CNS & PNS physiology

• Components of the central nervous system (CNS)


  • Brain
  • Spinal cord

  
• Connective tissue layers that cover the brain and spinal cord


  • Supply nutrients and oxygen, and provide some cushioning
  • 3 layers: dura mater, arachnoid mater, pia mater

  
• Cerebrospinal fluid (CSF)


  Fluid that occupies the subarachnoid space, central canal and the ventricular system around and inside the brain and spinal cord

  
• Blood-brain barrier (BBB)


  • Separates capillaries in the brain from nervous tissue
  • Prevents many drugs, proteins, ions, and other molecules from readily passing from the blood into the brain

  
• Structural components of the cerebrum

  • Cerebral cortex
  • Subcortical structures: basal ganglia, hippocampus, olfactory bulb

  
• Structures and functions of the brain

  • Cerebrum
  • Cerebellum
  • Diencephalon
  • Brain stem
  • Limbic system

  
• Spinal cord


  • Conveys information to and from the brain
  • Relay station for many reflexes
• Divisions of the nervous system


  • Central nervous system (CNS)
  • Peripheral nervous system (PNS)

  
• Components of the central nervous system

  • Neuron cell bodies
  • Myelinated and unmyelinated nerve fibers
  • Glial cells
  • Gray matter
  • White matter

  
• Layers of the meninges

  • Dura mater
  • Arachnoid mater
  • Pia mater
• Choroid plexus


  Specialized structure in the ventricles that secretes cerebrospinal fluid
• Circulation of cerebrospinal fluid

  Ventricular system → subarachnoid space via fourth ventricle → venous blood via arachnoid villi
• Cerebral cortex


  • Functions associated with analytical abilities and intelligence
  • Largest part of brain in advanced mammals
  • Can be divided into lobes based on functions

  
• Basal ganglia


  • Involved in movement initiation – complex direct and indirect pathways
  • Parkinson's disease
• Hippocampus


  • Involved in learning and memory
  • Important in transition between short-term and long-term memory
• Thalamus


  • Relay station for regulating sensory inputs to the cerebrum
  • Relay station for info from cerebellum and basal ganglia to cerebral cortex
• Hypothalamus


  • Interface between the nervous and endocrine systems
  • Regulation centre for autonomic nervous system
  • Regulation of body temperature, appetite, thirst, sex drive, osmolarity of body fluids
  • Part of the limbic system → emotions such as aggression & rage

  
• Limbic system


  • Group of brain regions responsible for emotional drives
  • Examples of emotions controlled: aggression, fear, hunger/satiety, sex drive, goal-directed behaviours
• Cerebellum


  • Balance, posture, coordination of movement
  • Influences motor coordination by inhibition on motor cortex
  • Works with the basal nuclei and motor cortex to coordinate movement
• Brain stem


  • Forms connection between the spinal cord and the rest of the brain
  • Pons links the cerebellum, forebrain and midbrain
  • Nuclei in medulla oblongata control heart activity, blood pressure & relative blood distribution to different organs
  • Nuclei in pons and medulla oblongata control respiratory movements & influence many digestive processes
  • Involved in reflexive control of eye movements and posture
  • Reticular formation: activation of cerebral cortex → switch from unconsciousness and sleep to consciousness

  
• Midbrain


  • Superior colliculi: visual reflexes
  • Inferior colliculi: auditory reflexes
  • Tegmentum: substantia nigra - important part of the motor circuit, red nucleus - important for voluntary movement
• Spinal nerves


  • Afferent nerves – towards the CNS
  • Efferent nerves – away from the CNS
  • Named according to corresponding level of vertebral column
  • Innervate muscles and skin in that particular segment
• Internal structure of spinal cord

  • Medulla: central part composed of gray matter
  • Cortex: outer part composed of white matter
• Dorsal and ventral nerve roots


  • Dorsal nerve roots - sensory (afferent) fibers
  • Ventral nerve roots - motor (efferent) fibers
• Dorsal and ventral horns


  • Dorsal horns - sensory nerve impulses to brain or other parts of SC
  • Ventral horns - motor nerve impulses to spinal nerves
• Cranial nerves


  • May be afferent only, efferent only or mixed (both)
  • Most originate from brainstem (III-XII)

  
• Sensory system


  • Sensory receptors - receive stimuli from the external or internal environment and transmits to the CNS
  • Neural pathways - conduct information to the CNS (ascending pathways)
  • Processing the information (parts of the brain)

  
• Sensory information that may not lead to conscious awareness

  • Muscle spindles
  • Receptors providing info about arterial blood pressure
  • pH of the cerebrospinal fluid

  
• Sensory receptors


  Either specialized endings of afferent neurons or separate cells that signal the afferent neuron

  
• Photoreceptors


  Receptors that can only be excited by a limited part of the electromagnetic spectrum - wavelengths 400-700 nm
• Transducers


  Convert various forms of energy (stimuli) in the environment into action potentials in neurons

  
• Types of sensory receptors


  • Photoreceptors - light
  • Chemoreceptors - chemical composition of local tissue environment
  • Proprioceptors - position of body in space at a given time
  • Mechanoreceptors - touch/pressure
  • Osmoreceptors - change in solute concentration
  • Thermoreceptors - temperature
  • Nociceptors - pain

  
• Producing a clear image

  1. Refraction of the light rays

  2. Accommodation of the eyes

  
• Diurnal animals


  • Sight is the dominant sense for perception of external environment
• Generator potential/receptor potential


  Graded potential
• Visual sense


  • Most advanced in birds
• Magnitude of the receptor potential

  Depends on: stimulus strength, temporal summation of successive receptor potentials, adaptation

  
• Orbit


  Bony cavity in the cranium where the eyes lie, cushioned by adipose tissue
• Adaptation: receptor adapts to a stimulus by no longer responding to the same degree
• Adaptation occurs when there is continuous stimulation of the same stimulus strength