Nervous Tissue

• CNS = brain & spinal cord connecting to PNS with inputs & outputs
• Efferent = output/effect

  • Somatic NS = voluntary (effect skeletal muscle)
  • Autonomic NS = involuntary (effect smooth & cardiac muscle & glands) - 2 types:

  1. Sympathetic NS = flight vs fight, the accelerator
  2. Parasympathetic NS = rest & digest, the break
• Neuron shapes:

  • Multipolar = multiple dendrites project from cell body, collect a lot of info
  • Bipolar = single dendrite opposite from axon
  • Pseudo-unipolar = axon & dendrite rise form common stem of cell body
• Cellular Structure of Neuron:

  • Large nucleus - reflecting metabolic demand
  • Many mitochondria - metabolic demand, in dendrites & axons as well
  • Lots of ER - particularly in large neurons, can be found in dendrites but not nucleus
  • Numerous neurofilaments - together with microtubules make up the cytoplasm
  • Synapses are found at dendrites & also cell body
• CNS divided into:

  • Grey matter - neuron = cell bodies, dendrites & axons
  • White matter - axons, many myelinated: contains lots of lipids why appears as white
• Glial cells support the CNS - Types:

  • Oligodendrocytes - myelinate axons (equivalent of Schwann cells of PNS)
  • Astrocytes - mechanical support, form blood-brain barrier
  • Microglia - immunological cells
  • Epenmdymal cells - ciliated cubodial epithelial cells line cavities of brain & spinal cord
• Peripheral Nerves:

  • Consists of 1 or more bundles of nerve fibres called fascicles
  • Axon inside fascicles surrounded by collagenous CT called endoneurium
  • Fascicles enclose by dense collagenous CT called perineurium
  • Fasciles bound together by losse collagenous CT called epineurium
• Peripheral Nerves (inner to outer):

  • Axon - Myelin sheath - Endoneurium - Bunch of axons covered by Perineurium - make a Fasicle - fasicles bunched together & covered by Epineurium
• PNS axons myelinated by Schwann cells, providing structural & metabolic support
• PNS (Non-)Myelinated:

  • Large & small diameter fibres different in degree to which enveloped by Schwann cells
  • Small = non-myelinated
  • Large = myelinated
• Non-myelinated nerves:

  • Small diameter
  • Autonomic system
  • Enveloped by cytoplasm of Schwann cells
  • Schwann cells provide metabolic support but not myelination
• Myelinated Nerves:

  1. Axon is invaginated into Schwann cell cytoplasm
  2. Outer membrane of Schwann cell fuses from a mesaxon (fuses together to itself)
  3. Then wraps around axon repeatedly until myelination formed
  4. Mesaxon rotates around the axon - wrapping the axon in concentric layers of membrane = myelin sheath

  • Myelin sheath - provides insulation for the axon
• Myelin sheath provides insulation for the axon
• Nodes of Raniver:

  • Gaps where axons aren't myelinated
  • Important in signal conduction along axon
  • Also found in CNS
  • Speeds up conduction of AP
• The Resting Membrane Potential (RMP):

  • Electrical potential exists across the plasma membrane of all cells
  • Fluid inside cell = neg; outside = pos
  • Allows for AP to travel down axon
  • Thru ion channels that are depolarised/opened to flow along axon
  • Differs weather is myelinated or not
  • Will swap neg to pos (inside) & pos to neg (outside) = depolarisation
• Will swap neg to pos (inside) & pos to neg (outside) = depolarisation
• AP of myelinated nerves quickly jumps bw/ nodes of Raniver

  • Localise ion channels at nodes
• Myelination speeds up conduction velocity
• Multiple Sclerosis - Myelin of CNS:

  • Autoimmine NS disease where immune system attacks the myelin (oligodendrocytes)
  • Slows down or blocks messages bw/ brain & body
  • Causes: visual disturbances, muscle weakness, trouble w/ coordination & balance, numbness, prickling, think & memory problems
  • Cause is unknown
• Synapses:

  • Synapses specialised intercelluar junctions which link neurons to each other & muscles
  • Allow electrical energy into chemical to activate electrical in other neuron
• Synaptic cleft = space between neurons
• Neurotransmitters include:

  • Noradrenaline
  • Glutamate
  • Dopamine
  • Acetylcholine
  • Serotonin
• Neurotransmitter disorders:

  • Loss of enzyme
  • Affects biomechanical pathways that synthesise neurotransmitters
• Motor neuron & Neuromuscular Junction:

  • Synapse bw/ motor neurons & muscle fibre
  • Also known as motor end plate
  • 1 motor neuron can divide into many branches each ending in a neuromuscular junction - 1 neuron may innervate thousands of muscle fibres
  • Each muscle fibre attached to 1 motor neuron
  • But 1 motor neuron can affect many fibres
• Neuromuscular Junction:

  • Each muscle fibre attached to 1 motor neuron
  • But 1 motor neuron can affect many fibres
• Motor neurons:

  • Allows you to be able to attach many fibres simultaneously
  • Where need more precision will be a motor neuron to just a few fibres
• Motor unit = motor neuron & connecting skeletal muscle fibres

  • Lots of fibres = Power
  • Fewer fibres = Endurance
  • Alter bw/ 2 for partial contraction
• Neuromuscular Junction also known as motor end plate
• Neuromuscular Junction:

  • Motor neurons branching to innervate a number of muscle fibres
  • Acetylchone released from synaptic vesicle - released in response of AP
  • Binds to nicotinic ion channels that cause membrane depolarisation
  • Secondary synaptic cleft caused by folding - allows increase SA for more ion channels for quicker depolarisation of muscle fibre
  • NMJ occupies a recess on the muscle surface = sole plate
• Botox to eliminate skin wrinkles

  • Botulinum regulates ACh release from nerve terminals & thus selectively inhibits muscle ability to contract
  • ACh not released so can't contract
  • Eliminates partial contraction
  • Existing lines smoothened
  • Too much get droopy eyelid muscles
• Botuinum toxin A (Botox) - mechanism of action

  1. Toxin cleaves protein involved in release of ACh (acetylcholine) from vesicle
  2. Stop vesicles w/ ACh from binding to membrane (thru proteins) to be released
  3. Botox cleaves protein so can't get effectice docking of vesicles with ACh so can't be released & can't bind to muscle cells for depolarisation
• Synaptic Transmission at Chemical Synapses:

  • Propagating axon terminates at the terminal bouton
  • AP from axon release neurotransmitter from synaptic vesicle into synaptic cleft
  • Neuromuscular diffuses across synaptic cleft & stimulates receptor on post-synaptic membrane (ligand-gated ion channels)
  • Stimulates response - AP in effector cell
• Neurons all have same basic structure but vary in size & shape

  • Axon may be from 1 mm to more than 1 m (PNS)
  • Have cell body & dendrites that feed info into neuron (connect to other neurons)
  • Output thru axon hillock, along axon to terminals to other neurons or effector organs
• Myasthenia Gravis

  • Autoimmune disease - body produces antibodies to nicotin receptor
  • Impact function of muscles at neuromuscular junction (NMJ)
  • Antibodies block ability of acetylcholine to bind receptor
  • Effects eyes, face, swallowing muscles most
• Guillian-Barre Syndrome - Myelin of PNS:

  • Autoimmune NS disease where immune system attack the myelin of the PNS (Schwann cells)
  • Characteristics: tingling hands & feet, progressing weakness of limbs & respiratory muscles
  • Effects on autonomic NS lead to altered HR & BP
  • Cause unknown
• Afferent = input

  • Sensory
  • Visceral (body sensing internal state of our organs)
  • Feeds up to brain (CNS) to be integrated
• Non-myelinated nerves are slower to conduct an AP