A&P Neurone Amanda Dunne

Lesson Objectives  At the end of this lesson you will be able to  Label a neurone and describe their organisation in nervous systems  Describe the mechanism and properties of a nerve impulse

Nerve Impulses

Nervous Tissue Neurones/ Nerve cells  Transmit nerve impulses. Neuroglia  A connective tissue which supports the neurons

Neurones Neurones are highly specialised nerve cells that receive and conduct nerve impulses from one part of the body to another. They are easily damaged by toxins and lack of oxygen. Unlike other cells in the body, they are not usually replaced when they die. Although they can differ structurally, and there are many, many different types of neurons they have common features. Each neurone has a  Cell body  Central nucleus  One or more slender processes called axons and dendrites  Axon Terminal

Common features Most neurons have a single axon – a long process designed to convey info away from the cell body.

Collective Function Individual neurones have the same function throughout the body, to transmit information, but collectively they make up 5 different types of nerves: Motor or Efferent nerves : carry impulses from the brain & spinal cord to muscles & glands which then act on the information producing movement r secretion. Sensory or Afferent nerves : carry impulses from all parts of the body to the brain. Mixed Nerves – carry both sensory & motor fibres. The only place mixed nerves are found in the body is in the brain & spinal cord as cranial & spinal nerves.

Collective Function White matter – on the inside of the brain and the outside of the spinal cord, this is made up of bundles of myelinated nerve fibres [ i.e with a sheath] Grey Matter –on the outside of the brain and the inside of the spinal cord, this is made up of cell bodies, unmyelinated axons and dendrites

Axons  Surrounded by myelin sheath  This wrapping is never complete.  The exterior wrapping is made up of Schwann cells.

Myelin sheath  The axons of many neurones are encased in a fatty myelin sheath (Schwann cells).  Where the sheath of one Schwann cell meets the next, the axon is unprotected.  The voltage-gated channels of myelinated neurons are confined to these spots (called nodes of Ranvier).  Neurilemma – delicate membrane surrounds the myelin sheath only found in Peripheral nerves. Na + Nodes of Ranvier Menu BiologyMad.com

What is a nerve impulse Nerve cells transmit and receive impulses throughout the body. Impulses do not continually run along each nerve but are created to respond to the internal or external stimuli including : Changes in temperature Pressure chemicals

Nerve Impulses  Neurones send messages electrochemically – this means that chemicals cause an electrical impulse.  Chemicals in the body are ‘ electrically charged ’ when they have an electrical charge, they are called ions. BiologyMad.com Menu

Nerve Impulse Positively charged sodium & potassium ions are present inside and outside the cell. In a resting axon, the concentration of sodium ions is lower inside the cell than in the tissue fluid outside, but the concentration of potassium ions is higher inside than outside. This is maintained by differences in membrane permeability to these ions. The inside of the cell has a more negative charge than the outside.

Nerve impulse cont Stimuli are detected by the sensory receptors. The axon membrane becomes temporarily more permeable to sodium ions which rush in, making the inside of the cell more positive. This electrochemical charge continues in waves along the length of the nerve cell – a nerve impulse. After it has passed the resting stage is restored.

Resting Membrane Potential

When a neurone is not sending a signal, it is at ‘ rest ’. The inside of the neurone is negative relative to the outside. There are also sodium and potassium ion channels in the membrane. These channels are normally closed, but even when closed, they ‘ leak ’, allowing sodium ions to leak in and potassium ions leak out – down their concentration gradients BiologyMad.com Menu

So really… how fast are impulses?  At this information allows for fast responses to stimuli but.....  Speed is affected by:  Temperature  Axon length  Myelin sheath Menu BiologyMad.com

Action Potential (AP)  The resting potential tells about what happens when a neurone is at rest.  An action potential occurs when a neurone sends information down an axon.  Is an explosion of electrical activity  The resting membrane potential changes BiologyMad.com Menu

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AP – All or nothing  AP only happens if the stimulus reaches a threshold value  Stimulus is strong enough to cause an AP  It is an ‘ all or nothing event ’ because once it starts, it travels to the synapse. Menu

Synapses BiologyMad.com

Synapses - Neurotransmitters  Neurotransmitters are made by the cell sending the impulse (the pre-synaptic neurone) and stored in synaptic vesicles at the end of the axon  The cell receiving the impulse (post-synaptic neurone) has chemical gated ion channels e.g Noradrenaline Menu BiologyMad.com

Nerve Stimulus  Skeletal muscles are stimulated by motor neurons.  Axons of these neurons travel in nerves to muscle cells  Axons of motor neurons branch repeatedly as they enter muscles  Neuromuscular junction is the term for the connection between Nervous system & the muscle fiber

In summary © The Institute for Optimum Nutrition