Nerves

• Stimulus


  A change inside or outside an organism which brings about a response in that organism e.g. light, sound, blood temperature
• Receptor


  A structure which detects a particular stimulus and initiates a nerve impulse. A receptor may be a single cell (receptor or sensory cell) or a group of cells (sense organ)
• Coordinator


  Where neurones are linked together in the CNS (Central nervous system) usually via connector neurones
• Effector


  A structure such as a muscle or a gland which responds to the arrival of a nerve impulse
• Response


  The change in an organism which results from a stimulus
• Receptors - Know the roles of mammalian sensory receptors in converting different types of stimuli into nerve impulses
• To include an outline of the roles of sensory receptors (e.g. Pacinian corpuscle) in responding to specific types of stimuli and their roles as transducers
• Organisation of the nervous system

  • PNS
  • CNS
  • Nervous system
  • Sensory
  • Motor
  • Somatic
  • Autonomic
  • Brain
  • Spinal cord
  • Sympathetic
  • Parasympathetic
• Stimulus


  Forms of energy, so sensory receptors act as transducers, converting stimulus energy into electrical energy in the form of a generator potential which, above a threshold value, will produce an action potential
• Types of sensory receptors

  • Pacinian corpuscle
  • Chemoreceptor
  • Thermoreceptor
  • Photoreceptor
• Pacinian corpuscle


  Found at the ends of sensory neurone axons, made of many layers of membrane around the end of the neurone separated by a gel, the gel contains positively charged sodium ions (Na+), the section of axon surrounded by layers of membrane contains stretch-mediated sodium ion channels which open when sufficient pressure is applied, allowing Na+ to flow into the neurone and establish a generator potential
• Pacinian corpuscle transduction


  1. Resting potential maintained by Na+/K+ pumps in the plasma membrane

  2. Pressure applied causes the corpuscle to change shape, stretching the membrane surrounding the neurone

  3. Stretch-mediated Na+ channels in the membrane open, increasing membrane permeability to Na+ ions

  4. Na+ ions diffuse into the neurone, down an electrochemical gradient, causing the potential difference to rise slightly and the membrane to depolarise, forming a generator potential

  5. If the generator potential is above a threshold value, it causes an action potential
• Complete the Qs on page 13 of the study guide
• Key terms relevant to the reaction time experiment

  • Accuracy
  • Anomaly (outlier)
  • Confidence
  • Error (of measurement)
  • Precision
  • Repeatability
  • Reproducibility
  • Resolution
  • Uncertainty
  • Validity
• Neurones


  Nerve cells with specialised extensions (dendrites & axons)
• Types of Neurones


  • Sensory Receptors
  • Sensory Neurone
  • Ganglia
  • Grey Matter
  • White Matter
  • Relay Neurone
  • Motor Neurone
  • Effector cells
• Sensory Neurone


  • Transmit impulses over long distances from sensory receptor to CNS
  • Have long dendron and axon
  • Cell body in ganglia
  • Myelinated
• Relay Neurone


  • Non-myelinated
• Motor Neurone


  • Transmit impulses over long distances from CNS to effector
  • Cell body in spinal cord
  • Long axon but short dendrites
  • Myelinated
• Effectors


  • Produce a response to a stimulus
  • Skeletal muscle
  • Salivary gland
  • Muscles contract to produce movement
  • Glands can secrete substances
• Neurones are adapted to carry electrochemical charges called impulses
• Cell body


  Contains nucleus and large amounts of rough endoplasmic reticulum, used in production of proteins and neurotransmitters
• Dendrites


  Smaller branches which extend from dendrons and transmit electrical impulses to the dendrons then to the cell body
• Axon


  Single fibre that carries electrical impulses away from the cell body
• Myelin sheath


  Electrical insulator that speeds up the nerve impulse, made up of lipid-rich Schwann cells
• Main types of neurones

  • Sensory neurones
  • Motor neurones
  • Relay neurones
• Sensory neurones send impulses from sensory receptors towards the CNS
• Motor neurones send impulses away from the CNS to muscles/glands
• Relay neurones send impulses between sensory and motor neurones, involved in integration
• Sensory receptors


  • Transducers that convert energy from one form to another
  • Chemoreceptors in tongue
  • Rods and cones in retina
  • Receptor molecules in cell surface membrane with complementary shapes to detected molecules
  • Light sensitive pigments to absorb light energy
• Coordination


  • Spinal cord
  • Brain
• Sensory neurones transmit impulses over long distances from sensory receptor to CNS
• Motor neurones transmit impulses over long distances from CNS to effector
• Effectors produce a response to a stimulus
• Skeletal muscle forms a neuromuscular junction at the motor end plate, causing the muscle to contract
• Salivary glands can secrete substances like saliva
• Reflex actions


  Conscious decision to respond does not occur, pathway goes through spinal cord or unconscious part of brain for very quick response, protective
• A simple reflex arc involves sensory receptors, sensory neurone, relay neurone, motor neurone, and effector cells
• Blinking reflex


  • Triggered by anything contacting the cornea, bright light, loud noise, or drying of the cornea
  • Protects the eye
  • Cranial reflex involving the brain, not the spinal cord
  • Doctors test for this to assess brain function
• Knee jerk reflex

  • Helps stop the knees buckling if carrying a heavy weight
  • Involves the spinal cord, so absence or exaggeration can indicate CNS problems
  • Stimulus is quadriceps muscle stretched, receptor is stretch receptors, sensory neurone to motor neurone (no relay), effector is quadriceps muscle contracting to move leg forward