5.2.3 - Neuronal Communication

What is the CNS and what does it consist of?

Central nervous system - the brain and spinal cord

What is the PNS and what does it consist of?

Peripheral nervous system - cranial nerves and spinal nerves

What is the function of the PNS?

Communicate between the CNS and the rest of the body

What are the two key divisions of the PNS?

Sensory and motor division

What is the function of the sensory division of the nervous system?

It conducts impulses from receptors to the CNS

What is the function of the motor division of the nervous system?

It conducts impulses from the CNS to effectors (muscles and glands)

What are the two branches of the motor division?

Autonomic and Somatic

What is the somatic nervous system?

It conducts impulses from the CNS to skeletal muscles

What is the autonomic nervous system?

It conducts impulses from the CNS to cardiac muscles, smooth muscles and glands

What are the two branches of the autonomic nervous system?

Sympathetic and parasympathetic

What is the sympathetic division of the autonomic nervous system, and what is it often known as?

It mobilises body systems during activity (fight or flight)

What is the parasympathetic division of the autonomic nervous system, and what is it often known as?

It conserves energy (rest and digest)

What is a receptor cell?

A cell that detects a stimulus

What is a transducer?

any device that converts one form of energy into another

What can receptor cells be described as and why?

They are transducers - they convert energy from one form into an energy in an electrical impulse within a sensor neurone

What are nociceptors?

They respond to tissue damage

How do mechanoreceptors work?

Pressure, bending or stretch opens an ion channel

How do thermoreceptors work?

Temperature affects an enzyme controlling an ion channel

How do chemoreceptors work?

A chemical binds to a receptor, initiating a signal cascade to control an ion channel

What is a pacinian corpuscle a type of?

It is a mechanoreceptor

Where are pacinian corpuscles found?

In the skin of fingers, soles of the feet and joints/tendons/ligaments

How are generator potentials created in pacinian corpuscles?

Pressure is applied, causing the stretch-mediated sodium channels to open. Sodium floods in and is they reach the threshold potential an action potential will be established

Describe the general structure of a neurone

They have a long fibre called an axon, a cell body that contains the nucleus, axon terminals to communicate with other neurones

What are the three main types of neurone

Sensory, relay and motor

What is the role of a motor neurone?

To carry impulses from the CNS to effectors

What is the role of a relay neurone?

To connect sensory and motor neurones

What is the role of a sensory neurone?

They carry impulses from receptors to the CNS

Describe the structure of a motor neurone

They have a large cell body at one end which lies within the spinal cord or brain, and any highly branched dendrites. The axon is outside the spinal cord.

Describe the structure of a relay neurone

They are found entirely within the spinal cord or CNS and have short but highly branched axons and dendrites

Describe the structure of a sensory neurone

They have a cell body that branches off in the middle of the cell, and a single long dendron which carries impulses towards the cell body and a single long axon which carries impulses away from the cell body

How are myelinated neurones formed?

Specialised cells known as Schwann cells wrap themselves around the axon

Describe the structure of a myelinated neurone

The axon is insulated by a myelin sheath with small uninsulated sections along its length called nodes of Ranvier

How are electrical impulses carried along myelinated neurones?

Via saltatory conduction - electrical impulses jump from one node to the next

How does the axon diameter of myelinated and unmyelinated neurones compare?

Myelinated neurones have small axon diameters whereas unmyelinated have much larger ones

What is the resting potential of a neurone?

-60 to -70mV

Why is there a negative resting potential in a neurone?

There are les Na+ ions inside a resting nerve fibre that there are outside

How is a resting potential maintained in a neurone?

Using a Na+/K+ pump

How many Na+ are transported out and how many K+ are transported in via a pump?

3 Na+ ions out for every 2 K+ ions in

Why might a weak stimulus not cause an action potential to be generated?

It may only cause a slight depolarisation so it remains below the threshold potential

Can the size of an action potential change?

No

Why are action potentials described as an all-or-nothing response?

The response is the same no matter what the size of the stimulus, as long as the threshold is reached

Explain how an action potential is generated (Step 1: Stimulus)

1A stimulus triggers ion channels in the membrane to open allowing sodium ions to flow in down the electrochemical gradient. When the stimulus is large enough and the threshold potential is reached, depolarisation is triggered

What is the threshold potential for neurones?

around -55mV

Explain how an action potential is generated (Step 2: Depolarisation)

Voltage gated Na+ channels open in the axon membrane, Na+ ions flow in and reduce the potential difference. This is depolarisation. This triggers more channels to open, allowing more Na+ to enter - positive feedback. This will reach a potential of about +30mV

Explain how an action potential is generated (Step 3: repolarisation)

Very shortly after the potential difference has reached around +30mV, all the Na+ channels close in that section. K+ channels in the section open, allowing K+ to diffuse out of the axon. This returns the potential difference to normal

Explain how an action potential is generated (Step 4: Hyperpolarisation)

K+ channels are slow to close so too many K+ ions diffuse out and result in a short period of hyperpolarisation where this section of the axon membrane becomes more negative than the resting potential

Explain how an action potential is generated (Step 5: Return)

Once the K+ channels have closed, the Na+/K+ pump restores the resting potential

What is the refractory period?

Following an action potential there is a need to redistribute the Na+ and K+ ions - during this time another action potential cannot be propagated

What are the benefits of a refractory period?

They ensure action potentials are separated and that there is a maximum frequency of propagation. They also ensure impulses can travel in only one direction

How does the density of channels in a myelinated neurone compare to an unmyelinated neurone?

They have a high density of channels within the nodes, and no channels in the myelin part of the neurone

Where are myelinated neurones often used ie in which part of the nervous system?

In the somatic nervous system

Where are unmyelinated neurones often used ie in which part of the nervous system?

In the autonomic nervous system

What is the gap between neurones called?

A synapse

What is the main neurotransmitter that we study?

Acetylcholine (Ach)

What are synapses that release Ach called?

Cholinergic synapses

What is the membrane of the neurone that delivers the neurotransmitter called

Presynaptic membrane

What is the membrane of the neurone that recieves the neurontransmitter called?

Postsynaptic membrane

Explain how synapses work

Action potential arrives at the synaptic knob. Depolarisation causes opening of Na+ and Ca2+ channels. Ca2+ ions diffuse into the synaptic knob, causing vesicles to move to the presynaptic membrane. The vesicles fuse with the membrane and release neurotransmitters into the synaptic cleft. The neurotransmitter diffuses across the cleft and binds to the specific receptors in the postsynaptic membrane. This causes Na+ channels to open.

How are neurotransmitters recycled (use Ach as an example)?

Acetylcholinesterase (AChE) breaks down the ACh, preventing it from being permanently bound and causing prolonged depolarisation in the postsynaptic membrane. The presynaptic membrane then reabsorbs the broken down transmitter and makes more ACh

How do neurotransmitters leave the presynaptic membrane ie vai what biological process?

Exocytosis

What is an EPSP?

Release of some neutrotransmitters e.g ACh lead to excitation caused by opening of sodium channels

What is an IPSP?

Some neutrotransmitters e.g GABA open chlorine channels which lead to hyperpolarisation

What is summation?

the process of adding up postsynaptic potentials and responding to their net effect

What are the two types of summation?

Temporal and spatial summation

What is temporal summation/when does it occur?

It occurs when a series of subthreshold EPSPs in one fibre produce an action potential in a postsynaptic cell - if multiple impulses arrive within quick succession the effect can be added together

What is spatial summation/when does it occur?

Multiple impulses arriving simultaneously at different synaptic knobs but stimulating the same cell body can also generate and action potential.

What are the advantages of synapses (3)?

They allow summation, they filter out low level signals and they allow one way transmission

How does temp affect impulse speed?

Colder temps slow down the conduction as there is less KE for the diffusion of K+ and Na+