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11      

Fundamentals  of  the  Nervous  System  and  Nervous  Tissue:  Part  A  

Func?ons  of  the  Nervous  System  

1.  Sensory  input  –  Informa?on  gathered  by  sensory  receptors  about  

internal  and  external  changes    

2.  Integra?on  –  Interpreta?on  of  sensory  input  

3.  Motor  output  –  Ac?va?on  of  effector  organs  (muscles  and  

glands)  produces  a  response  

Figure 11.1

Sensory input

Motor output

Integration

Divisions  of  the  Nervous  System  

•  Central  nervous  system  (CNS)    – Brain  and  spinal  cord  –  Integra?on  and  command  center    

•  Peripheral  nervous  system  (PNS)  – Paired  spinal  and  cranial  nerves  carry  messages  to  and  from  the  CNS  

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Peripheral  Nervous  System  (PNS)  

•  Two  func?onal  divisions  1.  Sensory  (afferent)  division  

•  Soma?c  afferent  fibers—convey  impulses  from  skin,  skeletal  muscles,  and  joints    

•  Visceral  afferent  fibers—convey  impulses  from  visceral  organs    

2.  Motor  (efferent)  division    •  Transmits  impulses  from  the  CNS  to  effector  organs  

Motor  Division  of  PNS  

1.  Soma?c  (voluntary)  nervous  system  –  Conscious  control  of  skeletal  muscles  

Motor  Division  of  PNS  

2.  Autonomic  (involuntary)  nervous  system  (ANS)  

–  Visceral  motor  nerve  fibers  –  Regulates  smooth  muscle,  cardiac  muscle,  and  

glands  –  Two  func?onal  subdivisions  

•  Sympathe?c  •  Parasympathe?c  

Figure 11.2

Central nervous system (CNS) Brain and spinal cord Integrative and control centers

Peripheral nervous system (PNS) Cranial nerves and spinal nerves Communication lines between the CNS and the rest of the body

Parasympathetic division

Conserves energy Promotes house- keeping functions during rest

Motor (efferent) division Motor nerve fibers Conducts impulses from the CNS to effectors (muscles and glands)

Sensory (afferent) division Somatic and visceral sensory nerve fibers Conducts impulses from receptors to the CNS

Somatic nervous system

Somatic motor (voluntary) Conducts impulses from the CNS to skeletal muscles

Sympathetic division Mobilizes body systems during activity

Autonomic nervous system (ANS)

Visceral motor (involuntary) Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands

Structure Function Sensory (afferent) division of PNS Motor (efferent) division of PNS

Somatic sensory fiber

Visceral sensory fiber

Motor fiber of somatic nervous system

Skin

Stomach Skeletal muscle

Heart

Bladder Parasympathetic motor fiber of ANS

Sympathetic motor fiber of ANS

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Histology  of  Nervous  Tissue  

•  Two  principal  cell  types  1.  ______________________—excitable  cells  that  

transmit  electrical  signals  

Histology  of  Nervous  Tissue  

2.  ______________________  (glial  cells)—suppor?ng  cells:  •  Astrocytes  (CNS)  •  Microglia  (CNS)  •  Ependymal  cells  (CNS)  •  Oligodendrocytes  (CNS)  •  Satellite  cells  (PNS)  •  Schwann  cells  (PNS)  

______________________  

•  Most  abundant,  versa?le,  and  highly  branched  glial  cells  

•  Cling  to  neurons,  synap?c  endings,  and  capillaries  

•  ______________________  and  brace  neurons  

Astrocytes  

•  Help  determine  capillary  permeability  •  Guide  migra?on  of  young  neurons  •  Control  the  chemical  environment  •  Par?cipate  in  informa?on  processing  in  the  brain  

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Figure 11.3a

(a) Astrocytes are the most abundant CNS neuroglia.

Capillary

Neuron

Astrocyte

______________________  

•  Small,  ovoid  cells  with  thorny  processes  •  Migrate  toward  injured  neurons  •  Phagocy?ze  microorganisms  and  neuronal  debris  

Figure 11.3b

(b) Microglial cells are defensive cells in the CNS.

Neuron

Microglial cell

______________________  Cells  

•  Range  in  shape  from  squamous  to  columnar  •  May  be  ______________________  •  Line  the  central  cavi?es  of  the  brain  and  spinal  column  – Separate  the  CNS  inters??al  fluid  from  the  cerebrospinal  fluid  in  the  cavi?es  

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Figure 11.3c

Brain or spinal cord tissue

Ependymal cells

Fluid-filled cavity

(c) Ependymal cells line cerebrospinal fluid-filled cavities.

______________________  

•  Branched  cells  •  Processes  wrap  CNS  nerve  fibers,  forming  insula?ng  myelin  sheaths    

Figure 11.3d

(d) Oligodendrocytes have processes that form myelin sheaths around CNS nerve fibers.

Nerve fibers

Myelin sheath

Process of oligodendrocyte

______________________  Cells  and  ______________________  Cells    

•  Satellite  cells  – Surround  neuron  cell  bodies  in  the  PNS  

•  Schwann  cells  (neurolemmocytes)  – Surround  peripheral  nerve  fibers  and  form  myelin  sheaths  

– Vital  to  regenera?on  of  damaged  peripheral  nerve  fibers  

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Figure 11.3e

(e) Satellite cells and Schwann cells (which form myelin) surround neurons in the PNS.

Schwann cells (forming myelin sheath)

Cell body of neuron Satellite cells

Nerve fiber

______________________    (Nerve  Cells)  

•  Special  characteris?cs:  – Long-­‐lived  (→  100  years  or  more)  – Amito?c—with  few  excep?ons  – High  metabolic  rate—depends  on  con?nuous  supply  of  oxygen  and  glucose  

– Plasma  membrane  func?ons  in:  •  Electrical  signaling    •  Cell-­‐to-­‐cell  interac?ons  during  development  

 Cell  Body  (Perikaryon  or  Soma)  

•  Biosynthe?c  ______________________  of  a  neuron  

•  Spherical  nucleus  with  nucleolus  •  Well-­‐developed  ______________________  •  Rough  ER  called  Nissl  bodies  (chromatophilic  substance)  

 Cell  Body  (Perikaryon  or  Soma)  

•  Network  of  neurofibrils  (neurofilaments)    •  _________  _____________—cone-­‐shaped  area  from  which  axon  arises  

•  Clusters  of  cell  bodies  are  called  nuclei  in  the  CNS,  ganglia  in  the  PNS  

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Figure 11.4b

Dendrites (receptive regions)

Cell body (biosynthetic center and receptive region)

Nucleolus

Nucleus Nissl bodies

Axon (impulse generating and conducting region)

Axon hillock Neurilemma

Terminal branches

Node of Ranvier Impulse direction

Schwann cell (one inter- node)

Axon terminals (secretory region)

(b)

Processes  

•  ______________________  and  axons  •  Bundles  of  processes  are  called    

– Tracts  in  the  CNS  – Nerves  in  the  PNS  

Dendrites  

•  Short,  tapering,  and  diffusely  branched    •  ______________________  (input)  region  of  a  neuron  

•  Convey  electrical  signals  toward  the  cell  body  as  graded  poten?als    

The  Axon  •  One  axon  per  cell  arising  from  the  axon  hillock  •  Long  axons  (nerve  fibers)  

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The  Axon  

•  Numerous  ______________________  branches  (telodendria)  

•  Knoblike  axon  terminals  (synap?c  knobs  or  boutons)    – ______________________  region  of  neuron  – Release  neurotransmifers  to  excite  or  inhibit  other  cells  

Axons:  Func?on  

•  ______________________  region  of  a  neuron  •  Generates  and  transmits  nerve  impulses  (ac?on  poten?als)  away  from  the  cell  body  

Axons:  Func?on  

•  Molecules  and  organelles  are  moved  along  axons  by  motor  molecules  in  two  direc?ons:  – ______________________—toward  axonal  terminal    

•  Examples:  mitochondria,  membrane  components,  enzymes  

– ______________________—toward  the  cell  body    •  Examples:  organelles  to  be  degraded,  signal  molecules,  viruses,  and  bacterial  toxins  

Figure 11.4b

Dendrites (receptive regions)

Cell body (biosynthetic center and receptive region)

Nucleolus

Nucleus Nissl bodies

Axon (impulse generating and conducting region)

Axon hillock Neurilemma

Terminal branches

Node of Ranvier Impulse direction

Schwann cell (one inter- node)

Axon terminals (secretory region)

(b)

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Myelin  Sheath  

•  Segmented  protein-­‐lipoid  sheath  around  most  long  or  large-­‐diameter  axons  

•  It  func?ons  to:  – Protect  and  electrically  ______________________  the  axon  

–  Increase  ______________________  of  nerve  impulse  transmission  

Myelin  Sheaths  in  the  PNS    

•  ______________________  cells  wraps  many  ?mes  around  the  axon    – Myelin  sheath—concentric  layers  of  Schwann  cell  membrane    

•  Neurilemma—peripheral  bulge  of  Schwann  cell  cytoplasm  

Myelin  Sheaths  in  the  PNS    

•  ______________________  of  Ranvier    – Myelin  sheath  gaps  between  adjacent  Schwann  cells  

– Sites  where  axon  collaterals  can  emerge  

Figure 11.5a

(a) Myelination of a nerve fiber (axon)

Schwann cell cytoplasm Axon

Neurilemma Myelin sheath

Schwann cell nucleus

Schwann cell plasma membrane

1

2

3

A Schwann cell envelopes an axon.

The Schwann cell then rotates around the axon, wrapping its plasma membrane loosely around it in successive layers.

The Schwann cell cytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath.

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Unmyelinated  Axons  

•  Thin  nerve  fibers  are  ______________________  

•  One  Schwann  cell  may  incompletely  enclose  15  or  more  unmyelinated  axons  

Myelin  Sheaths  in  the  CNS  

•  Formed  by  processes  of  ______________________,  not  the  whole  cells  

•  Nodes  of  Ranvier  are  present  •  No  neurilemma  •  Thinnest  fibers  are  ______________________    

Figure 11.3d

(d) Oligodendrocytes have processes that form myelin sheaths around CNS nerve fibers.

Nerve fibers

Myelin sheath

Process of oligodendrocyte

White  Mafer  and  Gray  Mafer  

•  ______________________  mafer  – Dense  collec?ons  of  myelinated  fibers    

•  ______________________  mafer  – Mostly  neuron  cell  bodies  and  unmyelinated  fibers  

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Structural  Classifica?on  of  Neurons  

•  Three  types:  1.  ______________________—1  axon  and  several  

dendrites  •  Most  abundant  •  Motor  neurons  and  interneurons  

2.  ______________________—1  axon  and  1  dendrite  •  Rare,  e.g.,  re?nal  neurons  

Structural  Classifica?on  of  Neurons  

3.  ______________________  (pseudounipolar)—single,  short  process  that  has  two  branches:  

•  Peripheral  process—more  distal  branch,  oien  associated  with  a  sensory  receptor  

•  Central  process—branch  entering  the  CNS  

Table 11.1 (1 of 3) Table 11.1 (2 of 3)

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Func?onal  Classifica?on  of  Neurons  

•  Three  types:    1.  ______________________  (afferent)  

•  Transmit  impulses  from  sensory  receptors  toward  the  CNS  

2.  ______________________  (efferent)  •  Carry  impulses  from  the  CNS  to  effectors  

Func?onal  Classifica?on  of  Neurons  

3.  ______________________  (associa?on  neurons)  •  Shufle  signals  through  CNS  pathways;  most  are  

en?rely  within  the  CNS  

Table 11.1 (3 of 3)