Nervous Tissue

Question 1

Functions

Answer 1

• controls and integrates all body activities (along with endocrine system)
• 3 basic functions:
  
  1. Sensory: sensing changes with receptors
  2. integration: interpreting and remembering those changes
  3. motor: reaction to those changes with effectors
     - muscular contractions
     - glandular secretions

Question 2

Divisions of Nervous System

Answer 2

1. Central nervous system: brain and spinal cord
2. Peripheral nervous system: cranial and spinal nerves that contain sensory and motor fibres. Connect CNS to muscles, glands, and all sensory receptors

Question 3

Subdivisions of the PNS

Answer 3

• somatic (voluntary NS
• sensory info from sensory receptors to CNS
• motor neurons to skeletal muscles
• autonomic (involuntary) nervous system
• sensory neurons from visceral organs to CNS
• motor neurons to smooth & cardiac muscle and glands
• divided into sympathetic & parasympathetic & enteric division (neurons in gut)

Question 4

Histology of Nervous Tissue

Answer 4

• consists of 2 main cell types
  
  • neurons
  • neuroglia

Question 5

Neuroglial Cells

Answer 5

• half the volume of CNS
• smaller cells than neurons
• 50x more numerous
• cells can divide
  
  • rapid mitosis in tumour formation (gliomas)
• 4 cells types in CNS
  
  • astrocytes, oligiodendrocytes, microglia, ependydymal
• 2 cell types in PNS
  
  • schwann and satellite cells

Question 6

Astrocytes

Answer 6

• star shaped cells
• form blood-brain barrier by covering blood capillaries
• metabolize neurotransmitters
• regular K+ balance
• provide structural support

Question 7

Oligiodendrocytes

Answer 7

• each forms myelin sheath around more than one axon in CNS

- analogous to Schwann cells of PNS

Question 8

Microglia

Answer 8

• small cells found near blood vessels
• phagocytic role: clear away dead cells
• derived from cells that also gave rise to macrophages and monocytes

Question 9

Ependymal Cells

Answer 9

• form epithelial membrane lining cerebral cavities and central canal of the spinal cord
• produces cerebrospinal fluid (CSF)

Question 10

Satellite Cells

Answer 10

• flat cells surrounding neuronal cell bodies in peripheral ganglia
• support neurons in PNS ganglia

Question 11

Shwann Cells

Answer 11

• cells encircling PNS axons

- each cell produces part of the myelin sheath surrounding an axon on the PNS

Question 12

Neurons

Answer 12

• functional unit of nervous system
• have capacity to produce action potential (electrical excitability)
• cell body is a single nucleons with prominent nucleolus
• cell body contains nissle bodies (chromatophilic substance)
  
  • rough ER and free ribosomes for protein synthesis
• neurofilaments give cell shape and support
• microtubules move material inside cell
• cell processes: dendrites and axons

Question 13

Dendrites

Answer 13

• conduct impulses toward the cell body (receptive zone)
• typically short, highly branched and unmyelinated
• surfaces specialized for contact with other neurons
• contains neurofibromas and Nissan bodies

Question 14

Axons

Answer 14

• conducts impulses away from cell body (conductive zone)
• long, thin cylindrical process of cell
• arises at axon hillock
• impulses arise from initial segment (trigger zone)
• side branches (collaterals) end in fine processes called axon terminals
• swollen tips called synaptic end bulbs contain vesicles filled with neurotransmitters

Question 15

Axon coverings in PNS

Answer 15

• axons surrounded by a lipid & protein covering produced by Schwann cells
• myelinated axons appear white (jelly-roll like wrapping made of lipoprotein myelin) acts as electrical insulator and speeds up conduction of nerve impulses
• regular spaces or gaps in myelin sheath called nodes of ranvier
• unmyelinated fibres: slow, small diameter fibres
  
  • surround glial cell processes but no myelin sheath wrapping
  • gray and white matter

Question 16

Myelination in PNS

Answer 16

• Schwann Cells myelinate axons in the PNS during fetal development
• shwann cell cytoplasm and nucleus forms outermost part with inner portion being myelin sheath
• tube guides growing axons that are being repaired themselves

Question 17

Parts of synapse

Answer 17

• presynaptic neuron
  
  • synaptic end bulb
  • synaptic vesicle
  • presynaptic membrane
• synaptic cleft
• postsynaptic neuron
  
  • postsynaptic membrane
  • neurotransmitter receptor sites

Question 18

Transmission of Nerve Impulses

Answer 18

• neuronal excitation and inhibition
  
  • axon terminals form synapses on dendrites and cell body (excitatory and inhibitory neurotransmitters)
• Nerve impulse conduction along axon
  
  • summation takes place at initial segment on axon hillock (concentration of Na ion channels)
• Nerve impulse conduction along axon
  
  • at threshold Na channels open and nerve impulse conducted down axon
  • myelinated axon, Na channels open at nodes of ranvier, Salvatore conduction
  • unmyelinated axon, Na channels open on adjacent patch of membrane, slower, continuous conduction

Question 19

Structural classification of neurons

Answer 19

• based on numbers of processes found on cell body
• multipolar: several dendrites and one axon (most common)
• bipolar: one main dendrite and one axon (found in retina, inner ear, and olfactory epithelium)
• unipolar: one process only develops from bipolar (always sensory neurons - dorsal root and ganglia)

Question 20

Functional classification of neurons

Answer 20

• sensory (affrent) neurons: transport sensory information from skin/muscles/joints/sense organs & viscera to CNS)
• motor (efferent): send motor nerve impulses to muscles and glands)
• interneurons (association) and projection neurons: connect neuron to neuron
  
  • 90% of neurons in the body

Question 21

Neuronal Circuits

Answer 21

• diverging: single cell stimulates many others (amplifying the signal)
• converging: one cell stimulated by many others
• reverberating: impulses from later cells repeatedly stimulate early cells in the circuit (short term memory)
• parallel-after-discharge: single cell stimulated a group of cells that all stimulate a common postsynaptic cell (precise activities such as math calculations)

Question 22

Regeneration and Repair

Answer 22

• plasticity maintained throughout life
  
  • sprouting of new dendrites
  • synthesis of proteins
  • changes in synaptic contacts with other neurons
• limited ability for regeneration (repair)
  
  • PNS can repair damaged dendrites or axons
  • CNS no repairs are possible

Question 23

Repair within PNS

Answer 23

• axons and dendrites may be repairs if:
  
  • neuron cell body intact
  • Schwann cells remain active and form a tube
  • scar tissue does not form too rapidly

Question 24

Repair within the PNS

Answer 24

• chromatolysis
  
  • 24-48 hours after injury, nissle bodies break up into fine granular masses
• by 3-5 days
  
  • Waller Ian degeneration occurs (breakdown of axon and myelin sheath distal to injury)
  • retrograde degeneration occurs back one node
• within several months, regeneration occurs
  
  • neuroglia on each side of injury repairs tube (Schwann cell mitosis)
  • atonal buds grow down the tube to reconnect (1.5mm per day)