-
Functions
*Figure 11.2, p. 371
- •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
-
Organization
*Figure 11.1, p. 370
*Figure 11.3, p. 372
- •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
-
Review: Cells of the Nervous System
- •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
-
Review: Neurons
*Figure 11.4, p. 373
- 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
-
Review: Neuron Processes
*Figure 11.4, p. 373
- 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
-
Types of Neurons
- Classification:
- •Structural– number of processes extending from cell body
- –multipolar
- –bipolar
- –unipolar
- •Functional – type and direction of information
- –sensory
- –motor
- –interneurons
-
Structural Classification
*Figure 11.5, p. 374
- Multipolar neurons
- •most abundant - ~99% of all neurons
- •single axon & many dendrites
- –may be myelinated or unmyelinated
- •most neuron in CNS (interneurons) & motor neurons
-
Structural Classification
*Figure 11.5, p. 374
- Bipolar neurons
- •two processes: one axon & one dendrite
- •sensory
- •in sensory organs (retina of eye & olfactory mucosa)
-
Structural Classification
*Figure 11.5, p. 374
- 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
-
Practice Question
- •Bipolar cells are commonly:
- a. motor neurons
- b. called neuroglia
- c. found in ganglia
- d. found in the retina of the eye
-
Functional Classification
- •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)
-
NS Cells: Neuroglia (Glial cells)
- •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)
-
Neuroglia of CNS
*Figure 11.6, p. 375
- 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)
-
Neuroglia of CNS
*Figure 11.8, p. 375
- •Microglia
- –specialized macrophages
- –phagocytize necrotic tissue & foreign substances
- –*cells of immune system can’t gain access to CNS
-
Neuroglia of CNS
*Figure 11.7, p. 375
- •Ependymal cells
- –epithelial lining of brain ventricles and central canal of spinal cord
- –choroid plexus = ependymal cells + blood vessels
- •produce CSF (cerebrospinal fluid)
-
Neuroglia of CNS
*Figure 11.9, p. 376
- •Oligodendrocytes
- –form cytoplasmic extensions that surround axons (produce myelin sheath)
- –single oligodendrocyte may form myelin sheath around portions of several axons
-
Neuroglia of PNS
*Figure 11.10, p. 376
- •Satellite cells
- –surround cell bodies in sensory ganglia
- –provide support & nutrients to cell bodies
- –protect neurons from heavy-metal poisons (mercury, lead)
-
Neuroglia of PNS
*Figure 11.4, p. 373
*Figure 11.11, p. 376
- •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
-
Let’s apply
•Predict the effect on the part of a severed axon that’s no longer connected to its neuron cell body.
•Explain
-
Let’s apply: Answer
•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 Sheath: Importance
- •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?
-
Myelin Sheath - Importance
*Figure 11.11, p. 376
- 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
-
Regeneration of Neurons
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
-
Neuron Regeneration in PNS - Steps
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
-
Regeneration of Neurons in CNS
- •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
-
Regeneration of Neurons in CNS
- •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
-
Organization of Nervous Tissue
• 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
-
Types of Nerves
- •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
-
Practice Questions
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
-
Let’s apply
•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?
-
Let’s apply: Answer
Demyelination results in interference with salutatory conduction (requires Nodes of Ranvier) which would result in a slowing down of nerve impulse propagation.
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