Neuronal Communication (5)

Question 1

What are neurones?

Answer 1

Specialised nerve cells which transmit electrical impulses rapidly around the body, so that the organism can respond to changes in its internal and external environment.

Question 2

Cell body

Answer 2

Contains organelles found in a typical animal cell. Proteins and neurotransmitters are made here.

Question 3

Dendrons

Answer 3

Short extensions which come from the cell body. Divide into smaller branches (dendrites). Transmit electrical impulses towards the cell body.

Question 4

Axon

Answer 4

Conductive. Long fibre which transmits impulse away from the cell body.

Question 5

Features of a myelinated neurone

Answer 5

Schwann cells wrap around the axon to from the myelin sheath.
Nodes of ranvier.

Question 6

How do myelinated neurones pass an impulse.

Answer 6

Schwann cells form the myelin sheath. The myelin sheath does not allow charged ions to pass through it because it is a lipid.
The nodes of ranvier are the gaps in the myelin sheath. The action potential jumps from node to node, which enables a more rapid response because it doesn’t have to generate an action potential along the whole axon, only at the nodes.
Depolarisation only occurs at the nodes.
This is saltatory conduction.

Question 7

Types of neurones.

Answer 7

Sensory neurone - carry impulse from sensory receptor to relay neurone (sometimes to motor neurones or the brain)

Relay neurone - carry impulse between sensory and motor neurones.

Motor neurone - carry impulse from a relay neurone (or sensory) to an effector.

Question 8

What is a sensory receptor?

Answer 8

Specialised cells that detect stimuli.
They are energy transducers that convert different types of energy into electrical energy.

Question 9

Types of receptors

Answer 9

mechanoreceptor - pressure and movement.
chemoreceptor - chemicals.
thermoreceptor - heat
photoreceptor - light.

Question 10

What is the pacinian corpsule?

Answer 10

A sensory receptor which detects mechanical pressure.
They are mainly found in the fingers and feet.
The neurone ending has a special type of sodium channel (stretch-mediated sodium channel)

Question 11

How the pacinian corpsule works.

Answer 11

1. in its ‘normal’ state, stretch mediated sodium channels in the sensory neurones membrane are too narrow for sodium ions to pass through. The neurone has a resting potential.
2. When pressure is applied, the corpuscle changes shape, causing the membrane surrounding its neurone to stretch.
3. the sodium ion channels widen so sodium ions can diffuse into the neurone.
4. the increase in positive sodium ions causes the membrane to depolarise. Resulting in generator potential.
5. the generator potential (if reaching threshold) creates an action potential that passes along neurones to the CNS.

Question 12

How is nervous transmission achieved?

Answer 12

An impulse is sent along the neurone by temporarily changing the voltage (potential difference) across the axon’s membrane.
Axon membrane switches between 2 states - resting potential and action potential.

Question 13

Resting potential.

Answer 13

This is when a neurone is not transmitting an impulse.
The potential difference across its membrane (difference in charge between the inside and outside of the axon) is resting potential.

The outside of the membrane is more positively charged than the inside of the axon.
The membrane is polarised.
-70mV

sodium potassium pump is pumping 3 sodium ion out and 2 potassium ions in by active transport. ATP required.
There is more sodium ions outside the membrane so Na+ ions diffuse in down its electrochemical gradient (conc of ion) and K+ diffuse out.
Some K+ channels open so some K+ ions diffuse out as the membrane is more permeable to K+.
This creates the resting potential across the membrane -70mV

Question 14

Action potential

Answer 14

When a stimulus is detected by a sensory receptor, the energy of the stimulus temporarily reverses the charges on the axon membrane.

Potential difference rapidly changes and becomes positively charged at +40mV.
The inside is more positive than the outside.
Depolarised.

Action potential occurs when protein channels in the axon membrane changes shape.
Voltage-gated sodium ion channels open and Na+ ions diffuse into the neurone (down the concentration gradient), causing more sodium channels to open and more Na+ move in.
This is the start of a nerve impulse.

Question 15

How do sensory receptors set off an action potential.

Answer 15

Membranes contain both voltage gated and non-voltage gated sodium ion channels.
Stimulus causes non-voltage gated sodium channels to open in receptor cells.
Na+ diffuses in causing depolarisation. This sets up a generator potential (small positive charge across the membrane).

If the stimulus is strong enough, enough voltage gated sodium channels will open to depolarise the membrane to a threshold potential (-50mV).
Once reached, voltage gated sodium channels open, which sets of an action potential.

This is the all or nothing response, the stimulus either sets of an action potential or it doesn’t.

Question 16

Repolarisation of neurones.

Answer 16

Voltage gated sodium channels close, voltage gated potassium channels open.
K+ diffuse out.
Neurone repolarises - inside more negative that the outside.

The potential difference goes slightly too far - this is hyperpolarisation (inside is more negative than resting potential)

Sodium potassium pump restores resting potential (Na+ out, K+ in).

During the recovery period, it is impossible to stimulate the the neurone membrane again as voltage-gated sodium channels are closed. This is the refractory period, it ensures that action potentials transmit in one direction and that action potentials do not overlap.

Resting potential is restores to -70mV.

Question 17

Transmission of a nerve impulse.

Answer 17

Initial stimulus causes a change in the sensory receptor which triggers an action potential in the sensory receptor, so the first region of the axon is depolarised.

Sodium ion channel open , Na+ diffuses in, neurone depolarises and an action potential is set up.
The influx of Na+ ions sets off the transmission of a nerve impulse along the neurone, this causes:
increase of Na+ ion concentration at the point of depolarisation sets up a local current of Na+ ions.
Na+ ions diffuse sideways along the inside of the neurone.
causes voltage gated sodium channels to open further down the neurone, setting off another action potential.
local current moves down the neurone as a wave of depolarisation, followed by a wave of repolarisation.

Question 18

What is a synapse?

Answer 18

Junction between the presynaptic and postsynaptic neurone.
Action potential is transmitted as neurotransmitters that diffuse across the synapse.

Question 19

Transmission of impulses across a synapse.

Answer 19

Action potential reaches the end of the presynaptic neurone.
Depolarisation of the presynaptic membrane causes calcium ion channels to open and calcium diffuses into the presynaptic knob.
This causes synaptic vesicles containing neurotransmitters to fuse with the presynaptic membrane and neurotransmitters are released into the synaptic cleft by exocytosis.

Neurotransmitters bind with specific receptors on the postsynaptic membrane, which causes sodium ion channels to open and sodium diffuses into the postsynaptic neurone. This triggers an action potential and the impulse is propagated along the postsynaptic neurone.

(see next card for next process)

Question 20

What happens after an impulse has reached the post synaptic neurone?

Answer 20

Neurotransmitters must be removed from receptors, so the stimulus is not maintained and a new stimulus can arrive and affect the synapse.
Na+ channels close, re-establishing resting potential.

Question 21

Cholinergic synapses.

Answer 21

Uses neurotransmitters which are acetylcholine (excitatory).
Acetylcholine binds to receptor sites on the surface of sodium ion channels.

Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid, which diffuse back across the synaptic cleft into the presynaptic neurone - recycling, this prevents a new action potential.

ATP recombines ach and stores it in vesicles. Na+ channels close.

Question 22

Role of synapses.

Answer 22

Ensure impulses are unidirectional.
Allow an impulse from one neurone to be transmitted to a number of neurones at multiple synapses.
A number of neurones may feed into the same synapse with a single postsynaptic neurone.

Question 23

What is summation.

Answer 23

If the amount of neurotransmitter builds up to reach the threshold, this will trigger an action potential.
This does not always happen.

Question 24

Spatial summation.

Answer 24

A number of presynaptic neurones connect to one postsynaptic neurone. Neurotransmitters build up in the synapse to trigger an action potential in the postsynaptic neurone.

Question 25

Temporal summation.

Answer 25

A single presynaptic neurone releases neurotransmitters as a result of an action potential several times over a short period of time, which builds up to trigger action potential.

Question 26

The peripheral nervous system

Answer 26

All neurones that connect the CNS to the rest of the body.

Question 27

Central nervous system

Answer 27

Brain and spinal cord.

Question 28

Autonomic nervous system.

Answer 28

works constantly under subconscious control (involuntary)

Question 29

Somatic nervous system.

Answer 29

conscious control, used when you voluntarily decide to do something.

Question 30

Sympathetic nervous system.

Answer 30

increases activity

Question 31

Parasympathetic nervous system.

Answer 31

decreases activity.

Question 32

Antagonistic.

Answer 32

the action of one opposes the action of another - sympathetic and parasympathetic branches.

Question 33

What is the brain?

Answer 33

An organ, made up of a collection of different tissues which are made up of cells specialised to perform a particular function.

Question 34

CFS.

Answer 34

Brain cells are bathed in a fluid called CFS.
This supplies the brain cells with oxygen and nutrients, also acts as a coolant and a shock absorber.

Question 35

Cerebrum.

Answer 35

controls voluntary actions.
Divided into cerebral hemispheres, connected via the corpus callosum.

It is highly folded, giving it a large surface area to pack as many neurones as possible for a variety of higher brain functions, such as thinking and learning.

Question 36

Corpus callosum.

Answer 36

connects 2 halves of cerebral hemispheres.

Question 37

Cerebellum.

Answer 37

controls unconscious functions such as posture and balance.
Processes sensory information and coordinates movement.
Carry out ‘autopilot’ activities such as walking.

Question 38

Hypothalamus.

Answer 38

Controls homeostasis (thermoregulation and osmoregulation).
regulates the secretion of hormones by the pituitary gland.

Question 39

Pituitary gland.

Answer 39

controls most glands in the body

anterior pituitary gland produces 6 hormones including FSH, involved in reproduction and growth hormones.

posterior pituitary gland stores and releases hormones produced by hypothalamus.

Question 40

Medulla oblongata.

Answer 40

controls many regulatory centres of the autonomic nervous system.
controls heart rate and breathing rate, peristalsis, coughing and swallowing.

Question 41

What is a reflex?

Answer 41

An involuntary response to a sensory stimulus.
The pathway of a neurone involved in a reflex is known as a reflex arc.

Question 42

Reflex arc

Answer 42

1. stimulus
2. receptor in the skin detects stimulus
3. sensory neurone passes impulse to spinal cord
4. relay neurone passes impulse across the spinal cord
5. motor neurone passes impulse to muscle
6. effector contracts
7. response in reaction to stimulus

Question 43

Knee jerk reflex

Answer 43

a spinal reflex (only goes up to the spinal cord, not the brain)

when the leg is tapped just below the knee cap it stretches the patellar tendon and acts as a stimulus.
This initiates the reflex arc and causes the extensor muscle on the thigh to contract.

Question 44

Blinking reflex

Answer 44

a cranial reflex (occurs in the brain)

involuntary blinking of the eyelids, occurs due to stimulation of the corneas to protect it.
dust, foreign objects, bright light, loud noise.

sensory neurone –> relay neurone in the brain stem –> motor neurone.

It can be used to detect brain function in unconscious patients.

Question 45

Survival importance of reflex arcs.

Answer 45

Involuntary - brain can deal with more complex response, prevents the brain from being overloaded.

innate - do not have to be learnt, provides immediate protection from birth.

extremely fast