body
  • axons --> transmit the impulse AWAY from the cell body
• Three types of neurone:
  • sensory --> transmit impulses from receptor to the relay neurone, motor neurone or the brain --> have one dendron and one axon
  • relay --> transmit impulses between sensory and motor neurones --> short dendrons and axons
  • motor --> transmit the impulse to the effector --> one long axon and many short dendrites
• Myelination:
  • Schwann cells produce layers of plasma membranes around the axon
  • The layers of plasma membrane act as an insulating layer which makes the impulse faster than non-myelinated as it has to "jump" between the gaps (aka the nodes of Ranvier)
  • This "jumping" = saltatory conduction

Sensory receptors

• Specific to a single type of stimulus
• Act as transducer --> convert stimulus into a nerve impulse by generating a generator potential
• Four types:
  • mechanoreceptor = pressure and movement
  • chemoreceptor = chemicals
  • thermoreceptor = heat
  • photoreceptors = light
• Pacinian corpuscle is a type of mechanoreceptor
  • has special sodium ion channels in its plasma membranes --> stretch-mediated channels
  • when these channels change shape / stretch the permeability to sodium ions changes too

Resting potential

• Resting potential = no impulse in the neurone
• -70mV
• Generated by:
  • sodium-potassium ion pump actively pumps 3 sodium ions OUT for 2 potassiums IN
  • the sodium ion channels are mostly closed so they can't diffuse in
  • the potassium ion channels are open so they diffuse back out of the axon
  • ^^ this combines to give an overall resting potential of -70mV

Action potential

1. The neurone has a resting potential (some K ion channels are open but the Na voltage-gated channels are closed)
2. Energy of the stimulus triggers some of the voltage-gated Na channels to open so Na ions diffuse into the axon down their electrochemical gradient
3. This change in charge triggers more of the voltage-gated Na channels to open (positive feedback)
4. When the potential difference reaches +40mV the voltage-gated Na channels close and the voltage-gated K channels open
5. K ions diffuse out the axon, down their electrochemical gradient to make the axon more negative
6. Lots of K ions move out, making the axon super negative (-90mV) so the voltage-gated K channels close
7. The Na-K pump takes over again to restore the resting potential of -70mV
8. This is followed by a refractory period where another impulse cannot be fired

• -70mV = resting potential
• -70mV --> +40mV = depolarisation
• +40mV --> -70mV = repolarisation
• -70mV --> -90mV = hyperpolarisation
• -90mV --> -70mV = restoration of resting potential

Propagation of action potentials

• The depolarisation of one region on the membrane of an axon acts as a stimulus for the next region along
• ^^ this continues along the length of the axon 
• Refractory period prevents the propagation of the action potential BACKWARDS down the membrane --> makes sure the impulse is unidirectional
• ^ also ensures the action potentials don't overlap
• Action potentials are sped up by:
  • axon diameter --> the bigger the diameter, the faster the impulse because there is less resistance to the flow of ions
  • temperature --> higher the temperature, the faster the impulse because the ions have more kinetic energy

All or nothing principle

• A certain level of stimulus must be achieved before a response is triggered (threshold value)
• No matter how large the stimulus, the same action potential is always triggered
• The size of the stimulus effects the number of action potentials generated at a given time (larger stimulus = more frequent action potentials)

Synapses

• Types of neurotransmitter
  • excitatory --> results in the depolarisation of the post synaptic neurone --> acetylcholine
  • inhibitory --> results in hyperpolarisation of the post synaptic neurone to prevent an action potential from being triggered --> GABA in the brain

1. Action potential reaches the end of the presynaptic neurone
2. Depolarisation of the presynaptic neurone causes Ca2+ channels to open
3. Influx of Ca2+ cause the vesicles containing the neurotransmitter to fuse with the presynaptic membrane --> neurotransmitter released by exocytosis
4. Neurotransmitter diffuses across the synaptic cleft and binds to the specific receptors in the membrane of the postsynaptic neurone
5. This causes the Na+ channels to open which triggers an action potential in the postsynaptic neurone
6. Once the neurotransmitter has triggered this action potential it is removed from the receptors and left in the cleft
7. They are often broken down by enzymes and the products diffuse back to the presynaptic neurone to be reformed into the neurotransmitter

• Acetylcholine synapses are often found between motor neurones and muscle cells
• The acetylcholine is hydrolysed by acetylcholinesterase which is also released from the presynaptic neurone
• The products of the hydrolysis are choline and ethanoic acid

Role of synapses

• Ensure impulses are unidirectional
• Allow a single stimulus to create a number of simultaneous responses - divergence
• Allows the results from different stimuli to create a single result - convergence

Summation and control

• When the neurotransmitter from a single impulse isn't enough to trigger an action potential, the neurotransmitter isn't removed from the cleft until it builds up to trigger an action potential --> summation
• Types:
  • temporal --> release of a neurotransmitter several times over a period to trigger an action potential in the postsynaptic neurone
  • spatial --> number of presynaptic neurones connect to one postsynaptic neurone --> each releases neurotransmitter which builds to a high enough level in the synapse to trigger an action potential

Organisation of the nervous system

• Central Nervous System --> brain and spinal cord
• Peripheral Nervous System --> neurones that connect the CNS to the rest of the body
• PNS splits to
  • Somatic nervous system --> concious control (move a muscle)
  • Autonomic nervous system --> subconcious control (peristalsis)
• Autonomic nervous system splits to:
  • Sympathetic nervous system --> "fight or flight"
  • Parasympathetic nervous system --> "rest and digest"

The brain

• Protected by the skull
• Surrounded by protective membranes --> meninges 
• Cerebrum --> voluntary actions (thinking, memory, personality, learning)
  • split into cerebral hemispheres which control each side of the body
  • outer layer of the hemispheres = cerebral cortex
• Cerebellum --> unconscious functions (posture, balance)
• Medulla oblongata --> autonomic control (heart and breathing rates)
• Hypothalamus --> regulatory centre for water balance and temperature (produces hormones like ADH)
• Pituitary gland --> stores and releases hormones
  • anterior produces 6 hormones including FSH
  • posterior stores and releases the hormones made in the hypothalamus (ADH)

Reflexes

• Stimulus, Receptor, Sensory, Relay, Motor, Effector, Response
• Silly, Rabbits, Sometimes, Rob, My, Enormous, Radishes
• Blinking reflex:
  • cornea is stimulated 
  • impulse along fifth cranial nerve (sensory neurone)
  • impulse passes to relay neurone in lower brain stem
  • impulses sent along seventh cranial nerve (motor neurone) 
  • the eyelids close
• Survival importance
  • being involuntary responses --> the brain is left to deal with more complex responses so its not overloaded
  • not having to be learned --> present at birth and provide immediate protection
  • extremely fast --> the reflex arc is very short
  • many are what we consider everyday actions like standing upright

Voluntary and involuntary muscles

• Skeletal muscle
  • striated
  • voluntary control
  • regularly arranged to get contraction in one direction
  • rapid contraction speed
  • short contraction
  • fibres tubular and multinucleated
• Cardiac muscle
  • specialised striated
  • involuntary control
  • cells branch and interconnect to simultaneously contract
  • intermediate contraction speed
  • fainted striations than skeletal muscle
  • fibres are branched and uninucleated
• Involuntary / smooth muscle
  • non-striated
  • involuntary control
  • different cells contract in different directions
  • slow contraction speed
  • no cross striations
  • fibres are spindle shaped and uninucleated

Structure of skeletal muscle

• Muscle fibres
  • enclosed in plasma membrane AKA sarcolemma
  • contain large number of nuclei
  • longer than normal cells
  • shared cytoplasm = sarcoplasm
  • parts of sarcolemma fold inwards to help spread electrical impulses = T tubules
  • lots of mitochondria to provide ATP for muscle contraction
  • modified endoplasmic reticulum = sarcoplasmic reticulum with calcium ions for muscle contraction
• Myofibrils
  • each muscle fibre contains many myofibrils
  • myofibrils = organelles made of protein and specialised for muscle contraction
  • lined parallel to provide maximum force when they contract together
  • two types of protein filament:
    • actin --> thinner filament (two strands twisted together)
    • myosin --> thicker filament (rod-shaped fibres with bulbous heads) 
  • alternatig light and dark bands
    • light bands = areas where actin and myosin DON'T overlap --> called the I bands
    • dark bands = areas where myosin are present, edges even darker because they overlap with the actin --> A bands
    • Z line = line found at centre of each light band --> distance between two is a sarcomere (when a muscle contracts, the sarcomere shortens)
    • H zone = lighter region at the centre of each dark band where only myosin are present (when a muscle contracts, the H zone decreases)

Sliding filament model

• Myosin pulls actin inwards towards the centre of the sarcomere so 
  • the light band becomes narrower
  • the Z lines move closer together
  • H zone becomes narrower
• Structure of myosin
  • globular heads which are hinged --> allow them to move back and forth
  • on the heads, there's one binding site for actin and one for ATP
• Structure of actin
  • filaments have binding sites for the myosin heads
  • binding sites often blocked by tropomyosin which is held in place by troponin

How muscle contraction occurs

1. Acetylcholine released into neuromuscular junction 
2. Causes depolarisation of sarcolemma
3. This travels deep into the fibre through the T tubules
4. When the action potential reaches the sarcoplasmic reticulum, the calcium ion channels are opened
5. The Ca2+ diffuse down their concentration gradient to flood the sarcoplasm with Ca2+
6. The Ca2+ bind to the troponin, causing it to change its shape, pulling the tropomyosin away from the binding sites on the actin
7. The myosin heads bind to the actin, forming a actin-myosin cross-bridge
8. The myosin heads flex, pulling the actin along
9. ATP binds to myosin head, causing it to detach because of the ATPase activity of the myosin
10. The myosin can now attach to the next binding site and the cycle continues until the Ca2+ detach from the troponin