nervous system

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

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

association neurone motor sensory neurone neurone effector receptor CNS association neurone motor neurone sensory neurone effector receptor response stimulus

structure of a myelinated motor neurone 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

the generation and transmission of nerve impulses

RP is approximately -70mV resting potential When a nerve cell is at rest positive ions are moved in and out of the axon across the membrane by facilitated diffusion using gated channel proteins and by active transport, using carrier proteins. More positive ions are moved out of the axon than in which 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 -70mV

Once the RP is achieved the gated channel proteins that allow facilitated diffusion to occur close, making the membrane impermeable to ions. This prevents ions moving back down their concentration gradient.

polarised membrane + + + + axon membrane - - - - inside - - - - outside + + + +

http://youtu.be/p0lmlWoEEKY

THIS CHARGE CAN BE MEASURED

the resting potential

sodium-potassium pump

all-or-nothing law 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 Action potentials generated Increasing intensity of stimulus Threshold intensity Successive stimuli

action potential Stimulation of the axon causes the membrane to become depolarised This causes some of the gated channel proteins to open and the membrane becomes permeable to ions. The positive ions diffuse down their concentration gradient through channel proteins. The pd across the membrane changes, causing more gated channel proteins to open in a positive feedback mechanism.

This reverses the potential difference across the membrane, to +40mV Making the inside positive in relation to the outside This change in charge evokes (starts) an ACTION POTENTIAL

depolarised membrane + + + - + - - - stimulus + - - - + + + - depolarisation + + + - + - - - stimulus + - - - + + + -

When the AP reaches 40mV the recovery phase starts. The positive ions are moved by facilitated diffusion through channel proteins and by active transport by carrier proteins to restore the RP. When the membrane is REPOLARISEDthe membrane becomes impermeable to ions again. The length of time it takes for the resting potential to be re-established is called the REFRACTORY PERIOD. This is necessary in order for further action potentials to pass along the axon.

The refractory period: Ensures the APs are propagated in one direction only, because the area behind the action potential is in a state of recovery.. Without it APs could be generated in both directions along the axon. Limits the number of APs that can be fired, ensuring each remains separate from subsequent Aps.

DIAGRAM & POINTS PAGE 53/54

propagation of a nerve impulse An action potential is generated at a specific part of the axon membrane and moves rapidly as a wave of depolarisation along the neurone. The area behind the depolarised part of the axon (AP) quickly repolarises. The membrane adjacent to the depolarised section will have opposite charges as they are polarised.

This results in the creation of local currents between the area where there is an action potential and the resting area next to it. Positive ions from the depolarised section pass along the inside of the membrane towards the polarised zone immediately in front of it. On the outside of the membrane positive ions move from the polarised zone to the depolarised zone. The flow of current in a series of these localised currents creates a wave of depolarisation that moves rapidly along the neurone. Similar circuits enable the RP to be restored directly behind the AP

+ + + + - + + + + - - - - - - - - + + - - - - - - - - + + + + - + + + Area behind the AP undergoes repolarisation during the refractory period so is unable to be depolarised, preventing the impulse moving backwards IMPULSE RP AP RP + + + + - + + + + - - - - - - - - + Stimulus generated this end + - - - - - - - - + + + + - + + + + local electrical circuit

factors which influence the speed of transmission 1. Diameter of the axon Thicker axons transmit impulses faster as there is proportionally less leakage of ions. (SA:VOL) Giant axons found in earthworms and squid are associated with the need for rapid escape responses.

2. Myelination of the axon and saltatory conduction The myelin sheath acts as an insulator in myelinated neurones. So 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, allowing depolarisation of the axon membrane, and AP are generated.

Local circuits form between the nodes only, allowing sections of the neurone to be by-passed. 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

There are relatively few ion channels under the myelin sheath, being concentrated at the nodes of Ranvier. This allows myelinated neurones to have relatively diameters. 3. Temperature also affects speed of the nerve impulse, as it affects the rate of diffusion of ions involved.

impulse ALWAYS moves from cell body away from the axon NB: impulse ALWAYS moves from cell body away from the axon draw diagram page 55 CCEA Text

structure and function of a synapse

the synapse Synapses are junctions between the axon of one neurone and the dendrite of the next. They also occur between neurones and muscle and are called neuromuscular junctions. The gap between 2 neurones is called the synaptic cleft. Chemicals called neurotransmitters diffuse across this very small gap. The neurone that releases the neurotransmitter is called the pre-synaptic neurone, whilst the neurone that it diffuses to is called the post-synaptic neurone.

PRE & POST SYNAPTIC NEURONES. ADAPTATIONS OF THE PRE & POST SYNAPTIC NEURONES. The end of the pre-synaptic neurone is thickened to form a synaptic bulb. Pre-synaptic bulbs contain many mitochondria (to release energy for neurotransmitter production) and synaptic vesicles, which store the neurotransmitter

The postsynaptic membrane contains receptors that are complementary to the specific neurotransmitter used in a particular synapse. These are linked to many ion channels will cause depolarisation of the post-synaptic neurone, allowing the impulse to continue to travel along the next neurone.

structure of the synapse 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

structure of the synapse 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

complementary receptor & ion channel postsynaptic cell axon myelin sheath action potential end of axon mitochondrion synaptic vesicle containing neurotransmitter substance synaptic bulb Draw diagram from page 332 Froggy synaptic cleft presynaptic membrane dendrite protein receptor postsynaptic membrane complementary receptor & ion channel postsynaptic cell

synapse at a neuromuscular junction

transmission of an impulse across a synapse An impulse arrives at the synaptic bulb Depolarisation of the membrane causes (voltage activated) calcium channels to open Ca2+ ions enter the synaptic bulb by diffusion The Ca2+ ions cause the vesicles containing neurotransmitter substance (usually acetylcholine) to move to the pre-synaptic membrane

Vesicles fuse with the pre-synaptic membrane releasing acetylcholine into the synaptic cleft (exocytosis) Acetylcholine diffuses across the synaptic cleft and binds to receptors on the post-synaptic membrane This causes the opening of ion channels and Na+ ions diffuse into the post-synaptic neurone.

The membrane gradually depolarises and an excitatory post-synaptic potential (EPSP) is generated. If sufficient depolarisation occurs (depending on the number of neurotransmitter molecules filling the receptors) The EPSP will reach the threshold intensity required to produce an AP in the post-synaptic membrane

The acetylcholine must be removed from the post-synaptic membrane, to prevent it continuously generating a new AP in the post-synaptic neurone. The enzyme acetylcholinesterase, which is attached to the post-synaptic membrane, breaks down acetylcholine into choline and ethanoic acid (acetyl) which are released into the synaptic cleft. They diffuse across the cleft and are reabsorbed by the pre-synaptic bulb, where they are resynthesised into acetylcholine and stored in the synaptic vesicles to be used again. The process requires ATP.

function of the synapse 1. Unidirectionality As the neurotransmitter is only made in the pre-synaptic neurone and the neurotransmitter receptors are present only on the post-synaptic neurone membrane, impulses can only cross from the pre-synaptic neurone to the post-synaptic neurone.

2. Prevent overstimulation of effectors Too many impulses over a short period will exhaust the supply of neurotransmitter more quickly than it can be resynthesised and the synapse becomes fatigued. 3. Integration A number of pre-synaptic neurones may synapse with a post-synaptic neurone, providing flexibility. Without synapses the impulse will follow the same route, producing automatic, never changing responses.

summation This process is important in providing integration. Sufficient neurotransmitter must diffuse across the synaptic cleft in order for an excitatory post-synaptic potential (EPSP) to be produced. An infrequent AP reaching the synapse may not be sufficient to generate an AP in the post-synaptic neurone.

The threshold level for the neurotransmitter can be reached in one of two ways: 1. 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, which together reaches the threshold level to produce an AP in the post-synaptic neurone

2. Temporal summation (time) A single pre-synaptic neurone fires a series of APs each releasing neurotransmitter over a short period of time. Although any one AP is not sufficient to result in an AP in the post-synaptic neurone, together there is sufficient neurotransmitter to reach the threshold potential, resulting in an EPSP.

spatial summation diagram p58 CCEA text

types of synapse Excitatory Neurotransmitter released causes an EPSP and AP in post-synaptic neurone. Inhibitory Make it difficult for synaptic transmission to occur e.g. by causing negative ions to move into the post-synaptic membrane. The inside becomes more negative than normal RP, hyperpolarisation, so that it is more difficult to reach the threshold level to develop an EPSP. Their function is to reduce background stimuli that could interfere with brain functioning or may prevent some reflex actions.

types of neurotransmitter Each synaptic bulb produces only one type of neurotransmitter. Acetylcholine Main neurotransmitter in CNS of vertebrates. Noradrenaline Used in involuntary nervous control, e.g. regulation of gut movement.

Drugs Many drugs stimulate the development of APs as they have a similar shape to the normal neurotransmitters, whilst others may cause the production/release of more neurotransmitter at the synapse. Some restrict the number of action potentials produced, by preventing the release of neurotransmitter or blocking receptor sites on the post-synaptic membrane. Nicotine Curare Opoids See p59 CCEA text

froggy page 347 Q 1,2,4

Cross section through an axon Schwann cell nucleus Layers of membrane containing fatty myelin mitochondrion axon

Cross section through a synapse

Cross section through a synapse Label: Pre-synaptic neurone Synaptic cleft Post-synaptic neurone Vesicles containing neurotransmitter Where EPSP generated Ca2+ ions diffusing across the membrane Location of receptors

Neuromuscular junction Cross section through a Neuromuscular junction