Control Systems of the Body

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The nervous system is just one of the control systems in the body.  The endocrine system is the other.   Of the two, the nervous system is by far the more rapid acting & complex.

Nervous cells communicate by means of electrochemical signals, which are rapid & specific, usually causing almost immediate responses.  It involves ions like Na+ (sodium) and K+ (potassium) crossing the membrane of neurons.   An action potential passes down the nerve axon and then is transmitted across the synapse to the next neuron by neurotransmitters, which are a chemical signal.  The neurotransmitter acetylcholine is widespread throughout the CNS & PNS.

 In contrast, the endocrine system typically brings about its effects in a more ‘leisurely’ way through the activity of hormones released into the blood that act on tissues.  A single hormone can alter the metabolic activity of many tissues simultaneously.  Chapter 7 discusses the endocrine system more thoroughly. 

[ Some kinds of signals, like the ones for muscle position, travel on extra-fast nerve fibers at speeds of up to 390ft/sec (119m/sec).  Close your eyes & wave your arms around—you can tell where they are at every moment because the muscle-position Nn are very fast.  Other messages, as in some kinds of pain signals, travel much more slowly.  If you stub your toe, you feel the pressure right away because touch signals travel at 250ft/sec.  But you won't feel the pain for another 2-3 seconds, because pain signals generally travel at only 2ft/sec. ]

Nervous System Function

1.   Processes & reacts to external & internal stimuli, i.e., the outside world & internal body systems. 

[ Encephalization or brain development in humans is highly developed; in fact it is the most complex object known to science so far.  A neuron may process information from 100,000 different sources, and there are estimated to be ‘billyuns & billyuns’ of neurons. ]

·   It uses its millions of sensory receptors scattered throughout the body (skin, organs, glands, etc) to monitor changes occurring both inside and outside the body.  This gathered information is called sensory input.

·   Sensory receptors are specialized to respond to environmental changes (stimuli).  Most sensory receptors consist of modified dendrites or dendritic end organs of sensory neurons. The general sensory receptors include the simple receptors for pain, touch, pressure, and temperature found in the skin, those found in skeletal muscles and tendons that monitor stretch and position, and in the visceral organs.  Complex sense organs serve the special senses (vision, equilibrium, hearing, smell, and taste).

2.   It processes and interprets the sensory input and makes “decision” about what should be done at each moment, a process called integration.  Thus it acts as the major control center for body functions.

3.   It affects a response by activating muscles, organs, or glands: the response is called motor output.

EXAMPLE:Let’s say you are driving down Johnson Street.  You are late for class already and you can’t find a place to park.  Just before you reach the first stop light at the cross walk, you see it turn yellow (sensory input).  What do you do?  Your nervous system integrates this information (red light means “stop” ), and your foot goes for the brake (motor output).

Can you still diagram the General Organization of the Nervous System?:

NERVOUS TISSUE

Nervous tissue is specialized for the conduction of impulses.  There are two types of nerve cells, neurons and neuroglia.  Neurons are specialized to conduct signals/impulses, and neuroglial cells support & maintain the neurons    [ glia = glue ].  Neuroglia separate & protect the neurons, provide a supportive framework for neural tissue, act as phagocytes [ phago = eat ], & help regulate the composition of the interstitial fluid.  One type of neuroglial cell produces cerebrospinal fluid, which supports, physically protects, chemically protects, & nourishes the CNS inside & out.  The csf is the medium of exchange of nutrients & wastes between the CNS cells & the blood.  The cerebrospinal fluid also must maintain a strict ionic composition for optimal neuron signaling.  Physically, the nerve tissue floats in cerebrospinal fluid & the fluid acts as a shock absorber.  What’s the blood-brain barrier??

[ The working class of glial nerve tissue outnumbers the elite neuron class around 10:1 and accounts for about half the volume of the nervous system. ]

A typical neuron contains a cell body with a nucleus & root-like processes called dendrites [ = branch ].  Dendrites conduct an impulse toward the cell body.  From the body, a long slender process called the axon conducts the impulse away.  At the end of the axon are the axon terminals, which transfer the impulse to another cell, gland, or neuron.  Myelin is expanded cell membrane (which is 80% lipid) that wraps around an axon, providing electrical insulation (“plastic around telephone wires”), and these myelinated Nn have an increased speed of action potential propagation along an axon.

Do you remember the structure of a typical neuron?  Diagram a Typical Neuron here for review.

[ If cancer occurs originally in the brain it is because the neuorglial cells are dividing uncontrollably.  Neurons are rarely cancerous because they are what are called fixed post mitotic cells.  They don’t ever divide again, so if you lose them you’ll never get them back…thus the severity of brain and spinal cord damage. ]

[Neuroglia of the PNS - neuron cell bodies & axons are completely wrapped in glial cells

1. Schwann cells (aka neurilemmal cells)  - sheath axons of the PNS; some produce myelin sheaths (which are formed from part of the Schwann cell). Schwann cells form 1mm long “beads” along myelinated axons, with gaps in between called nodes of Ranvier. This node/internode construction speeds up the nerve impulse. 

2. Satellite cells [ aka amphicytes ]  - surround the neuron cell bodies in peripheral ganglia.  ]

[ Demyelination is the progressive destruction of myelin sheaths in the CNS & PNS.  The result is a gradual loss of sensation & motor control that leaves affected regions numb & paralyzed.  Possible causes include heavy metal poisoning, diphtheria, multiple sclerosis, Guillain-Barré syndrome, or starvation.  Once myelin is stripped from the nerve, the body cannot replace it. ]

  [Nerve Impulse/Action potential]

1. the resting neuron has a membrane potential; the cell membrane is polarized due to high concentrations of sodium ions inside & potassium ions outside   [ Na-K pump maintains a 3:2 ratio—costs a lot of ATP energy; what’s rigor mortis? ]

2. stimulate the membrane & its permeability increases

3. ions move from the area of high concentration to low concentration, and there is a local depolarization of the membrane

4. if the stimulation is strong enough, depolarization of the adjacent membrane occurs, which leads to a wave of depolarization, or a “domino effect” of an electrical potential, or rather, an electrical impulse

5. the myelin insulators cause depolarization to jump over them from node to node and the impulse “skips down the axon,” resulting in faster impulses   [ up to about 390ft/sec ]

Functional Classification of Neurons: neurons are classified by what they do.

1.       Afferent sensory neuron - these neurons carry nerve impulses from receptors to the CNS  [ ad + fero = to lead towards, as in adduct ]

2.       Efferent motor neuron - carry nerve impulses away from the CNS to effectors [ ex + fero = to lead away, as in exit ]

3.       Interneurons - connect neuron to neuron & are only found in the CNS.  Interneurons are responsible for the distribution of sensory information & coordination of motor activity.  The more complex the response to a given stimulus, the greater the number of interneurons involved.

Do the neurons above sound familiar?  If they are repeated again here….they must be important.

SYNAPSES

Neurons are separated by a gap, a synapse, which is the small space between two neurons or the space between a neuron and a muscle cell, gland, or organ.  In a typical synapse between two neurons the neuron before the synapse is called the presynaptic neuron and the neuron after the synapse is called the postsynaptic neuron. 

A nerve impulse causes a release of neurotransmitters (a chemical signaler) into the synapse at the end of the axon, and the neurotransmitters stimulate the postsynaptic neuron. Thus the signal is transmitted along its path to wherever it’s going.   Neurotransmitters (and there are many different types in the body), such as acetylcholine, are then broken down into non-reactive parts (e.g., acetate & choline) by enzymes (e.g. acetycholinesterase). The non-reactive parts are picked up by the presynaptic neuron and put back together again.  Neurotransmitters are stored in vesicles until they are needed again.  Neurotransmitters can stimulate or inhibit.  Acetylcholine is the usual neurotransmitter to stimulate muscle contraction.  There are many others.  The brain uses some like dopamine, serotonin, melatonin, etc.  Almost all synapses are triggered chemically, but there are a few that are electrical in which the cells are directly connected. [ e.g., in some centers of the brain, including the vestibular nuclei, and are found in the eye, and in at least one pair of PNS ganglia (the ciliary ganglia) ].   It does take time for the neurotransmitters to diffuse across the synapse (called synaptic delay), the delay last only fractions of a second (1/1000th or less to be precise) but signal conduction times are still extremely rapid reaching speeds of up to 300 mph. [ Flea collars have an inhibiting compound that breaks down acetylcholine, and some nerve gases work in the same manner.  What would happen if you  could not  remove acetylcholine from it’s receptor sites? ]

Diagram a typical synapse and a myoneural junction (aka neuromuscular junction)….These must be important; here they are again!!!!

REFLEXES   are automatic, fast responses of the nervous system to stimuli and serve as the basic functional unit of the nervous system.

Diagram: A typical Reflex Arc

1.  Monosynaptic reflex arc

2.  Polysynaptic Reflex Arc

A. Spinal Reflex Arcs  - automatic response system that consists of a receptor, afferent neuron, synapse, efferent neuron, & effector (such as a muscle or gland) that involves the spinal cord.  Arcs may include more than one synapse.

1. stretch reflex arc – automatic response in which a muscle stretch receptor runs to spinal cord, synapses with a motor neuron that twitches a muscle; a good example is the knee jerk when the doctor hits your patellar tendon with his Thor-like (Terence)  hammer.

2. flexor reflex arc – automatic response that contracts flexor Mm to pull the body part away from the stimulus; a good example is when you place your hand on a hot stove and jerk it away.

3. crossed extensor reflex arc - at the same time as the flexor reflex, the automatic response of the extensor reflex happens on the opposite side of the body; extensors contract to ward off the stimulus; for example, you place your left manus on the stove burner, and without conscious thought, your left arm flexes to pull the searing manus away from the heat (flexor reflex), and at the same time your right arm extends to ward off & protect yourself from the maniacal stove. Another example is when you step on a nail with your right foot.  The right leg flexes to pull your foot off the painful stimulus, and at the same time your left leg extends to help you keep your balance.

[ Bruce Lee was so fast that they actually had to SLOW the film down so you could see his moves.  In general, women have faster reflexes than men.  ]

4. tendon reflex arc - inhibits muscular action to reduce tendon tension to prevent damage.

B. Cranial Reflex Arcs - automatic response system that consists of a receptor, afferent neuron, synapse, efferent neuron, & effector (such as a muscle or gland) that involves the brain  Arcs may include more than one synapse and in these instances the appropriate response may need to be determined after several inputs have been evaluated; hence, integrative function of the central nervous system is required. Some examples follow:

1. pupillary reflex = automatic response that controls light entering the eye through the pupil

2. corneal reflex arc = automatic response that blinks the eyelid when the cornea is touched

3. accommodation reflex = automatic response that changes the shape of the lens when focusing on objects at various distances

4. labyrinthine reflex arc = automatic response to maintain balance.

CENTRAL NERVOUS SYSTEM aka CNS - is composed of the brain & spinal cord, which integrate, process, & coordinate sensory data & motor commands.  The expanded & specialized anterior nerve cord, or the brain, is the seat of higher functions, such as memory, learning, & emotion.   What are the made of?  Both are composed of white matter and gray matter.  White matter is collections of  myelinated axons.  Gray matter is collections of unmeylinated axons and neuron cell bodies.  Some general CNS terms.

center - collection of neuron cell bodies with a common function.  A center with a discrete anatomical boundary is a nucleus.

neural cortex - a thick layer of gray matter covering portions of the brain surface.  The term higher centers refers to the most complex integration centers, nuclei, and cortical areas of the brain.         

tract - white matter of the CNS contains bundles of axons that share common origins, destinations, and functions.  Tracts in the spinal cord form larger groups called columns.         

pathways - centers & tracts that link the brain with the rest of the body.  For example, sensory pathways distribute information from peripheral receptors to processing centers in the brain, & motor pathways begin at CNS centers concerned with motor control and end at the effectors they control.   

PERIPHERAL NERVOUS SYSTEM aka PNS – sensory receptors, nerves, & ganglia, i.e., all nerve tissue outside the CNS

ganglia (ganglion is singular)   - masses of neuron cell bodies

nerves, aka peripheral nerves - bundles of neuron axons that carry sensory information & motor commands in the PNS;  12 pairs of cranial Nn, 31 pairs of spinal Nn, 1 pair of sympathetic trunks

plexus (pl. plexi) – a braided group of nerve branches, namely braids of ventral rami of spinal Nn:

Cervical Plexus: phrenic N (C3-5) & others;  Brachial Plexus: many branches we do later;

Lumbar Plexus: obturator N, femoral N, & others;

Sacral Plexus: sciatic N, sup. gluteal N, inf. gluteal N., & others

[In some tissues, neural stem cells persist throughout life.  Their divisions produce daughter cells that differentiate into highly specialized neurons, such as olfactory receptors.  However, stem cells are very rare inside the CNS, and CNS neurons generally lose their centrioles during differentiation.  As a result, typical CNS neurons cannot divide and will not be replaced if lost to injury or disease :(     Though nerve repair is minimal to none, there are cases where it’s possible to learn new motor pathways, bypassing damaged ones, and even though the brain is highly specialized, it does retain some plasiticity, being able to learn new pathways for some tasks.  Growth hormone treatment, genetic manipulation, & embryonic tissue implantation are areas of nerve repair science. ]

Functional Parts of Nervous System

SOMATIC (VOLUNTARY) NERVOUS SYSTEM - consists of sensory Nn from extremities, body wall, etc., & motor nerves to skeletal Mm

AUTONOMIC (INVOLUNTARY) NERVOUS SYSTEM - consists of sensory nerves from viscera & motor nerves to glands, smooth (involuntary) Mm,  & cardiac M. 2 types of autonomic nerves: sympathetic - for “fight or flight” responses & parasympathetic - for relaxed, normal operations, or rather, vegetative functions   ||||

The  ANS provides the homeostatic adjustments in physiological systems regardless of our state of consciousness. [ A person suffering brain damage can survive for years in a state of coma, because the ANS coordinates functions of cardiovascular, respiratory, digestive, excretory, & reproductive systems.  In doing so, the ANS adjusts internal water, electrolyte, nutrient, & dissolved gas concentrations in body fluids—and it does so without instructions or interference from the conscious mind. ]

Usually the sympathetic division & parasympathetic division have opposing effects; if the sympathetic division causes excitation, the parasympathetic causes inhibition. But this is not always the case, because:

- the 2 divisions may work independently, with some structures innervated by only one division or the other

- the 2 divisions may work together, each controlling one stage of a complex process.

Sympathetic subdivision of the ans

The sympathetic division prepares the body for heightened levels of somatic activity. When fully activated, this division produces what is known as the "fight, fright, or flight" response, which prepares the body for a crisis that may require sudden, intense physical activity.  [ Sympathetic Ginny says, “RUN, FORREST, RUN!!! ]

To understand the nature of this response, imagine walking down a long, dark alley & hearing strange noises in the darkness ahead.  Your body responds immediately, and you become more alert & aware of your surroundings.  Your metabolic rate rises quickly, up to twice the resting level.  Your digestive & urinary activities are suspended temporarily, and blood flow to your skeletal muscles increases.  You begin breathing more quickly & deeply.  Both your heart rate & blood pressure increase, circulating your blood more rapidly.  You feel warm & begin to perspire.  Why do many people enjoy scary or gory movies?

The general pattern of sympathetic innervation:

1. heightened mental alertness

2. increased metabolic rate

3. reduced digestive & urinary function

4. activation of energy reserves

5. increased respiratory rate & dilation of respiratory passageways

6. increased heart rate & blood pressure

7. activation of sweat glands

8. dilation of pupils (for focusing on near objects)

9. heavy sexual arousal & ejaculation

In extreme fear, both systems may act simultaneously, causing involuntary micturation & defecation reflexes along with the sympathetic actions listed above.

PARASympathetic subdivision of the ans

The parasympathetic division stimulates visceral activity, innervating areas serviced by the cranial Nn & organs in the thoracic & abdominopelvic cavities.  For example, it is responsible for the state of "rest, digest, & repose" that follows a big dinner.  Your body relaxes, energy demands are minimal, and both your heart rate & blood pressure are relatively low.  Meanwhile, your digestive organs are highly stimulated.

General Fxs of the Parasympathetic Subdivision:

1. Constriction of the pupils (to restrict the amount of light for focusing on far away objects)

2. Secretion by digestive glands, including salivary glands, gastric glands, duodenal glands, intestinal glands, pancreas, & liver

3. Secretion of hormones that promote the absorption & utilization of nutrients by peripheral cells

4. Increased smooth muscle activity along the digestive tract

5. Stimulation & coordination of defecation

6. Contraction of the urinary bladder during urination

7. Constriction of the respiratory passageways

8. Reduction in heart rate & in the heart’s contraction force

9. initial sexual arousal & erection

Sympathetic vs. Parasympathetic Pathways

è  Presynaptic Sympathetic neurons (originating in CNS) release ACh at autonomic ganglion (nicotinic receptors) and Norepinephrine at the target tissue/effector (adrenergic receptors).

è  Presynaptic Parasympathetic neurons (originating in CNS) release ACh at autonomic ganglion (nicotinic receptors) and ACh at the target tissue/effector (muscarinic receptors)

è  See Fig. 11-17

Autonomic Neurotransmitters

	Sympathetic Division	Parasympathetic Division
Neurotransmitter	Norepinephrine	Acetylcholine
Synthesized (made) from	Tyrosine	Acetyl CoA + choline
Inactivated by (ENZ)	Monoamine oxidase (MAO)	Acetylcholinesterase AChE
ENZ location in	Mitochondria of varicosity	Synaptic cleft
Varicosity of reuptake	Norepinephrine	Choline

*Varicosity = swollen regions along autonomic axons that store and release neurotransmitters.  See Fig. 11-8

SYMAPTHETIC EFFECTS OF NOREPINEPHRINE (neurotransmitter) AND EPINEPHRINE (hormone)

SENSITIVITY OF PERIPHERAL ADRENERGIC RECEPTORS TO CATECHOLAMINES

Receptor	Found In	Sensitivity	Second Messenger
a1	Most sympathetic target tissue	NE > E	Activates phospolipase C
a2	Gastrointestinal tract & pancreas	NE > E	Inhibits cAMP
b1	Heart muscle, kidney	NE = E	Activates cAMP
b2	Certain blood vessels and smooth muscle of some organs	E > NE	Activates cAMP
NE = Norepinephrine   (neurotransmitter)                 E = Epinephrine (hormone from adrenal medulla)

Alpha Receptor Stimulation

Norephineprine binds to receptor à a₁à Activates phospholipase C à Release of Ca⁺² à muscle contraction & gland

   cell secretion

                                                à a₂ à Reduction in cAMP levels à smooth muscle relaxation or decrease

gland secretion

Beta Receptor Stimulation

Epinephrine binds to receptor àb₁ à Activates adenylate cyclase à Activates cAMP àStimulation of metabolism,

  cardiac muscle stimulation

                                            à b₂ à Activate adenylate cyclase à Activates cAMP àInhibition and relaxation of smooth

muscle in respiratory passageways and in blood vessels of skeletal muscles

AGONISTS AND ANTAGONISTS OF NEUTROTRANSMITTER RECEPTORS

Direct agonists and antagonists act by altering secreting, reuptake, or degradation of neurotranmitters

Receptor	Agonists (mimics)	Antagonists (blockers)	Indirect Agonists/Antagonists
Cholinergic	Acetylcholine		AChE* inhibitors: neostigmine; parathion Inhibits ACh release: botulinus toxin
Muscarinic	Muscarine	Atropine; scopolamine
Nicotinic	Nicotine	a-bungarotoxin (muscle only), tetraethylammonium (TEA) (ganglia only), curare
Adrenergic	Norepinerphrine; Epinephrine		Stimulates NE release: ephedrine, amphedimines Prevents NE uptake: cocaine
a	Pheylephrine	“alpha-blockers”
b	Isopreterenol	“beta-blockers”; propranolol (b1 and b2); metoprolol (b1 only)
*AChE = acetylcholinesterae

Where do we come from and what do we do?  Notice how cool plants are!!!

Nicotine = Alkaloid found in Tobacco plants (Nicotiana tabacum).  ACh mimic on nicotinic receptors.

Curare = Alkaloid found in plants (Strychnos toxifera & Chondrodendron tomentosum ).  ACh receptor blocker at nicotinic receptors.

Muscarine = Found in mushrooms particularly in Inocybe and Clitocybe species.  ACh mimic on muscarinic receptors

Atropine =  Alkaloid from plants (Atropa belladonna).   ACh blocker on muscarinic receptors

Scopolamine, also known as hyoscine, is an alkaloid drug obtained from plants of the Solanaceae family (Nightshade), such as henbane or jimson weed (Datura stramonium).  ACh blocker at muscarinic receptors

Alpha-bungarotoxin is derived from snake venom.  ACh blocker on nicotinic receptors.

Botulinus toxin comes from the bacteria, Clostridium botulinum).  Inhibits ACh release.  The cause of  Botulism, the most poisonous substance known.

Ephedrine is an alkaloid derived from various plants in the genus Ephedra (family Ephedraceae).

Cocaine is an alkaloid obtained from the leaves of the coca plants.  Prevents uptake of NE.

Phenylephrine hydrochloride is an α-agonist used medically to increase blood pressure, as a nasal decongestant and also to dialate the pupil.

Propranolol (Inderal®) is a non-selective beta blocker (i.e. it blocks the action of adrenalin on both β1- and β2-adrenoreceptors).

COMPARISION OF SNS & ANS

	SOMATIC	AUTONOMIC
Number of neurons in efferent pathway	1	2
Neurotransmitter/receptor at neuron-target synapse	ACh (nicotinic)	ACh (muscarinic) or NE (a or b)
Target tissue	Skeletal Muscle	Smooth and cardiac muscle; some endocrine and exocrine glands; some adipose tissue
Structure of axon terminal regions	Boutons	Boutons and varicosities
Effects on target tissue	Excitatory only:  muscle contracts	Excitatory or Inhibitory
Peripheral components found outside the CNS	Axons only	Preganglionic axons, ganglia, postganglionic neurons
Summary of function	Posture and movement	Visceral function, including movement in internal organs & secretion; control of metabolism