Nerves…..and how they work LCSC06 Emily Burtenshaw

Nerve and muscle physiology

Anatomical components of the nervous system Central nervous system: – Brain and spinal cord Peripheral nervous system: – Cranial nerves – Spinal nerves – Nerve cells or neurons are the functional ‘building blocks’ of the nervous system – Glial cells are primarily supporting tissue

Neurons hubpages.com

Structural classification of neurons

Glial cells in the PNS: Schwann cells and satellite cells dictionary.thefreedictionary.com/neurilemma

Glial cells in the CNS Manatomy.com

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Facepunch.com

Myelinated v unmyelinated neurons The majority of neurons are myelinated Even so-called unmyelinated neurons have a thin coat of neuroglial plasma membrane Myelin acts like a form of insulation It speeds up the transmission of the tiny electrical current in the neuron So…..axons that are myelinated conduct faster than unmyelinated axons

Functional classification of neurons A,B and C classes (A and B are myelinated) A fibres are further SUBDIVIDED Into: α β γ δ Further subdivision of sensory fibres by Roman numeral and a letter For example: A α, A δ III, C IV

Directed study 1: Peripheral nerve classification and function The thickness of the neuron, and whether or not it is myelinated, determines its specific function in the nervous system.

Which are the fastest and slowest conduction neurons? Order the following from FASTEST to SLOWEST conducting neuron (and which are MOTOR - efferent and which are SENSORY -afferent): a)Pain, temperature (warmth) b)Skeletal muscle (extrafusal muscle fibres) c)Smooth muscle d)Touch, pressure, pain, temperature (coolness) e)Muscle spindle/intrafusal muscle fibres

Useful resources for this task: Page 73 in Atkinson & McHanwell (2002) or page 539 in Seikel

MS: a demyelinating disease Progressive destruction of the myelin sheath in the CNS myhealth.alberta.ca

Neurons hubpages.com

Excitable cells A neuron is one of the types of excitable cell in the body A muscle cell is the other Communication between a neuron and another neuron, or a neuron or muscle/glandular cell, is in the form of electrical signals – called action potentials and graded potentials

The story so far: O 2, CO 2, H 2 0 and certain lipid soluble substances can pass directly through the membrane Channels exist in the cell membrane to allow the passage of substances including ions and water – Integral proteins contain water-filled pores that allow certain substances through with their concentration gradient eg K+ (these are also called leakage channels) – Other proteins act as transporters

Resting membrane potential Humanbiologylab.pbworks.com

Most body cells are polarised…. In neurons, the resting membrane potential is around -70mV A cell that has a membrane potential is said to be ……………………… The job of the sodium pump is to expel Na+ and thus maintain this potential across the cell membrane When neurons ‘fire’, the membrane potential is suddenly reversed –this is called depolarisation

What causes depolarisation? The membrane is stimulated by changes in the membrane potential initiated by neurotransmitters Special membrane channels open, which allow Na+ to rush into the cell (and then K+ to leave) This changes the potential inside the cell to + 50mV ie the cell membrane is depolarised

What are these special membrane channels? Four types: 1) Voltage-gated ion channels 2) Ligand or chemically gated ion channels 3) Mechanically gated ion channel 4) light-gated ion channels

Voltage gated channels Open in response to changes in voltage (changes in the current flowing along the axon)

Ligand or chemically gated channels Opened by extracellular chemical transmitters

Graded potentials The presence of specialased ion gated channels in the cell membrane results in small flows of current across the cell membrane Graded potentials can make the membrane more polarised (more negative = hyperpolarised) or less polarised (less negative = depolarised) than the resting level Graded = vary in amplitude (strength) These are very localised changes BUT…….

Nerve impulses or Action potentials When a graded potential is strong enough ie reaches threshold Then this threshold stimulus causes a SUDDEN depolarisation of the membrane at the trigger zone in the neuron ie the membrane potential is decreased and eventually reversed Followed by repolarisation ie restored to its previous state

How neurons work, and the action potential oPY oPY ih2o ih2o Absolute and relative refractory periods: wQdU wQdU

Action potential: propagation Depolarisation is local at first Spreads down the axon in one direction current flows passively but as it moves along the axon it opens the voltage-gated Na + channels Action potential is propagated along the axon Na + channels close quickly and the membrane is then unresponsive or refractory This means the action potential can only travel in one direction

Action potentials are all-or-nothing events: Action potential is self-propagating ie once it starts in carries on moving down the axon Action potential stays at the same strength as it travels down the axon Speed of propagation is not related to stimulus strength, is it related to…..?

Above is the method of propagation in unmyelinated neurons What class of neuron is unmylelinated? What functions to they subserve?

Myelinated neurons The myelin sheath forms an insulating layer At intervals (the nodes of Ranvier) there are many Na + channels Depolarisation at one node passes current to next node without any loss of current as the neuron is so well insulated This apparent ‘leaping’ of the current is called saltatory conjunction As a result, rapid conduction velocity in myelinated neurons

Intensity of response: Two factors: the frequency of impulses generated at the trigger zone (hillock) The number of individual neurons activated by the stimulus

A good summary of the action potential: 6t_n6kTj1A&feature=youtube_gdata_player 6t_n6kTj1A&feature=youtube_gdata_player

Communication between neurons..and between neurons and target tissue Neurons do not directly touch one another Neither to their directly touch their target tissue ie a muscle cell or glandular tissue There is a minute gap called the …………………? The action potential does not jump across the gap or cleft Rather, it is converted into a chemical ‘signal’ by a neurotransmitter Then the action potential is re-activated in the post- synaptic neuron if the neurotransmitter is excitatory

Neurotransmitters When sufficiently stimulated, pre-synaptic axons discharge neurotransmitter substances into the synaptic cleft The neurotransmitter is a chemical substance Acts like a key The post-synaptic neuron will respond if the ‘key’ fits its ‘lock’ ie the receptor site

Synapses

What happens when the action potential reaches the end of the axon?

Summary: Depolarisation wave arrives at …..…terminal axon …………-gated Ca 2+ open Results in influx of …………. Triggers release of neurotransmitter Neurotransmitter crosses the synaptic …….. And binds to ………….gated channel on the …….synaptic membrane

Neurotransmitters There are many types Some are found in the CNS (the …?..& the ……?…..) Others in the peripheral NS (the …?..& the ……?…..) Some in both CNS & PNS Some are EXCITATORY and some are INHIBITORY

Directed study 2: Neurotransmitter Where foundExcitatory/inhibitory Acetylcholine motor end plate excitatory Dopamine Adrenaline Noradrenaline Glutamate γ-aminobutyric acid (GABA)

Excitatory and inhibitory actions of neurotransmitters Excitatory post-synaptic potential (EPSP) is set up if the neurotransmitter activates Na+ channels on the post synaptic membrane An EPSP depolarises the post-synaptic membrane Inhibitory post-synaptic potential (IPSP) is set up if the neurotransmitter activates Cl- channels in the post-synaptic membrane An IPSP hyperpolarises the post-synaptic membrane

In summary: EPSPs depolarise the post-synaptic membrane and the action potential is generated in the next cell IPSP’s hyperpolarise the post-synaptic membrane, and no AP is generated