Nervous System

• Action potentials are rapid changes in membrane voltage that allow neurons to send messages quickly over long distances.
• Key Words:

  • Stimulus: something detected by an organism, can be internal in multicellular organisms or external in any organism
  • Receptor: an organ or specialised cell that detects the change causing the stimulus
  • Response: a reaction caused by the stimulus detected by the receptor, can be movement or a change in behavior
• Taxes and Kinesis:

  • Taxis: movement in a specific direction, positive taxis is towards the stimulus, negative taxis is away from the stimulus
  • Example: positive chemotaxis is mobile bacteria moving to an area with a higher concentration of glucose
  • Kinesis: movement in random directions, speed and frequency of direction change increase to increase the chance of entering different conditions rapidly
  • Example: woodlouse in a dry area speeds up and changes direction frequently to find damp conditions
• Plants response to stimuli:

  • Plants exhibit tropisms, growth responses controlled by directional stimuli
  • Example: phototropism where shoots grow towards light (positively phototropic) and roots grow away from light (negatively phototropic)
  • Plant growth controlled by indoleacetic acid (IAA), an important auxin produced in tips and shoots of flowering plants
  • In shoots, IAA causes cell elongation, however in roots, IAA inhibits cell growth
• Photoreceptors in the eye:

  • Light receptors in the eye, cones for color vision and rods for monochromatic vision
  • Cones: sensitive to bright light, provide good visual acuity, each cone cell has its own synapse via a bipolar neurone
  • Rods: sensitive to low light, provide low visual acuity, multiple rods share the same synapse with a bipolar cell
• Control of heart rate:

  • Heart is myogenic, sinoatrial node is the pacemaker initiating electrical stimulation for atrial contraction
  • Accelerator nerve (sympathetic) increases heart rate, vagus nerve (parasympathetic) decreases heart rate
  • Factors increasing heart rate: changes in pH from high CO2 concentration and changes in blood pressure
• Nerve impulses:

  • Neurones have a cell body with a nucleus and organelles like mitochondria
  • Dendrites conduct impulses towards the cell body, axons conduct impulses away from the cell body
• Neurones have a polarised structure with axons conducting electrical impulses away from the cell body
• The resting potential of neurones is -70mV, maintained by the sodium-potassium pump
• Action potentials in neurones involve depolarisation, repolarisation, and hyperpolarisation
• Factors affecting the speed of nerve impulse include the presence of myelin sheath, diameter of the axon, and temperature
• The refractory period in neurones ensures action potentials pass in one direction as discrete signals
• Synaptic transmission involves the release of neurotransmitters across synapses to stimulate action potentials in the post-synaptic neurone
• Skeletal muscles are under voluntary control and are attached to bones by ligaments and tendons
• Muscle contraction requires ATP and involves the interaction of myosin and actin filaments
• Muscles can be slow or fast twitch fibres, adapted for different types of exercise
• Slow twitch muscle fibers are adapted for endurance activities like marathon running
• Adaptations for aerobic exercise in slow twitch fibers include:

  • Large store of myoglobin
  • Rich supply of blood vessels
  • Numerous mitochondria
• Fast twitch muscle fibers are adapted for rapid release of energy during intense exercise like sprinting
• Adaptations for intense exercise in fast twitch fibers include:

  • Thick and numerous myosin filaments
  • High concentration of glycogen
  • High concentration of enzymes for anaerobic respiration
  • Store of phosphocreatine for rapid ATP generation
• Damage to the CNS can result in paralysis or death.
• The central nervous system (CNS) consists of the brain and spinal cord.
• Spinal cord - transmits information between the CNS and other parts of the body through spinal nerves.
• Intermediate neurons are located within the brain and spinal cord and connect different parts of the nervous system to process and integrate information.
• The nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS).
• Neurons have three main parts: cell body, dendrites, and axon.
• Sensory neurons have dendrites at one end and axon terminals at the other end.
• Neurons communicate with one another by releasing neurotransmitters at synapses.
• Sensory neurons carry sensory input from receptors throughout the body to the CNS, while motor neurons transmit signals from the CNS to effector organs such as muscles and glands.
• The PNS includes all nerve tissue outside the brain and spinal cord, including sensory receptors, cranial nerves, and autonomic nerves.
• Autonomic nervous system (ANS): Regulates involuntary activities such as heart rate, breathing, digestion, and sweating.
• Axons can be myelinated or unmyelinated.
• Myelin sheath is formed by Schwann cells in PNS and oligodendrocytes in CNS.
• Autonomic motor neurons are further subdivided into sympathetic and parasympathetic divisions that work together to maintain homeostasis.
• Sympathetic division: Prepares the body for physical activity or stressful situations.
• Parasympathetic division: Restores normal functions when sympathetic response is no longer needed.
• Nodes of Ranvier are gaps between adjacent segments of myelin sheath that allow faster conduction of action potentials along the axon.
• Unmyelinated axons have slower conduction speeds but can still conduct impulses.
• Action potentials occur when an electrical charge moves along the membrane of a neuron.
• Neuromuscular junction: The point where an axon connects with a muscle fiber.