Neuronal Communication

• Role of sensory receptors as transducers


  Convert mechanical, chemical, thermal, or electromagnetic stimuli into electrical signals that can be processed by the nervous system
• Myelinated neurons


  Have a myelin sheath that increases the speed of nerve impulse conduction
• Unmyelinated neurons


  Lack a myelin sheath
• Frequency of impulse transmission

  Indicates the strength of a stimulus or the intensity of a sensory experience
• Excitatory synapses


  Excitatory neurotransmitters make an action potential more likely to fire by depolarising the post-synaptic membrane.
• Inhibitory synapses


  Inhibitory neurotransmitters make an action potential less likely to fire by hyperpolarising the post-synaptic membrane
• Summation


  Occurs when multiple excitatory or inhibitory signals converge at the postsynaptic membrane, leading to either the generation or inhibition of an action potential
• Transducers


  Devices or systems that convert one form of energy or signal into another
• Electromagnetic stimuli


  Forms of energy that travel in waves, such as light or radiation
• Stimulus detection


  The sensory receptor detects the environmental stimulus
• Signal transduction


  The sensory receptor converts the stimulus into an electrical signal
• Signal transmission


  The electrical signal is transmitted to the nervous system
• Sensory receptors


  Specialized cells or organs that detect changes in the environment
• Chemical stimuli


  Changes in the chemical composition of the environment
• The Pacinian corpuscle is a mechanoreceptor found in the skin.

  
  

  Mechanoreceptors respond to changes in pressure to establish a generator potential

  
• A cell that responds to a stimulus is called a Receptor cell
• • Receptor cells are transducers – they convert energy from one form (such as light, heat or sound) into energy in an electrical impulse within a sensory neurone
• • Pacinian corpuscles are a type of mechanoreceptor found deep in the skin
  • They are present in the skin of fingers, soles of the feet as well as in joints, tendons and ligaments
  • They respond to changes in pressure
  • When these receptors are stimulated by pressure on the skin it leads to the establishment of a generator potential
• Cells specialised for detection of stimuli are known as receptors.

  Sense organs such as the ear, eye and skin are composed of groups of receptors.
• When a Pacinian corpuscle is squashed the plasma membrane is deformed and this makes it more permeable to sodium ions which starts depolarization in the sensory neurone.

  
• • The resting potential is the difference in charge across the neuronal membrane when the neurone is at rest. When neurones are at rest, the outside of the cell is more negative than the inside of the cell – this is caused by differences in ion concentrations and maintained by proteins called ion channels.
• Sodium-potassium pumps actively transport sodium ions out of the cell and potassium ions into the cell, resulting in a polarised membrane with a resting potential of approximately -70 mV.

  
• What is spatial summation

  • Spatial summation takes place when multiple presynaptic neurones form a junction with a single neurone.
  • Each presynaptic neurone releases neurotransmitters. Overall there are many neurotransmitters that bind to the receptors on one postsynaptic membrane.
  • Together the neurotransmitters can establish a generator potential that reaches the threshold value and an action potential is generated.

  
• What is temporal summation

  • Temporal summation takes place when multiple nerve impulses arrive at the same synaptic knob within a short period of time.
  • More neurotransmitter is released into the synaptic cleft, so more neurotransmitter is available to bind to receptors on the postsynaptic membrane.
  • Together the neurotransmitters can establish a generator potential that reaches the threshold value and an action potential is generated.

  
• How does synaptic plasticity contribute to learning and memory?

  Synaptic plasticity plays a key role in learning and memory by allowing the strength and efficiency of synapses to be modified through repeated use. This helps to encode and consolidate memories and improve information processing in the nervous system.
• Sensory receptors



  Play a crucial role in detecting various stimuli and converting them into nerve impulses, which are then transmitted to the central nervous system (CNS)

  
• Pacinian corpuscle (sensory receptors)?

  Mechanoreceptor in skin that responds to pressure and movement stimuli

  
• Motor neurons



  carry action potentials from the CNS to an effector. They have lots of short dendrites and one long axon.

  
• Relay neurons


  carry action potentials between the sensory and motor neurons and are found within the CNS. They have lots of short dendrites.

  
• Sensory neurones 


  carry action potentials from receptors to the central nervous system. They consist of one long dendron and a short axon.

  
• Generation of an action potential


  1. When a stimulus exceeds the threshold level, voltage-gated sodium ion channels open, allowing sodium ions to enter the cell, depolarizing the membrane and generating an action potential
  2. This depolarization triggers a positive feedback loop, leading to the propagation of the action potential along the axon

  
• Transmission of nerve impulses in a myelinated neuron


  Action potentials are conducted rapidly through saltatory conduction, where the depolarization occurs only at the nodes of Ranvier

  
• Neurotransmission at a cholinergic synapse


  1. When an action potential arrives at the synaptic bulb, voltage-gated calcium ion channels open, allowing calcium ions to enter and trigger exocytosis of ACh-filled vesicles
  2. ACh diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane, causing sodium ion channels to open and depolarize the postsynaptic membrane, generating a new action potential
  3. Acetylcholinesterase breaks down ACh to terminate the signal

  
• What is the first step in generating an action potential

  Stimulus – A stimulus triggers an inflow of Na+ ions into the cell. This increases the potential difference from -70mv to a less negative value. During this period, there are some failed initiations of the action potential.

  
• What is the second step in the generation of action potential

  2. Depolarisation – The threshold potential (-55mV) is reached, and voltage-gated Na+ ions open. Therefore Naᐩ ions rapidly diffuse in. The potential difference increases to +40mV, meaning the inside of the cell is more positive than the outside.

  
• What is the third step in the generation of action potential


  3. Repolarisation – Voltage-gated potassium channels open and sodium channels close. Kᐩ ions quickly diffuse out of the axon, repolarising the membrane.

  
• What is the refractory period?



  The refractory period is a brief period of time after an action potential during which the neuron is less responsive to additional stimuli. This is due to the inactivation of sodium channels and the slow repolarisation of the membrane.