topic 8

• how does the nervous system work
• Nervous system


  Detects changes in our environment (known as stimuli) through cells called receptors
• Receptors


  • Sensitive to light, pressure (touch) and chemicals in the air (smell)
• Receptor detecting stimuli


  Signals to the central nervous system (CNS) through initiating an electrical impulse through a neuron (nerve cell)
• Sensory neuron


  Sends an electrical impulse from the receptor within the sense organ and the coordination centre
• Coordination centre


  Receives impulses from various receptors around the body, processes the information and coordinates a response by signalling to other parts of the body
• Coordination centres


  • Brain
  • Spinal cord
  • Pancreas
• Coordination centre signalling


  Releases an electrical impulse along a motor neuron to an effector (a muscle or gland)
• Effector


  Produces a response such as muscle contraction or hormonal release
• Endocrine (hormonal) system


  A group of glands which secrete chemicals called hormones
• Hormone secretion


  1. Glands are stimulated by neurons

  2. Glands are stimulated by a change in concentration of a chemical substance (such as another hormone)
• Hormones


  Chemicals that travel around our body in our bloodstream to their target organs where it produces an effect
• Target cells


  Possess complementary receptors on their cell surface membranes that allow it to respond to the hormone, triggering a response inside the cell
• Types of hormones


  • Proteins/peptides (e.g. insulin)
  • Steroids (a type of lipid) (e.g. testosterone)
• Compared to the nervous system

  The endocrine system is slower to bring about an effect, longer-lasting, has a widespread response (can act on multiple target tissues), and involves the use of chemicals rather than electrical impulses
• Receptors


  Used by the nervous system to detect stimuli (changes in the environment) and pass on this information to the CNS
• Receptors


  • Can be whole cells (e.g. photoreceptors are cells which are sensitive to light)
  • Can be protein molecules found on the cell surface membrane
  • Each receptor is specific to a single type of stimulus, such as light, temperature or glucose concentration
• Receptor function


  1. When not stimulated, there is a charge difference between the inside and outside of the membrane and it is said to be polarised

  2. When the receptor detects a stimulus, the permeability of its cell membrane changes which changes the charge difference (potential difference) across the membrane

  3. If the change in potential difference is large enough (i.e. it exceeds the threshold level), it will trigger an action potential (an electrical impulse) in a sensory neuron
• types of receptors

  • Chemoreceptors receptors which detect chemicals
  • Thermoreceptors - receptors which detect heat
  • Mechanoreceptors - receptors which detect pressure (e.g. Pacinian corpuscles)
  • Photoreceptors - receptors which detect light (e.g. rods and cones) 
• Photoreceptors


  Receptors which detect light and are found in the retina of the eye
• Fovea


  • Area of the retina containing a cluster of photoreceptor cells
• Photoreceptor function


  1. Detect light as it hits the retina

  2. Send nerve impulses to the brain along the optic nerve
• Blind spot


  Region of the eye containing the optic nerve where there are no photoreceptors so light can't be detected
• Bipolar neurone


  Photoreceptors are connected to the optic nerve through this
• Types of photoreceptors


  • Rods
  • Cones
• Rods


  • Mostly located along the outside of the retina
  • Responsible for black-and-white vision
  • Can function in lower light levels than cones
  • Much more sensitive than cones
• Cones


  • Clustered together in the fovea
  • Responsible for colour vision
  • Sensitive to blue, green or red light
  • Provide good visual acuity
• Rod cell membrane in dark conditions

  1. Depolarised

  2. Sodium ions actively transported out of cell

  3. Sodium ions flow straight back in through channels

  4. Triggers release of neurotransmitters which inhibit bipolar neurone

  5. Bipolar neurone cannot fire action potential, no information sent to brain
• Rod cell membrane in light conditions

  1. Rhodopsin pigment splits into retinal and opsin

  2. Sodium ion channels close

  3. Sodium ions continue to be actively transported out but cannot flow back in

  4. Rod cell becomes hyperpolarised

  5. Stops releasing neurotransmitters

  6. Inhibition of bipolar neurone stops

  7. Bipolar neurone can become depolarised

  8. Depolarisation exceeds threshold, information passed to brain via optic nerve
• types of neurons

  • Sensory neurons carry action potentials from receptors to the central nervous system. They consist of one long dendron and a short axon.
  • Relay neurons carry action potentials between the sensory and motor neurons and are found within the CNS. They have lots of short dendrites.
  • Motor neurons carry action potentials from the CNS to an effector. They have lots of short dendrites and one long axon.
• Resting potential


  The difference in charge between the inside and the outside of the membrane when a neurone is not firing (not transmitting an action potential)
• The resting potential is usually around -70 mV
• Polarisation of neuronal cell membranes at rest

  1. Sodium-potassium ion pumps actively transport sodium and potassium ions into and out of the neurone

  2. For every three sodium ions pumped out of the cell, two potassium ions are pumped into the cell

  3. This ensures there are more positive ions out of the cell compared to inside the cell

  4. This makes sure there is a charge difference across the membrane
• The charge difference across the membrane is referred to as the resting potential
• Action potential


  When a neurone is stimulated, the charge difference between the inside and outside of the cell membrane is lost and the membrane is depolarised. If enough charge is lost and depolarisation exceeds -55 mV, an action potential will occur in that neurone.
• Threshold potential


  The -55 mV 'limit' - any depolarisation above this number will result in an action potential whereas anything less than that will result in nothing
• Action potentials


  An "all-or-nothing" response
• Action potential generation

  1. Sodium ion channels close

  2. Potassium ion channels open

  3. Potassium ions move out of the neurone down their concentration gradient

  4. Charge difference across the membrane - repolarisation

  5. Neurone becomes hyperpolarised

  6. Sodium-potassium ion pump restores balance and returns neurone to resting potential of -70 mV
• Refractory period


  Immediately after an action potential, a brief period where the neurone cannot be stimulated and an action potential cannot occur
• Refractory period


  • Ensures action potentials do not overlap and are unidirectional