neurons

• Neurons

  • Highly specialised nerve cells of the nervous system
  • Generate electrochemical nerve impulses, to carry information from one part of the body to another
  • Consist of a cell body and 2 different types of extensions, axon and dendrite
• Structure of neuron

  • Cell body
  • Dendrites
  • Axons
  • Myelin sheath
• Cell body

  • Contains the nucleus
  • Granular cytoplasm is due to clusters of ribosomes
  • There is an abundance of organelles, especially mitochondria, endoplasmic reticulum, Golgi apparatus
  • Responsible for controlling and functioning of the cell, site of chemical reactions
• Dendrites

  • Usually short and highly branched
  • Synapse with other neurons or receptors
  • Short extensions of the cytoplasm of the cell body
  • Receives messages from other neurones and carry them TOWARDS cell body
• Axons

  • Single long nerve fibre ( generally longer than dendrites)
  • Brain axons shorter than spinal cord axons
  • Terminate at synaptic end bulbs into axon terminals
  • Connect with muscles (neuromuscular junction), glands (neuroglandular junction) or other neurons
  • Carry impulses AWAY from cell body
• Myelin sheath

  • White fatty sheath surrounding the axon of most neurons
  • Produced by Schwann cells ( glial cells) in the PNS
  • In CNS myelin is produced by oligodendrocytes
  • Nerve fibres with a myelin sheath are said to be myelinated fibres
  • Those without are called unmyelinated fibres
  • At intervals along the axon are gaps in the myelin sheath called Nodes Of Ranvier
  • The outermost coil of the Schwann cell forms a structure called the neurilemma around the myelin sheath
  • Speeds up nerve transmission (impulse)
  • Acts as an insulator
  • Protects axon from damage
• Classification of neurons by structure

  • Multipolar
  • Bipolar
  • Unipolar
  • Pseudo unipolar
• Classification of neurons by function

  • Connectors
  • Sensory
  • Motor
• Efferent (motor) neurons


  • Take nerve impulses from CNS to effectors
  • Mostly multipolar with a single long axon
  • Cell body in grey matter of spinal cord
  • Pass through ventral root of spinal nerves
  • Effector structures ( muscles or glands) occur at end of axons
  • Dendrites synapse with connector neurons in spinal cord
  • Can be somatic ( voluntary) or autonomic (involuntary)
• Afferent (sensory) neurons


  • Take nerve impulses from receptors to CNS
  • Mostly unipolar with the cell body lying off to one side of the axon
  • Cell body in dorsal root ganglion
  • Pass through dorsal root of spinal nerves
  • Sensory receptors occur at end of dendrites
  • Axons synapse with connector neurons in spinal cord
• Nerve fibres


  • The axons and dendrites of nerve cells
  • Outside brain and spinal cord, nerve fibres are grouped together to form a nerve
  • Nerve fibres are arranged into bundles held together by connective tissue, with multiple bundles joining together to form a nerve
  • Inside brain known as tracts-bundles of nerves
• Nerve impulse

  • The message that travels along a nerve fibre
  • Nerve impulses are transmitted very quickly, making it possible for the body to respond rapidly to any change in the internal or external environment
• Conduction of nerve impulse

  1. polarisation
  2. depolarisation
  3. repolarisation
  4. hyperpolarisation
  5. refractory period with sodium potassium pump
• Electrical charge and potential difference


  • Two types of electrical charge pos & neg
  • When opposite charges are separated, an electrical force tends to pull them together
  • The potential, or potential difference between two places can be measured- voltage ( volts/millivolts)
• Potential difference across a cell membrane

  • When some chemical substances are dissolved in water, they break up into electrically charged particles called ions
  • The extracellular fluid, contains high concentration of sodium chloride and so most of its charges particles are positive sodium ions (Na+) and negative chloride ions (Cl-)
  • The intracellular fluid, has low concentration of sodium ions and chloride ions, its main positive ions are potassium (K+) and the negative ions come from a variety of organic substances made by the cell
  • Differences in the concentration of ions mean that there is a potential between inside and outside of cell membrane
  • Membrane potential
  • Occurs in all body cells, but particularly large in nerve and muscle cells
  • The resting membrane potential (unstimulated nerve cells) is around -70mV
  • Ions are usually unable to diffuse through the phospholipid bilayer of the cell membrane, they move through protein channels
  • The resting membrane potential of neurons is due mainly to difference in the distribution of potassium and sodium ions on either side of the cell membrane, making the extracellular fluid more positively charged than the intracellular fluid
  • Sodium and potassium ions move across the cell membrane through a carrier protein known as the sodium-potassium pump
• Action potential


  • Rapid depolarisation and repolarisation of the membrane ( due to the opening and closing of voltage gated channels-changing permeability to Na+, lasts approx 1 millisecond
  • All or none response
  • Polarised
  • Depolarisation
  • Repolarisation
  • Refractory period
• Transmission of the nerve impulse

  1. A single action potential occurs in one section of a membrane, however it triggers an action potential in the adjacent membrane. This process continues along the length of the neuron and is called a nerve impulse

  2. An action potential doesn't travel along the nerve fibre, it's the message or nerve impulse that travels along the fibre
• Conduction along an unmyelinated fibre

  1. Depolarisation of 1 area of the membranes causes a movement of Na+ into adjacent areas. This stimulates the opening of the voltage-gated Na+ channels in the next part of the membrane which initiates an action potential in that area of the membrane

  2. Repeats itself along the length of the membrane, so that the action potential moves along the membrane away from the point of stimulation

  3. If stimulus was to occur in the middle of a fibre, the impulses would travel in both directions along the fibre AWAY from the point of stimulation

  4. The refractory period prevents the nerve impulse to go backwards and during this period another action potential cannot be generated at that point on the fibre
• Conduction along myelinated fibres


  1. Myelin sheath insulates the nerve fibre. This does not occur at the nodes of ranvier (myelin sheath is absent)

  2. Action potential only occurs at the nodes of ranvier-action potential jumps from one node to the next ( saltatory conduction)

  3. Speeds up to 140m/s
• A nerve impulse that travels along a fibre is always the same size, regardless of the size of the stimulus
• A weak stimulus, provided it exceeds the threshold, produces the same action potential as a strong one- ALL OR NONE RESPONSE
• Steps that take place in myelinated fibres

  1. Depolarisation occurs at the nodes of ranvier

  2. Impulse jumps from one node to the next (saltatory conduction)

  3. Cell membrane becomes permeable to Na

  4. Na diffuses into cell

  5. Inside the cell becomes more positive relative to the outside

  6. K diffuse out of cell (across membrane)

  7. Inside membrane becomes negative relative to outside

  8. Needs to reach threshold -55mV

  9. Na-K pumps acts to transport Na out of cell and K into cell

  10. Returns to a polarised/resting state/neurons repolarised

  11. Hyperpolarisation occurs ( when the membrane potential becomes more negative)

  12. Action potential triggers depolarisation in adjacent membrane region
• Nerve impulse that travels along a fibre

  • Always the same size, regardless of the size of the stimulus
  • A weak stimulus, provided it exceeds the threshold, produces the same action potential as a strong one- ALL OR NONE RESPONSE ( stimulus either strong enough to trigger an impulse or its not)
  • A strong stimulus causes depolarisation of more nerve fibres than a weak stimulus
  • Strong stimulus produces more nerve impulses in a given time than a weak stimulus
• Saltatory conduction


  Speeds up to 140m/s
• Effects of chemicals on the transmission of nerve impulses


  • Stimulants such as caffeine and benzedrine stimulate transmission at the synapse
  • Other drugs like anaesthetics or hypnotics depress the transmission
  • Venom also affects the neuromuscular junction
  • Nerve agents ( nerve gases) contain organophosphates, which causes the build-up of acetylcholine at the neuromuscular junction
• Synapse


  • Junction/small gap between the branches of two neurons, or between a neuron and an effector ( muscle or gland)
  • Messages have to be carried across the synapse. To transmit the message from a neuron, across a space to the next neuron, the message changes from electrical to chemical back to electrical
• Neurotransmitters


  • Special chemicals released into the tiny gap ( the synaptic cleft), which separates the two nerve cells
  • Acetylcholine and noradrenaline are the neurotransmitters of the peripheral nervous system
• Neurotransmission


  1. Action potential arrives at the axon terminal, which activates voltage gated calcium ion channels

  2. Triggers Ca2+ influx into the axon terminal

  3. Vesicles of neurotransmitters ( e.g. Acetylcholine) are exoctyosed into the synapse

  4. Neurotransmitters diffuse across the synapse ( synaptic cleft) and bind to receptors on the postsynaptic membrane of the next neuron or effectors

  5. Open gated channel proteins

  6. Attach to the motor end plate ( region where receptors are located on muscles)

  7. Stimulates ligand-gated protein channels to open which allows the influx of sodium ions and initiates an action potential in the post-synaptic membrane

  8. Neurotransmitters are removed from the synapse by being reabsorbed by the presynaptic membrane, by being degraded by enzymes or by moving away through diffusion
• Neurotransmitters


  • Adrenaline
  • Noradrenaline
  • Dopamine
  • Serotonin
  • Acetylcholine
  • Endorphin
• Adrenaline


  Produced in stressful situations, increased HR and blood flow, leading to physical boost and heightened awareness
• Noradrenaline


  Affects attention and responding actions in the brain, contracts blood vessels and increases blood flow
• Dopamine


  Feelings of pleasure, addiction, movement and motion, people repeat behaviours that lead to a dopamine release
• Serotonin


  Contributes to wellbeing and happiness, helps sleep cycle and digestive system regulation, affected by exercise and light exposure
• Acetylcholine


  Involved in thought, learning and memory, activates muscle action in the body, associated with attention and awakening
• Endorphin


  Released during exercise, excitement and sex, produces euphoria and wellbeing, reducing pain
• Reflex


  Rapid automatic response to a change in the external or internal environment, tries to restore homeostasis
• Properties of a reflex


  • Stimulus required to trigger a reflex- reflex is not spontaneous
  • Is involuntary- occurs without any conscious thought
  • Reflex response is rapid- only a small number of neurons are involved
  • Reflex response is stereotyped- it occurs in the same way each time it happens
• Spinal reflex


  Reflex is carried out by spinal cord alone
• Reflex arc


  Pathway a nerve impulse follows in travelling from a receptor to an effector
• Examples of reflexes


  • Patellar reflex ( knee jerk)
  • Pupillary reflex
  • When a stimulus activates sensory receptors, sensory neurons transmit nerve impulses to the spinal cord or brainstem, initiating a reflex arc
  • In the spinal cord , interneurons process the sensory input and generate motor responses, which are transmitted via motor neurons to effectors such as muscles or glands
  • Reflex actions are characterized by their speed and involuntary nature, providing immediate responses to potentially harmful stimuli to minimize damage