Topic 15: Control and Coordination

Compare the nervous and endocrine system

Nervous System

• signal is a nerve impulse

• transmission through neurones

• fast transmission

• specific part of the body

• rapid, localised and lasts a short time

Endocrine System

• hormone

• via bloodstream

• slow transmission

• only acts on target cells with a specific receptor

• slow, widespread and lasts much longer

Describe the structure of a sensory neurone

• long axon

• long dendrites

• cell body in the middle of the axon

Describe the structure of a motor neurone

• long axon

• short dendrites

• cell body usually found at one end of the axon

What are the functions of sensory and motor neurones?

sensory neurones transmit impulses from sensory receptors to the CNS

motor neurones transmit impulses from the CNS to the effectors

State the role of a sensory receptor

to detect stimuli and stimulate a sensory neuron to transmit a nerve impulse. e.g. a chemoreceptor taste bud

Define resting potential

potential difference across the membrane of a neurone when a cell is not being stimulated to conduct an action potential, usually around -70mV

Explain how resting potential is established

• sodium-potassium pimp actively transports 3Na+ 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 ion channels

• cytoplasm is now more negative so an electrochemical gradient is formed

What is an action potential?

a rapid change in the potential difference across the neurone membrane, resulting in a propagating wave of depolarisation along the axon

Name the 4 stages of an action potential

1. depolarisation

2. repolarisation

3. hyperpolarisation

4. re-establishment of the resting potential

Describe how depolarisation occurs in an action potential

a stimulus causes voltage-gated Na+ channels to open. Na+ ions enter the axon and increase the potential difference. if the change reaches -55mV, more Na+ channels open to allow a large influx of Na+ ions. 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 further Na+ influxes

Explain the process of repolarisation

at 40mV, voltage-gated K+ channels open. there is an outflux of K+ ions diffusing down their electrochemical gradient- this decreases potential difference of the neurone

Why does hyperpolarisation occur?

as K+ ions move out of the neurone, there is a temporary overshoot. this causes the potential difference to become more negative then normal

How is the resting membrane potential restored?

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

What is the myelin sheath?

a lipid covering around the axon of a neurone, there are gaps called nodes of Ranvier

Name the cell that forms the myelin sheath

Schwann cells - the membranes surround the axon to form the myelin sheath

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

nerve impulses are conducted faster due to saltatory conduction

What is saltatory conduction?

the myelin sheath around the axon acts as an insulator, preventing depolarisation occurring at that point. therefore, action potentials can be initiated at the nodes of Ranvier. the impulse then ‘jumps’ to the next node, and can pass faster along the myelinated neurone.

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

State why a refractory period is necessary

• ensures action potentials only propagate in one direction along the neurone

• impulses are discrete and separate from each other

• limits how many action potentials can be produced in a given time

What is a synapse?

the junction between two neurones

Describe the structure of a synapse

• ends are separated by a synaptic cleft

• the end of the presynaptic neurone is called a synaptic knob, containing lots of mitochondria and endoplasmic reticulum

• neurotransmitters are stored in vesicles

• postsynaptic neurone has receptors for the neurotransmitters

Explain the process of synaptic transmission at a cholinergic synapse

• action potential arrives at the presynaptic neurone , Ca2+ channels open

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

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

• Na+ ions enter the postsynaptic neurone and cause depolarisation. if -55mV is reached, an action potential occurs in the postsynaptic neurone

What happens to ACh in the synaptic cleft?

ACh is hydrolysed by acetlycholineesterase in the synaptic cleft. products are choline and acetyl, 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 - many presynaptic neurones may contact a postsynaptic neurone

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 sarcoplasm

the cytoplasm of the muscle fibre

Name the contractile unit of muscle

the sarcomere

Describe how and 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

Thich 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 the binding sites on the actin filament

Describe the sliding filament model of filament contraction

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

• ADP molecules on the myosin heads are released when the myosin head pulls on the actin filament

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

• the myosin head can now repeat the cycle

What is needed to activate the ATPase enzyme for muscle contraction?

Ca2+ ions

Name the sources of ATP in the muscle

• aerobic respiration in mitochondria

• anaerobic respiration

• Phosphocreatine (provides phosphate to combine with ADP)