bio chp#03

• Nervous coordination


  The system of the body that provides communication through electrical and chemical signals, called the nervous system
• Neurons


  • Highly specialized cells that are the fundamental unit of the nervous system
  • Perform five functions: receive information, integrate information, conduct signals, transmit signals, coordinate metabolic activities
• Receptors


  Specialized neurons or epithelial cells that detect energy from the environment and convert it into electrical signals
• Types of receptors


  • Mechanoreceptors
  • Pain receptors (Nociceptors)
  • Thermoreceptors
  • Chemoreceptors
  • Photoreceptors
• Transducers


  Structures that convert signals from one form to another form
• Effectors


  Muscles or glands that produce a response to a stimulus based on commands from the central nervous system
• Pathway of nervous coordination

  1. Sensory input

  2. Integration

  3. Motor output
• Neuron


  Functional and structural unit of the nervous system specialized for transmitting signals
• Structure of neuron


  • Cell body (soma) with nucleus and organelles
  • Dendrites for receiving signals
  • Axon for transmitting signals
  • Myelin sheath for insulation and faster transmission
• Types of neurons


  • Sensory (afferent)
  • Motor (efferent)
  • Interneuron (association)
• Reflex action


  Simple, automatic response to a stimulus involving a reflex arc with sensory, integration, and motor neurons
• Nerve impulse


  Electrical signal that depends on the flow of ions across the neuron membrane
• Resting membrane potential


  Negative charge inside the neuron membrane when not stimulated, maintained by ion gradients and pumps
• Action potential


  Change in membrane potential triggered by a stimulus, causing an influx of sodium ions and depolarization
• Threshold stimulus is required to produce an action potential, sub-threshold stimulus does not produce a response
• Depolarization


  Change in membrane potential from resting state to more positive inside, caused by influx of sodium ions
• Action potential follows an "all or none" principle - it is either fully generated or not at all
• Potassium channels open


  1. Potassium diffuses out of the membrane

  2. Membrane potential becomes negatively charged once again

  3. Repolarization occurs

  4. Takes 10 to 30 milliseconds to return to resting state
• Threshold stimulus


  Stimulus capable of producing an action potential in a neuron
• Sub-threshold stimulus


  Stimulus not capable of exciting or producing a response
• Depolarization


  1. Threshold stimulus of about -55 mV causes a change in the membrane potential

  2. Threshold stimulus must be strong enough to change the resting membrane potential into action membrane potential

  3. Influx of sodium ions (electropositive ions) 10 times more than in the resting state

  4. Sodium voltage-gated channels open
• If the stimulus is not more than the threshold value, then there will be no action potential state across the length of the neurolemma
• If the stimulus is more than the threshold value, then it will generate a nerve impulse that will travel across the entire length of the neurolemma
• Voltage-gated sodium channel


  1. Open channel carries an influx of Na+ ions, giving rise to depolarization

  2. As the channel becomes closed or inactivated, the depolarization ends
• After a cell has established a resting potential, that cell has the capacity to undergo depolarization
• Depolarization


  1. Membrane potential rapidly shifts from negative to positive

  2. Sodium ions rush back into the cell, adding positive charge to the cell interior

  3. Membrane potential changes from negative to positive

  4. Channels close again once the interior of the cell becomes more positively charged
• Repolarization


  1. Electrical balance is restored inside and outside the neurolemma

  2. Potassium channels open due to high concentration of sodium ions inside the axoplasm

  3. Efflux of potassium ions through the potassium channels

  4. Sodium voltage-gated channels are closed, no sodium ions move inside the membrane

  5. Helps maintain or restore the original membrane potential state
• Refractory period


  1. Brief period after successful transmission of a nerve impulse

  2. Membrane prepares itself for conduction of second stimulus

  3. Sodium-potassium pump restores original distribution of ions

  4. Requires ATP as ions move against concentration gradients
• Hyperpolarization


  1. Repolarization causes an overshoot in cell potential

  2. Potassium ions continue to move out, making the potential more negative than resting potential

  3. Resting potential ultimately re-established by closing of all voltage-gated ion channels and sodium-potassium pump activity
• Velocities of nerve impulse

  • Larger diameter axons have less resistance, increasing velocity
  • Myelinated axons conduct impulses more rapidly than unmyelinated axons due to saltatory conduction
• Synapse


  Junction that controls communication between a neuron and another cell
• Synaptic transmission


  1. Electrical synapse involves direct cytoplasmic connections

  2. Chemical synapse involves release of neurotransmitters into synaptic cleft
• Excitatory neurotransmitters


  Cause depolarization and excitation of postsynaptic cell, e.g. acetylcholine, biogenic amines
• Inhibitory neurotransmitters


  Cause hyperpolarization and inhibition of postsynaptic cell, e.g. GABA, glycine, glutamate, aspartate
• Components of the human nervous system

  • Central nervous system (brain and spinal cord)
  • Peripheral nervous system (nerves projecting from CNS)
• The CNS is protected by bony armor consisting of the skull and vertebral column
• Ion


  Charged particle
• Occipital Lobe


  • Vision
• Cerebellum


  • Movement co-ordination
• Somatic


  • Voluntary, muscle movement