UNIT 3:  HOMEOSTASIS – Nerve Signals and Homeostasis

 A. The Nervous System
·                     the nervous system, together with the endocrine system controls actions of the body

·                     endocrine effects take longer to occur, and last longer, whereas nervous system responses are faster and don’t last very long

·                     the body is constantly adjusting to maintain the internal environment within safe limits

·                     responses take place either through the actions of electrochemical messengers relayed to or from the brain, or through a series of chemical messengers, called hormones, that are carried by the blood

·                     since the hormones are produced by glands, they require more time for response than nerves require

·                     the nervous system controls memory, learning, and language

·                     regeneration of nervous tissue is limited

·                     scientists continue to look for chemical factors that both stimulate and inhibit the growth of new nerve cells

·                     one such factor that prevents growth of axons in tissue fibres is MAG (myelin-associated glycoprotein) – if this were “turned off” the regeneration (mitosis) pathway of nerve cells could occur

·                     the use of stem cells has showed some promise – these are cells that have not yet specialized into tissue cells

·                     in October of 2000, scientists announced that they had reconnected severed nerves in the spinal cords of rats using spore-like cells from the nervous system of adult rats

·                     when spore-like cells were seeded into the spinal cords of injured rats, new cells began to grow in the area of the severed cord, which helped some of the rats stand on their hind legs

 

VERTEBATE NERVOUS SYSTEM

 

·         there are two main divisions of the nervous system:

 

1. central nervous system (CNS)

§         consists of the nerves of the brain and spinal cord

§         acts as the processing or coordinating centre for incoming and outgoing information

 

2. peripheral nervous system (PNS)

§         consists of nerves that carry information between the organs of the body and the central nervous system

§         further divided into

 

a.  somatic nerves    – control skeletal muscles (striated muscles), bones and skin

                                -- conscious or voluntary control

            (i)  sensory somatic nerves -- relay information about the environment to the central nervous system

            (ii) motor somatic nerves – initiate an appropriate response

 

b.  autonomic nerves   – control internal organs of the body

                                    -- subconscious or involuntary control

                                    -- further divided into “on-off” switches:

 

            (i)  sympathetic nervous system – prepares the body for stress

            (ii) parasympathetic nervous system – restores normal balance

 

·         Figure 1, p. 412 illustrates the major divisions of the nervous system

 

1.  Anatomy of Nerve Cells

 

·         the nervous system has two types of nerve cells:

 

1.        glial cells (also known as neuroglial cells)

 

§         non-conducting cells that have a structural support and metabolic function

 

2.         and neurons -- functional units of the nervous system

                                -- organized into three groups:

 

                                    a.  sensory neurons (also known as receptors) – see Figure 2, p. 413 for an illustration its structure

 

o        also known as afferent neurons

o        sense and relay information from the environment to the CNS for processing

o        types include – thermoreceptors, chemoreceptors, mechanoreceptors

o        located in clusters called ganglia

 

                                        b.  interneurons (also known as association neurons) – see Figure 2, p. 413 for its structure

 

o        link neurons within the body

o        found mostly throughout the brain and spinal cord

o        integrate and interpret the sensory information and connect neurons to outgoing motor neurons

 

                                    c.  motor neurons (also known as efferent neurons) – see Figure 2, p. 413 for its structure

 

o        relay information to the effectors – muscles, organs, glands

 

·         all neurons contain:

 

1.  dendrites  –  the part of a neuron that receives information, either from sensory receptors, as in the case of sensory

                            neurons, or from other nerve cells, as in the case of motor neurons

                        -- conduct nerve impulses toward the cell body

 

2.  cell body   -- the portion of the neuron that contains the nucleus and the majority of the cytoplasm

 

2.  axon          -- an extension of the cytoplasm

                        -- projects nerve impulses from the cell body

                        -- extremely thin -- approx. 100 axons can be placed inside the shaft of a single human hair (Figure 3, p. 414)

                        -- carries the nerve impulse toward other neurons or to effectors

                        -- covered with a white coat of fatty protein called myelin sheath – insulation for the neurons to prevent the

                            loss of charged ions from the nerve cell

                        -- the myelin sheath is formed by special glial cells called Schwann cells

                        -- the areas between the sections of myelin sheath are known as the nodes of Ranvier

                        -- nerve impulses jump from node to node, which speeds up the conduction process

                        -- impulses move much faster along myelinated nerve fibres than they do along non-myelinated nerve fibres

                        -- the smaller the diameter of the axon, the faster the speed of the nerve impulse

 

·         all nerve fibres within the PNS contain a thin membrane called neurilemma – which surrounds the axon

·         nerves within the brain that contain myelinated fibres and a neurilemma are called white matter

·         the neurilemma promotes the regeneration of damaged axons

·         not all nerve cells contain neurilemma and a myelin sheath – nerve cells within the brain and spinal cord are examples of this

·         these kinds of unmyelinated nerve cells are called grey matter – they don’t regenerate after injury, therefore, damage to such cells is permanent

 

2.  Neural Circuits

 

·         when you touch something hot, the immediate response is pulling away

·         the response occurs before the brain gets the message

·         this is called a reflex arc (Figure 6, p. 416)

·         reflexes are involuntary – the sensory neuron passes the impulse on to an interneuron, which, in turn, relays the impulse to a motor neuron

·         the motor neuron cause the muscles in the hand to contract and the hand pulls away

·         the reason that the brain is not involved in the coordination or processing is because it would take too long for an effect to take place, that could result in worse damage

 

·                     the importance of the nervous system:

 

Structure	Function
neuron	·                     nerve cell that conducts nerve impulses
sensory neuron (afferent neuron)	·                                   carries impulses to the central nervous system
interneuron	·                                   carries impulses within the central nervous system
motor neuron (efferent neuron)	·                                   carries impulses from the central nervous system to effectors
dendrite	·                                    projection of cytoplasm that carries impulses toward the cell body
axon	·                                    extension of cytoplasm that carries nerve impulses away from the cell body
myelin sheath	·                                    insulated covering over the axon of a nerve cell ·                                    composed of Schwann cells
nodes of Ranvier	·                                    regularly occurring gaps between sections of myelin sheath along the axon where nerve cells are transmitted
neurilemma	·                     delicate membrane that surrounds the axon of some nerve cells
reflex arc	·                                                                                                                     neural circuit that travels through the spinal cord ·                                                                                                                     provides a framework for a reflex action

 

Homework:       1-6, p. 417

 

B. The Impulse

 

• in the late 18^th century, Galvani realized that the leg muscle of a frog could be stimulated to contract with an electric shock
• although his finding that it was that the muscle itself which produced “animal electricity” was incorrect, it sparked research in this area that ultimately lead to the understanding that nerve impulses were basically electric waves that traveled throughout the body
• this concept lead to the development of such technologies as the ECG (electrocardiogram), used to monitor heart contractions, and the EEG (electroencephalogram), used to monitor brain-wave activity
• there are differences between current in wire and those that travel through nerves:

 

1. current in a wire travels much faster
2. the cytoplasmic core of a nerve cell offers great resistance to the movement of electric current
3. electric currents diminish as they move through a conductor
4. nerve impulses remain as strong at the end of the transmission as they were at the beginning of the impulse
5. nerves use cellular energy to generate current, whereas the electric conductor relies on an external force to push the current through the conductor

 

• Figure 2, p. 418 shows how nerve impulses are electrochemical messages created by movement of ions through the nerve cell
• when a miniature electrode is placed inside the giant axon of a squid, a rapid change in the electrical potential difference across the membrane was detected every time the nerve became excited
• the resting membrane normally had a potential near – 70 mV and registered + 40 mV when the nerve became excited
• the voltage difference across a nerve cell membrane during the resting stage is called the resting potential
• the reversal of potential is described as an action potential – the voltage difference across a nerve cell membrane when the nerve is excited
• this action potential did not last very long – only a few milliseconds before the potential returned to - 70 mV
• the membranes become charged due to the movement of positive ions – not negative ions
• an unequal concentration of positive ions across the nerve cell membrane establishes the gradient
• potassium is high inside, and sodium is high outside – this creates a tendency for each to move down the gradient
• as potassium diffuses out, sodium diffuses in
• Figure 4, p. 420 illustrates the process of depolarization:

 

1. the relative rates of diffusion of each is unequal – the resting membrane is 50 times more permeable to potassium than it is to sodium -- this means that more potassium ions diffuse out of the nerve cell than sodium ions diffuse into the nerve cell (Figure 3, p. 419) -- the result is that the nerve cell loses a greater number of positive ions than it gains, making the exterior of the membrane positive relative to the interior
2. ion gates (protein channels) in the membrane of nerve cells control the movement ions across the cell membrane, where excess positive ions accumulate along the outside of the nerve membrane, while excess negative ions accumulate along the inside of the membrane -- the resting membrane is charged and is called a polarized membrane
3. the separation of an electrical charge has the potential to do work, which is measured in millivolts (mV) -- the potential of a resting neuron is – 70 mV, which indicates the difference between the number of positive charges found on the inside of the nerve membrane relative to the outside….. – 90 mV on the inside of the nerve membrane would indicate even fewer positive ions inside the membrane relative to the outside
4. when the nerve is excited because of a stimulus, the cell membrane becomes more permeable to sodium than potassium (see Figure 4, p. 420) – sodium gates open, while potassium gates close
5. sodium ions rush into the nerve cell because of the electrochemical gradient -- this rapid influx of sodium ions causes a charge reversal known as a depolarization
6. once the charge on the inside becomes positive, the sodium gates close, stopping the influx of sodium
7. a sodium-potassium pump, located in the cell membrane, restores the condition of the resting membrane by transporting sodium ions out of the neuron, while moving potassium ions inside the neuron, in a ratio of 3 Na⁺ to 2 K⁺ ions (see Figure 5, p. 420) – the energy comes from ATP, and the process of restoring the original polarity of the nerve membrane is called repolarization

 

• until the condition of the resting membrane potential is restored, nerves cannot be activated again
• the nerve must repolarize before the next action potential can be conducted
• the time required for the nerve cell to become repolarized is called the refractory period, which usually lasts 1 to 10 ms

 

 

I.  MOVEMENT OF THE ACTION POTENTIAL

 

·         the movement of sodium ions into the nerve cell causes the depolarization of the membrane

·         this signals an action potential in that area, but in order for this impulse to be conducted along the axon, it must move from the point of depolarization to adjacent regions (see Figure 6, p. 421)

·         when the nerve gets excited, the difference across the membrane changes at that region causing an action potential

·         in the region of the action potential two things happen: (see Figure 7, p. 421)

 

1. the positively charged sodium ions are attracted to the adjacent negative ions, which are aligned along the inside of the nerve membrane
2. the positively charged sodium ions of the resting membrane are attracted to the negative charge that has accumulated along the outside of the membrane in the area of the action potential

 

·         the flow of positively charged ions from the area of the action potential toward the adjacent regions of the resting membrane causes a depolarization in the adjoining area

·         this creates an electric disturbance, which causes adjacent sodium channels to open

·         the result is a wave of action potential that moves along the cell membrane

·         the wave of action potential and depolarization is followed by a wave of repolarization

 

II.  THRESHOLD LEVELS AND THE ALL-OR-NONE RESPONSE

 

·         nerve cells respond to changes in pH, changes in pressure, and to specific chemicals

·         mild electric shock is most often used to stimulate nerve cells since it can be intensified and easily controlled

·         Figure 8, p. 422 illustrates an experiment that Mr. Laudari did in university

·         the experiment shows that the stimulus must be above a certain voltage in order to produce a response in the muscle

·         the critical level is know as the threshold level

·         the data shows that increasing the intensity of the stimuli above the critical threshold value will not produce an increased response – the intensity of the nerve impulse and speed of transmission are constant

·         the results indicate an all-or-none response – neurons either fire maximally or not at all

·         even though responses to stimuli are all or none, the brain is still able to recognize the differences between a two stimuli of different intensities

·         the body has neurons of various threshold levels

·         the more intense the stimulus, the greater the number of nerve cells stimulated, thus the frequency of impulses increases

·         for example, a glass rod at 50°C causes more neurons to fire than a glass rod at 40°C

·         Figure 9, p. 422 shows how the 50 °C rod stimulates more neurons than the 40°C rod

·         the greater the number of impulses reaching the brain, the greater the intensity of the response

 

III.  SYNAPTIC TRANSMISSION

 

·         the small spaces between neurons, or between neurons and effectors, are known as synapses

·         Figure 10 a, p. 423 shows how nerves can branch out to one another (in 3-D)

·         at the end of the axon is an end plate – it is here that small vesicles containing chemicals called neurotransmitters are released (see Figure 10 b, p. 423)

·         neurotransmitters are released from the presynaptic neuron and diffuse across the synaptic cleft, creating a depolarization of the dendrites of the postsynaptic neuron

·         as nerve impulses travel from neuron to neuron, they slow down at the synaptic cleft – the more synapses, the slower the conduction

·         this explains why reflex arcs are significantly faster than regular motor responses

·         acetylcholine is a typical neurotransmitter found in the end plates of many nerve cells

·         Figure 11, p. 424 shows how acetylcholine acts as an excitatory neurotransmitter on many postsynaptic neurons by opening the sodium ion channels, which in turn, causes depolarization in the postsynaptic neuron

·         after depolarization has occurred, to make the neuron repolarize, and to stop it from being permeable to sodium ions, the enzyme cholinesterase is released from the postsynaptic membrane to destroy acetylcholine

·         once acetylcholine is destroyed, the sodium channels are closed, and the neuron begins its recovery phase

·         the reason why insecticides are very effective is because they block cholinesterase – the heart of an insect continues to contract and never relaxes

·         some drugs that treat myasthenia gravis, a disease of progressive fatigue and muscle weakness caused by the impaired transmission of nerve impulses, prevent cholinesterase from working

·         not all neurotransmitters are excitatory – some make the postsynaptic membrane more permeable to potassium

·         this causes even more sodium to move out of the neuron, which increases the number of positive ions outside the cell relative to the number found inside the cell

·         when this happens, the neurons are said to be hyperpolarized because their resting potential is even more negative than – 70 mV

·         a hyperpolarized membrane means that more sodium channels than normal need to be opened in order for depolarization to be achieved

·         such inhibitory neurotransmitters prevent postsynaptic neurons from becoming active

·         sometimes, a postsynaptic neuron requires more than one presynaptic neurons to depolarize it

·         Figure 12, p. 424 shows how the excitation of neuron D requires both neuron A and neuron B to send out neurotransmitters

·         this way, a sufficient amount of neurotransmitter is released to cause depolarization of neuron D

·         this principle is called summation

·         Figure 12 shows that when neuron C fires, neuron D becomes hyperpolarized

·         other neurotransmitters, other than acetylcholine, are present

·         serotonin, dopamine, gamma-aminobutyric acid (GABA), and glutamic acid are all neurotransmitters

·         norepinephrine is excitatory in the peripheral nervous system, but is both inhibitory and excitatory in the central nervous system

·         the interaction of excitatory and inhibitory neurotransmitters is what allows you to perform various physical acts involving your  tricep and bicep muscles – when the bicep contracts, the tricep relaxes, because the bicep receives excitatory neurotransmitters, and the tricep receives inhibitory neurotransmitters

·         inhibitory impulses of the CNS are just as important as excitatory impulses, because they help you prioritize information

·         for example, you don’t respond to every single stimulus in the room, instead, you focus your attention onto those things that are necessary for that particular moment

·         Parkinson’s disease is associated with involuntary muscle contractions and tremors – it is caused by inadequate production of dopamine

·         Alzheimer’s disease is associated with the deterioration of memory and mental capacity – it has been related to the decreased production of acetylcholine

 

Homework:       1-13, p. 426

 

C. The Central Nervous System

 

·                     includes the brain and the spinal cord

·                     the brain is a concentration of nervous tissue that acts as the coordinating centre of the nervous system

·                     the brain is covered by a three-layer protective membrane knows as meninges

·                     the outer membrane is called the dura mater, the middle layer is called arachnoid matter, and the inner layer is called pia matter

·                     these three membrane layers form the blood-brain barrier, which determines what chemicals will reach the brain

·                     between the innermost and middle meninges of the brain and through the central canal of the spinal cord, is a fluid called cerebrospinal fluid – it functions as both a shock absorber and a transport medium

·                     the c.s.f. carries nutrients to the brain cells while it relays wastes from the cells to the blood

·                     in a procedure called a spinal tap, the fluid is extracted to diagnose bacterial or viral infections such as poliomyelitis and meningitis

 

A.  The Spinal Cord

 

ü       carries sensory nerve messages from receptors to the brain

ü       relays motor nerve messages from the brain to muscles, organs, and glands

ü       Figure 1, p. 427 illustrates the anatomy of the spinal cord and its association with vertebrae

ü       a cross-section shows that the spinal cord consists of two types of nerve tissue – white matter and grey matter

ü       the core of the cord consists of unmyelinated interneurons, and the periphery, consisting of both sensory and motor neurons is myelinated

ü       the interneurons consist of nerve tracts that connect the spinal cord with the brain

ü       dorsal nerve tracts bring sensory information into the spinal cord, and ventral nerve tracts carry motor information from the spinal cord to the peripheral muscles, organs, and glands

 

B.  The Brain

 

ü       the human brain is what makes us unique

ü       our hearing, vision, and sense of smell are relatively elaborate when compared to other species

ü       our ability to conceptualize and reason is extremely unique

ü       despite our uniqueness, the human brain shares developmental links with other chordates (see Figure 2, p. 428)

ü       the human brain is made of three distinct regions:  the forebrain, the midbrain, and the hindbrain

 

Ø       The Forebrain:

 

o        contains olfactory lobes – sense smell

o        contains cerebrum – coordinating centre where speech, reasoning, memory, and personality reside

o        the surface of the cerebrum is called the cerebral cortex – made of grey matter of many folds and fissures

o        the right side of the forebrain has been associated with visual patterns or spatial awareness

o        the left side of the brain is linked to verbal skills

o        research indicates that your ability to learn is heavily dependent on the dominance of on of the hemispheres

o        a bundle of nerves called the corpus callosum allows both hemispheres to communicate

o        each hemisphere can be further subdivided into 4 lobes:  frontal, temporal, occipital, and parietal

o        Figure 3, p. 429 illustrates the location of these lobes, and the following table lists the functions of each: