The Cells of the Nervous System

• A neuron contains several parts:

  • Cell body

  • Dendrites

  • Axon

  • Myelin sheath

  • Axon terminal

  • Synapse
• Cell Body -

  • Contains the nucleus, cytoplasm and cell membrane.

  • Produces essential chemicals called neurotransmitters.
• Dendrites -

  • Receive nerve impulses and transport them towards the cell body

  • look like plant roots.

  • They are cytoplasmic extensions also known as nerve fibres.
• Axon -

  • A single fibre

  • Transmits electrical impulse

  • Away from the cell body

  • Tiny or up to a metre in length
• Myelin Sheath -

  • Fatty material

  • Forms an insulating jacket

  • Derived from closely packed layers derived from plasma membranes

  • Neurone without myelin is described as

  • Unmyelinated

  • Speeds up the transmission of nerve impulses
• • Myelination continues from birth to adolescence.

  • Responses to stimuli in the first two years of life are not as rapid or co-ordinated as those of an older adult.

  • In infants the hypothalamus is not fully myelinated

  • Accounts for poor thermoregulation in infants
• Certain diseases destroy the myelin sheath causing loss of co-ordination.

  • Multiple sclerosis causes de-myelination of nerve fibres

  • Results in poor impulse transmission causing:

  • Numbness

  • Blurred vision

  • Walking difficulties
• Glial cells produce the sheath and support the neurons.
• Axon terminal

  • the end of the axon

  Synapse

  • area between the axon terminal and the dendrites

  • Neurons do not actually touch each other.
• Adjacent neurones come very close to each other but don’t touch. Tiny gap between neurones is called a synapse.
• Chemical Transmission At Synapse -

  • Electrical impulses do not cross synapses

  • Neurons connect with other neurons or muscle fibres at a synaptic cleft.

  • Communication across the synaptic cleft is by means of a chemical called a neurotransmitter.

  • The nerve cell before the synapse is the presynaptic neurone

  • The nerve cell after the synapse is the postsynaptic

  neurone

  • Neurotransmitters are stored in vesicles in the axon endings of the presynaptic neuron. They are released into the cleft and bind to receptors on the membrane of the postsynaptic neuron.
• Mechanism

  • Nerve impulse arrives at axon end plate

  • Vesicles containing neurotransmitter

  • migrate to plasma membrane

  • Merge with it

  • Release neurotransmitter molecules into the synaptic cleft (the gap between the cells)

  • Neurotransmitter diffuses across gap

  • Attaches to receptor of next cell

  • Triggers a response
• Threshold -

  • Synapses can filter out weak stimuli arising from insufficient secretion of neurotransmitters.

  • A minimum number of neurotransmitter molecules must attach to receptors in order to reach the threshold on the postsynaptic membrane to transmit the impulse. (Threshold stimulus) Lower than this value is called the sub-threshold stimulus

  • Weak nerve impulses can therefore be filtered out as they remain sub-threshold.

  • Summation of a series of weak stimuli can release enough neurotransmitter to trigger an impulse. (convergent neural pathways)
• Neural Transmitter Fate

  • Neural transmitter molecules must be removed as soon as possible after they have been released and completed their task.

  • This is done by enzymes or reuptake of the neurotransmitter and prevents continuous stimulation of the postsynaptic neurons.

  • To be effective nerve impulses must be very short lived and able to be repeated very soon afterwards if needed
• If neural transmitters were not rapidly removed:

  • Receptors would be blocked

  • Post-synaptic cells would be unable to reach their threshold and

  respond quickly

  • Sequences of impulses would be lost

  • Only a limited number of impulses could be passed on

  • e.g. the ability to differentiate the loudness of sounds would be lost
• Frequency Of Impulses

  • Every nerve impulse transmitted on by a post synaptic neurone will have the same intensity

  • Remember this is triggered by the neurotransmitter molecules attaching to their receptor sites.

  • A strong stimulus will result in a rapid sequence of impulses each releasing a dose of neurotransmitter.

  • The rapid clearance of neurotransmitter molecules allows the next impulse in the sequence to be received.

  • The frequency of these nerve impulses allows us to interpret them into an impression of the strength of a stimulus

  • e.g. the loudness of a sound
• Excitatory And Inhibitory Signals

  • Each neurone in the Central Nervous System has synapses with many other neurones.

  • It receives impulses from many different neurones:

  • Some are excitatory others inhibitory.

  • The neurone balances these against each other.

  • If the sum of excitatory impulses is greater then impulses are passed on.

  • If the sum of the inhibitory impulses is greater no impulses are passed on.

  Summation of a series of weak stimuli can trigger enough neurotransmitter to fire an impulse.