Neuronal Communication

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Glossary

Neurotransmitters- chemicals which are used to pass signals from one neurone to the next

Transducer- something that converts a stimulus into a nerve impulse

Depolarised- a change in the potential difference from negative to positive

Polarised- the process or act of producing positive and negative electric charge values to opposite ends, such as the electrical charge inside a (nerve) cell relative to the surrounding environment

Hyperpolarisation- a change in a cell's membrane potential that makes it more negative. It is the opposite of a depolarization. It inhibits action potentials by increasing the stimulus required to move the membrane potential to the action potential threshold

Positive feedback- the enhancing or amplification of an effect by its own influence on the process which gives rise to it

Refractory period- a period immediately following stimulation during which a nerve or muscle is unresponsive to further stimulation

Neuromuscular junction- where a motor neurone and a muscle cell (effector) meet

Nervous Systems

Structural Organisation

CNS- Brain and spinal chord

PNS (peripheral)- The rest of the nervous system apart from the brain and spinal chord e.g stimulus, response, neurones, effectors and sensory organs

Functional Organisation

Somatic NS- (conscious control) used when you voluntarily decide to do something e.g. moving a muscle in your arm

Autonomic NS- (subconscious control) used when the body does something automatically e.g. causing the heart to beat

Autonomic Nervous System

Sympathetic motor system- fight or flight responses, neurotransmitter is noradrenaline, increases activity e.g. increased heart rate

Parasympathetic motor system- relaxing responses, neurotransmitter is acetylcholine, decreases activity e.g. decreased breathing rate

Neurones

stimulus -> receptor -> sensory -> relay -> motor -> effector -> response

Structure

Cell body- the nucleus surrounded by cytoplasm, large amounts of ER and mitochondria which help to produce neurotransmitters

Dendrons- short extensions that come from the cell body which divide into smaller and smaller branches called dendrites which transmit electrical impulses towards the cell body

Axons- singular elongated nerve fibres that transmit impulses away from the cell body, consists of a very narrow region of cytoplasm (1 micrometre) surrounded by a plasma membrane

Stimulus

Detect a stimuli (change in the environment) e.g. heat, light or pressure

Sensory neurones

Transmit impulses from a sensory receptor cell to a relay or motor neurone or the brain

They have one dendron which carries the impulse to the cell body and one axon which carries the impulse away from the cell body

Relay neurones

Transmit impulses between neurones

Many short axons and dendrons

Motor neurones

Transmit impulses from a relay or sensory neurone to an effector e.g. muscle or gland

One long axon and many short dendrites

Myelination

The axons of some neurones are covered in a myelin sheath, many layers of plasma membranes

Produced by Schwann cells which grow around the axon many times and each time they grow around the axon a double layer of phospholipid bilayer is laid down

Much faster conduction of the electrical impulses

100m/s instead of 1m/s

Nodes of Ranvier are the gaps in between Schwann cells (2-3 micrometres), this creates gaps in the myelin sheath (every 2-3mm in humans), the electrical impulse 'jumps' from one node to the next

Sensory receptors

Features

Specific to a single type of stimulus

Act as a transducer- convert a stimulus into a nerve impulse

Types

Mechanoreceptor- stimulus is pressure and movement occurring in the skin, detects changes in pressure e.g. a pacinian corpuscle

Chemoreceptor- stimulus is chemicals occurring in the nose, detects smells e.g. olfactory receptor

Thermoreceptor- stimulus is heat occurring on the tongue, detects temperature changes e.g. end-bulbs of krause

Photoreceptor- stimulus is light occurring in the eye, detects different light wavelengths e.g. cone cell

Pacinian Corpuscle

The neurone ending in a pacinian corpuscle has a stretch-mediated sodium channel and when these change shape their permeability to sodium also changes

Converts mechanical pressure into a nervous impulse by:

1. In its resting state the sodium stretch- mediated sodium channels are too narrow to allow the sodium ions to pass through, this is its resting potential

2. When pressure is applied to the pacinian corpuscle, it changes shape which causes the membrane surrounding its neurone to stretch which in turn widens the sodium ion channels so the sodium ions can now diffuse into the neurone

3. The influx of positive sodium ions changes the potential of the membrane-depolarised- which results in a generator potential

4. The generator potential creates an action potential (nerve impulse) that passes along the sensory neurone

Nervous Transmission

Resting potential

The potential difference across a neurones membrane (inside and outside of axon) when its not transmitting an impulse

In this state the outside of the membrane is more positively charged than the inside of the axon, said to be polarised as there is a potential difference across it, -70mV

Occurs as a result of the movement of sodium and potassium ions across the axon membrane, have to be transported by channel proteins because of the phospholipid bilayer, some are gated and others allow the ions through constantly

A resting potential is created by:

1. Na+ are actively transported out of the axon, K+ ions into the axon by a sodium potassium pump. 3 Na+ ions are pumped out for every 2 K+ ions

2. There are more Na+ ions outside the membrane and the3re are more K+ ions inside the cytoplasm so sodium ions diffuse back into the axon down their electrochemical gradient whereas potassium ions diffuse out of the axon

3. Most of the gated sodium ion channels are closed, preventing the movement of sodium ions whereas most of the potassium ion channels are open so potassium ions can diffuse out of the axon, therefore there are more positively charged ions outside the axon than in the cell which creates a resting potential across the membrane of -70mV, with the inside negative relative to the outside

Action Potential

1. The stimulus causes sodium voltage-gated ion channels to open, allowing the Na+ ions into the cell, this makes the cell more positively charged in relation to the outside of it

2. When the signal is strong enough and the voltage reaches a threshold an action potential is triggered, more gated ion channels open allowing more Na+ ions inside the cell, the cell then depolarises so that the inside of the cell is even more positively charged when compared to the outside

3. The peak voltage of the action potential causes the gated sodium channels to close and potassium channels to open. The potassium ions move out of the cell repolarising it, with Na+ ions on the inside of the cell and K+ ions on the outside

4. The neurone becomes hyperpolarised when more potassium ions are on the outside than sodium ions are on the inside, when the cells potential has dropped slightly lower than resting potential

5. The neurone then enters a refractory period which returns potassium to the inside of the cell and sodium to the outside

Propagation of action potentials

Occurs in non-myelinated neurones

Electron flow from - to +

Local current (electron flow) makes the next section of the membrane permeable to sodium ions and so is depolarised, this continues along the membrane so the impulse passes along the membrane

Saltatory conduction

Occurs in myelinated neurones

When the impulse jumps between the nodes of Ranvier (gaps in the myelin sheath)

Factors affecting speed of conduction other than myelination:

Axon diameter, the bigger the diameter the faster the impulse is transmitted

Temperature, the higher the temperature the faster the impulse is transmitted

All or nothing principle

A certain level of stimulus (threshold value) triggers a response, as long as this threshold is reached then an action potential will be triggered

Synapses

Structure

Synaptic cleft- the gap which separates the axon of one neurone from the dendrite of the next, approx 20-30nm across

Presynaptic neurone- neurone along which the impulse arrived

Postsynaptic neurone- neurone that receives the neurotransmitter

Synaptic knob- the swollen end of the presynaptic neurone, contains lots of mitochondria and ER to enable it to produce neurotransmitters

Synaptic vesicles- vesicles containing neurotransmitters, the vesicles fuse with the presynaptic membrane and release their contents into the synaptic cleft

Neurotransmitter receptors- receptor molecules which the neurotransmitter binds to in the postsynaptic membrane

Types of neurotransmitter

Excitatory- these neurotransmitters result in the depolarisation of the postsynaptic neurone, if the threshold is reached in the postsynaptic membrane then an action potential is triggered e.g. acetylcholine

Inhibitory- result in hyperpolarisation of the postsynaptic membrane, the prevents an action potential being triggered e.g. GABA (gamma-aminobutyric acid)

How impulses are transmitted across synapses

The action potential reaches the end of the presynaptic neurone

Depolarisation of the presynaptic membrane causes calcium ion channels to open

Calcium ions diffuse into the presynaptic knob which causes synaptic vesicles containing neurotransmitters to fuse with the presynaptic membrane and the neurotransmitter is released into the synaptic cleft by exocytosis

Neurotransmitter diffuses across the synaptic cleft and binds with its specific receptor molecule on the postsynaptic membrane which causes sodium ion channels to open which triggers an acton potential and the impulse is propagated along the postsynaptic neurone

Transmission across cholinergic synapses

Use the neurotransmitter acetylcholine

Common in the CNS of vertebrates and at neuromuscular junctions

If the neurotransmitter reaches the receptor on a muscle cell it will cause the muscle to contract. Acetylcholine is released from the vesicles in the presynaptic knob, it then diffuses across the synaptic cleft an binds with specific receptors in the postsynaptic membrane which triggers and action potential in the postsynaptic neurone or muscle cell. Acetylcholine is the hydrolysed by acetylcholinesterase to give choline and ethnic acid, the breakdown products are taken back into the presynaptic knob to be reformed into acetylcholine and the postsynaptic membrane is ready to receive another impulse

Role of synapses

Ensure impulses are unidirectional

Allows an impulse from one neurone to be transmitted to a number of neurones at multiple snaps creating numerous responses from one stimulus

Alternatively a number of neurones may feed in to the same synapse with a single postsynaptic neurone so stimuli from different receptors interact to produce a single result

Summation and control

In some synapses the amount of neurotransmitter isn't enough to trigger an action potential as the threshold level isn't reached however if the amount of neurotransmitter builds up sufficiently to reach the threshold then this will trigger an action potential by:

Spatial summation- when a number of presynaptic neurones connect to one postsynaptic neurone, each releases neurotransmitter which builds up to a high enough level in the synapse to trigger an action potential in the single postsynaptic neurone

Temporal summaton- when a single presynaptic neurone releases neurotransmitter as a result of an action potential several times over a short period, this builds up in the synapse until the quantity is sufficient enough to trigger an action potential

Structure and function of the brain

Cerebrum- controls voluntary actions, e.g. learning memory, personality and conscious thought

Cerebellum- controls unconscious functions such as posture, balance and non voluntary movement

Medulla oblongata- used in autonomic control, e.g. controls heart and breathing rate

Hypothalamus- regulatory centre for temperature and water balance

Pituitary gland- stores and releases hormones that regulate many body functions

Reflexes

Reflex arc

receptor (detects stimulus and creates an action potential in the sensory neurone) -> sensory neurone (carries the impulse to the spinal chord) ->

relay neurone (connects the sensory and motor neurones together within the spinal chord or brain) ->

motor neurone (carries impulse to the effector to carry out the appropriate response)

Survival importance

being involuntary- the fact that conscious thought is not required to form a response means that the brain can deal with more complex responses

not having to be learnt- present at birth and provide immediate protection

extremely fast- very short reflex arc

Voluntary and involuntary muscles

Types of muscle

Skeletal- make up the bulk of body muscle tissue, cells responsible for movement e.g. biceps and triceps

striated, conscious control, regularly arranged so muscle contracts in one direction, rapid and short contraction

Cardiac- found only in the heart, myogenic (contract without a nervous stimulus)

specialised striated, involuntary control, cells branch and interconnect resulting in simultaneous contraction, intermediate contraction in both length and speed

Involuntary- (smooth muscle) e.g. the walls of hollow organs such as the stomach or bladder, walls of blood vessels and digestive tract

non striated, involuntary control, no regular arrangement so different cells can contract in different directions, slow contraction but can remain contracted for a relatively long time

Structure of skeletal muscle

Made up of bundles of muscle fibres enclosed within a plasma membrane called the sarcolemma

The muscle fibres contain a number of nuclei and are much longer than normal cells, formed as a result of many embryonic muscle cells fusing together which makes the muscle stronger. The shared cytoplasm within a muscle fibre is known as the sarcoplasm

Parts of the sarcolemma fold inwards to help spread electrical impulses throughout the sarcoplasm which ensures that the whole of the fibre receives the impulse to contract at the same time

Lots of mitochondria to provide the ATP necessary for muscle contraction, also have sarcoplasmic reticulum which contains the calcium ions necessary for muscle contraction

Each muscle fibre contains many myofibrils which are long cylindrical organelles made of protein and specialised for contraction

Myofibrils are made up of 2 types of protein filament:

Actin- the thinner filament consisting of 2 strands twisted around each other

Myosin- the thicker filament consisting of long rod shaped fibres with bulbous heads that project to one side

Myofibrils have alternating light and dark bands which results in their striped appearance

Light bands- these areas appear light as they are the region where the actin and myosin filaments don't overlap (isotopic/ I bands)

Dark bands- these areas appear dark because of the presence of thick myosin filaments, the edges are particularly dark as the myosin is overlapped with actin (anisotropic/ a bands)

Z-line- a line found at the centre of each light band, the distance between adjacent z lines is called a sarcomere which is the functional unit of the myofibril and when the muscle contracts the sarcomere shortens

H-zone- a lighter coloured region found in the centre of each dark band, only myosin filaments are present at this point and when the muscle contracts the H-zone decreases

Sliding filament model

Structure of myosin

These filaments have globular heads that are hinged which allows them to move back and forwards, on the head is a binding site for each of actin and ATP. The tails of several hundred myosin molecules are aligned together to form the myosin filament

Structure of actin

Actin filaments have binding sites for myosin heads, these are called actin-myosin binding sites. These sites are often blocked however by the presence of another protein called tropomyosin which is held in place by the protein troponin

When a muscle is in a resting state (relaxed) the actin-myosin sites are blocked by tropomyosin so the myosin heads cannot bind to the actin and the filaments cannot slide past each other

How muscle contraction occurs at a neuromuscular junction

Muscle contraction is triggered when an action potential arrives at a neuromuscular function, the point at which a motor neurone and a skeletal muscle fibre meet. There are many neuromuscular junctions along the length of a muscle to ensure that all the muscle fibres contract simultaneously. If only one existed, the muscle fibres wouldn't contract together therefore the contraction of the muscle wouldn't be as powerful as a wave of contraction would have to travel across the muscle to stimulate the individual fibres to contract

All the muscle fibres supplied by a single motor neurone are called a motor unit, if a strong force is needed, a large number of motor units are stimulated, whereas only a small number are stimulated if a small force is required

When an action potential reaches the neuromuscular junction, it stimulates calcium ion channels to open, calcium ions then diffuse from the synapse into the synaptic knob, where they cause synaptic vesicles to fuse with the presynaptic membrane. Acetylcholine is released into the synaptic cleft by exocytosis and diffuses across the synapse and binds to receptors on the postsynaptic membrane (sarcolemma) opening sodium ion channels and resulting in depolarisation

Sarcoplasm

The depolarisation of the sarcolemma travels deep into the muscle fibre by spreading through the t-tubules. These are in contact with the sarcoplasmic reticulum which contains stored calcium ions which it actively absorbs from the sarcoplasm

When the action potential reaches the sarcoplasmic reticulum it stimulates calcium ion channels to open, the calcium ions then diffuse down their concentration gradient flooding the sarcoplasm with calcium ions

The calcium ions bind to troponin causing it to change shape which pulls on the tropomyosin moving it away from the actin-myosin binding sites on the actin filament. The binding sites are now exposed so the myosin heads bind to the actin filament forming an actin-myosin cross-bridge

Once attached to the actin filament the myosin head flexes, pulling the actin filament along. The molecule of ADP bound to the myosin head is released and an ATP molecule can now bind to the myosin head which causes the head to detach from the actin filament

The calcium ions present in the sarcoplasm also activate the ATPase activity of the myosin. This hydrolyses the ATP to ADP and phosphate, releasing the energy which he myosin head uses to return to its original position

The myosin head can now attach itself to another actin-myosin binding site further along the actin filament and the cycle is related. The cycle continues as long as the muscle remains stimulated. During the period of stimulation many actin-myosin bridges form and break rapidly, pulling the actin filament along, this shortens the sarcomere and causes the muscle to contract

Energy supply during muscle contraction

Aerobic respiration- most of the ATP used by muscle cells is regenerated from ATP during oxidative phosphorylation which takes place in the mitochondria which are plentiful in the muscle. This can only occur in the presence of oxygen and is therefore used for long periods of low intensity exercise

Anaerobic respiration- in a very active muscle oxygen is used more quickly than the blood supply can replace it therefore ATP has to be generated anaerobically. ATP is made by glycolysis but because no oxygen is present the pyruvate which is also produced is converted into lactate which can quickly build up in muscles resulting in muscle fatigue. Used for short periods of high intensity exercise

Creatine phosphate- stored in muscles and used to form ATP by the addition of a phosphate group to ADP. Creatine phosphate acts as a reserve supply of phosphate and is immediately available to combine with ADP forming ATP. This system generates ATP rapidly but the store of phosphate is used up quickly and as a result is used for short bursts of vigorous exercise. When the muscle relaxes the creatine phosphate store is replenished using phosphate from ATP