Coordination and Response

Describe the structure of a sensory neurone

Long axon that carries impulses away from the cell body

Long dendrites carries information towards the cell body

Cell body in the middle of the axon

Describe the structure of a motor neurone

Long axon

Short dendrites

Cell body usually found at the end of the axon

What are the functions of motor and sensory neurone?

Sensory: transmit impulses from sensory receptors to the CNS

Motor neurone: transmits impulses from CNS to effector

State the role of a sensroy receptor

To detect internal or external stimuli and stimulate the sensory neurone to transmit a nerve impulse

Describe the roles of the neurones in the reflex arc

Sensory: sends the impulse from receptor to relay neurone

Relay: transmits impulse to a motor neurone by-passing the brain so there is no conscience decision making

Motor: sends impulse to an appropriate effector

What is the function of the reflex arc?

It is a fast, automatic response to stimuli that may cause harm and this can protect the body

Define term ‘Resting potential’

The potential difference across the membrane of a neurone when the cell is not being stimulated to conduct an action potential

It’s usually -70mv in humans

Explain how the resting potential is accomplished

Sodium-potassium pump actively transports 3 Na+ ions out of the neurone for every 2K+ ions in

The axon is more permeable to K+ ions so they can diffuse out of the axon through the channels

This makes the cytoplasm of the neurone more negative than the outside making an electrochemical gradient

This results in a potential difference across the membrane

What is action potential?

A rapid change in potential difference across the neurone membrane resulting in the depolarisation of the membrane

What are the 4 stages of action potential?

1. Depolarisation

2. Repolarisation

3. Hyperpolarisation

4. Reestablishment of resting potential

Describe how depolarisation occurs in an action potential

A stimulus causes the voltage gated channels on the neurone membrane to open

This allows Na+ ions to enter the axon and increase the potential difference slightly making it less negative

If the change in potential difference is high enough to reach threshold level, more Na+ channels open to allow more ions to enter the axon

This depolarises the axon to +40mv

What prevents the potential difference of the axon increasing beyond +40mv?

The voltage gated Na+ channels close to prevent any further influx of Na+ ion

Explain the process of repolarisation

At +40mV, voltage gated K+ channels open and K+ ions leave the neurone down their electrochemical gradient

This decreases the potential difference of the neurone making it more negative

Why does hyperpolarisation occur?

As K+ ions move out of the neurone, there is a temporary overshoot and this causes the potential difference of the neurone to become more negative than normal (below -70mV)

How is the resting membrane potential restored?

The sodium-potassium pumps remove the Na+ ions from the cell and bring in K+ ions and this restores the electrochemical gradient and brings the potential difference to -70mV

What is a myelin sheath?

A lipid covering around the axon of a neurone

The gaps between the sheath are called Nodes of Ranvier

Name the cell that forms the myelin sheath

Schwann cells - the membranes of these cells surround the axon to from the myelin sheath

How does the speed of an action potential differ in a myelinated neurone compared to unmyelinated neurone?

Nerve impulses are conducted faster in myelinated neurones due to saltatory conduction

What is saltatory conduction?

The myelin sheath around the axon acts as an insulator, preventing depolarisation occurring at that point

So action potentials can only be initiated at the nodes of Ranvier. The impulse then ‘jumps’ to the next node and can pass faster along the myelinated neurone than in unmyelinated

What is the refractory period?

A time after an action potential when voltage-gated Na+ channels are closed, so another action potential cannot be generated until the channels recover

Why is the refractory period necessary?

It ensures action potentials only go spread in one direction

It means impulses are discrete and separate from each other and it limits how many action potentials can be produced in a given time

What is a synapse?

The junction between two neurones

Describe the strcutre of a sysnapse

The ends of the neurones are separated by a synaptic cleft

The synaptic knob contains lots of mitochondria and endoplasmic reticulum

Neurotransmitters in the synaptic knob are stored in vesicles

The postsynaptic neurone has receptors for the neurotransmitters on its surface

Explain the process of synaptic transmission of a cholinergic synapse

The action potential at the synaptic knob causes Ca2+ channels to open

Ca2+ enters the presynaptic neurone and causes vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft

Acetylcholine diffuses across the cleft and binds to the receptors on the ligand-gated Na+ channels on the postsynaptic membrane

Na+ ions enter the postsynaptic neurone and causes depolarisation. If the threshold is reached an action potential occurs in the postsynaptic neurone

What happens to the acetylcholine in the synaptic cleft?

The acetylcholine is hydrolysed by acetylcholinesterases in the synaptic cleft

The products are choline and acetate, which diffuse back into the presynaptic neurone

State the roles of the synapse

Ensures that nerve impulses can only travel in one direction

Allows connections between many neurones in one place

What is a neuromuscular junction?

The junction between a motor neurone and a motor end plate on a skeletal muscle fibre

What are transverse tubules (T-tubules)?

A system of infoldings of the cell surface membrane of the muscle fibre

The tubules extend throughout the cell and are in contact with sarcoplasmic reticulum of the muscle fibre

Define the term sarcoplasm

This is the cytoplasm of the muscle fibre

Name the contractile unit of muscle

The sarcomere

Describe how an action potential initiates muscle contraction

An action potential travels into the T-tubules and contacts the sarcoplasmic reticulum.

This opens Ca2+ channels, leading to the entry of Ca2+into the sarcoplasm

Ca2+ acts on the sarcomere to induce contraction

Outline the ultrastructure of striated muscle

Thick myosin filaments and thin actin filaments

A-band=region of overlap between actin and myosin

H-zone=myosin only

M-line=middle of sarcomere

I-band=actin only

Z-line= boundary between sarcomeres

Describe how Ca2+ ions allow contraction to occur

The Ca2+ ions enter the sarcoplasm and bind to troponin. This pulls on tropomyosin changing its shape to expose binding sites on the actin filaments

Describe the sliding filament model of filament contraction

The actin binding sites are exposed, which allow myosin heads to attach, forming a cross-bridge

There are ADP molecules on the myosin heads and they are released when the myosin head pulls on the actin filament

An ATP molecule attaches to the myosin head and the head detaches from the actin

The ATP molecule is hydrolysed, which provides energy to re-cock the myosin head back to its original position

What is needed to activate ATPase enzyme for muscle contraction?

Calcium ions (Ca2+)

Name the sources of ATP in the muscle

Aerobic respiration in the mitochondria

Anaerobic respiration

Phosphocreatine (provides a phosphate to combine with ADP)

Explain how the Venus fly trap catches its prey?

• Sensory hairs on the lobe are deflected by insects and Ca2+ channels open at the base of the hair and create a receptor potential

• If two hairs or one hair twice are stimulated in a short time an action potential is creates and spreads across the lobe causing them to close

• Further stimulation of the hairs causes the edges of the lobes to seal and digestive enzymes are released

State the function of the auxin

It coordinates and controls the growth of the plant

Explain how auxin works

Auxin stimulate the proton pump within cells to pump H+ ions from the cytoplasm to the cell wall

The decrease in pH in the cell wall activates expansin proteins which loosen the cell wall and allow the cell to elongate

K+ channels open and K+ ions move from the cell wall to the cytoplasm, water potential of the cytoplasm is lowered so water moves in via osmosis

Describe the role of gibberellin in germination

The seed absorbs water causing the release of gibberellins in the embryo

Gibberellin stimulates the aleurone cells to synthesis amylase

Amylase can then hydrolyse starch into maltose and glucose for respiration to produce ATP