5.3 - Neural Communication

Question 1

What is a sensory receptor?

Answer 1

Specialised cell that detects change in an environment. An energy transducer that converts energy from one form to another, e.g. light energy into biochemical energy of the action potential.

Question 2

Name stimuli, their receptor and the energy change.

Answer 2

Stimulus: Light intensity. Receptor: Rods and cones in eye. Energy change: Light to electrical.
Stimulus: Temperature. Receptor: Skin receptors and hypothalamus. Energy change: Heat to electrical.
Stimulus: Pressure. Receptor: Pancinian corpuscles in skin. Energy change: Movement to electrical.
Stimulus: Sound. Receptor: Vibration receptors in cochlea. Energy change: Movement to electrical.
Stimulus: Muscle length. Receptor: Spindle fibres. Energy change: Movement to electrical.
Stimulus: Chemicals in air. Receptor: Olfactory cells in nose. Energy change: Chemical to electrical.
Stimulus: Chemicals in food. Receptor: Receptors in taste buds. Energy change: Chemical to electrical.

Question 3

What is the Pacinian corpuscle?

Answer 3

Pressure sensor in skin.

Question 4

Describe the structure of the Pacinian corpuscle.

Answer 4

Oval shaped, concentric rings of connective tissue.
Wrapped around nerve ending.
Pressure distorts rings of connective tissue.
Sensitive only to changes in pressure.
If pressure is constant, cells stop responding.

Question 5

Name the proteins associated with the nervous system and describe their action.

Answer 5

Protein: Sodium ion channels. Action: Ion specific channels, sensitive to small movements in nerve membrane, distortion increases permeability of membrane to sodium ions allowing diffusion into cell, may be gated.
Protein: Potassium ion channels. Action: Ion specific channels, may be gated.
Protein: Sodium/ potassium pump. Action: Active transport of 3 sodium ions out of nerve cell; active transport of 2 potassium ions into nerve cell.

Question 6

Describe the nerve cell membrane when at rest.

Answer 6

Polarised. More positive sodium ions leave the membrane then positive potassium ions enter. Negatively charged on inside compared to outside.

Question 7

Describe the three types of neurones.

Answer 7

Motor - carry action potential from CNS to an effector - gland or muscle.
Sensory - carry action potential from receptor to CNS.
Relay- connect motor and sensory neurones.

Question 8

Describe the features common to all neurones.

Answer 8

Long.
Cell surface plasma membrane high in ion channels.
Sodium potassium pump uses ATP for active transport of ions.
Potential difference maintained across cell surface plasma membrane.
Cell body rich in mitochondria and ribosomes.
Dendrites connect neurones and carry impulses towards cell body.
Axon carries impulses away from cell body.
Many neurons wrapped in fatty sheath of myelin.

Question 9

Describe the differences between neurones.

Answer 9

Motor neurones - cell body in CNS, long axon carries impulse to effector.
Sensory neurones - long dendron carries impulse from receptor to cell body, short axon from cell body to CNS.
Relay neurone - many dendrites, short axons, interconnect motor and sensory neurones.

Question 10

Describe the myelin sheath.

Answer 10

Formed from Schwann cells wrapped tightly around neurone.
Several layers within cytoplasm.
Restricts diffusion of ions in/out of neurone.
Gaps in sheath at 1-3mm intervals called nodes of Ranvier.
Ions diffuse in/out of neurone at nodes of Ranvier.
Action potential jumps from node to node.

Question 11

What are the advantages of myelination?

Answer 11

Action potential transmitted more rapidly than in non-myelinated. 100-120 m/s, non-myelinated 2-20 m/s. Myelinated neurones longer.

Question 12

Describe non-myelinated neurones.

Answer 12

Associated with Schwann cells. Several neurones loosely wrapped by on Schwann cell. Ions able to diffuse in/out of neurone. Action potential moves in waves along neurone.

Question 13

What is the action potential?

Answer 13

A brief reversal of potential across the membrane of a neurone. +40mV.

Question 14

What is the resting potential?

Answer 14

The potential difference across the membrane. -60mV.

Question 15

Describe the neurone at rest.

Answer 15

Active transport of ions - 3 Na+ out, 2 K+ in. Sodium potassium pump uses hydrolysis of ATP to ADP + Pi.
Gated sodium ion channels closed. Some potassium ion channels are open.
Membrane more permeable to potassium ions so some diffuse from cell, across membrane down concentration gradient - passive diffusion.
Interior of cell is negative compared to outside.
Membrane is polarised, difference across membrane is -60mV. This is resting potential.

Question 16

Describe the generation of an action potential.

Answer 16

At rest higher concentration of sodium ions outside cell than within cell. Higher concentration of potassium ions inside cell than outside.
Stimulus distorts membrane making it more permeable to sodium ions. Sodium ions diffuse down concentration gradient into cell from tissue fluid.
Membrane is depolarised - it becomes less negative. Threshold value of -50mV reached.
Voltage gated sodium ion channels open, more sodium ions diffuse into cell-positive feedback. Membrane continues to depolarise. Potential difference across membrane reaches +40mV.
Sodium ion channels close, potassium ion channels open. Potassium ions diffuse out of cell.
Potential difference inside cell returned to more negative relative to outside, repolarisation. Potential difference overshoots, hyperpolarised.
All channels close, sodium/potassium ion pump restores resting potential difference -60mV.

Question 17

What is the refractory period?

Answer 17

After an action potential passes sodium ions are inside the cell and potassium ions are in the tissue fluid outside the cell. The sodium/potassium ion pump restores the balance. During this refractory period the cell membrane cannot be stimulated. Ensures action potential can only be transmitted in one direction.

Question 18

What is the all or nothing response?

Answer 18

All action potentials are equal. Once threshold has been reached an action potential is transmitted. The strength of a stimulus is indicated by the frequency of action potentials.

Question 19

What is a local current?

Answer 19

The movement/ flow/ current of ions into the cytoplasm of the neurone. Generated by the flow of sodium ions into the neurone following a stimulus.

Question 20

Describe the formation of local currents and the transmission of the action potential.

Answer 20

Following a stimulus, sodium ion channels open and ions diffuse into membrane. Inside the neurone sodium ions diffuse down their concentration gradient. Point of diffusion = high concentration, diffusion is away from this point. Movement of charged ions is a local current. Local current causes depolarisation of membrane causing voltage gated sodium ion channels to open. Influx of sodium ions crosses threshold potential and generates action potential. Full depolarisation occurs, action potential propagated along neurone.

Question 21

How do local currents ensure action potential moves only in one direction?

Answer 21

Concentration of sodium ions is high at point of diffusion and remains high. Rate of generation of action potential is rapid compared to rate of diffusion.

Question 22

What is saltatory conduction?

Answer 22

Occurs in myelinated neurones. Schwann cells wrapped tightly around neurone creating insulating sheath. Ions cannot diffuse through myelin sheath. Gaps in sheath occur at nodes of Ranvier. Ions diffuse in/out of neurone at nodes. Leads to extended local currents. Action potential jumps between nodes. Rapid transmission of action potential.

Question 23

Describe the link between stimulus strength and action potential.

Answer 23

All action potentials are generated when the potential difference across the cell membrane reaches +60mV. The stronger the stimulus, the more frequently the action potential is propagated along the neurone. Our brain translates the direction of frequency of the action potential into the type and strength of stimulus.

Question 24

What is a synapse?

Answer 24

The junction between two or more neurones.

Question 25

What is a neurotransmitter?

Answer 25

A chemical used as a signalling molecule between one neurone and another, crossing the synapse.

Question 26

What is the cholinergic synapse?

Answer 26

A synapse that uses acetylcholine as neurotransmitter. Used in the parasympathetic nervous system.
Remember that noradrenaline acts as a neurotransmitter during the sympathetic nervous system.

Question 27

What is the presynaptic bulb?

Answer 27

Swelling at the neurone end before the synapse. High in mitochondria - ATP for active transport. High in smooth endoplasmic reticulum - packaging of neurotransmitter into vesicles. Large number of calcium voltage gated channels in membrane cell surface membrane.

Question 28

What is the postsynaptic neuron?

Answer 28

Cup shape at the start of the neurone after the synapse. Contains specialised sodium ion channels activated by binding of the neurotransmitter. Five polypeptides - tertiary structure. Two polypeptides have receptor site specific to acetylcholine. When acetylcholine binds to receptor sodium ion channel in proteins opens. Sodium ions enter postsynaptic neurone.

Question 29

Outline the sequence of events at the synapse.

Answer 29

Action potential arrives at presynaptic bulb. Voltage gated calcium channels open. Calcium ions diffuse into bulb. Calcium ions cause vesicles containing neurotransmitter to move to, and fuse with, presynaptic membrane. Neurotransmitter released by exocytosis and diffuses across cleft. Neurotransmitter binds to receptors on postsynaptic neurone and sodium ion channels open. Sodium ions diffuse into postsynaptic neurone. A generator potential, excitatory postsynaptic potential (EPSP) is generated. If threshold is crossed new action potential is generated.

Question 30

What regulates the transmission of the impulse across the synapse?

Answer 30

Acetylcholinesterase is located in the synaptic cleft. This enzyme hydrolyses acetylcholine into acetic acid and choline. The impulse is halted. Acetic acid and choline diffuse back into the presynaptic neurone and recombine to form acetylcholine. This requires energy in the form of ATP. Acetylcholine stored in vesicles.

Question 31

Describe the features that ensure the action potential is transmitted only in one direction across the synapse.

Answer 31

Only in presynaptic neurone: Calcium voltage gated channels, vesicles containing acetylcholine, enzymes for condensation of acetic acid and choline to form acetyl choline.
Only in postsynaptic neurone: Acetylcholine regulated sodium ion channels.
Concentration gradients established in cleft ensures molecules diffuse down concentration gradients from one side of the cleft to the other.

Question 32

What is summation?

Answer 32

The combining of several excitatory postsynaptic potentials that result in threshold in the postsynaptic neurone. It can be spatial or temporal.

Question 33

What is temporal summation?

Answer 33

Several action potentials arrive at the presynaptic neurone. Generates one action potential in the postsynaptic neurone.

Question 34

What is spatial summation?

Answer 34

Several presynaptic neurones reach a synapse. Contribute toward generating an action potential in one postsynaptic neurone.

Question 35

What are inhibitory postsynaptic potentials, IPSPs?

Answer 35

Presynaptic neurone reduces the effect of summation. No action potential generated in postsynaptic neuron.

Question 36

What causes IPSPs?

Answer 36

GABA neurotransmitter triggers influx of chloride ions or potassium ions into postsynaptic neurone. Membrane is hyperpolarised. No action potential is generated.

Question 37

How do synapses regulate neuronal communication?

Answer 37

Spatial summation allows coordinated response to more than one stimuli - often signs of danger.
Interactions between EPSPs and IPSPs regulate generation of action potentials.
Acetylcholine only present in presynaptic bulb/receptors only present in postsynaptic neurone so impulse can only be transmitted in one direction.
Low level action potentials may not trigger release of sufficient neurotransmitter to cross synapse so low grade stimuli filtered out. Low level action potentials can be amplified by summation.
Repeated stimulation may deplete neurotransmitter so response to stimulus ceases - habituation.
Synapse membranes are adaptable; postsynaptic membrane concentration of neurotransmitters variable to increase/decrease sensitivity.