Nervous System II: CNS/ Brain

• Receptor cells


  Dozens of different, specialised types that respond to changes in the environment
• Most receptor cells are not neurons, but are directly connected to a sensory neuron
• Sensory adaptation


  Getting used to a specific stimulus
• We only sense when things are changing
• Sensory neurons


  • Have long, myelinated axons
  • Neurons transmitting precisely localised information send axons to the top of the spinal cord
  • Neurons transmitting poorly localised information synapse immediately with other neurons upon entering the spinal cord
• Sensory neurons from the head send axons directly into the brain via cranial nerves
• Sensory signal transmission


  1. Signals transmitted via several 'relay stations'

  2. At each, integrated with other incoming signals from 'lower', 'higher', and same-level processing stages
• A lot of information processing takes place even before a signal has reached the brain
• Brain stem


  • Hindbrain (medulla, pons, cerebellum)
  • Midbrain (functions include combination of information from different sense modalities; direction of attention)
• Thalamus


  • Main 'relay station' for incoming sensory signals
  • Receives downward-going input from higher areas, modulating the relay of sensory signals
• Hypothalamus


  'Gateway' to endocrine system: nervous system can influence endocrine system via hypothalamus - pituitary connection
• Cerebral hemispheres


  • Divided into two highly similar (but not identical) hemispheres
  • Each covered in cerebral cortex (thin layer of neurons covering each hemisphere), also contains several groups of sub-cortical nuclei
• Grey matter


  Cortex & sub-cortical nuclei
• White matter


  Myelinated axons of neurons
• Basal ganglia


  • Group of nuclei surrounding the thalamus
  • Involved in motor control process
  • Consist of globus pallidus, putamen, & caudate
• Limbic system


  • Several interconnected cortical & sub-cortical areas, playing a crucial role in memory & emotion
• Cerebral cortex


  • Thin layers of neurons covering the whole hemisphere, i.e., not just the outside, but the inner ('medial') surface as well
• Corpus callosum


  Thick bundle of axons connecting the two hemispheres
• Virtually all signal transfer between the cortices of the hemispheres done via corpus callosum
• Cerebral lobes


  • Occipital lobe (visual perception)
  • Temporal lobe (auditory perception)
  • Parietal lobe (somatosensory perception; inter-sensory & sensory-motor integration)
  • Frontal lobe (planning & motor output)
• Sensory input from the right side of the body (or the right visual field) is processed in the left half of the brain (and vice versa)
• Motor output to the right side of the body is generated in the left half of the brain (and vice versa)
• Contralateral


  The technical term for "on opposite side"
• Sensory signal transmission and interpretation


  Sensory signals from the diencephalon are relayed to their appropriate primary sensory cortex (visual cortex, auditory cortex, somatosensory cortex)
• Topographic maps


  Signals arrive at positions corresponding to the position of the receptor cells
• Neurons transmit signals only in one direction (from the dendrites to cell body [soma] to the end of the axon)
• Combination of feed-forward and feedback signal transmissions means that signals are never just passively 'forwarded' - every 'input' is already modified by everything else going on in the brain
• We CANNOT perceive the world 'objectively'
• Cortical motor areas


  • Located in the frontal cortex, at the boundary to the parietal cortex
  • Supplementary motor cortex & premotor cortex: involved in planning, monitoring, & sensory guidance of movements
  • Primary motor cortex: final execution stage - its motor neurons send axons directly down the spinal cord (the pyramidal tract)
• Motor control circuits


  • Cortical motor areas are massively interconnected with basal ganglia and cerebellum
  • Basal ganglia: modulate movements, particularly involved in selective inhibition of movements
  • Cerebellum: involved in maintaining posture & balance, timing of movements, & motor learning
• Motor signals are ultimately sent down the spinal cord