Compendium Review: Nervous System and Senses

Nervous System

Table of Contents:

Overview of the Nervous System
The Central Nervous System
The Limbic System and Higher Mental Functions
The Peripheral Nervous System
Drug Abuse

Overview of the Nervous System:

The nervous system has 2 major divisions: the central nervous system (CNS) which consists of the brain and spinal cord and the peripheral nervous system (PNS) which consists of the nerves.

http://www.invanet.org/pics/central%20nervous%20system.jpg

The nervous system has 3 specific functions:

• Receives sensory input
• CNS performs integration
• CNS generates motor output

Nervous tissue contains 2 types of cells: neurons and neuroglia. Neurons transmit nerve impulses between parts of the nervous system; neuroglia support and nourish neurons.

http://www.readingeagle.com/blog/tullio/archives/neurons.jpg
Neurons are classified according to function: sensory neuron (takes nerve impulses from sensory receptor to CNS), interneuron (receive input from sensory neurons and from other interneurons) , and motor neurons (takes nerve impulses away from the CNS to the effector.)

Neurons vary in appearance, but all of them have 3 parts: a cell body (contains nucleus), dendrites (short extensions that receive signals from sensory output), and an axon (portion of neuron that conducts nerve impulses.)

http://www.neurevolution.net/wp-content/uploads/Synapse.JPG

Many axons are covered by a protective myelin sheath. In the PNS this covering is formed by a type of neuroglia called Schwann cells and in the CNS oligodendrocytes perform this function. The gaps where there is no myelin sheath are called teh nodes of Ranvier.

http://kvhs.nbed.nb.ca/gallant/biology/schwann_myelin.jpg

Nerve impulses convey information within the nervous system. The nerve impulse is studied by using excised axons and a voltmeter to measure voltage, which allows us to measure the potential difference between 2 sides of an axonal membrane.

• Resting potential: -65mV (inside of the neuron is more negative than the outside
• Action potential: +40 mV (rapid change in polarity across the axonal membrane)

http://cache.eb.com/eb/image?id=54737&rendTypeId=4

Every axon branches into many fine endings, tipped by a small swelling called an axon terminal, which lies very close to either the dendrite or the cell body. Transmission across a synapse is carried out by molecules called neurotransmitters, which are stored in vesicles in the axon terminals.

http://fig.cox.miami.edu/~cmallery/150/neuro/c7.48.1.neurotransmitters.jpg

A single neuron has many dendrites plus the cell body, and all of the can have synapses with many other neurons. Therefore, a neuron is on the receiving end of many signals which can be either excitatory or inhibitory. Neurons integrate these incoming signals. Integration is the summing up of excitatory and inhibitory signals.


http://mcdb.colorado.edu/courses/3280/images/synapse/integration.gif


The Central Nervous System

The spinal cord and the brain make up the CNS. Both are protected by bone and wrapped in a protective membrane known as meninges. The spaces between the meninges are filled with cerebrospinal fluid, which cushions and protects the CNS. The brain has 4 ventricles that serve as a reservoir for cerebrospinal fluid. The CNS is composed of 2 types of nervous tissue: gray matter (contains cell bodies and short non-myelinated fibers) and white matter (contains myelinated axons that run together in bundles called tracts.)


http://www.octc.kctcs.edu/gcaplan/anat/images/Image432.gif

The spinal cord extends from the base of the brain through a large opening in the skull called the foramen magnum and into the vertebrae canal. The spinal nerves project from the cord between the vertebrae. The central canal contains cerebrospinal fluid. The gray matter is centrally located and shaped like the letter H. The white matter occurs in areas around the gray matter.


http://www.laesieworks.com/spinal/pict/SpinalCord.jpg

The spinal cord functions as a means of communication between the brain and the peripheral nerves that leave the cord. The spinal cord is the center for thousands of of reflex arcs. Depending on an injury to the spinal cord, paralysis may occur.

http://www.apparelyzed.com/_images/content/spine/spinenerves.jpg

The human brain has been called the last great frontier of biology. The brain is divided into different sections: the cerebrum, the diencephalon, the cerebellum, and the brain stem.

http://www.ahaf.org/alzdis/about/brain_headBorder.jpg The cerebrum is the largest portion of the human brain and the last center to receive sensory input and carrt out integration. The cerebrum has 2 hemispheres; the left and right. The longitudinal fissure divides the hemispheres. Shallow grooves divide the hemispheres into lobes with each lobe carrying out particular functions:

• Frontal lobe: primary motor area
• Temporal lobe: primary auditory area
• Parietal lobe: primary somatosensory area
• Occipital lobe: primary visual area

http://static.howstuffworks.com/gif/adam/images/en/brain-picture.jpg

The cerebral cortex is a thin but highly convoluted outer layer of gray matter that covers the cerebral hemispheres. The cerebral cortex contains over 1 billion cell bodies and is the region of the brain that accounts for sensation, voluntary movement, and all the though processes we associate with consciousness.
Association areas are places where integration occurs.

http://www.princetonbrainandspine.com/photos/brain_anat/brain_areas_illus19a.jpg The diencephalon contains the hypothalamus and the thalamus. The hypothalamus integrates homeostasis by regulating hunger, sleep, thirst, body temperature, and water balance. The thalamus receives all sensory input except for smell. The thalamus also participates in higher mental functions such as memory and emotion. The pineal gland (which secretes melatonin) is located in the diencephalon.

http://kybele.psych.cornell.edu/~edelman/Psych-231/diencephalon.gif
The cerebellum lies under the occipital lobe of the cerebrum and is separated from the brain stem by the 4th ventricle. The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts. It also receives motor output from the cerebral cortex about where the parts should be located.

http://www.nlm.nih.gov/medlineplus/ency/images/ency/fullsize/18008.jpg The brain stem contains the midbrain, the pons, and the medulla oblongata. The midbrain acts as a relay station for tracts passing between the cerebrum and the spinal cord or cerebellum. The pons contains bundles of axons traveling between the cerebellum and the rest of the CNS. The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction. Reticular formation is a complex network of nuclei and fibers that extend the length of the brain stem. The reticular formation os a major component of the reticular activating system (RAS) which receives sensory signals and sends them up to the higher centers, and motor signals which sends it ti the spinal cord.


http://library.thinkquest.org/C005704/media/brain_stem.gif


The Limbic System and Higher Mental Functions

The limbic system is an evolutionary ancient group of linked structures deep within the cerebrum that is a functional grouping rather than an anatomical one. The limbic system blends primitive emotion and higher mental function as a united whole. Two significant systems in the limbic system are the amygdala (causes experiences to have emotional overtone) and the hippocampus (crucial to learning and memory.)


http://universe-review.ca/I10-41-limbic.jpg


The Peripheral Nervous System

The peripheral nervous system (PNS) lies outside the CNS and contains the nerves. Nerves are designated as cranial when they arise from the brain and spinal nerves when they arise from the spinal cord. Nerves take impulses to and from the CNS. Nerves are composed of axons.


http://zenith-designs.com/files/nerves.jpg

Humans have 12 pairs of cranial nerves attached to the brain. They are designated with roman numerals. Some are sensory, some are motor, and some are mixed.


http://can-do.com/uci/ssi2001/title_artcranial.jpg

Spinal nerves emerge in pairs from either side of the spinal cord. The roots of the spinal nerve physically separate the axons of the sensory neurons from the motor neurons. The cell body of a sensory neuron is called a dorsal-root ganglion. A ganglion is a collection of cell bodies outside the CNS.


http://www.infovisual.info/03/img_en/039%20Nervous%20system%20spinal%20nerves.jpg

The PNS has different divisions The somatic system serves the skin, skeletal muscles, and the tendons and includes nerves that take sensory information from the external sensory receptors to the CNS and motor commands away from the CNS to the skeletal muscles. Automatic responses to a stimulus in the somatic system are called reflexes.


http://www.daviddarling.info/images/reflex_arc.jpg

The autonomic nervous system is also in the PNS. The autonomic nervous system regulates the activity of cardiac and smooth muscles and glands. The system is divided into the sympathetic and the parasympathetic systems.

The sympathetic nervous division is important during emergency situations. The "fight or flight" mechanism occurs from this division. It accelerates heartbeat and dilates the bronchi by releasing norepinephrine. The parasympathetic division promotes internal responses associated with relaxation: pupil contraction, food digestion, and slowed heartbeat by releasing acetlycholine.


http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/autonomic.gif

Drug Abuse

Drugs alter mood and affect emotional state. Drugs either affect the limbic system or promote or decrease the action of a particular neurotransmitter. Drug abuse occurs when a person takes a drug at an improper dose level or takes an illegal drug and becomes physiologically and/or psychologically dependent on the drug.

Alcohol is a CNS depressant and the most commonly abused drug in the world. Alcohol increases GABA and the release of beta-endorphins. Alcohol crosses the blood-brain barrier causing damage to several tissues. Alcohol dehydrogenase is the liver enzyme that metabolizes alcohol.


http://www.thelivercentre.com.au/development/images/alcohol-Effects.jpg

Nicotine is a CNS stimulant that binds to nicotine receptors in the midbrain. Nicotine works by releasing dopamine and is highly addictive. The newest drug Chantix (varenicline) works by interrupting and blocking the uptake of nicotine from the nicotine receptors in the brain.


http://www.sfn.org/skins/main/images/brainbriefings/nicotine.illus.jpg

Cocaine and methamphetamine are both powerful CNS stimulant drugs. Cocaine and meth are primarily dopamine reuptake inhibitors, but have varying degrees of effect and longevity on the human body. Cocaine and methamphetamine have medicinal uses (cocaine as an anesthetic and vasoconstrictor for eye, nose, ear, and throat surgery and meth for ADHD, obesity, Parkinson's disease, and narcolepsy) but are abused world-wide as an illegal substance. They are both highly addictive.


http://www.deadiversion.usdoj.gov/pubs/brochures/pseudo/images/methuse.jpg


MDMA (3,4,methylenedioxymethamphetamine) or Ecstasy closely mimics methamphetamine, but is actually classified as a hallucinogenic drug because it does cause hallucinations (contrary to the text book.) Methamphetamine primarily causes users to see things from a lack of sleep, or from a psychotic episode from overuse, but does not cause synesthesia (transposing of the senses) as true hallucinogenic substances do. One of the primary symptoms of MDMA is synesthesia.


http://fusionanomaly.net/mdma.jpg

Heroin is derived from the opium poppy plant. It has no medicinal value, but is quickly converted to morphine after delivery to the human body. Morphine itself is a narcotic analgesic (pain killer). Narcotics differ from CNS depressants in that narcotic analgesics lower body temperature, cause pupil constriction, limit the pupillary reaction due to the constriction, and ptosis.


http://drugrecognition.com/images/SmallPupil.jpg

Marijuana is an illegal and impairing substance from the THC (Δ⁹-tetrahydrocannabinol) within the plant. THC belongs to a group of chemicals called cannabinoids. Cannabinoid receptors have been located in the brain. Cannabinoids affect the neurotransmitter anandamide and cause problems with motor skills, perception, memory, and orientation. Although cannabis causes euphoria and relaxation, it raises both blood pressure and pulse rate, causes severe pupil dilation, and reddening of the conjunctiva. Federal law prohibits medicinal marijuana in the US, however 14 states have passed laws allowing medicinal marijuana use. Dr. Robert Melamede has conducted much research on cannabinoids for medical use. The following website has interesting information: http://www.thc-ministry.net/cannabisinfo.htm


http://upload.wikimedia.org/wikipedia/commons/e/e1/Cannabis_sativa.jpg


Senses

Table of Contents

Sensory Receptors and Sensations
Proprioceptors and Cutaneous Receptors
Senses of Taste and Smell
Sense of Vision
Sense of Hearing
Sense of Equilibrium

Sensory Receptors and Sensations
Sensory receptors are dendrites specialized to detect certain types of stimuli. Exteroceptors are sensory receptors that detect stimuli outside the body, such as taste, smell, vision, hearing, and equilibrium. Interoceptors receive stimuli from inside the body.

• Chemoreceptors respond to chemical substances in the immediate vicinity and monitor blood pH.
  
• Nociceptors respond to pain and the chemicals released by damaged tissue.
• Photoreceptors respond to light energy.
• Mechanoreceptors are stimulated by mechanical forces.
• Thermoreceptors are stimulated by changes in temperature.

Sensory receptors respond to environmental stimuli by generating nerve impulses. When the nerve impulses arrive at the cerebral cortex, sensation occurs.


http://www.rci.rutgers.edu/~uzwiak/AnatPhys/Blood_Vessels_files/image012.jpg

Proprioceptors and Cutaneous Receptors


Proprioceptors are mechanoceptors involved in reflex actions that maintain muscle tone and thereby the body's equilibrium and posture. Muscle spindles act to increase the degree of muscle contraction and Golgi tendon organs act to decrease it. Cutaneous receptors are contained in the dermis layer of the skin. Cutaneous receptors make the skin sensitive to touch, pressure, pain, and temperature. Three types of cutaneous receptors are sensitive to fine touch, two types of cutaneous receptors are sensitive to pressure. Temperature receptors are simply free nerve endings in the epidermis.


http://www.unmc.edu/Physiology/Mann/pix_5/skin.gif

Nociceptors are pain receptors sensitive to chemicals released by damaged tissue. Some pain medication works by inhibiting the synthesis of certain chemicals. Sometimes, stimulation of the internal pain receptors is felt as pain from the skin as well as internal organs. This is called referred pain. For example, pain from the heart may be felt in the shoulder.


http://www.sfn.org/skins/main/images/brainbriefings/pain_illus_large.gif

Senses of Taste and Smell

Taste and smell are called chemical senses because their receptors are sensitive to molecules in the food we eat and the air we breathe. Taste cells and olfactory cells bear chemoreceptors. Olfactory cells act from a distance and taste cells act directly.

An adult human has about 3000 taste buds located on the tongue, hard palate, pharynx, and epiglottis. There are 4 primary types of tastes: sweet, salty, sour and bitter, and a 5th, umami may exist. Taste buds open at a taste pore and the taste is interpreted and weighted at the gustatory cortex.


http://www.nature.com/nature/journal/v444/n7117/images/nature05401-f1.2.jpg

Approximately 80-90% of what we perceive as taste is actually due to our sense of smell. Our sense of smell depends on between 10 and 20 million olfactory cells located within the olfactory epithelium. Olfactory cells are modified neurons. An odor's signature is determined by which neurons are stimulated. The neurons communicate this information to the cerebral cortex. The olfactory bulbs have direct connections to the limbic system, which accounts for the aroma-memory connection.


http://www.scienceclarified.com/images/uesc_09_img0525.jpg

Sense of Vision

Vision requires the work of the eye and the brain. The eyeball has 3 layers: the sclera, the choroid, and the retina.


http://library.thinkquest.org/J002508/images/eyeball.gif

The outer layer, sclera, is white and fibrous except for the cornea, which is made of transparent collagen fibers. The middle, thin, darly pigmented layer, the choroid, is vascular and absorbs stray light rays that photoreceptors have not absorbed. The iris regulates the size of the pupil, a hole in the iris center where light enters. The ciliary body contains the ciliary muscle, which controls the shape of the lens for near and far vision. The lens is attached to the ciliary body. The third layer, the retina, is filled with a clear, gelatinous material called the vitreous humor. The retina has a special area called the fovea centralis where cone cells are densely packed. The cornea, assisted by the lens and the humors focuses images on the retina.


http://www.lebct.org/donoreye.jpg (cornea)

The photoreceptors are called rods and cone cells. The visual pigment in rods is a deep purple pigment called rhodopsin. Rods are best suited to night vision because they are very sensitive to light. They also provide peripheral vision and perception of motion. The cones are activated by bright light. Color vision depends on 3 different types of cones which contain different pigments. The optic nerve carries nerve impulses to the optic chiasma. Fibers from the chiasma make up the right and left optic tracts, which take nerve impulses to the visual cortex.


http://www.eyedesignbook.com/ch3/fig3-61retinarods-conesBIG.jpg

Sense of Hearing

The ear has 2 functions: hearing and balance. The sensory receptors are located in the inner ear. The ear itself has 3 divisions: outer, middle, and inner. The outer ear consists of the pinna and auditory canal. The middle ear begins at the tympanic membrane and ends at a bony wall containing 2 small openings; the oval and round window. The bones in the ear are the malleus, the incus, and the stapes. The eustachian tube extends from them iddle ear to the nasopharynx and permits equalization of air pressure. The inner ear has 3 areas: the semicircular canals and the vestibule (equilibrium) and the cochlea (balance).


http://www.hearingprofessionals.co.nz/Images/The-Human-Ear.gif

Sound waves enter the auditory canal and strike the tympanic membrane. THe malleus takes the pressure from the inner surface of the tympanic membrane and passes it, by means of the incus, to the stapes in such a way that the pressure is multiplied about 20x as it moves. The pressure is passed to the fluid within the cochlea. The spiral organ in the cochleas is sensitive to different wave frequencies. The nerve impulses are passed from the cochlear nerve to the auditory cortex.

Sense of Equilibrium

The vestibular nerve arises in the semicircular canals, saccule, and utricle. It takes nerve impulses to the brain stem and cerebellum. Mechanoreceptors in the semicircular canals detect rotational and/or angular movement of the head (rotational equilibrium). Mechanoreceptors in the utricle and saccule detect movement of the head in the vertical or horizontal planes (gravitational equilibrium).


http://cache.eb.com/eb/image?id=14297&rendTypeId=4


References:

Brain (2008)
http://www.med.harvard.edu/AANLIB/home.html

Drugs (2008)
www.drugs.com

Mader, S. (2008) Nervous System and Senses. Human Biology 10th Ed. (p 247-94)

Nervous System (2008)
www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookNERV.html