Unit 4: Functional Organization of Nervous Tissue

• •Sensory – recognize changes in environment (stimuli) 
• •Integration – analyze sensory information, stores information, makes decisions
• •Motor – initiates impulses to effectors (muscles or glands)
• •Maintaining homeostasis
• •Establishing & maintaining mental activity 

• •Central Nervous System (CNS) 
•     –brain and spinal cord
• •Peripheral Nervous System (PNS)
•    -Sensory (afferent) division
•    –Motor (efferent) division
• •Somatic Nervous System
• •Autonomic Nervous System (ANS)
•    –Sympathetic division
•    –Parasympathetic division
•    –Enteric NS

• •Neurons = nerve cells 
• •Neuroglia = glial cells = nonneural cells 

• Neurons (Nerve Cells) 
• •most are amitotic (no cell division) 
• •high metabolic rates (aerobic respiration) 
• •long-lived 
• •produce impulses to transfer information 
• •communicate with each other at synapses 
•    –junction of nerve cell with another cell

• Consists of: - Cell body 
• - Dendrites 
• - Axons 

• Cell body (perikaryon) 
• •contains nucleus, mitochondria, neurofilaments,  & microtubules 
• •rich in ribosomes 
• •extensive rough ER (Nissl bodies) & Golgi apparatus 
•    –protein synthesis & export to axon or dendrite 
• •no myelin 

• Dendrites 
• •short, highly branched with dendritic spines (extensions)
• •bring depolarization (message) toward cell body
• •no myelin

• Axons
• •take action potential (impulse) away from cell body
• •myelinated or unmyelinated
• •contain trigger zone (axon hillock & initial segment)
• •form presynaptic terminals

• Classification: 
• •Structural– number of processes extending from cell body
•    –multipolar
•    –bipolar
•    –unipolar

• •Functional – type and direction of information
•    –sensory
•    –motor
•    –interneurons

• Multipolar neurons 
• •most abundant - ~99% of all neurons
• •single axon & many dendrites 
•    –may be myelinated or unmyelinated 
• •most neuron in CNS (interneurons) & motor neurons 

• Bipolar neurons 
• •two processes: one axon & one dendrite
• •sensory 
• •in sensory organs (retina of eye & olfactory mucosa)

• Pseudounipolar neurons 
• •single process extending from cell body
•   –divides into 2 branches: 
•     •to CNS 
•     •to periphery – dendritelike sensory receptors (conduct action potential toward cell body) receptive
•     •sensory neurons in ganglia of PNS

• •Bipolar cells are commonly: 
• a. motor neurons
• b. called neuroglia
• c. found in ganglia
• d. found in the retina of the eye

• •Sensory: 
•    –most unipolar or bipolar 
•    –afferent (brings sensory info to CNS) 
• •Interneurons (Association): 
•    –most multipolar 
•    –integration between sensory & motor in CNS 
• •Motor: 
•    –most multipolar 
•    –efferent (goes toward effector)

• •more numerous than neurons 
• •support and protect neurons 
• •Neuroglia of CNS 
•    –astrocytes 
•    –oligodendrocytes 
•    –ependymocytes (ependymal cells) 
•    –microglia 
• **Tumors due to abnormal divisions of glial cells of CNS 

• •Neuroglia of PNS
•    –satellite cells  
•    –Schwann cells (neurolemmocytes)

• Astrocytes - “star cells” 
• •most abundant
• •form foot processes that cover surfaces of blood vessels, neurons & pia mater (blood-brain barrier)  
•    –control composition of interstitial fluid (regulate ions & gases)
• •may influence synaptic sygnaling by secreting/ removing neurotransmitters (essential for learning &  memory)

• •Microglia 
•    –specialized macrophages 
•    –phagocytize necrotic tissue & foreign substances 
•    –*cells of immune system can’t gain access to CNS

• •Ependymal cells 
•    –epithelial lining of brain ventricles and central canal of spinal cord
•    –choroid plexus = ependymal cells + blood vessels
• •produce CSF (cerebrospinal fluid)

• •Oligodendrocytes 
•    –form cytoplasmic extensions that surround axons (produce myelin sheath)
•    –single oligodendrocyte may form myelin sheath around portions of several axons

• •Satellite cells 
•    –surround cell bodies in sensory ganglia
•    –provide support & nutrients to cell bodies
•    –protect neurons from heavy-metal poisons (mercury, lead)

• •Schwann cells (neurolemmocytes) 
•    –form myelin sheaths around larger nerve fibers 
•    –play role in regeneration of nerve fibers 
•    –may be useful to treat damaged regions of spinal cord 
•       •Schwann cell transplants are tried in lab experiments

•Predict the effect on the part of a severed axon that’s no longer connected to its neuron cell body. 

•Explain

•Distal portion --> detached from cell body --> no access to enzymes & proteins (for repair) -->axon degenerates & dies 

•Proximal portion --> attached to cell body --> nucleus & new proteins available --> remains alive -->may grow & replace severed distal axon

• •myelin protects & electrically insulates axons from one another à more rapid spread of action potential 
• • formed by:
•    –oligodendrocytes in CNS
•    –Schwann cells in PNS – guide neuron regeneration

Multiple sclerosis – destruction of myelin sheath in CNS --> diminishes impulse conduction

•What cells of the nervous system might be affected?

• Myelinated axons 
• •white appearance (lipids & proteins)
• •nodes of Ranvier = gaps between sheath cells
• •allow faster speed of impulse conduction (less energy required)

• Unmyelinated axons
• •axons rest in invaginations of oligodenrocytes or Schwann cells
•    –plasma membrane surrounds but not wraps around axon many times

cell body - if cell body is damaged à cell dies, neurons “downstream” from damaged neuron may die as well 

• peripheral axons (PNS)
• •portion of axon distal to site of damage is degraded within 1 week
• •neurolemma usually remains intact (depends on severity of damage)
• •axon regrows at about 1-5 mm per day
• •when two ends of injured axon are not aligned in close proximity --> regeneration is unlikely

1.axon & its myelin sheath distal to site of injury disintegrate 

2.macrophages enter area to phagocytize debris & release mitosis-stimulating chemicals

• 3.Schwann cells:
• •proliferate in response to mitosis-stimulating chemicals & nerve growth factor (NGF)
• •form column of cells which axon follows during regeneration
• •Schwann cells that form cord will eventually remyelinate regenerated axon

• •limited & poor compared to nerves of PNS
•  
• •oligodendrocytes 
•    –cell body a distance from the axons they myelinate 
•    –fewer oligodenrocytes than Schwann cells 
•    –myelin sheaths (oligodendrocytes) of nearby axons secrete inhibitory proteins or die 

•nearby astrocytes (reactive astrocytes) may proliferate to form wall around the injury --> scar-forming astrocytes limit regeneration

•CNS macrophages - phagocytize debris more slowly than peripheral macrophages do

• •in experiments 
•    –macrophages transplanted to CNS can secrete proteins that inhibit inhibitory proteins
•    –ALS - mice or rat models are being used to replace damaged motor neurons with stem cells

•Nerve: bundle of neuron fibers in PNS 

•Tract: bundle of neuron fibers in CNS

•Ganglion (ganglia): cluster of cell bodies in PNS

•Nucleus (nuclei): cluster of cell bodies in CNS

• •White matter: myelinated neuron fibers in CNS
•    –outside in spinal cord; inside in brain

• •Gray matter: cell bodies, dendrites, and unmyelinated neuron fibers in CNS
•    –inside in spinal cord, outside in brain

• •Cranial nerves – originate from brain 
•    –12 pairs 

• •Spinal nerves – originate from spinal cord
•    –31 pairs 
•    –Sensory nerves - carry afferent fibers only
•    –Motor nerves - carry efferent fibers only
•    –Mixed nerves - carry both kinds of fibers

1.The oligodendrocytes can myelinate several axons. 

TRUE/FALSE

2.In the neuron, the rough ER is also known as Nissl bodies.

TRUE/FALSE

3. After axonal injury, regeneration in peripheral nerves is guided by:

• a.Oligodendrocytes
• b.Schwann cells
• c.Dendrites
• d.Golgi organs

•Multiple sclerosis (MS) is a disease in which the myelin sheaths are destroyed. 

–With what process does this interfere and what would be the consequence?

Demyelination results in interference with salutatory conduction (requires Nodes of Ranvier) which would result in a slowing down of nerve impulse propagation.