Biology 3.8 The Nervous System

Components of a nervous response 

Detection of stimuli by receptors

Response by the effector 

How is information transferred from a receptor to an effector 

The nervous system

Hormones via the blood

Components of the central nervous system

Brain

Spinal cord

Components of the peripheral nervous system

Cranial nerves

Spinal nerves

Peripheral nerves

Neurone type in central nervous system

Relay

Neurone type in peripheral nervous system

Sensory

Motor

Purpose of the reflex arc

Rapid

Protective 

Involuntary 

The transmission of a nerve impulse along a reflex arc

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

Hydra

Sense receptors respond to a limited number of stimuli and so the number of effectors is small

Nerve net

Simple nerve cells with short extensions join to each other and branch in a number of different directions

Dendrites

Receive and carry information to the cell body

Cell body

Carries out normal cell functions

Contains organelles such as mitochondria

Nucleus

Controls cell functions

Myelin sheath

Electrical insulation

Speeds up conduction of electrical impulse

Schwann cell

Forms the myelin sheath

Node of Ranvier

Allows rapid transmission of impulses

Axon endings

Transmit impulses to another neurone or effector

Axon

Carry information from cell body to axon endings

How is the resting potential maintained 

Sodium-potassium pumps actively transport sodium ions out of the neurone and potassium ions into the neurone 

Voltage gated Na+ channels are closed but some K+ channels allow K+ to ‘leak’ out

Large protein anions and organic phosphates (ATP4-) remain in the cytoplasm and produce a negative potential difference across the membrane

How many Na+ and K+ ions are transported per molecule of ATP hydrolysed 

3 Na+ out 

2 K+ in

Depolarisation 

A stimulus causes a change in voltage across the membrane, opening the Na+ channels so that Na+ flood in and depolarise the axon to +40mv

Repolarisation

Na+ channels close and K+ channels open

K+ flood out of the axon and reduce the potential difference across the axon membrane

Hyperpolarisation

An overshoot of K+ out of the axon

Relative refractory period

Concentrations of Na+ and K+ are restored to that of the resting potential

During this time the axon cannot transmit another action potential

This ensures the transmission is in one direction only

The all or nothing law 

The size of the action potential is independent of the size of the stimulus 

Factors affecting the speed of conduction

Temperature

Axon diameter

Myelin sheath

Effect of temperature

Higher temperature, faster the speed

Effect of axon diameter

Larger diameter, faster the speed

Who has a myelin sheath 

Vertebrates 

Where are ion channels found in myelinated axons

The nodes of Ranvier

What does the myelin sheath act as

A good electrical insulator

Saltatory propagation

The action potential jumps large distances from node to node

Dramatically increasing the speed of propagation 

100m/s compared to 1m/s in unmyelinated neurones 

Describe impulse propagation

Local currents are set up as Na+ move laterally through the axon, depolarising the adjacent section of the membrane

How are chemical synapses transmitted

Neurotransmitters

What happens when an action potential arrives at the axon terminal

Voltage gated Ca2+ channels open and Ca2+ diffuse in

What happens when Ca2+ diffuse in

Vesicles containing acetylcholine migrate to pre synaptic membrane

What happens when vesicles containing acetylcholine reach the pre synaptic membrane 

Release acetylcholine into the synaptic cleft by exocytosis 

Acetylcholine diffuses across the synaptic cleft

What happens to acetylcholine after it diffuses across the synaptic cleft

Acetylcholine binds to receptors on the post synaptic membrane, triggering it to open as an Na+ channel

Na+ diffuses into the post synaptic membrane

What happens if the threshold potential is reached

The post synaptic neurone will be depolarised, causing an action potential

How is the merging of impulses prevented

Direct uptake of acetylcholine from the cleft across the pre synaptic membrane

Ca2+ are actively transported back out of the synaptic end bulb

Acetylcholine is hydrolysed by acetylcholinesterase in the synaptic cleft, producing choline and ethanoic acid which diffuse back into the axon terminal through the pre synaptic membrane

2 equations to learn

Choline + Acetyl COA = Acetylcholine + COA

Acetylcholine = Choline and Ethanoic Acid (catalysed by acetylcholinesterase)

How do organophosphates affect the synapses

Prevent the breakdown of acetylcholine

Acetylcholine not hydrolysed so remains in synaptic cleft, causing repeated depolarisation across the post synaptic membrane

Examples of organophosphates

Insecticides

Herbicides

Psychoactive drugs 

Drugs that affect neurotransmitters and receptors in the central nervous system