Nervous System

• Nervous systems are needed to allow us to move in response to stimuli from the world
• INVERTEBRATES, such as sponges, jellyfish have nerve net

  • sponges don't have a NS: they react to stimuli but this involves a 'pre-neurons'
  • origin of vertebrate brain: small central collection of neuronal control circuits in amphioxus
  • brain divisions common to all vertebrates:

  1. olfactory bulb
  2. optic tectum
  3. medulla oblongata
  4. cerebral hemispheres
  5. cerebellum

  
• THE NEURAL TUBE

  • Three layers of cells

  1. endosperm (linings of organs; viscera)
  2. mesoderm (bones and muscles)
  3. ectoderm (nervous system and skin)

  • the neural plate folds and fuses to form the neural tube
  • CNS develops from the walls of the tube. PNS derives from the neuronal crest
• SPINA BIFIDA

  • failure of the posterior neural tube to close
  • supplementing diet with folic acid in early pregnancy can reduce neural tube defect incidence by 90%
  • some anti epilepsy/ bipolar drugs interfere with folate metabolism and increase risk of SB\
• DEVELOPMENT DIFFERENTIATION

  • Three swellings at the rostral end pf the neural tube become the primary vesicles: fore, mid and hind brain

  
• VENTRICLES

  • The rain is hollow and bathes in CSF (cerebrospinal fluid)

  1. CSF acts to cushion the brain and also has a role maintaining chemical stability and removing waste products
• THE SPINAL CORD

  • Protected by spinal column, surrounded by meninges and cerebrospinal fluid (CSF)
  • primary channel for messages from skin, joints and muscles to brain to periphery

  1. Dorsal roots of the spinal cord contains sensory, afferent neurons
  2. ventral roots contain motor, efferent neurons

  
• BRAINSTEM

  • oldest part of the brain- decision matrix
  • controls vital functions
  • contains:

  1. midbrain: movement, sensory input, eyes, ears
  2. hindbrain- pons, medulla, cerebellum
• PONS: swells out from ventral surface of brain stem (important relay between cortex and cerebellum)

  MEDULLA: important in control of blood pressure and respiration

  • brain stem damage (hydrocephalus or haemorrhage)

  1. severe cases can lead to 'cloning'
  2. damage to the medulla causes respiratory arrest
• DIENCEPHALON AND MESENCEPHALON

  • Midbrain: linkages between components of motor systems: eye movements, sleep, temperature regulation
  • Diencephalon: thalamus (relay and gating roles) in sleep, conscious movement and hypothalamus (homeostasis and reproduction)]
• CEREBELLUM

  • movement control centre
  • extensive connections to cerebellum and spinal cord. Contains at least as many neurons as both cerebral hemispheres
  • diseases include: ataxias- aberrant movement coordination
  • you can live without the cerebellum
  • alchohol affects the cerebellum profoundly
• CEREBRAL CORTEX

  • Clear division between two halves along sagittal fissure
  • controls:

  1. voluntary actions
  2. cognition
  3. perception/ awareness

  • mammals have a more complex 6-layer structure of the cortex = neocortex
  • number of the neurons related to 'intelligence'
  • cortical lobes clockwise from the front: frontal (actions, motor, speech control, emotion); parietal (sensory, language); occipital (visual) and temporal (memory, sensory, language)
• CORTICAL FOLDING

  • to increase intelligence, you need to increase processing power
  • cortical neurons represent processing power = increase number of cortical neurons (thin layers at surface)
  • but the skull is confined structure, want to keep volume + mass to minimum
  • big heads are harder to protect than little ones
  • high ends of folds = gyrus
  • low ends of folds = sulcus
• CELLS OF NERVOUS SYSTEM

  GOLGI AND THE RETICULAR THEORY

  • reduced silver stain
  • picks our random individual cells in great detail
  • Golgi believed that neuritis were fused together to form a network

  CAJAL AND THE NEURON DOCTRINE

  • each neuron is a discrete cell (neuron doctrine)
  • principle of dynamic polarisation
  • principle of connectional specificity
• ELECTRON MICROSCOPY

  • can examine cell ultrastructure
  • confirmed existence of synapses
  • resolution = 0.1 nm
  • disadvantages = cells fixed (dead)
• IMMUNOFLUORESCENCE LABELLING METHODS

  1. prepare selective antibody (or drug), tagged with fluorescent label
  2. add to tissue and allow to bind strongly
  3. wash off any free labelled antibody (or drug)
  4. image distribution of fluorescence

  • DISADVANTAGE: limited by range of antibodies available
• CONFOCAL MICROSCOPY

  • includes: lasers; high sensitivity cameras; imaging software
  • can examine live cells
  • DISADVANTAGES: resolution = 0.1 micrometer
• BRAINBOW

  • technique whereby the genome of an organism is modified spy that cells express random combinations of several different fluorescent proteins

  1. used to trace the path of individual axons
• CELLS OF NERVOUS SYSTEM: GLIA

  1. neurons
  2. glia = supporting cells

  • outnumber neurons in some regions of the brain: 17:1 in the thalamus and 1:1 in the cerebral cortex
  • many mediate some signalling in the brain
  • primary role is to support neurons
  • can divide (unlike neurons)
  • SATELLITE CELLS = help regulate the chemical environment around neuronal cell bodies in the autonomic nervous system (present in ganglia)
• GLIA- ASTROCYTES

  • Astrocytes direct the proliferation and differentiation of neural stem cells

  1. majority of glia
  2. star-shaped
  3. fill space between neurons
  4. regulate composition of extracellular fluid
• OLIGODENDROCYTES/ SCHWANN CELLS

  • myeline axons of neurons
  • oligodendrocytes = CNS, many axons
  • Schwann cells = PNS, single axons
• MICROGLIA

  • microglia act as the brain scavengers

  1. phagocytic/ immune function
  2. they can migrate
• EPENDYMAL

  • ependymal cells line ventricles and also direct cell migration during developed of the brain
  • produce CSF
• NEURONAL STRUCTURE

  • can be in different forms but most have a cell body and two types of processes (neurites)

  1. DENDRITES = specialised for receipt of info
  2. AXONS = specialised for transmission of info
  3. CELL BODY WITH CYTSOL and organelles, including nucleus
  4. cell membrane (PLASMALEMMA)
  5. cannot divide
  6. can trigger action potentials
• CYTOSOLIC ORGANELLES

  • peroxisomes, mitochondria
  • ribosomes
  • vascular apparatus (secretory/ endocytic pathway)

  1. endoplasmic reticulum
  2. secretory vesicles
  3. Golgi complex
  4. endoscopes
  5. lysosomes

  • DIVISIONS AT AXON HILLOCK:

  1. synaptic vesicles
  2. mitochondria
  3. smooth ER
• ROLE OF NEURONAL CYTOSKELETON

  • structural support- shape and calibre of axons and dendrites
  • transports cargo to and from axons and dendrites
  • tethering of components at membrane surface
• MICROTUBULES in cytoskeleton

  • ROLE= structural and transport
  • run longitudinally down axons and dendrites
  • big, 20nm, wide, tubilin polymer
  • polymerisation/ depolymerisation- shape change

  1. KINESINE (motor protein): moves cargo enclosed in vesicles down the microtubule towards the axon terminal
  2. DYNEIN: moves stuff back from terminal to cell body
• NEUROFILAMENTS in cytoskeleton

  • ROLE = mechanical strength
  • 10nm wide filamentous protein threads

  MICROFILAMENTS in cytoskeleton

  • ROLE = mediate shape change
  • 5nm wide, actin polymer
  • tethered to membrane
• NEURONAL CLASSIFICATIONS

  • Based on the direction of conducted impulses:

  1. SENSORY (afferent; somatic or visceral): neurons originate from sensory receptors in the CNS
  2. MOTOR ( efferent; somatic or visceral): neurons conduct signals that originated in the CNS
  3. INTERNEURONS are between sensory and motor neurons