Nervous Coordination

Nervous System

• fast and specific

Neurones

• adapted to rapidly carry a nervous impulse
• cell body - contains organelles
• Dendrons subdivide into dendrites
• Axons end in axon terminals

Motor neurone

• schwann cells make myelin sheath 
• node of Ranvier - gaps between myelin sheath
• myelin sheath insulates neurone 

Sensory neurone - one long dendron and one axon

Relay neurone - multiple dendrons and one axon

Resting potential

• Sodium-potassium pump actively transports sodium ions out and potassium ions into axon
• Membrane more permeable to potassium ions than sodium ions
• more potassium ions channels open than sodium ion channels
• more potassium ions diffuse out than sodium ions diffuse in
• axon is polarised - more posisitve potential outside axon than inside axon

Axon potential

• Stimulus opens voltage gated sodium ion channels, sodium ions diffuse in 
• more sodium channels open causing influx of sodium ions, membrane is depolarised

Repolarising

• Sodium ion channels close, Potassium ion channels open and potassium ions diffuse out causing hyperpolariation
• K⁺ Potassium ion channels close and sodium potassium pump repolarises membrane

Unmyelinated neurone

• Stimulus causes depolarisation
• localised electrical ciruits established opening sodium ion channels further along axon
• behide depolarisation - sodium ion channels close and potassium ion channels open
• axon membrane is repolarised

Myelinated neurone

• Myelin sheath acts as an electrical insulator preventing action potentials from forming
• localised circuits arise between nodes of ranvier
• saltatory conduction - action potential jumps between nodes

Synapse featues

• unidirectional
• spacial summation - multiple neurones for one synapse
• temporal summation - single knob releases many neurotransmitters over a short time period

Passage of action potential

• fast
• size remains constant 
• intense stimulus increases frequency of action potentials

Factors affecting speed of action potential

• myelin sheath 
• diameter - greater = faster
• temperature - higher =faster

All or nothing principle

• action potentials only occur if threshold value reached

Refractory period

• time after depolarisation when no new action potential can be created
• produces discrete, directional impulses 

Cholinergic synapse

• Action potential depolarisess synaptic knob
• calcium ion channels open and calcium ions diffuse in
• synaptic vesicles fuse with membrane and release acetylcholine into synaptic cleft
• acetylcholine binds to receptors on postsynaptic knob
• sodium ion channels open and sodium ions diffuse in
• Action potential if above threshold potential
• acetylcholinesterase hydrolyses acetylcholine into acetyl and choline
• Sodium ion channels close stopping depolarisation

Inhibition of synapses

• Neurotransmitters bind to chlorine ion channesl on postsynaptic membrane 
• chlorine ion channels open and chlorine diffuses in
• potassium ion channels opne and potassium ions diffuse out
• neurone becomes hyperpolarised so more sodium ions required to produce action potential

Muscle structure

• made of parallel myofibrils which are made of sarcomeres
• myofibrils grouped into muscle fibre, then bundles of muscle fibres, then muscle tissue
• muscle cells fused together and share nuclei and sarcoplasm

Sarcomeres

• made of myosin (thicker) and actin filaments (thinner)

Slow twitch fibres

• less powerful contractions over a long time period
• aerobic respiration, many mitochondria and blood vesseles

Fast twitch fibres

• powerful contractions over short time period
• anaerobic respiration, more myosin, high glucogen/phosphocreatine stores

Neuromuscular junction

• connection between motor neurone and muscle
• motor unit - muscle fibres stimulated by one neuromuscular junction

How it works:

1. Action potential reaches many neuromuscular junctions

2. Acetylcholine released

3. Binds to sarcolemma

4. Causes depolarisation

Sliding filament theory

1. Action potentials result in acetylcholine being released

2. Acetylcholine binds to ligand-gated receptors on muscle fibres causing depolarisation

3. depolarisation spreads via transverse tubules

4. sarcoplasmic reticulum releases calcium ions

5. calcium ions bind to troponin molecules which change shape and move tropomyosin

6. actin-myosin cross bridges formed 

7. ATP hydrolyses by ATPase on myosin head causing myosin head to bend and pull actin

8. ATP binds again to myosin head causing mysoin head to detach