Transcript Neurons
Neurons HBS3B Neurons - types A neuron is There are ___ types of neuron: Explain the major differences in appearance and functions between • receptor (sensory) neurons • effector (motor) neurons • connector neurons (inter-neurons, association neurons, relay neurons) White matter consists of Grey matter consists of Neurons can be myelinated or unmyelinated. Explain the major differences between • myelinated neurons • unmyelinated neurons A nerve fibre is A nerve is Ganglia are Neurons Neurons - types A neuron is a nerve cell There are 3 types of neuron: • receptor (sensory) neurons– send information from receptors towards the brain/spinal cord. Have long dendrites (usually myelinated), shorter axons (often unmyelinated) , and cell body positioned off to one side. • effector (motor) neurons – send information from the brain/spinal cord to muscles/glands (effectors). Have long axons (usually myelinated), shorter dendrites (usually unmyelinated), and cell body positioned towards the dendrite end of the neuron. • connector neurons (inter-neurons, association neurons, relay neurons) – found in the brain/spinal cord, connecting motor and sensory neurons. Cell body is central, axons and dendrites are usually the same size, and often unmyelinated. White matter consists of myelinated fibres Grey matter consists of unmyelinated fibres and cell bodies Neurons can be myelinated or unmyelinated. Explain the major differences between • myelinated neurons – have a myelin sheath, and a faster transmission of impulses • unmyelinated neurons – lack myelin sheaths and are slower in transmission of impulses A nerve fibre is an extension of neuron (ie axon or dendrite) A nerve is a collection of nerve fibres Ganglia are groups of nerve cell bodies outside the central nervous system Motor neuron Neurons - structures Describe the functions of the following • cell body • Dendrite • Axon • myelin sheath • Schwann cell • nodes of Ranvier • axon terminal • Neurilemma • Neuromuscular junction Neurons - structures Describe the functions of the following • cell body- general functioning of cell + metabolism – respiration, synthesis, etc, contains the nucleus, which controls cell activities. • Dendrite - receive impulses + pass them on to the cell body • Axon - carry impulses away from the cell body • myelin sheath - protects neuron, and speeds up transmission of impulse • Schwann cell – makes the myelin sheath • nodes of Ranvier – speed up transmission of impulses • axon terminal – passes impulse to a muscle/gland by means of chemicals (neurotransmitters), thus causing movement or change in activity (eg secretion) • Neurilemma – helps repair and protect the neuron • Neuromuscular junction – passes impulse to a muscle by means of chemicals (neurotransmitters), thus causing movement How nerve impulses are generated in a neuron The transmission of a nerve impulse along a neuron from one end to the other occurs as a result of chemical changes across the membrane of the neuron. The membrane of an unstimulated neuron is polarized there is a difference in electrical charge between the outside and inside of the membrane. The inside is negative with respect to the outside. Normally there is more sodium ions (Na+) on the outside and more potassium ions (K+) on the inside. Na+/K+ pumps in the membrane actively keep these ions on the correct side. The cell contains other ions, eg large, negatively charged proteins and nucleic acids, that contribute to the negative charge on the inside of the cell membrane Gated channels open in response to neurotransmitters, changes in membrane potential, or other stimuli to let in ions. This changes the difference in electrical charge (depolarisation) and triggers an impulse. The change must be large enough to overcome the threshold, or no impulse will be generated. This is known as the ‘all or none rule’ Nerve impulses Nerve impulses are transmitted down a neuron The waves of depolarisation travel down the axon until the reach the axon terminal, where they trigger the release of neurotransmitters. These chemicals act as the stimulus to the nerve at the other end of the synapse (sensory or connector neurons) or trigger a change in an effector (motor neurons) In unmyelinated fibres, the impulse travels slowly along the entire fibre. In myelinated fibres, the impulse ‘jumps’ from one node of Ranvier to the next, and therefore moves more quickly. Nerve transmission Define and explain the importance of the following in the generation and transmission of nerve impulses: • Potential difference • Membrane potential • Resting membrane potential • Sodium potassium pump • Polarisation • Depolarisation • Action potential • Refractory period • Saltatory conduction • All-or-nothing response Nerve transmission Define and explain the importance of the following in the generation and transmission of nerve impulses: • Potential difference – differences in electrical charges between two places • Membrane potential - differences in electrical charges between inside and outside of membrane • Resting membrane potential - describes the unstimulated, polarized state of a neuron (at about −70 millivolts). • Sodium potassium pump – special protein gates in cell membrane that use energy to transport sodium ions out and potassium ions in • Polarisation – the situation where the membrane separates a more negative inside from a positive outside (ie has a potential difference) • Depolarisation – when a stimulus is applied, the membrane is more permeable to Na+ and it floods in, reversing the polarisation, and generating an impulse • Action potential – the rapid polarisation – depolarisation which causes a change in membrane voltage. The waves of action potentials along the nerve fibre form the impulse • Refractory period - the axon can not respond to a new stimulus while it is re-establishing the original concentrations of the Na+ and K+ using the Na+/K+ pumps in the cell membrane. • Saltatory conduction – describes the conduction of nerve impulses along a myelinated fibre – as it ‘jumps’ from one node of Ranvier to another. This is faster than in unmyelinated fibres. • All-or-nothing response – a new impulse is only generated if the stimulus is greater than the threshold. Once the impulse is generated the size is the same regardless of size of initial stimulus. Synapses A synapse is Messages are transmitted across the synapse by n_____________________ which attach to r____________ in the membrane of the d_________ of the next neuron. Examples of neurotransmitters include Enzymes Messages are transmitted to muscles across the by means of Many nerve poisons work by disrupting synapse function – eg Synapses A synapse is the gap between two neurons Messages are transmitted across the synapse by neurotransmitters which attach to receptors in the membrane of the dendrite of the next neuron. Examples of neurotransmitters include acetylcholine, adrenaline, dopamine, histamine. Enzymes remove the neurotransmitters and so clear the receptor sites for another message Messages are transmitted to muscles across the neuromuscular junction by means of neurotransmitters (usually acetylcholine) Many nerve poisons work by disrupting synapse function – eg curare, sedatives, insecticides A – dendrite B – nucleus C – myelin sheath D – axon E – Schwann cell F – side or colateral branch of axon G – node of Ranvier H - motor end plate/axon terminal I - neurilemma A – motor neuron B – connector neuron C – sensory neuron 1 – cell body of the sensory neuron 2 – synapse 3 – cell body of connector neuron 4 – synapse 5 – axon of motor neuron 6 dendrite of motor neuron 7 – muscle fibre 8 – motor end plate 9 – dendrite of sensory neuron 10 – receptor cell The spinal reflex arc Describe the functions of the five components of a spinal reflex arc • R__________________: • S__________________: • S__________________: • M_________________: • E_________________: Spinal reflex arcs are important Spinal reflex arc The spinal reflex arc Describe the functions of the five components of a spinal reflex arc • Receptor: responds to change by initiating nerve impulse in sensory neuron • Sensory neuron: takes impulses into the central nervous system • Synapse: at least 2 neurons will be involved – sensory and motor. Sometimes there is more than 1 synapse as connector neurons may also be involved • Motor neuron: takes impulses to the effector • Effector: receives impulse and carries out a response (eg movement, change in activity) Spinal reflex arcs are important in protecting the body as they allow very fast responses to potentially harmful changes (eg removal of hand from hot object)