CH 8 The Nervous System

Central Nervous System

• Central Nervous System- includes the brain and spinal cord
• Brain and Spinal Cord
• Brain- protected by the cranium, the principal organ of the nervous system; contains higher centers of thought and is the chief organ of the mind
• Spinal Cord- a thick bundle of nerve fibers located within the spinal cavity
• Spinal nerves- transmit nerve signals to and from the rest of the body
• Cranial nerves- another 12 pairs of nerves that branch directly from the brain stem and transmit nerve signals to and from the eyes, ears,mouth, face, and scalp
• Meninges- triple layer of protective tissues that covers the brain and spinal cord; composed of 3 layers
• Dura mater- outer layer
• Arachnoid- middle layer
• Pia amter- inner layer
• Cerebrospinal fluid- a clear fluid that circulates through the fibers of the arachnoid serving to cushion the brain when you bump your head
• Neurons- actual nerve cells that compose brain and spinal cord
• Cell body- contains a nucleus and a cytoplasm
• Dendrite- short branched extension of the cell that receives nerve impulses from other neurons and conducts them toward the cell body
• Axon- long extension which relays nerve impulses from the cell body to other neurons.
• Nerve impulses travel in one direction in a nerve fiber- from the dendrites toward the cell body and from the cell body toward the axon

• Gray matter- found within the brain and spinal cord consists largely of the cell bodies of neurons and is gray because the cell bodies lack the white covering known as myelin
• White matter-found within the brain and spinal cord composed of axons and glial cells that are white because they have the white covering myelin.
• Ganglion- mass of cell bodies
• Plexus- ganglion grouped together
• Nerve Center- group of cell bodies in the brain or spinal cord
• Types
• Sensory neurons-neurons that transmit information to the CNS from the senses
• Motor neurons- neurons that relay signals from the CNS to the other parts of the body
• Interneurons- found in the CNS only; relay signals between neurons or group of neurons.
• Blood-brain barrier- tightly sealed capillary walls that prevent bacteria, viruses, and harmful molecules from entering the brain or spinal cord.
• Diseases
• Meningitis- disease caused by microorganisms entering(slipping through) the nerve tissue and infect the meninges.
• Poliomyelitis- a serious disease that attacks the spinal cord; polio

Peripheral Nervous System

• Peripheral nervous system- consists of nerves (bundles of nerve fibers, axons, branching from the brain and spinal cord and connecting the CNS to the extremities of the body)
• Cranial and spinal nerves
• Cranail nerves- 12 pairs; branching directly from the brain
• Spinal nerves- 31 pairs; connected to the brain by the way of the spinal cord.
• Sensory nerve fibers- carry impulses from the senses from other parts of the body to the spinal cord and brain for analysis
• Motor nerve fibers- carry impulses from the brain and spinal cord to produce action in muscles and organs.
• Automatic nervous system- part of the PNS that controls the heart and other internal organs; controls body functions that are not controlled consciously
• Sympathetic and parasympathetic divisions- divisions of automatic nervous system , controlled by the hypothalamus
• sympathetic- responds to your body's needs during increased activity and in emergency situations
• parasympathetic division- acts as a balance to counteract the actions of the sympathetic division

Structure

• Median nerve- controls the muscles of the forearm and the muscles and skin of the hand
• Consists of bundles of nerve fibers imbedded in connective tissues
• Within bundles lies axons imbedded among glial cells and protected by a myelin sheath
• Each axon relays either a sensory or motor impulse(never both)
• Nerve cells- responsible for the process of nerve impulse conduction; rarely reproduce
• Schwann cells- glial cells which produce layers of myelin sheathing that acts much like the insulation on an electrical wire; transmit nerve impulses much faster than those which lack a covering
• Defective impulse transmission
• Multiple sclerosis- a disease of the brain or spinal cord; occurs when the body's immune system attacks the glial cells that provide myelin sheaths for nerve cell axons, sheaths deteriorate and are replaced by scar tissue.
• Process of neurons
• Neuron is triggered to fire, a brief pulse of positive charge sweeps through the neuron and races down the axon creating an action potential( opens and closes sodium and potassium ion gates, allowing them briefly to flow in and out of the cell). After the action potential has passed the cell membrane's ion pumps restore the cell's original ion balance, preparing the cell to fire again in as little as 1/2000 second.

• Synaptic transmission
• Synapse- enclosed junction between the axon of one neuron and the dendrites or cell body of another.
• Neurotransmitter- chemical that is released into the synapse that makes the neuron generate electrical signals that trigger the neuron to fire.
• Inhibitors of nerve impulses- some substances can inhibit the transmission of nerve impulses.
• Parkinson's disease- serious disease that affects the nervous system; affects the patients control of posture and movement, characterized by tremors and stiffness of the limbs.

Reflex

• Reflex- simplest act of the nervous system; quick automatic response
• Reflex arc- pathway of a reflex; short circuit that allows nerve impulses to bypass the brain when a speedy response is necessary.
• Interneurons transmit an emergency signal to the appropriate motor neurons which cause the muscles to respond

Cerebrum

• Cerebrum- largest part of the brain and is the physical organ that helps you in the areas of consciousness, memory, voluntary actions, thinking, and intelligence.
• Spilt into right and left hemispheres
• Hemispheres split by the longitundinal fissure.
• Right hemisphere controls the left side of the body
• Left hemisphere controls the right side of the body
• Right hemisphere- specializes in "big picture", creativity, and intuitive approaches to problems
• Left hemisphere- specializes in details, known procedures, and formal logic
• Corpus callosum- mass of nerve fibers located at the base of the cerebrum that allow the two hemispheres to communicate.
• Gray and white matter
• Cerebral cortex- the outer layer of the cerebrum; contains most of the brain's gray matter
• White matter lies in the interior and consists largely of bundles of myelin-covered nerve fibers that extend from the cell bodies in the cerebral cortex

• Cerebral Lobes- regions caused by the convolutions of each of the cerebral hemispheres that are divided by shallow groves; correpsond to the major bones of the cranium
• Frontal lobes- associated with personality, judgment, and self control.
• Rear portion is the motor area that controls voluntary movement of the skeletal muscles
• Parietal lobes- located behind the frontal lobe; analyze the senses on their respective sides of the body that allow us to feel sensations
• Temporal lobes- associated with the senses of hearing, taste, and smell
• Occipital lobes- associated with the sense of vision.
• Damage
• Cerebral palsy- damage to the cerebral motor area

Cerebellum

• Cerebellum- second largest region of the brain.
• Divided into right and left hemispheres
• Function and purpose
• Function- of the cerebellum is primarily to coordinate skeleteal muscle activity; relieves the cerebrum
• Most complex muscle coordniatrion is handled by the cerebellum

Brain Stem

• Brain stem- located between the cerebrum and the spinal cord; all nerve fibers which connect the brain and the spinal cord must pass through it.
• Medulla oblongata- lowest part of the brain stem; contains nerve centers that monitor and regulate breathing, heartbeat, blood pressure, and other vital bodily functions
• Pons- links the cerebrum with the cerebellum
• Midbrain- help coordinate the movements of both eyes
• Recticular formation- intricate network of neurons that function as the master switch of the cerebrum; activates your cerebral cortex to a state of "high alert" to protect you from danger

Limbic System

• Limbic system- number of complex brain structures that lie clustered around the brain stem at the core of the brain, surrounded by the cerebrum
• Involved in coordinating the activity of the different parts of the brain; generate and regulate emotions and desires in coordination with incoming sensory information and the powers of reason in the cerebrum
• Thalamus and hypothalamus
• Thalamus- uppermost end of the brain; acts like a switchboard routing activation signals from the reticular formation and sensory impulses from various parts of the body to the appropriate areas of the cerebral cortex
• Hypothalamus- serves as the control unit for your body's automatic systems; responsible for the physical effects of emotions

Alcohol and the Nervous System

• Alcohol- one of(if not the most) commonly abused drugs
• Effects
• When it reaches the brain it interferes with specific neurotransmitter systems in the thought and motor areas of cerebral cortex.
• Acts as a depressant of the CNS by slowing down the functions of the neurons
• Breaking down alcohol
• Liver is the only organ able to break down alcohol
• Alcohol is removed from the bloodstream by slowly converting it into another poison responsible for the "hangover" that follows
• Long-term effects
• Neuritis- degeneration and inflammation of nerves
• Alcohol can also cause permanenet decline in brain function
• Alarming facts
• Most widely used drugs in the world
• Average age of a person who starts drinking is 12(actually 10 by now)
• At least 100,000 alcohol related deaths occur in USA every year
• Nearly a 3rd of all suicides are alcohol related
• There are perhaps 20 million alcoholics in the US
• Fewer than 5% of all alcoholics are found on "skid row"

Injuries and Death

• Spinal cord and nerve injuries
• Sciatica- condition characterized by pain in the lower back and the back of the thigh, caused by a pinching of the sciatic nerve of the lower spinal cord.
• Whiplash
• Brain injuries
• Stroke- leading cause of death throughout the world; internal blockage or rupture of a blood vessel in the brain
• Concussion-a period of paralysis of the CNS accompanied by a short period of unconsciousness
• Amnesia- loss of memory
• Coma- prolonged unconscious state that may not be reversible
• Diseases
• Tetanus- lowckjaw; sever spasms of jaw muscles
• Senile Dementia- disorder that results from the slow process of natrual cell loss
• Arteriosclerotic dementia- caused by the death of brain cells
• Alzheimer's disease- symptoms similar to dementia but more severe
• Epilepsy- brain cells of the cerebral cortex fire in seemingly random patterns as they help process the normal thoughts and actions of life.

Sensory Receptors

• Sensory receptors- living sensors that receive information relayed to the spinal cord or brain
• Specialized neurons are divided into 2 groups
• Somatic senses- include all the sense of your skin
• Special senses- sight, hearing,m smell, taste and balance; neurons are located in the sense organs

Sensations

• Pain receptors- a bare dendrite that reacts to a certain type of strong stimulus
• Mechanoreceptors- sensory receptors responsible for sensations and pressure; lie at different depths in the layers of the skin
• Thermoreceptors- respond to temperature changes
• Heat receptors- react to above normal body temperature
• Cold receptors- sensitive to temperatures below normal body temperature

Chemical Senses

• Chemical senses- senses of taste and smell; result from stimulation of chemoreceptors on the tongue and in the nose
• Taste
• Organs: taste buds- chemoreceptors on the back, sides, and front of the tongue that detect dissolved chemicals in the mouth
• 4 identities: sweet, sour, salty, or bitter
• Most flavors we taste are strongly affected by the odors that we smell
• Smell
• Organ: olfactory nerve- connect the brain to the nose sensory receptors
• 7 primary odors: camphorlike(mothballs), ethereal (ether), floral, minty, musky, pungent(vinegar and spices), and putrid(rotten)

Ears

• Outer ear- part of the ear that we see
• Pathway- from outer ear sound waves travel through a 1in canal that leads to the eardrum- a taut membrane stretched across the canal like the surface of a drum
• Lining the canal is wax an dhairs that trap any foreign invaders
• Middle ear- chamber that contains the malleus, incus and stapes
• Vibrations from eardrum are relayed to the inner ear by 3 tiny bones( malleus, incus, and stapes) the hammer, anvil, and stirrup
• Inner ear- actual organ of hearing
• Cochlea- main component of the fluid filled chamber; a coiled tube resembling a snail's shell
• Auditory nerve- electrical messages sent by whte eaving of cells caused by the vibrations
• Semicircular canals- serce as balance sensors
• 3 fluid filled chambers
• One loop detects forward motion, one side-to-side motion, and one up-and-down motion
• Bone conduction- type of hearing received from the vibrations of the jawbones
• Disease
• Punctures- threat to hearing
• Tinnitus- ringing of the ears
• Ear infections- occur when microbes from the throat travel up the eustachian tube to the middle ear.

Eyes

• Sockets, eyelids, and tears
• Socket and eyelid protect the eye
• Tears- product of tear glands(lacrimal glands) which are located under the upper eyelid on the side way from the nose; more than 90% water, also contain lysozyme which destroys bacteria
• Eye movement- extrinsic muscles that move the eye(6 muscles total)
• General descriptions
• Outer part of eyes- composed of 3 layers
• Sclera- opaque; white of the eye
• Uvea- middle layers includes the iris, ciliary body, and choroid
• Retina- innermost layer composed of 3 more separate layers
• Retina broken down
• Rods and cones- cells found in the retina
• Fovea- small depression of the retina; area that produces the clearest vision
• Optic nerve- bundle of nerves that caries processed signals to the vision centers of the brain
• Persistence of vision- image or picture of the object on the retina persists(remains) about a tenth of a second after the object is gone

• Defective vision
• Nearsightedness- myopia, the ability to see only near objects clearly; can be corrected by wearing concave lenses
• Farsightedness- hyperopia, the ability to see only far away objects clearly; can be corrected by wearing convex lenses
• Astigmatism- focusing problem; result of an symmetrically curved cornea, which causes light rays entering the eye to be bent incorrectly
• Colorblind- people who cannot distinguish one or more of the primary colors of light
• Night blindness- when the eyes cannot adjust to darkness
• Glaucoma- condition in which the pressure of the fluid inside the eye becomes much higher than normal causing permanent damage to the cells of the retina.