Anatomy & Physiology Lecture Outline: Week #6

• The Nervous System -- Ch.12

LSc 107 Anatomy and Physiology Spring 1999

Thibodeau Ch 12: Nervous Tissue

Systems which control and communicate -- Nervous and Endocrine

Nervous System

• unifies body functions so they work together
• detects changes in internal and external environments
• evaluates info
• initiates appropriate response

Overview of the Nervous System - Fig. 12-1

General Organization

Central Nervous System (CNS)

• brain
• spinal cord

Peripheral Nervous System (PNS)

• 12 cranial nerves
• 31 pairs of spinal nerves
• outer nerves

Organized by function Fig.12-7:

Afferent (refers to incoming sensory, transmits to brain or spinal cord)

Efferent (refers to outgoing motor, transmits away from CNS to glands or muscles)

Interneurons or Association neurons (in CNS connecting afferent and efferent)

Organized by organs innervated:

Somatic (SNS) : to skeletal muscles

• motor: info to effector skeletal muscles
• sensory: feedback to somatic integration center of CNS

Autonomic (ANS) : to smooth muscle, heart, glands, blood vessels

• sympathetic (efferent) mediate immediate threat "fight or flight"
• parasympathetic (efferent) coordinate normal resting activities "rest and repair"
• visceral: sensory feedback to ANS integration center in CNS

 

Cell Types of the Nervous System

Neurons- nerve cells- do the work of conducting impulses (~100 billion)

Neuroglial cells - ~10X the number of neurons (900 billion) - supporting function

Neuroglial cells - Fig. 12-3

Functions

• Schwann cells: give fatty sheath to neurons of CNS and PNS
• Astrocytes: form the blood-brain barrier (Box 12-1)
• Microglia: phagocytosis
• Ependymal: lining the cavities of the brain and spinal cord
• Oligodendrocytes: physical support for neurons (also produce myelin)

"brain tumor" is actually a tumor of neuroglial cells since neurons can not divide

Neurons

Structure - Fig. 12-4

• cell body, nucleus, other cellular organelles
• axon: conduct impulses away from cell body, can be myelinated, distal tip synaptic knob
• dendrites: conduct impulses to cell body, distal tip has receptors

Different structures of neurons - Fig. 12-6

• multipolar: most common; one axon, several dendrites
• bipolar: least common; one axon, one dendrite
• unipolar: usually sensory; one process from cell body divides into central and peripheral fibers

 

Reflex arc (feedback loop)

• afferent neuron
• interneuron (integration center)
• efferent neuron

Examples Fig. 12-8 spinal reflexes

ipsilateral: same side, contralateral: opposite side

can involve more than one segment of the spinal cord

examples of spinal reflexes: adjust postural muscles in response to sensory information from joints, muscles, etc.

Neurons (nerve fibers) and nerve tracts (bundles of nerve fibers in CNS) Fig. 12-9

 

White matter and grey matter in the nervous system

• White matter - myelinated fibers in bundles some appear white to the naked eye, interior (medulla) of the brain is composed of white nerve fibers
• Grey matter - unmyelinated fibers and cell bodies, exterior of the brain (cortex) is composed of cell bodies which appear grey

Collections of cell bodies within interior of brain appear as grey patches: "nuclei". In PNS grey patches are ganglia

 

Repair of nerve fibers

• If the cell body is damaged (by trauma, toxins , ischemia etc.) then the cell dies, whether CNS or PNS, and NO regeneration takes place
• peripheral nerve can regenerate a new axon IF the cell body is intact, and IF the nerve sheath provides a guide for growth, and IF there is no scar tissue blocking the way; Slow process, about 1 mm/day on average
• CNS neurons do not regenerate.
• Nerve cells do not reproduce themselves, incapable of cell division

 

Nerve Impulses

• Neurons initiate and conduct electrical impulses
• Polarized membranes have a difference in electrical charge across a membrane because cells maintain difference in ion concentration across their membranes
• Resting Membrane Potential : Na⁺ / K⁺ pump creates unequal distribution of ions on either side of the cell membrane - most K⁺ is inside, and most Na⁺ is outside. Net effect: difference of 70mV between the inside of the cell (lower) and the outside (higher)
• The resting membrane potential of -70 mV continues as long as Na⁺ / K⁺ pump operates, and cell membrane permeability is not altered

 

Action potential Fig 12-16 and Table 12-1

• something happens which alters permeability of the cell membrane (the application of an electrical stimulus)
• sodium channels: protein molecules in the cell membrane change shape on the application of electricity change allowing Na⁺ ions to rush into the cell
• depolarization: a change in the electrical balance across the cell, the interior becomes more positive than exterior (+30mV)
• first change in electrical charge is slow, cell may not continue from this, if it reaches a threshold, then depolarization proceeds rapidly (the cell "fires")
• repolarization: K⁺ channels also open, allowing K⁺ to come out of the cell reestablishes an electrical balance similar to the resting potential (-70mV)
• K⁺ channels close, and the Na⁺ / K⁺ pump restores original ionic distribution

 

Refractory Period (Fig. 12-17)

• absolute RP: time during which the neuron can not be stimulated again
• relative RP: time during which the neuron can be stimulated but only by a very strong stimulus

 

Conduction along a neuron

• each action potential of each part of the neuron cell membrane acts as a stimulus for the next section
• the change of electrical charge spreads along the membrane, electrical impulse or "message"
• impulse spreads in both directions from the original point of stimulation
• impulse can not spread back to an area it just left , one-way movement of action potentials along axons, because of refractory period

All or none

• If the original stimulus does not change the charge of the cell enough to make it "fire" (to raise the potential to the threshold potential) nothing happens
• If the stimulus is enough to reach threshold, then the cell will continue to rapidly depolarize, and "fire"

Saltatory conduction Fig. 12-19

• Nerves with myelin sheath do not conduct along entire surface of membrane
• conduct only in unmyelinated nodes (Ranvier) which "show through" between the cells providing the myelin
• impulse skips from node to node, traveling faster than an unmyelinated nerve

Speed of conduction in nerves

• Depends on: presence of myelin sheath and size of nerve fiber
• Fastest: large and myelinated (skeletal muscle ~300 mph)
• Slowest: small unmyelinated (sensory receptors in skin ~1 mph)

The synapse: junction, can be electrical (between cells) or chemical (Fig. 12-20)

Chemical (use neurotransmitters):

• junction between a nerve axon ending and another nerve
• junction between a nerve axon ending and a gland
• junction between a nerve axon ending and a muscle

Nerve impulse, which has been electrical , now becomes chemically transmitted

Effect - one way traffic

Events at the synapse - Fig. 12-21, 12-22

• electrical impulse (action potential) reaches synaptic knob, causes Ca⁺⁺ to rush into the presynaptic cell
• Ca⁺⁺ triggers release of neurotransmitter chemicals from storage vesicles
• neurotransmitters diffuse across the synaptic cleft, bind specific receptors on the other side (postsynaptic membrane PSM)

One of two effects:

• it can build up a potential in PSM until cell "fires" (excittatory postsynaptic potential)

OR

• it can have the opposite effect, inhibiting the potential, making it less likely to fire (inhibitory postsynaptic potential, or hyperpolarization)

How neurons link together

• Connected in networks or "pools"
• Each neuron has thousands of connections with other neurons
• Some neuron axons bring excitatory messages
• Some neuron axons bring inhibitory messages
• The neuron either fires or does not fire (all or none) according to the sum total of the messages it receives in a given time

Neurotransmitters Table 12-2

• chemical messengers which travel across the synaptic gap
• many different compounds throughout the body with this function
• classified according to function or chemical structure
• neurotransmitters action is terminated when it releases from the receptor very quickly, and is either reabsorbed by the presynaptic cell and recycled, or broken down by enzymes in the synaptic gap

 

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