7. Nervous System: Introduction, Neurons, Neuroglia

neurons

cell of nervous system

• electrical excitability

  • can respond to stimulus

  • transmit propagating electrical signals (APs) along length (axon)

• high metabolic rate

  • requires continuous supply of O2 + glucose

• long-lived

• cannot undergo mitotic divisions

neuroglia

cell of nervous system

• support cells

  • no electrical excitability

• retains ability to replicate

• CNS

  • astrocytes

  • microglia

  • ependymal cells

  • eoligodendrocytes

• PNS

  • satellite cells

  • Schwann cells

central nervous system (CNS)

• brain

• spinal cord

peripheral nervous system (PNS)

• cranial nerves

• spinal nerves

• nerve plexuses

• peripheral nerves

functional organization of nervous system

neurons transmit in different directions

• sensory → CNS

• CNS = integrates info → motor neurons → motor components

afferent

neurons going towards CNS

• sensory neurons

efferent

neurons going away from CNS

• motor neurons

somatic nervous system (SNS)

conscious nervous system

• voluntary

  • targets skeletal muscles

    • muscles you can control

autonomic nervous system (ANS)

unconscious nervous system

• involuntary → sympathetic + parasympathetic

  • targets smooth/cardiac muscles + glands

    • muscles you cannot control

  • can target enteric motor neurons

    • muscles of GI tract

enteric nervous system (ENS)

subdivision of ANS

• targets muscles of digestive system

  • sympathetic + parasympathetic

    • smooth/cardiac muscle + glands

  • ____

    • smooth muscle, glands, endocrin cells of GI tract

neuron cell body

• has typical cell organelles

• has short branches processes = dendrites

• usually organized in discrete collections:

  • PNS → ganglia

  • CNS → nuclei

dendrites

short branched processes on cell bodies of neurons

• receive signals from neighbouring neurons/sensory receptors

  • determine if action potential is initiated at initial segment

axon terminals

end of axon that meets dendrite of next neuron

• enlarged distal end = synaptic end bulbs

synaptic end bulbs

enlarged distal ends of axon

• contain synaptic vesicles → store NTs

  • NTs released into synaptic cleft → bind to receptors on another cell

    • excite/inhibit neuron/myofiber/gland cell

neural synapse

point of contact b/w pre-synaptic + post-synaptic neuron

nerves

PNS axons

• contain many axons packaged within connective tissue layers

  • epineurium = entire nerve

  • perineurium = fasicle of neurons

  • endoneurium = individual neurons

• can be:

  • sensory

  • motor

  • mixed

epineurium

connective tissue packaging surrounding entire nerve

perineurium

connective tissue packaging surrounding fascicle of neuron

endoneurium

connective tissue packaging surrounding individual neurons

tracts

CNS axons

• bundles of axons transmitting similar info within CNS = white matter

  • white matter

    • primarily myelinated axons

  • gray matter

    • primarily neuron cell bodies

white matter

tracts of myelinated axons in CNS

• spinal cord → outside layer

• brain → inside layer

gray matter

primarily neuron cell bodies

• spinal cord → inside layer

• brain → outside layer

multipolar neurons

(1) several dendrites + 1 axon

• most neurons of brain + spinal cord

  • ie. interneurons

• motor neurons

  • ie. efferent neurons

bipolar neurons

(2) 1 main dendrite + 1 main axon

• neurons involved in special senses

  • eye, inner ear, olfactory area

pseudounipolar neurons

(3) short single process leaving cell body → dendrite + axon continuous

• sensory neurons in PNS

  • ie. afferent neuron

neuron interaction

astrocyte

CNS neuroglia

• strength + support

• BBB

microglia

CNS neuroglia

• phagocytes

ependymal cell

CNS neuroglia

• production of cerebrospinal fluid

oligodendrocytes

CNS neuroglia

• myelination of axons in CNS

  • possess processes responsible for myelin sheath formation

  • 1 can myelinate multiple axons

satellite cells

PNS neuroglia

• regulates extracellular environment of PNS neurons

  • similar to astrocytes in CNS

Schwann cells

PNS neuroglia

• myelinating = myelination of axons in PNS

• unmyelinating = nutritional support → nerve development + regeneration

• 
  

myelination

multilayers lipid + protein covering = myelin sheath

• myelinated axons → has myelin sheath

• unmyelinated axons → no myelin sheath

• made by 2 types of neuroglia

  • CNS = oligodendrocytes

  • PNS = Schwann cells

• 
  

myelin sheath

multilayered lipid + protein covering

• electrical insulation for axon = ⬆ speed of AP propagation → saltatory conduction

nodes of Ranvier

gaps in myelin sheath along axon

multiple sclerosis

autoimmune inflammatory demyelinating disease of CNS

• formation of scars/lesions (plaques)

• sensory symptoms in limbs/face

  • vision loss

  • motor weakness

  • gait + balance problems

myelinating Schwann cells

wraps around PNS axons to form myelin sheath

• 1 Schwann cell myelinates 1 axon

  • given axon can have multiple Schwann cells contributing to myelin sheath along its length

unmyelinating Schwann cells

encases unmyelinated neurons + hold them in place

• forms remak bundles

  • unmyelinated Schwann cell + unmyelinated axon

• provides nutritional support for neurons

• role in nerve development + regeneration

neuron plasticity

plasticity maintained throughout life

• sprouting of new dendrites

• modifying proteins synthesized

• altering synaptic contacts w/ other neurons

neuron regeneration

• PNS = limited capacity

• CNS = not possible for most part

PNS neuron regeneration and repair

0. damage to peripheral nerve axon

1. distal to injury site → degeneration of axon + myelin via macrophages

2. Schwann cells proliferate along path of original axon

   1. macrophages move into area → remove degenerating debris distal to injury site

3. regeneration of axon into injury site + distally → along path created by Schwann cells

   1. Schwann cells help guide axon regrowth

4. axon elongates → Schwann cells wrap around

   1. axon re-establishes normal synaptic contacts = normal function regained

   2. axon stops growing/wanders off = normal function will not return

ganglia

neuron cell bodies in PNS

nuclei

neuron cell bodies in CNS