The nervous system allows the body to respond

• The nervous system allows the body to respond to changes in the internal and external environment • Receptors detect changes/stimuli which are rapidly transmitted along neurones to effectors that bring about a corrective response

STIMULI RECEPTORS PNS peripheral nervous system changes in the environment sensory cells Cranial and spinal nerves (sensory + motor neurones) CNS brain + spinal cord central nervous system processes info muscle or gland EFFECTOR RESPONSE action

• Adapted to transmit information throughout the nervous system. • 3 types: Type of neurone Sensory Stimulated by Stimulus Transmits impulses to Association Sensory Motor Association Effector

CNS sensory neurone receptor stimulus association neurone motor neurone effector response

• Large cell body: containing a nucleus, nucleolus, mitochondria and ribosomes • Dendrons: long thin strands of cytoplasm that carry impulses towards the cell body and connect to many neurones in the CNS • Axon: a long extension that carries impulses away from the cell body and terminates in motor end plates that connect to muscles or glands

• Schwann cells: wrap around the axon of myelinated neurones forming the fatty myelin sheath that insulates the impulse. • Nodes of Ranvier: small gaps between adjacent Schwann cells that aid transmission of impulses Schwann cell axon

• When a nerve cell is at rest ions are moved in and out of the axon across the membrane • More positive ions are moved out of the axon than in • This causes the inside of the axon to become negative in relation to the outside; the membrane is polarised • The charge across the membrane, the (potential difference) is called the RESTING POTENTIAL RP is approximately -70 m. V

• A stimulus must reach a specific level in order for an impulse to be generated • A more intense impulse will not produce a bigger impulse; all impulses are the same size • Strong stimuli produce a greater frequency of action potentials (impulses) NB: below threshold, no AP above threshold AP all the same size

Below threshold intensity: no action potentials Increasing intensity of stimulus Action potentials generated Threshold intensity Successive stimuli

• Stimulation of the axon causes the membrane to become depolarised • This is caused by the ions moving in the opposite direction across the axon membrane • This reverses the potential difference across the membrane, to +40 m. V • Making the inside positive in relation to the outside • This change in charge evokes (starts) an ACTION POTENTIAL

depolarisation

• When an action potential is generated at a specific part of the axon membrane • the areas on either side will have opposite charges as they remain polarised • This difference in charge sets up a local current between the area where there is an action potential and the resting area next to it. • The flow of current in a series of these localised currents results in an action potential moving along an axon

local electrical current

• In order for further action potentials to pass along the axon the original part of the axon must recover its resting potential • A process called REPOLARISATION • The length of time it takes for the resting potential to be re-established is called the REFRACTORY PERIOD.

• There are two parts to the refractory period: • The absolute refractory period is the interval during which a second action potential absolutely cannot be initiated, no matter how large a stimulus is applied • It is caused by the closing of carrier proteins which transport ions across the axon membrane • The relative refractory period is the interval immediately following during which initiation of a second action potential is inhibited but not impossible.

• In this way the action potential can only travel in one direction down the neurone because the area behind the action potential is in a state of recovery. • The transfer of action potentials along the length of an axon represents an impulse Diagram page 330

1. Diameter of the axon Thicker axons transmit impulses faster as they have a greater surface area over which exchange of ions can occur. Giant axons found in earthworms and squid are associated with the need for rapid escape responses.

2. Myelination of the axon and saltatory conduction • Areas of the axon that are myelinated cannot be polarised or depolarised • This is because myelin is a fatty substance that does not allow movement of ions across it • Myelin is absent at the nodes of Ranvier where the Schwann cells meet

• Action potentials can occur at these points • APs jump from one node to the next, speeding up transmission by about 100 times. • This is called SALTATORY CONDUCTION • and is found only in the myelinated axons of vertebrates • Saltatory conduction also saves energy as less is required for the active transport of ions across the axon membrane

• The axons of neurones end in swellings called synaptic bulbs. • Present in the bulbs are many mitochondria and synaptic vesicles which contain a chemical messenger (a neurotransmitter substance)

• The gap between two neurones is called the synaptic cleft. • The membrane before the cleft is called the pre-synaptic membrane

• On the other side of the synaptic cleft is the postsynaptic membrane. • This contains many ion channels and has a large number of protein molecules on its surface which act as receptor sites for the neurotransmitter

• Synaptic cleft: gap between the pre and post synaptic membranes • Presynaptic membrane: neurone membrane before the synapse • Postsynaptic membrane: neurone membrane after the synapse • Neuromuscular junction: synapse between a motor neurone and a muscle • Neurotransmitter: chemical that carries the impulse across the synaptic cleft, found in synaptic vesicles

You will be expected to identify and label the following structures from LEM and TEM photographs and diagrams • Synapse: gap between 2 neurones • Synaptic bulb: swelling at the end of an axon • Synaptic vesicles: vesicles containing neurotransmitter found in the synaptic bulb

axon myelin sheath action potential end of axon mitochondrion synaptic vesicle containing neurotransmitter substance synaptic bulb synaptic cleft presynaptic membrane dendrite postsynaptic membrane protein receptor postsynaptic cell ion channel

• An impulse arrives at the synaptic bulb • Depolarisation of the membrane causes (voltage activated) calcium channels to open • Ca 2+ ions enter the synaptic bulb by diffusion • The Ca 2+ ions fuse with vesicles containing neurotransmitter substance (acetylcholine) and cause them to move to the pre-synaptic membrane

• Vesicles fuse with the pre-synaptic membrane releasing neurotransmitter into the synaptic cleft (exocytosis) • Neurotransmitter diffuses across the synaptic cleft • and binds to receptors on ion channels on the post-synaptic membrane causing them to open • Na+ ions diffuse into the post-synaptic cell

• This causes the development of an excitatory post-synaptic potential • resulting in depolarisation and an action potential in the post-synaptic membrane • The neurotransmitter must be removed from the synaptic cleft to prevent continued stimulation of the post-synaptic membrane

• This is carried out by the enzyme acetylcholinesterase • The products (choline & ethanoic acid) are reabsorbed by the pre-synaptic cell and recycled, using ATP to resynthesise the neurotransmitter

• Sufficient neurotransmitter must diffuse across the synaptic cleft in order for • an excitatory post-synaptic potential (EPSP) to be produced. • In some synapses this is achieved through summation.

There are 2 ways in which summation may occur Temporal summation (time) • A high frequency of APs need to arrive at the pre-synaptic membrane, each resulting in the release of neurotransmitter. • The post-synaptic membrane depolarises only when sufficient neurotransmitter builds up

Spatial summation • 2 or more pre-synaptic neurones synapse with a single post-synaptic neurone. • Each releases small quantities of neurotransmitter into the synaptic cleft. • When sufficient is released the • post-synaptic membrane is depolarised

K channel K+ Na channel

depolarisation