Biology 12 - Kevan Krugerkevankruger.com/.../nervous_system___brain_notes.docx · Web viewBiology...

Biology 12 Mr. Kruger

NERVOUS SYSTEM PART I: NeuronsNERVOUS SYSTEM PART I: NeuronsI.I. Axons, Dendrites, & Cell Bodies:Axons, Dendrites, & Cell Bodies:

A. The Dendrites are the short processes that extend out from the cell body. They usually ‘branch out’. This is the part of the neuron that conducts an impulse TOWARDS the cell body.

B. The Cell Body contains the nucleus and therefore maintains the cell.

C. The Axon is the long process that extends out from the cell body. It is the part of the neuron that conducts an impulse AWAY from the cell body.


II. Neurons: There are three types of neurons

(Sensory neurons, Interneurons, and Motor neurons) One of each of these neurons makes up the REFLEX ARC. The reflex arc

is the basic functional unit of the nervous system. A somatic reflex arc is one in which there is the simplest possible

arrangement of elements to permit a response to stimuli, and in which the final element in the chain is skeletal muscle. In the crude sketch given here, you see the basic elements of this system:

1. This is some sensory receptor (ie: a touch sensor in the skin). 2. This is the sensory neuron in the circuit. Its cell body is physically located in a

dorsal root ganglion (but it could also be on a cranial nerve). 3. This is an interneuron, whose cell body is found in the CNS.4. This is a motor neuron whose cell body is in the ventral horn of the gray ‘H’ of

the spinal cord.

Back of the Spine Back of the Spine = Dorsal

Front of the Spine= Ventral

MOTOR NEURON

SENSORY NEURON INTERNEURON


5. This is the last element involved, is the effector organ, which in the case of this type of arc, will always be skeletal muscle.

Here's how the system works: Something stimulates the sensory receptor, which causes the afferent fiber of the

sensory neuron to fire.

That signal is transmitted via its efferent fiber into the CNS, specifically into a synapse with an interneuron in the dorsal horn of the gray butterfly.

That neuron then sends a signal to a synapse with the motor neuron in the ventral horn. The afferent motor fiber (axon) of the motor neuron--which may actually be several meters in length--leaves the CNS and terminates at a motor end plate on some effector (ie: muscle cell).

When it fires it initiates contraction of the muscle.

o The brain is not involved with REFLEXIVE ACTIONS. Notice that this loop is completely independent; it's not necessary to have CNS involvement beyond the "relay" at the interneuron. The stimulus was able to cause an effect without first going to the brain.

o This is not to say that a second interneuron didn’t conduct an impulse through the CNS to the brain to alert the conscious mind, but the key is that the response occurred without even thinking.

o Reflexive actions like these are protective mechanisms. Let's say you inadvertently put your hand on a hot stove burner. You will of course immediately remove it, and in doing so you are making use of this type of arc, bypassing conscious thought.

o In fact, the sensation of uncomfortable heat makes it to the CNS after the motor response to withdraw your hand is initiated. In other words, you move your hand away before you "know why" you're doing it.

o Of course, it's possible to override this loop with direct CNS input. As is always true, the Brain is The Boss. If you really, really want to, it's possible to hold your hand on that hot burner, ignoring the somatic reflex. Some people have done things like this, but most of us haven't got that level of willpower (or ‘stupidity’)!

Type of Neuron

Length of fibres

Location Function Picture

SENSORY

NEURON

Long dendrite

Short Axon

Cell body and dendrite are outside of spinal cord. The cell body is located in a DORSAL ROOT GANGLION

Conducts the impulse to the SPINAL CORD


The RECEPTOR of the sensory neuron

No fibres Peripheral part of body; such as skin

Detects stimuli that surpass a threshold.

Initiates the impulse

Stretch receptor in the alveoli

Pain receptor in the skin

INTERNEURONShort dendrite

Long or short Axon

Entirely within the spinal cord (or Central Nervous System-CNS)

Interconnects the sensory neuron with an appropriate motor neuron

MOTOR NEURON

Short dendrite

Long Axon

Dendrites and the cell body are located in the spinal cord; the axon is outside of the spinal cord

Conducts the impulse towards an EFFECTOR

The EFFECTOR of the motor neuron

No fibres Peripheral part of body; such as muscles around a joint

Causes the responseA muscle causing movement

A gland causing secretion


The dendrite of a sensory neuron can be located in the same nerve as the axon of a motor neuron.

These long fibres of neurons make up ‘mixed nerves’. Within these mixed nerves there can be hundreds of the long fibres from different neurons taking messages, electrical impulses in both directions to different places. A NERVE then, is simply a bundle of long fibres from neurons.

The long fibre of the sensory and the motor neurons are covered with a fatty sheath called MYELIN SHEATH.

This myelin sheath is composed of Schwann cells that wrap around the nerve fibre. This has two functions:

1. It insulates the neurons from each other as they pass through the nerve

2. It helps to speed up the impulse The points between the Schwann cells are called NODES OF RANVIER.

III. THE NATURE OF AN IMPULSE:

Nerve impulses are electrical in nature. When a nerve impulse travels along a nerve fibre, there is actually a wave of ionic changes that occur. These ionic changes create a very small shift in the electrical nature of the fibre.

It is important to understand the nature of the axon when it is “at rest” before considering the changes that occur to it when it is conducting its impulse.


A. At Rest(Resting Potential) (-60 to -70 mV) there are three ions that are significant:1) Sodium – in abundance on the outside of the axon2) Potassium and negative organic ions – are in abundance on the inside of

the axon (in the axoplasm) At rest, the membrane of the nerve cell is not permeable to these ions.

Because of the distribution of the ions, the OUTSIDE of the neuron is slightly POSITIVE when compared to the outside.

B.

Depolarization: (+30 mV at peak) The effect of a stimulus that surpasses threshold (-55mV) is to disrupt this resting permeability (and the polarity of the membrane is ‘reversed’). The membrane is suddenly made permeable to sodium (the ‘gates’ for sodium are opened), and the sodium ions flood to the inside of the axon.

Outside neuron Inside neuron


C. REPOLARIZATION : Next the potassium floods to the outside (as the potassium ‘gates’ open). This causes the initial polarity to be restored, but the ions are in the reverse positions. At the same time as the potassium gates are opening, the sodium gates are closing.

D. RECOVERY PERIOD: Finally, potassium and sodium gates all close and the sodium potassium pump (with the help of ATP) actively pumps the sodium and potassium ions back to their original condition. This re-establishes the resting condition so the neuron can conduct another impulse.


Another way to represent these changes is with a graph of the voltage in the axon membrane over time as the impulse passes a given point in the axon. The magnitude of these changes is always the same.

This means that, as long as the threshold stimulus has been reached, there will be an impulse (or action potential) and each impulse is equal to each other impulse. This is called the “ALL OR NONE” response. In other words, a stronger stimulus does not mean a bigger impulse, rather it means a greater number of impulses (more nerves involved or a single nerve conducting a series of impulses) will give bigger results.



SYNAPTIC TRANSMISSION:How does the impulse travel from one neuron to another (or from a neuron to an effector and from a receptor to a neuron) if they are not physically connected? The SYNAPSE.

When an impulse arrives at the end of an axon, it must make a connection to the next nerve cell (or to the muscle or gland, as the case may be). The axon of one nerve cell does not actually come in direct contact with the membrane of the receiving cell. There is a small space, termed the SYNAPTIC GAP, which the impulse must cross. This is the story of synaptic transmission.

1. The gap is bordered by two membranes: PRESYNAPTIC MEMBRANE POSTSYNAPTIC MEMBRANE

Encloses synaptic vesicles filled with neurotransmitters (manufactured by the axon)

Contains protein receptor sites within in the membrane to recognize specific neurotransmitters.

Presynaptic membrane

Postsynaptic membrane


2. When an impulse arrives at the end of an axon, the sodium from outside of the axon floods to the inside (as it normally would). The difference here is that the synaptic space is home to another ion; CALCIUM, and a unique sequence of events is triggered.

a. the calcium (Ca+2) passes to the inside of the axon along with the sodium.

b. The calcium binds with CONTRACTILE PROTEINS attached to the vesicles.

c. The contractile proteins contract, pulling the vesicles to the membrane surface.

3. EXOCYTOSIS occurs as the neurotransmitters are spilled across the synaptic gap.

a. Neurotransmitters bond with receptor sites on the post-synaptic membrane.

b. The reception of the neurotransmitters causes a permeability shift in the post-synaptic membrane and the impulse (or contraction, or secretion) is generated in the next cell.

c. The synaptic gap contains enzymes that are specific to the neurotransmitters. The enzymes will destroy the neurotransmitters, thus returning the synapse to its original condition prior to the arrival of the impulse.

Note… The calcium ions are returned to the synaptic gap by active transport The energy for this process comes from the mitochondria that can be

found in abundance in the synaptic knob The synaptic region is returned to its pre-imulse condition so that it

may be used again Synaptic transmission may only occur in one direction because of the

location of the vesicles and neurotransmitter and receptor sites.


STEP ONE:

STEP TWO:

Calcium channel with a calcium ion

Calcium ions


STEP THREE:

STEP FOUR:

STEP FIVE:

Acetylcholinesterase

breaks down acetylcholine and,

thus, closes the channel

Acetylcholine

Sodium and potassium ions

(sodium moves in and potassium moves out)


STEP SIX:

The topic of the autonomic nervous system (ANS) is a part of Unit N, but it makes sense to make reference to its neurotransmitters here. There are two different neurotransmitters (both are proteins) that are well-known.

NORADRENALIN *the excitatory transmitter

Calcium ions


(norepinephrine) *it almost always increases the activity of the receiving cell/tissue/organ… *involved in ‘fight or flight’ situations*it is destroyed by the enzyme monoamine oxidase

ACETYLCHOLINE *it is responsible for promoting all internal responses in a relaxed state AND for all motor neurons that control sketetal muscles.*it is destroyed by the enzyme acetylcholinesterase

*the short existence of neurotransmitters in the synapse prevents continuous stimulation (or inhibition) of post synaptic membranes.

*Drugs, such as pain killers, function as inhibitors to the neurotransmitters. They occupy the receptor sites so that the sensation of pain cannot be transmitted between the nerve cells. Other drugs have different effects.

The Brain and the Divisions of the Nervous

System


The nervous system monitors and controls almost every organ system through a series of positive and negative feedback loops.

o The Central Nervous System (CNS) includes the brain and spinal cord.

o The Peripheral Nervous System (PNS) connects the CNS to other parts of the body, and is composed of nerves (bundles of neurons).

o You will remember from Unit M, that the dendrites of sensory neurons and the axons of motor neurons are a part of the PNS, whereas the interneurons are always part of the CNS. There are also subdivisions of the PNS, which are illustrated below.

I. The central nervous system The Central Nervous System (CNS) is composed of the brain and spinal cord. The CNS is entirely enclosed 3 layers of membranes (meninges) and in bone: the skull and vertebrae. Cerebral Spinal Fluid and tissue also insulate the brain and spinal cord. After puberty, we lose ~10,000 neurons in the CNS every day, and no new neurons are made.

NERVOUS SYSTEM

Central NervousCentral Nervous System (CNS)System (CNS)

Peripheral Nervous System

(PNS)

Brain Spinal CordAutonomic N.S.

(involuntary)

Somatic N.S.

(voluntary)

Parasympathetic Nervous

System

Sympathetic Nervous

System


The Brain

The brain has 3 main parts:

The Cerebrum The

Cerebellum The Medulla

oblongata

1. The cerebrum is the largest portion of the brain. It is divided into a right and left cerebral hemispheres, which are separated by the corpus callosum. The hemispheres are covered by a thin layer of gray matter known as the cerebral cortex; the most recently evolved part of the brain, which is responsible for intelligence. The cerebrum coordinates sensory data and motor functions. This is the processing area for the body. It is where memory is kept; where conscious thought processes are made; where a lot of interneural, non-reflexive connections are made. When impulses require some processing before responses are made, this is where they go (ie: responding to a verbal question). Overall, the cerebrum governs intelligence and reasoning, planning, learning, memory, and personality.

The cerebrum is divided into lobes.

Saggital View of the Brain

Corpus Callosum


No region of the brain functions alone, although the major functions of the lobes are:

a) The occipital lobe (back of the head) receives and processes visual information.

b) The temporal lobe receives auditory signals, olfactory signals, processing language and the meaning of words.

c) The parietal lobe is associated with the sensory cortex and processes information about touch, taste, pressure, pain, and heat/cold.

d) The frontal lobe conducts 3 functions: (1) Motor activity & integration of muscle activity (2) Speech (3) Thought processes

2. The corpus callosum is the dense tissue that holds the two hemispheres of the cerebrum together. It conducts impulses from one side of the brain to the other. It is through this part of the brain that the activities of one side of the brain are co-ordinated with the other.

3. The cerebellum is the 2nd largest part of the brain. This is the place where the impulses that give rise to movements are coordinated. The impulses originate in the frontal lobe of the cerebrum, but are co-ordinated here. As a result, movements can be smooth and small (as in fine motor control). The cerebellum also maintains normal muscle tone and posture, and coordinates balance (proprioception).

4. The brain stem is the smallest and the oldest and most primitive part of the brain. The brain stem is continuous with the spinal cord, and is composed of the medulla oblongata and pons. The medulla oblongata is closest to the spinal cord, and is involved with the regulation of heartbeat, breathing, vasoconstriction (blood pressure), and reflex centers for vomiting, coughing, sneezing, swallowing, and hiccuping. The medulla oblongata contains receptors that are receptive to the conditions of the blood (such as too much CO2 and H+


which causes inhalation). Has control over the internal organs; is the ‘unconscious’ part of the brain.

5. The midbrain connects the hindbrain and forebrain. It controls our eye reflexes.

6. The Thalamus directs the impulses that travel up the spinal cord to the correct region of the cerebrum. It is often referred to as the ‘sorting centre’ or ‘gatekeeper of the cerebrum’. It lies below the corpus callosum and redirects impulses to the right or left sides of the brain. It filters out extraneous and unimportant sensory stimuli to the higher brain.

7. The Hypothalamus regulates homeostasis (maintaining a constant internal environment). It has regulatory areas for thirst, hunger, body temperature, water balance, and blood pressure, and links the Nervous System to the Endocrine System. This part of the brain also has control over the internal organs. The hypothalamus samples the blood that travels through it and responds, either through the initiation of nerve impulses or by its association with the pituitary gland, which extends below it.

8. The Pituitary Gland is also known as the ‘Master Gland’. This is a small gland with two parts: the anterior and posterior lobes. It produces a large number of hormones, many of which control the release of hormones from other glands in the body.

a) Posterior Pituitary : This part of the gland releases the hormones that are made in the Hypothalamus, but are stored in the Posterior Pituitary. The hormones are transferred and stored in special hollow nerve fibres that run from the hypothalamus to the posterior pituitary. (eg. ADH and Oxytocin)

b) Anterior Pituitary : This part of the gland makes and releases its own hormones. It is stimulated to release its hormones by hormones released from the hypothalamus. There is a blood connection between the hypothalamus and the anterior pituitary.

Hormones made in the Ant. Pit. include: Growth hormoneProlactinFSH & LH


Thyroid Stimulating Hormone (TSH)Adrenal Cortex Stimulating Hormone (ACTH)Melatonin

The levels of all the Pituitary hormones in the blood are monitored by the hypothalamus and are controlled by a negative feedback system (except Oxytocin!), so the level of these hormones remains relatively constant.

*The sex hormones FSH & LH vary in the female.

The Spinal Cord: The spinal cord (dorsal nerve cord)runs along the dorsal side of the body and links the brain to the rest of the body. Vertebrates have their spinal cords encased in a series of (usually) bony vertebrae that comprise the vertebral column.

The gray matter of the spinal cord consists mostly of cell bodies and dendrites. The surrounding white matter is made up of bundles of interneuronal axons (tracts).

Some tracts are ascending (carrying messages to the brain), others are descending (carrying messages from the brain). The spinal cord is also involved in reflexes that do not immediately involve the brain.Between most of the vertebrae, there are peripheral nerves that extend out into the body. The spinal cord is easily considered in 4 parts as illustrated below:


1. Cervical : the neck region. *No peripheral nerves originate from this region.

2. Thoracic : the thoracic cavity (lungs etc…)

3. Lumbar : the abdominal cavity(guts, kidneys, liver…)

4. Saccral : the tail bone area

II. The PERIPHERAL nervous system

The Peripheral Nervous System (PNS) contains only nerves and connects the brain and spinal cord (CNS) to the rest of the body. There are 12 pairs of Cranial nerves in the PNS that take impulses to and from the brain (CNS). These Cranial nerves can be either sensory (optic/olfactory) or motor (facial nerves) or a mixture of the two (tongue, facial skin). There are also 31 pairs of Spinal nerves which take impulses to and away from the spinal cord.


There are two main components of the PNS:1. Somatic Nervous System(skeletal): This system includes all of the nerves controlling the voluntary musculo-skeletal region, the exterior sense organs (including the cranial nerves) and the reflex arcs. These are all under VOLUNTARY control. Most sensory input carried in the PNS remains below the level of conscious awareness. Input that does reach the conscious level contributes to perception of our external environment.

External sense organs (including skin) are receptors. Muscle fibers and gland cells are effectors. The reflex arc is an automatic, involuntary reaction to a stimulus. The reaction to the stimulus is involuntary, with the CNS being informed but not consciously controlling the response. Examples of reflex arcs include balance, the blinking reflex, and the stretch reflex.Motor neurons of the somatic system are distinct from those of the autonomic system. Inhibitory signals cannot be sent through the motor neurons of the somatic system!

Senses: Input to the nervous system is in the form of our five senses: pain, vision, taste, smell, and hearing.

o Vision, taste, smell, and hearing input are the special senses.

o Pain, temperature, and pressure are known as somatic senses.


Sensory input begins with sensors that react to stimuli in the form of energy that is transmitted into an action potential and sent to the CNS.

Sensory Receptors: these are classified according to the type of energy they can detect and respond to: Mechanoreceptors: detect hearing and balance, stretching. Photoreceptors: detect light. Chemoreceptors: detect smell and taste mainly, as well as internal sensors in the

digestive and circulatory systems. Thermoreceptors: detect changes in temperature. Electroreceptors: detect electrical currents in the surrounding environment.

2. Autonomic Nervous System (smooth muscle, cardiac muscle, and glands). This system is that part of PNS consisting of motor neurons that control internal organs. The autonomic system controls muscles in the heart, and the smooth muscle in internal organs such as the intestine, bladder, and uterus. These are all under Involuntary control; they happen automatically and without awareness.

The autonomic system is subdivided into:

a) The Sympathetic Nervous System: involved in the ‘fight or flight’ response. This system is stimulatory and prepares the body for action. These nerves come off the ganglia from the mid-region of the spinal cord.

Sudden, simultaneous release of noradrenalin from all the sympathetic neurons (as in times of fright) has a critical effect. It causes the release of the hormone adrenalin from the medulla of the adrenal glands (located on top of the kidneys). The noradrenalin and adrenal initiate and sustain what is known as the ‘fight or flight’ response, in which they prepare the body in the following ways:

1. Increases heart rate and blood pressure; more blood supplied to the body quickly

2. Widens air passageways so that more air can be exchanged with each breath


3. Sudden contraction of some muscles to tense the body up for action. Included in this is the contraction of the diaphragm causing a ‘gasp’.

4. The iris of the eye contracts to widen the pupil, thus maximizing visual alertness.

5. Increased blood flow to the skeletal muscles so they are more able to act.6. Decreased digestive activity, circulation and control.

b) Parasympathetic Nervous System: involved in relaxation (‘vegetative’ body functions). Releases acetylcholine to cause the opposite of all of the items listed above. ie: decreases heart rate and increases digestive rate. This system returns the body to its normal state after stress and ultimately maintains a relaxed state. These nerves come off the ganglia from the lower part of the spinal cord.

*Each of these subsystems operates in the reverse of the other (antagonism). Both systems innervate the same organs and act in opposition to maintain homeostasis. *Motor neurons in this system do not reach their targets directly (as do those in the somatic system) but rather connect to a secondary motor neuron, which in turn innervates the target organ. The ganglia of the sympathetic fibres are closer to the CNS thatn those of parasympathetic neurons.

Basis of Comparison

Sympathetic Neurons

Parasympathetic Neurons

Effect Active body function Vegetative body functionSpinal origin Thoracic and lumbar Cranial and sacralNeurotransmitter Noradrenalin AcetylcholineRestoring enzyme Monoamine oxidase AcetylcholinesteraseLocation of motor ganglion

Closer to the CNS Farther from the CNS

III. THE BRAIN AND DRUGS: Some neurotransmitters are excitory, such as acetylcholine,

norepinephrine, serotonin, and dopamine.

Some are associated with relaxation, such as dopamine and serotonin. Dopamine release seems related to sensations of pleasure. Endorphins are natural


opioids that produce elation and reduction of pain, as do artificial chemicals such as opium and heroin.

A. Neurological diseases, for example Parkinson's disease and Huntington's disease, are due to imbalances of neurotransmitters. Parkinson's is due to a dopamine deficiency. Huntington's disease is thought to be cause by malfunctioning of an inhibitory neurotransmitter. Alzheimer's disease is associated with protein plaques in the brain.

B. Drugs are stimulants or depressants that block or enhance certain neurotransmitters. Dopamine is thought to be involved with all forms of pleasure. Cocaine interferes with uptake of dopamine from the synaptic cleft. Alcohol causes a euphoric "high" followed by a depression.

1) Marijuana, material from the Indian hemp plant (Cannabis sativa), has a potent chemical THC (tetrahydracannibinol) that in low, concentrations causes a euphoric high (if inhaled, the most common form of action is smoke inhalation). High dosages may cause severe effects such as hallucinations, anxiety, depression, and psychotic symptoms.

2) Cocaine is derived from the plant Erthoxylon coca. Inhaled, smoked or injected. Cocaine users report a "rush" of euphoria following use. Following the rush is a short (5-30 minute) period of arousal followed by a depression. Repeated cycle of use terminate in a "crash" when the cocaine is gone. Prolonged use causes production of less dopamine, causing the user to need more of the drug.

3) Heroin is a derivative of morphine, which in turn is obtained from opium, the milky secretions obtained from the opium poppy, Papaver somniferum. Heroin is usually injected intravenously, although snorting and smoking serve as alternative delivery methods. Heroin binds to ophioid receptors in the brain, where the natural chemical endorphins are involved in the cessation pain. Heroin is physically addictive, and prolonged use causes less endorphin production. Once this happens, the euphoria is no longer felt, only dependence and delay of withdrawal symptoms.


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