Mod 4

• Nervous system organisation


  • Cerebral cortex = conscious mind
  • Initiate movement
  • Sensation
  • Communication
  • Memory
  • Understanding

  
• Grey matter


  • Cell bodies, dendrites, glia, blood vessels
  • No fibre tracts
  • 40% of brain mass
  • Only 2-4mm thick!
• Cerebral White Matter


  • TRACT– collection of neural fibres in CNS
  • NERVE – collection of neural fibres in PNS

  
• These two brain areas are the precommand areas


  • Spinal cord and midbrain
  • Midbrain and basal nuclei
  • Basal nuclei and cerebellum
  • Cerebellum and spinal cord

  
• Blood-Brain Barrier (BBB)


  • Almost impermeable protective membrane separating the blood from cerebrospinal fluid
  • Maintains chemical environment around brain
  • Blood borne compounds pass through 3 layers
  • Capillary endothelium – tight junctions
  • Thick basal lamina
  • Bulbous feet of astrocytes cling to capillaries
  • NOT the meningeal layers!
• Blood-Brain Barrier (BBB)


  YES - glucose, electrolytes, essential amino acids

  NO - waste metabolites, proteins, toxins, drugs

  Ineffective against fats, O2, CO2, alcohol, caffeine, morphine (via passive diffusion)

  Astrocyte feet don't actually provide the barrier they regulate the tight junctions provided by the endothelial cells in the walls of the capillary allowing certain lipophilic substances through, increased permeability with certain drugs.
• Pia mater is NOT the Blood Brain Barrier!
• Meninges


  • Dura mater
  • Pia mater
  • Arachnoid mater
• Epidural anaesthesia


  Epidural space located between bony vertebrae and spinal dura mater filled with fat padding
• Lumbar puncture


  Subarachnoid space located between arachnoid mater and pia mater filled with?
• Phineas Gage – Medical Miracle
• Spinal Cord : cross section

  • Central canal runs through centre of spinal cord contains CSF

  Grey matter found in centre of spinal cord

  White matter surrounds grey matter

  Identify dorsal vs ventral surface

  Hint – dorsal root ganglion

  Spinal nerve = mixed afferent and efferent fibres

  Each spinal nerve splits into a ventral and a dorsal ramus (mixed fibres)

  Dorsal rami supply the posterior body trunk

  Thicker ventral rami supply the rest of the body trunk and limbs (plexi)
• CNS

  PNS
• Pathways of the Central Nervous System

  Name of pathway?

  Afferent or efferent?

  What function does it mediate?

  How many neurons?

  How many synapses?

  Location of synapses?

  Location of decussation?

  How can you draw these simply?

  
• Pathways


  • DCML System

  Spinothalamic Pathway

  Corticospinal Pathway
• Sensory receptor classification


  • Mechanoreceptors

  Thermoreceptors

  Photoreceptors

  Chemoreceptors

  Nociceptors
• Dorsal Column

  Visual Pathway

  Spinothalamic Tract

  Spinothalamic Tract

  Olfactory Pathway and Gustatory Pathway
• Stretch reflexes


  • Monosynaptic

  Fastest possible response

  Ensures muscle stays same length

  e.g: patellar reflex (knee jerk)

  Somatic reflex (skeletal muscle)

  Can be modified by conscious control
• Clinically why test reflexes? To check the integrity of the nervous system at a particular level of the spinal cord
• Pupillary light reflex is an example of an autonomic reflex and is under involuntary control.
• Peripheral Nervous System
• Sympathetic division


  • "Fight or flight" response

  Excitement, emergency, threat

  Increases energy use

  ↑ HR

  Vasodilation coronary blood vessels

  Vasoconstriction visceral blood vessels

  ↑ blood pressure

  Bronchiolar dilation

  ↑ respiration , ↑ oxygen delivery to muscles

  Pupil dilation

  Clammy skin

  Sweating
• Parasympathetic division


  • "Rest and digest"

  Conserves energy use

  Promotes energy storage

  Digestion

  Elimination of waste

  eg: after consuming dinner → read and relax

  HR ↓

  BP ↓

  Bronchiolar constriction

  Gastrointestinal motility ↑

  Pupil constriction
• Special senses


  • Light transduction

  Visual pathways

  Sound pathway

  Equilibrium: static vs dynamic
• Drawing

  Cornea → aqueous humor (anterior segment) → pupil → lens → vitreous humor (posterior segment) → inner segment neural layer retina → outer pigmented neural layer → photoreceptors (rods and cones)
• Special senses


  • Light transduction
  • Visual pathways
  • Sound pathway
  • Equilibrium: static vs dynamic
• Pathway of a photon of light

  Cornea → aqueous humor (anterior segment) → pupil → lens → vitreous humor (posterior segment) → inner segment neural layer retina → outer pigmented neural layer → photoreceptors (rods and cones)
• Photoreceptors unusual: depolarise in response to absence of stimuli
• In the dark cGMP levels are high. Keeps cGMP-gated Na+ channels open allowing a steady inward 'dark' current → dark current keeps the cell depolarized at about -40 mV → Ca2+ channels open releasing glutamate onto bipolar neuron. Glutamate (normally excitatory) here acts as an inhibitory neurotransmitter.
• Light CLOSES cGMP Na+ channels, Switches OFF dark current, ↓ glutamate release, ↓ inhibition of ganglion cell
• Myopic eye


  • too long, near-sighted, unable to see distance, focal point in front of retina, corrected with a concave lens to diverge light rays
• Emmetropic vision


  • normal eye with light focused properly (objects > 6m = 20 feet away), if object is closer, lens needs to accommodate
• Hyperopic eye


  • too short, far-sighted, unable to see close, focal point behind retina, corrected with a convex lens to converge light rays
• Accommodation


  • contraction of ciliary muscles causes lens to become more round/convex
• Convergence


  • contraction of both medial recti muscles, eyes look inwards to maintain focus
• Miosis (pupillary constriction)


  • reduces the amount of light entering the eye through the pupil
• Two optic nerves form optic chiasm at base of the brain, Medial fibres of the optic nerve decussate at optic chiasm, Most fibres of the optic tracts continue to the lateral geniculate body of the thalamus, Some optic tract fibers end in superior colliculi → contribute to visual reflexes, Optic radiations from thalamus to visual cortex
• Outer ear – conduction pathway, External acoustic meatus, Middle ear – conduction pathway, Tympanic membrane, Auditory ossicles: Malleus, Incus, Stapes, Oval window
• Inner ear – sensory component, Receptors hearing (cochlea), Organ of Corti, Activate Cochlear nerve fibres, Receptors of static equilibrium (vestibule), Maculae (in utricle and saccule of vestibule), Activate Vestibular nerve fibres, Receptors of dynamic equilibrium (semicircular canals), Crista ampullaris in semicircular ducts, Activate Vestibular nerve fibres
• Basilar membrane


  • Frequency = pitch, Amplitude = loudness