Biology 30Biology 30NERVOUS SYSTEMNERVOUS SYSTEM

1.    Nervous System OverviewNervous System Overview2.    The Neuron2.    The Neuron3.    Reflex Arc3.    Reflex Arc4.    The Action Potential4.    The Action Potential5.    The Synapse / Neurotransmitters5.    The Synapse / Neurotransmitters6.    Nervous System Diseases6.    Nervous System Diseases7.    Drugs7.    Drugs8.    PNS8.    PNS9.    CNS9.    CNS10. The Brain10. The Brain

General FunctionsGeneral Functions  

1. Reception

2. Conduction

3. Interpretation and Organization

4.  Transmission

The nervous systems main function is to maintain homeostasis: a maintenance of the internal environment i.e. pH, body temp, glucose levels, BP, water levels, etc

Nervous System OrganizationNervous System Organization

The Neuron- The functional unit The Neuron- The functional unit of the nervous system, it of the nervous system, it conducts nerve impulsesconducts nerve impulses

There are 3 kinds of neurons:– 1) sensory neurons (afferent)- take impulses from the

body receptors to the CNS– 2) motor neurons (efferent)-take impulse away from the

CNS and to the muscles and glands– 3) interneurons- are actually in the CNS (in the brain and

spinal cord)– ***Neuroglia (glial cells)- supportive cells that aid the

interneurons of the brain. Nourish neurons, removes waste from neurons, and protect neurons.

The Neuron has 3 basic Basic parts:

1) cell body- nucleus and cytoplasm2) Dendrites-finger-like projections of cytoplasm

of cell body. They receive information3) Axon-extension of cytoplasm. Transmit impulse

away from the body

Some neurons contain the following additional parts:

Myelin Sheath-a white fatty covering that insulates the axon. Schwann cells produce the myelin sheath– Schwann cells- a special kind of glial cell that

produces a myelin sheath that wraps around the axons as insulation

– On top of protection, the myelin sheath allows for faster conduction of impulses and greater power of regeneration

– The myelin sheath is NOT continuous but rather forms intermitted gaps called the Nodes of Ranvier. Impulses will now ‘jump’ from Node to Node rather then slowly moving through the entire length of the axon. (faster impulses)

All nerves of the PNS, as well as all motor and sensory neurons are myelinated, only some of the nerves in the CNS are myelinated – Non- Myelinated neurons in the brain= grey

matter– Myelinated nerves in the brain are called white

matter

Neuron structure

Nerves

Individual neurons are organized into tissues called nerves.

Repairing Damaged NervesNerves in the PNS are surrounded by a thin

membrane called the neurilemma which helps to regenerate damaged axons

Nerves in CNS lack neurilemmas and cannot be repaired

Area of research: stem cells, brain band-aid

Reflex ArcReflex Arcautomatic, quick, involuntary responses to

internal or external stimuli.

does not immediately involve the brain.

allows quicker reaction times to a potentially harmful stimulus

Stretch ReflexStretch Reflex

5 Components of a Reflex Arc:5 Components of a Reflex Arc:

1. Sensory receptor -senses something

2. Sensory neuron -sends signal to the CNS

3. Interneuron- registers signal

4. Motor neuron -sends response back

5. Effector- caries out the action

Action PotentialAction Potential

A nerve impulse is an action potentialAn action potential is an

electrochemical event with a rapid change in polarity (relative electrical potential) down a nerve cell that results in the conduction of a nerve impulse.

Steps of a Nerve Impulse (Action Potential

1. Resting PotentialPolarization: voltage difference of -70mV across a nerve

cell membrane

caused by the sodium potassium pump (gate): more sodium is pumped out than potassium is pumped in, resulting in a negative charge inside the axon of the neuron

also potassium moves out by diffusion more easily than sodium moves in

Result: excess positive charge outside the membrane and negative charge inside the membrane

2. Stimulation / Depolarization A threshold stimulus must be applied (a change in pH,

pressure, or an electrical stimulus) This causes the sodium potassium pump to cause Na+ gates to open and tons of Na+ rush into the cell, and small amounts of K+ to move out

membrane becomes depolarized (+ 40 mV)

3. Re-polarization After the wave of impulse has move through

Na+ gates close to stop inflow

change in electrical potential causes K+ channels to open and K+ ions rush out of the cell

Restores the polarized state but now is hyperpolarized – more positively charged on the outside than the resting state (over-shoot)

(also the ion concentrations are reversed from the resting state )

4. Refractory period resting potential (-70mv) must be

restored before the neuron can fire againNa+ are pumped out and K+ are pumped

back into the cell using ATP energy.The

The Action PotentialThe Action Potential

The Action Potential in ActionThe Action Potential in Action

Neuron Action Potential

Propagation

Saltatory Action

• the speed of the nerve impulse is increased by jumping from node of Ranvier to node of Ranvier (gated channels are found only at the nodes)

Propagation of the Action Potential

Threshold level – minimum depolarization that must be reached (usually around +30mV) before sufficient Na+ gates open to continue the action potential

All or None Response – if the threshold level is not reached, the action potential will not occur at all. If the threshold is reached or exceeded a full action potential will result.

The wave of depolarization is self propagating, meaning that it will continue without any aid until its passed on to the next neuron

How do we differentiate intensity? Ex hot vs warm?

Intensity is determined by: 1. the number of neurons that fire simultaneously 2. the frequency at which the neurons fire 3. the threshold level of different neurons (lower threshold neurons are more likely to fire, and are found in more “sensitive” areas)

The Synapse and The Synapse and NeurotransmittersNeurotransmitters

Neurons are NOT physically attached to each other, but are separated by a gap (synaptic cleft), the electrical impulse cannot just simply cross the gap to the next dendrite, something is needed

Neurotransmitters are stored in synaptic vesicles of the axon and are released to carry the information across this synaptic gap

The SynapseThe Synapse

Terminal Axon

Structures in the SynapseStructures in the SynapsePre-synaptic membrane – membrane

found at the synaptic ending of the neuron sending information

Post-synaptic membrane- membrane found at the dendrite of the neuron receiving information

Synaptic cleft – space between the pre and post synaptic membranes.

Neurotransmitters continue the Neurotransmitters continue the impulse across the synaptic cleftimpulse across the synaptic cleft

Crossing the impulse across the gap is a chemical reaction– 1) The end of pre-synaptic axon contains vesicles

that have specialized neurotransmitters (NTs), as the impulse gets to the end of the axon the NTs are released in to the gap

– 2) The NTs diffuse across the gap and attached to specialized receptors found on the post synaptic dendrite.

– 3) The wave of depolarization continues on the through the next neuron

Neurotransmitters

1. excitatory neurotransmitters – cause the opening of Na+ channels to cause depolarization

2. inhibitory neurotransmitters –block Na+ channels and open K+ channels ions which causes hyper-polarization

-inhibits action potentials

Summation – at any given time there are many neurons acting and releasing NTs into the synaptic cleft, the net effect of excitatory and inhibitory neurotransmitters is called summation

– ONLY if there is adequate excitation to reach the threshold, the neuron will fire.

--may require more than one neuron may require more than one neuron to release neurotransmittersto release neurotransmitters

A response may involve both excitatory and inhibitory neurotransmitters

Ex) Throwing a ball: Triceps contracts and bicep relaxes

Integration – the degree of sensation felt or the degree of response created by the brain depends on the number of neurons that fire

There are 9 universally recognized neurotransmitters: aspartate, glycine, GABA, glutamate, dopamine, nor-epinephrine, epinephrine, seratonin, and acetylcholine.

Some of the more common neurotransmitters Some of the more common neurotransmitters (and their enzymes) include:(and their enzymes) include:

Neurotransmitter Enzyme Function of Neurotransmitter

Acetylcholine Cholinesterase

Involved with muscle contraction of the skeletal muscles

Dopamine Monoamine oxidase enzyme

Responsible for voluntary movement and emotions of pleasure

Serotonin Monoamine oxidase enzyme

Regulates temperature, sensory perception, sleep and involved in mood stabilization and control

Nor-epinephrine Regulates the stress “fight or flight” response

GABA Inhibitory action of motor behavior

Removing NeurotransmittersTo be effective, the NTs must NOT linger around

in the gap, other wise successive impulses will occur. NTs are removed from the gap by:

1. Degradation by enzymes in the synaptic cleft2. Re-uptake by the pre-synaptic membrane3. Diffusion out of the synaptic cleft4. Inability to bind due to competitive inhibitors

The Effects of DrugsDrug – anything that is not food that alters

the normal bio-chemistry of the body in some way.

Stimulant – mimics neurotransmitter, decreases rate of breakdown of neurotransmitter or increases release of neurotransmitter

Depressant – blocks receptor site, decreases production of neurotransmitter, or increases the breakdown of neurotransmitter

Alcohol: - depressant -seems to enhance GABA -leads to lack of coordinated response, and

loss of normal social inhibitions. -may also weaken the effect of glutamine, an

excitatory neurotransmitter, leading to sluggishness and lack of co-ordination.

Close to Home Animation: Alcohol

Marijuana:

- a depressant and hallucinogen

-acts on the canniboid receptors of the brain that affect concentration, perception and movement.

-may have an impact on the activity of seratonin, GABA and norepinephrine in the brain

not physically addicting, however this is a gateway drug and may be psychologically addicting

Cocaine: -a stimulant

-blocks the re-uptake of dopamine, causing an adrenaline like effect from the dopamine

-as dopamine levels increase in the synapse, the body produces less, thus making cocaine very physically addicting

Close to Home Animation: Cocaine

Crystal meth:

-a stimulant

-passes directly through neuron membranes and causes excessive release of dopamine

-leads to feelings of euphoria, psychosis, delusions and extreme aggressiveness.

Ecstasy: - a stimulant and hallucinogen

-affects neurons in the brain by causing an over-production of serotonin.

-creates shorter feelings of pleasure, however use can result in brain damage, and cardiac arrest.

The venom of the black widow spider is called “latrotoxin”. This toxin results in a massive release of the neurotransmitter acetylcholine from the neuromuscular junctions of victims and may cause muscle spasms, pain, increased blood pressure, nausea and vomiting.

Diseases of the Nervous Diseases of the Nervous SystemSystem

Parkinson’s Disease: wide-eyed, unblinking expression, involuntary tremor, muscle rigidity, shuffling gait. Ex. Muhammad Ali

-dopamine deficiency caused by the degeneration of dopamine producing cells in the brain

-

-caffeine may offer protection against Parkinson’s disease as it prevents loss of dopamine

Alzheimer’s Disease: characterized by loss of memory, senility, deterioration of cells in the basal nuclei, presence of tangles and plaques

-possibly due to a malfunction of acetylcholine

- seems to be linked to a gene located on chromosome #21

Schizophrenia: delusions, random thoughts, disjointed thoughts, sensory hallucinations

- may be the result of excessive activity of brain neurotransmitters such as dopamine

Huntington’s Disease: progressive deterioration of the nervous system that leads to writhing movements, insanity and eventually death

- seems to be caused by the malfunction of the inhibitory neurotransmitter GABA

Depression: low affect, feeling blue, lack of or excessive sleep and eating patterns

- seems to be linked to malfunctions in dopamine and seratonin, perhaps caused by an excess of monoamine oxidase enzymes

Stroke: caused by interruption of blood flow to the brain which causes brain cells to perish.

Epilepsy: is a seizure disorder where there is a sudden, un-explained surge of electrical activity through the brain with no specific known cause.

Epilepsy.com

Chemical WarfareChemical Warfare

1) Nerve Gas: inhibit acetylcholinesterase (enzyme that removed acetylcholine from the synaptic gap). Causes spastic paralysis which is continues muscle spasms

2) Strychnine: similar to nerve gas, causes convulsions and spastic paralysis

Central Nervous System (CNS)

Is primarily responsible for the processing and organization of information.

The CNS consists of two major structures:

1. The Brain

2. The Spinal Cord

Spinal Cord

•Made of 31 segments

•Protected by the vertebrae

Spinal CordCentral Cavity – contains cerebrospinal fluid

White Matter – contains myelinated nerve cells

Grey Matter – contains un-myelinated nerve cells

Spinal CordDorsal Root Ganglion – entry of

sensory neurons to spinal cord and CNS, ganglion is the collection of cell bodies

Ventral Root – exit of motor neurons from the spinal cord

Meninges – 3 protective membranes surrounding the spinal cord and brain (dura mater, arachnoid, pia mater)

Meningitis is an infection of the meninges (A spinal tap is a needle that is inserted between 2 vertebrates into the meninges to check for meningitis)

Cerebrospinal Fluid – circulates between the inner and middle membranes of the brain and spinal cord. – Provides protection, nutrient / waste

exchange, etc.

Spinal Cord

Spinal Cord Functions 1. center for reflex action

2. provides a pathway for communication between the brain and peripheral nerves

The Brain

Hindbrain - The Unconscious Brain

– important for autonomic functions required for survival

Cerebellum – responsible for muscle co-ordination, posture, coordinated muscle movement and balance

Medulla oblongata – controls heartbeat, respiration, blood pressure, reflex center for vomiting, sneezing, hiccupping, coughing and swallowing

Pons – connects the cerebrum to other parts of the brain, regulates breathing rate

Midbrain – reflex center for head movements in response to visual stimuli, connects cerebrum to other parts of the brain

Forebrain – responsible for conscious and unconscious actions

Thalamus – central relay station

- directs incoming sensory information to the cerebrum

Hypothalamus – contains cells that produce some hormones, controls thirst, hunger, and controls many of the pituitary hormones

Also aids in sleep regulation, sexual arousal, emotions (anger, fear, pain, pleasure)

Cerebrum – largest part of the brain (80% of brain mass), left and right hemispheres. – responsible for intellect, memory,

consciousness and language.

Lobes of the Cerebral Cortex

Frontal Lobe –voluntary motor muscle movement, higher intellectual processes, personality/behavior, language

Temporal Lobe – hearing

Parietal Lobe –perceptions of touch, temperature, pressure, pain, etc from the skin

Occipital Lobe –vision

Olfactory Lobe –smell

Other parts of the brainLimbic System –emotions, associated

with hypothalamus

Pituitary Gland- Master Gland– attaches to hypothalamus

Corpus Callosum – Bundle of nerves that connects the

two halves of the brain– allows for integrated thoughts and

coordinated responses

– Left brain – verbal, linguistic dominant

– Right brain – spatial, artistic, visual dominant

PET – Positron Emission Tomography– Radioactive chemicals are injected into the

bloodstream

– data is used to produce 2D or 3D images of the distribution of the chemicals throughout the brain and body.

SPECT-Single Photon Emission Computed Tomography

– radioactive tracers and a scanner record data

– computer constructs 2D or 3D images of the active brain regions.

MRI-Magnetic Resonance Imaging- magnetic fields and radio waves produce high-quality 2D or 3D images of brain structures without injecting radioactive tracers.

EEG-Electroencephalography

- electrodes placed on the scalp detect and measure patterns of electrical activity from the brain.

CT-Computed Tomography Scan

- a series of X-ray beams passed through the head.

-images are then developed on sensitive film.

-creates cross-sectional images of the brain

Peripheral Nervous System (PNS)

Cranial nerves – 12 pairs of sensory, motor and mixed nerves that control the face, neck and shoulders

Spinal Nerves – 31 pairs of nerves that emerge from the spinal cord by two roots (one pair for each segment)

Dorsal root nerves – contain sensory neurons and ganglia

Ventral root nerves – contain motor neurons

All other nerves not part of the CNS

Spinal Cord Injuries

The PNS is subdivided into two major parts:

1. The Somatic Nervous System

-contains all the nerves that serve the muscular-skeletal system and the sensory organs.

-conscious and deliberate.

2. The Autonomic Nervous System

-non-voluntary actions

-contains all the nerves that serve the internal organs.

-unconscious and automatic.

-made of two parts:

A. Sympathetic nervous system

-responsible for the 4 Fs: fight, flight, fright, or having sex response

-ex) dilation of the pupils, increased heart rate, increased breathing rate, slowed digestion, enhanced performance, increase in blood sugar

B. Parasympathetic nervous system – responsible for the relaxation response (after fight, flight, fright, having sex)

- brings the body back to normal levels

– http://itc.gsw.edu/faculty/gfisk/anim/autonomicns.swf

Fig 2 p 434Fig 2 p 434