Biology (5.3): Neuronal communication

sensory receptors

cells/sensory nerve endings that respond to a stimulus in the internal or external environment of an organism and can create action potentials

transducer

a cell that converts one form of energy into another

What is the sensory receptor and energy change involved with a change in light intensity? (2)

1. light sensitive cells (rods and cones) in the retina

2. light to electrical

What is the sensory receptor and energy change involved with a change in temperature? (2)

1. temperature receptors in the skin and hypothalamus

2. heat to electrical

What is the sensory receptor and energy change involved with a change in pressure on the skin? (2)

1. Pacinian corpuscles in the skin

2. movement to electrical

What is the sensory receptor and energy change involved with a change in sound? (2)

1. vibration receptors in cochlea of the ear

2. movement to electrical

What is the sensory receptor and energy change involved with movement? (2)

1. hair cells in inner ear

2. movement to electrical

What is the sensory receptor and energy change involved with a change in length of muscle?

1. muscle spindles in skeletal muscles

2. movement to electrical

What is the sensory receptor and energy change involved with chemicals in the air?

1. olfactory cells in epithelium lining the nose

2. detect a chemical and create an electrical nerve impulse

What is the sensory receptor and energy change involved with chemicals in food? (2)

1. chemical receptors in taste buds on tongue

2. detect a chemical and create an electrical nerve impulse

Pacinian corpuscles

a pressure sensor that detects change in pressure on the skin

What kind of receptor is a Pacinian corpuscle?

mechanoreceptor

Characteristics of a corpuscle (2)

1. oval-shaped structure

2. series of cocentric rings of connective tissue wrapped around the end of a nerve cell

What happens to a Pacinian corpuscle when the pressure on the skin changes?

it deforms the rings of connective tissue, which push against the nerve ending

What happens to the corpuscle when the pressure is constant?

it stops responding

What are the two kinds of specialised channel proteins found in cells associated with the nervous system?

sodium and potassium channels

What kind of sodium channels are found in a Pacinian corpuscle?

stretch-mediated

What happens when the rings are deformed?

the sodium ion channels open and sodium ions diffuse into the cell, creating a generator potential

When is an action potential triggered?

If the generator potential reaches the threshold value

What do sodium potassium pumps do?

actively pump sodium ions out of the cell and potassium ions into the cell

How many sodium ions are pumped out for every two potassium ions that are pumped into the cell?

3 Na ions out for every 2 K ions in

Why do potassium ions leak out of the cell?

because the membrane is permeable to potassium ions

What is the potential gradient formed across the membrane?

cell more negatively charged than outside the cell

What is the cell called when it is inactive?

polarised (-vely charged inside compared with the outside)

depolarisation

the inside of the cell becomes less negative compared with the outside (due to the movement of ions across the membrane)

What happens to the membranes to cause an action potential?

if enough gates are opened and enough sodium ions enter the cell, the potential difference across the membrane changes significantly and will initiate an impulse (action potential)

sensory neurones

carry the action potential from a sensory receptor to the CNS

relay neurones

connect sensory and motor neurones

motor neurones

carry action potentials from the CNS to an effector

What type of neurone is this?

sensory neurone

What type of neurone is this?

motor

What type of neurone is this?

relay

Axon

carries impulses away from the cell body

What does the cell body of a neurone contain?

nucleus, many mitochondia and ribosomes

Dendrites

carry impulses towards the cell body

myelinated neurones

are insulated by the myelin sheath

motor neurone (structural differences) (2)

1. cell body in CNS

2. long axon (that carries AP to effector)

sensory neurone (structural differences) (2)

1. long dendron

2. short axon

relay neurone (structural differences) (2)

1. many short dendrites

2. short axon

Schwann cells

make up the myelin sheath

Nodes of Ranvier

gaps in between the Schwann cells

What does myelination do to the AP?

Makes the AP jump from one node to the next, so conduction is more rapid

Advantages of myelination (2)

1. can transmit AP quickly over long distances

2. enables rapid response to stimulus

action potential

a brief reversal of the potential across the membrane of a neurone causing a peak of +40 mV compared to the resting potential of -60 mV

resting potential

the potential difference across the membrane while the neurone is at rest

What kind of response are action potentials?

all-or-nothing

Stages of an action potential (13)

1. membrane in resting state (polarised), Na⁺ conc outside>inside & K⁺ conc inside>outside

2. Na ion channels open

3. Na ions diffuse into cell

4. membrane depolarises

5. voltage-gated sodium ion channels open

6. cell becomes more positively charged inside

7. p.d reaches +40(ish)mV (inside of cell positive compared to outside)

8. Na ion channels close

9. K ion channels open

10. K ions diffuse out of cell

11. p.d back to -ve inside compared with outside (repolarisation)

12. p.d overshoots, cell hyperpolarised

13. original p.d restored & cell returns to resting state

refractory period

period of recovery during hyperpolarisation when the ion channels are closed

How are local currents formed?

when Na ions move along the neurone towards regions where their conc is lower (causing slight depolarisation of the membrane)

Steps in the formation of local currents (6)

1. Na ion channels open

2. Na ions diffuse into neurone

3. localised increases in Na ion conc inside neurone (AP)

4. Na ions diffuse along axon/dendron

5. Na gate open due to movement of Na ions

6. AP moves along neurone as more Na ions enter

What is it called when the action potentials appear to jump from one node to the next?

saltatory conduction

Advantage of saltatory conduction

speeds up transmission of AP along the neurone

How does the brain determine the intensity of a stimulus?

from the frequency of APs arriving in the sensory region of the brain

A higher frequency of APs means …

a more intense stimulus

Synapse

junction between 2 or more neurones where one neurone can communicate with, or signal to, another neurone

What is the small gap between 2 neurones called?

synaptic cleft

Cholinergic synapse

a synapse that uses acetylcholine as its neurotransmitter

neurotransmitter

a chemical used as a signalling molecule between two neurones in a synapse

Specialised features of the pre-synaptic bulb (4)

1. many mitochondria

2. large amount of SER (packages neurotransmitter into vesicles)

3. lots of vesicles

4. voltage-gated Ca ion channels

What does the post-synaptic membrane contain? (2)

1. Na ion channels

2. receptor sites with complementary shape to neurotransmitter

Stages of the transmission across a synapse (12)

1. AP arrives at synaptic bulb

2. voltage-gated Ca ion channels open

3. Ca ions diffuse into synaptic bulb

4. Ca ions cause synaptic vesicles to fuse with pre-synaptic membrane

5. ACh released by exocytosis

6. ACh molecules diffuse across cleft

7. ACh binds to receptor sites on Na ion channels in post-synaptic membrane

8. Na ion channels open

9. Na ions diffuse across post-synaptic membrane into post-synaptic neurone

10. GP created

11. if sufficient GPs combine potential reaches threshold potential

12. new AP created in post-synaptic neurone

acetylcholinesterase

an enzyme found in the synaptic cleft that hydrolyses acetylcholine to ethanoic acid and choline

What happens to the ethanoic acid and choline?

They are recycled (re-enter the synaptic bulb by diffusion and are recombined to ACh using ATP)

When does summation occur?

when the effects of several excitatory post-synaptic potentials (EPSPs) are added together

excitatory synapse

neurotransmitters depolarise the postsynaptic membrane, making it fire an AP if the threshold is reached

inhibitory synapse

when neurotransmitters bind to the receptors on the postsynaptic membrane, they hyperpolarise the membrane (more -ve) preventing an AP from being fired

synaptic divergence

when one neurone connects to many neurones so information is dispersed to different parts of the body

synaptic convergence

when many neurones connect to one neurone so information is amplified

spatial summation

when neurones converge, so several pre-synaptic neurones contribute to producing an AP in the post-synaptic neurone

temporal summation

two or more nerve impulses arrive in quick succession from the same presynaptic neurone

Habituated

After repeated stimulation a synapse may run out of vesicles containing the neurotransmitter so the nervous system no longer responds to the stimulus