chapter 18 - nervous tissue

• human nervous system: a physically connected network of cells, tissue, and organs that allow us to communicate with and react to the environment and perform life activities
• organ system: collection of organs working together for a common function
• organs of nervous system:

  1. brain
  2. spinal chord
  3. nerves
• the nervous system is broken into:

  1. CNS
  2. brain
  3. spinal cord
  4. integrating and control center
  5. PNS
  6. nerves
  7. cranial nerves (12)
  8. spinal nerves and their branches (31)
  9. spinal cord
  10. ganglia
  11. neuron cell bodies
• three primary functions

  1. sensory input: gather information
  2. integration: interpreting the input
  3. motor output: activates effector organs; causes a response
  4. muscles and glands
• central nervous system: filters and integrates sensory coordinates appropriate responses
• somatic sensory division nerve ending in finger detects heat --> sends an afferent signal to the CNS via spiral nerves --> CNS filters and integrates signals & coordinates appropriate response --> efferent signal sent to effector organs via the somatic motor division --> skeletal muscle in hands contract to pull finger away from heat source
• PNS is broken into:

  1. sensory (afferent) division
  2. somatic sensory division: carries general sensory stimuli from muscles, bones, joints, and the skin as well as special sensory stimuli
  3. visceral sensory division: carries stimuli from organs
  4. motor (efferent) division
  5. somatic motor division: carries stimuli to skeletal muscles
  6. autonomic nervous system (ANS): carries stimuli to smooth muscle, cardiac muscle, and glands
  7. sympathetic division
  8. parasympathetic division
• nervous tisse

  • 80% of tissue = cells
  • 20% of tissue = extracellular matrix
  • ground substance and glycoproteins
  • 10% of cells= neurons
  • 90% of cells = neuroglia
• neurons: range from 1mm to 1m in length most are amiotic long lived, excitable cells
• cell body of neuron

  • contains nucleus & maintains cytoplasm
  • produces all proteins required for nerve signaling
  • numerous mitochondria
  • nissl bodies (dark staining associations of ribosomes and rough ER)
  • golgi apparatus
  • no centrioles
  • cannot undergo mitosis
  • neurofibrils
  • intermediate filament cytoskeleton
• dendrites

  • cell body and dendrites make up main receptive region and conveys signals to the axon body
  • numerous, short, highly branched processes
  • large surface area for receiving signals
  • generate local potentials but not action potentials
  • local potentials stimulated by signals from other neurons or sensory receptors
• axons

  • processes than generate & conduct action potentials (conduct signals)
  • action potentials are turned into chemical signals at synaptic knobs (secretion)
  • range from micrometers to < meter in length
  • axon plasma membrane = axolemma
  • often wrapped in myelin sheath
  • always only 1 (if present)
• axon structure:

  • hillock: narrowing of body to axon
  • collaterals: branches off of the axon
  • terminal branches: 10,000+ branches at end of axon
• function of neuron processes:

  • generates and transmits nerve impulses
  • along axolemma
  • initiated at trigger zone
  • junction of hillock and axon
  • conducted to terminals
  • triggers release fo neurotransmitters to be released
• transport of neuron processes:

  • anterograde: away from cell body
  • retrograde: towards cell body
  • uses ATP-dependent motor proteins
  • kinesin or dynenin
• myelin sheath of neuron process

  • protein-lipid
  • extension of glial plasma membrane
  • acts as insulation and protection
  • increases transmission speed
• PNS has schwann cells that wrap around axon and gaps between cells are called nodes of ranvier
• CNS has oligodendrocytes that can wrap around many axons at once and white and gray matter

  • white matter: myelinated cell bodies
  • gray matter: unmyelinated cell bodies
• axonal transport

  1. slow axonal transport
  2. anterograde only
  3. 1-3 mm / day
  4. cytoskeletal and other proteins
  5. slow because it is "stop & go"
  6. fast axonal transport
  7. anterograde & retrograde
  8. 200-400 mm / day
  9. vesicles containing substances (e.g. acetylcholine) & membrane bound organelles
• poliovirus, herpes simplex virus and tetanus toxin also use retrograde transport to infect the CNS
• neuron classification

  • multipolar (most common)
  • 3+ processes
  • one axon, many dendrites
  • bipolar (rare)
  • 2 processes
  • one axon, one dendrite
  • found in the retina
  • unipolar
  • 1 process
  • "T" shaped
  • distal peripheral process
  • sensory receptor
  • central process enters the CNS
  • receptive endings instead of dendrites
  • found primarily in ganglia of PNS
• multipolar neurons

  1. spinal motor neuron: cell body at one end and long axon
  2. pyramidal cell: cell body in middle with axons on both sides
  3. purkinje cell: cell body in middle, dendrites on one side and oxon on the other side
• functional classification of neurons

  1. sensory neurons
  2. transmit impulses from receptors to CN
  3. unipolar, mostly
  4. cell bodies found outside the CNS
  5. motor neurons
  6. carry impulses away from the CNS
  7. cell bodies located in the CNS
  8. interneurons
  9. connect sensory and motor neurons
  10. mostly confined to the CNS
  11. primarily multipolar
• the cell bodies and axons of neurons in common signaling pathways tend to group together

  • CNS: nuclei --> CNS: tracts
  • PNS: ganglia --> PNS: nerves
• neuroglia: hold neurons together, maintain extracellular fluid, assist neural function, repair damaged tissue
• glial cell types

  1. CNS
  2. astrocytes: anchor neurons & blood vessels in place; maintain extracellular environment; assist in blood-brain barrier formation; repair damaged tissue; most abundant
  3. oligodendrocytes: flattened end processes form myelin sheaths around some CNS axons
  4. microglial cells: act like macrophages in CNS tissue; monitor neuron cell health
  5. ependymal cells: line brain ventricles; produce cerebrospinal fluid with cilia; range in shape
  6. PNS
  7. schwann cells: wrap around thicker nerve fibers to form myelin sheath
  8. satellite cells: surround & support cell bodies; similar to astrocytes
• nerve cells

  • live long
  • mostly occur in the CNS
  • amitotic: lose their ability to divide
  • high metabolic rate
  • need much energy and oxygen
• myelin sheaths

  • electrical insulators: numerous phospholipid bilayer
  • CNS
  • no neurolemma
  • nucleus & organelles in centralized location outside of wrapping
  • myelination begins after birth
  • PNS
  • outer layer forms a neurolemma
  • containing the nucleus & organelles
  • myelination begins in early fetal period
• small axons in CNS and PNS are usually unmyelinated
• PNS myelin

  • all PNS nerve cells are associated with Schwann cells
  • for most, Schwann cells form myelin sheaths
  • for some, multiple axons are bundled in Schwann cells like a multi-dog hot dog (unmyelinated)
• axon repair

  • CNS
  • astrocytes form scar tissue
  • PNS
  • axons regeneration is possible as long as cell body & part of myelinated axon is present
• neurons, once formed, cannot undergo mitosis (lose their centrioles); amitotic
• voltage (V): measure of potential energy by separate electrical charges
• volts (V): 1000 mV = 1 V
• current (I): flow of charge from one point to another
• resistance (R): hindrance to flow of electrical charge

  • insulators: high resistance
  • conductors: low resistance
• Ohm's law: current (I) = voltage (V) / resistance (R)

  • greater voltage = greater current
  • less resistance = greater current
• ion channels in membrane potential

  • leak channels
  • always open
  • chemically gated/ligand gated
  • open when the appropriate chemical binds
  • e.g. acetylcholine on ligand gated sodium channels on motor end plate
  • voltage gated
  • open and close in response to changes in membrane potential
  • e.g. voltage gated sodium channels in sarcolemma and axolemma
  • mechanically gated
  • open by physical deformation
  • channels that release water under extreme osmotic pressure
  • ions diffuse down their concentration gradient
  • ions move toward areas of opposite electrical charge
  • electrical gradient
• Na+/K+ pump

  • 3 Na+ out, 2 K+ in