neurons structure of a neuron
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Lecture 7, Part A: Chapter 11: The Nervous System
• The Nervous System
• Coordinates many of the body’s functions
• Central Nervous System (CNS):
– Brain and spinal cord
• Peripheral Nervous System (PNS)
– Nervous system outside of central nervous system
– 2 parts:
• Sensory Division of PNS – carries information to CNS
• Motor Division of PNS – carries information from CNS to other parts of body – Somatic Division: controls skeletal muscle (voluntary and
sometimes involuntary)
– Autonomic Division: controls smooth muscle, cardiac muscle, and glands
Neurons • Neurons (nerve cells) and nervous system
connective tissue make up all parts of the nervous system
• Neurons generate and transmit impulses through the body
• 3 Types: – Sensory Neurons of PNS: respond to stimuli and
transmit info to CNS
– Interneurons of CNS: transmit impulses between parts of CNS
– Motor Neurons of PNS: transmit impulses away from CNS
Structure of a Neuron
• Cell Body
• Dendrites
• Axon
Structure of a Neuron
– Cell Body • Large, round, central part of cell
• Contains nucleus and most of organelles
– Dendrites • Extensions that transmit signals (electrical impulses)
toward cell body
• More than 1 per neuron
– Axons • Extensions that transmit signals away from cell body
• Only 1 per neuron
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• Myelin sheath
– Composed of fatty cells called Schwann Cells (in peripheral nervous system) which wrap around the axon
– Forms protective covering
– “Gaps” in Myelin = “Nodes of Ranvier”
How are Signals Sent?
– All living cells have an electrical charge across their membranes (“membrane potentials”) • Anions (-) are more concentrated inside cell and cations
(+) are more common outside cell
– Only “excitable” cells (muscle cells and nerve cells) can generate large changes in their membrane potentials
– These excitable cells have gated ion channels that can regulate movement of ions across membrane in response to stimuli • Stimulus may be chemical (via neurotransmitters) or
electrical (via changes in membrane potential)
Neurons Initiate Action Potentials
• Neurons at rest have a “resting potential” of -70mV
– Meaning, the inside of the cells are negatively charged relative to the outside of the cells
• The sodium-potassium pump removes three Na+ ions for every two K+ ions it allows inside the cell
– This helps maintain the negative membrane potential
• Resting potential of neuron can change in response to signals from other neurons
• These transient changes in membrane potential can vary in size and are called graded potentials
– They fade away from the region of the cell membrane that is affected
– They may also be summed in space and time (summation)
• The cell membrane may become:
– Depolarized = having a membrane potential closer to zero
– Repolarized = returning to resting potential
– Hyperpolarized = having a membrane potential that is more negative than the resting potential
– If enough ion channels are opened up to depolarize the membrane, an “action potential” is generated • Occurs once the “threshold” is
met through summation of graded potentials
• In neurons, only occurs in axons
• Spreads rapidly down axon
• The result: the inside of the membrane rapidly becomes temporarily more positive than the outside
• Followed by a gradual re-polarization
– Signals are regenerated along axons • Sodium ions (Na+) entering cell depolarize membrane
• Action potential is generated in the neighboring region, opening more sodium gates
• Is spread forward, “jumping” from node to node (gaps in the Myelin sheath where plasma membrane is in contact with extracellular fluid) – = “saltatory” conduction
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• Myelin-coated axons transmit signals more rapidly
– Signals transmitted at approximately 1 mile/hour in nonmyelinated axons
– 250 miles/hour in myelinated axons
Action Potentials are All-Or-None and Self-Propagating
• An action potential (or “impulse”) is like the firing of a gun – once you reach the threshold (“pull the trigger”), the nerve “fires”
– Pulling the trigger lightly, or pulling it harder than necessary, won’t affect the firing of the gun
• Action potentials travel down the axon at a constant rate and amplitude
Neuroglial Cells
• Support and protect neurons
– Some help maintain composition of extracellular fluid
– Some provide physical support
– The Schwann Cells of the myelin sheath of axons in the PNS are a type of neuroglial cell
– In the CNS, the protective sheath is composed of oligodendrocytes
• This sheath degenerates once an axon is destroyed
• Make up about 80% of cells in the nervous system
• Nerve cells cannot be replaced
– Highly specialized
– Lose ability to divide
– Nerve damage is difficult to deal with
• Axons that have been cut in the PNS can potentially grow back together and/or reestablish connections to muscle tissue – Schwann Cells do not degenerate
• Severed axons in brain or spinal cord cannot regenerate, however – Oligodendrocytes degenerate after axon is destroyed
– We do generate new brain cells into adulthood
• Discovered in 1998; new cells found in the hippocampus – Area involved with memory and learning
• Mice living in stimulating environments and exercised vs. those in standard cages – Those in the enhanced environment had more new brain cells
in the hippocampus
– Performed better on learning tasks
– Application to humans: physical exercise and mental activity may result in greater learning capacity
– New neurons have also been identified in human brain
– Must come from adult stem cells in brain
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• Severity of spinal cord injury depends on where the spinal cord is injured – High in neck (2nd or 3rd cervical
vertebrae) – disrupts signals to muscles controlling breathing
– Below 5th vertebra in neck paralyzes legs and arms (quadriplegia)
– Below nerves that control arms, only legs are paralyzed (paraplegia)
– Brain damage
• Neurons are energetically expensive and require much oxygen
• Biggest single-organ oxygen consumption: 1) liver (20.4%)
2) brain (18. 4%)
3) heart (11.6%)
• Lack of oxygen to brain cells (within 4-5 minutes) often results in death of nerve cells
• The longer the amount of time without oxygen, the greater the damage
Synapses • How are impulses transmitted from neuron to
neuron?
– Small gaps (synapses) separate adjacent neurons
– Ends of neurons are branched, ending in a terminal bouton
– Postsynaptic and presynaptic neurons
• When nerve impulse reaches the terminal bouton, stimulates release of neurotransmitters
• Travel across the gap (synaptic cleft) to receptors on the next (postsynaptic) neuron
• When enough neurotransmitters bind to receptors, an action potential is generated
Neurotransmitters Video: http://www.youtube.com/watch?v=haNoq8UbSyc&feature=BFa&list=PL801A75AA4ED9C39D&index=41
– Many pesticides work by disrupting the nervous system • Cause build-up of neurotransmitters in synaptic cleft,
making the neuron continue to fire
• In people exposed to pesticides, can cause headache, blurred vision, rapid pulse, & sweating
• Exposure to organophosphates in children can result in learning disorders and behavioral problems
• Children living near an agricultural area where malathion and parathion were used experienced reduced visual acuity (Ishikawa 1970)
• Organophosphate pesticides have also been shown to reduce intellectual functioning, abstract thought, and simple motor skills
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– Malathion used extensively to control crop pests and mosquitos
– Over 600,000 pounds applied annually in California
– EPA regulates pesticides, and has not found Malathion to pose “unreasonable” risks
– But evidence is mounting against it (EPA was even prepared to list it as a carcinogen)
– Anesthetics – also work by temporarily disrupting neurotransmitters associated with producing pain signals
• May block neurotransmitter production (pre-synaptic)
• May block post-synaptic receptors
• Brain
– Housed inside skull
• Spinal cord
– Housed inside vertebral canal
• Meninges
– three layers of connective tissue that surround brain and spinal cord
– Space between middle and innermost layer filled with cerebrospinal fluid; provides cushioning
• The Brain
– Cerebrum
• Largest part of brain
• Left and right cerebral hemispheres
• Thin outer layer composed of gray matter: the cerebral cortex; highly folded
• Each cerebral hemisphere divided into four lobes: frontal, temporal, occipital, and parietal lobes – The types of processing that occur in each lobe differ between
the left and right hemispheres
• During development, left hemisphere becomes specialized for – Language
– Math
– Logic
– Speed-optimized activities
– Processing of visual and auditory details
• Right hemisphere becomes specialized for – Pattern recognition
– Face recognition
– Emotional processing
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– Cerebral cortex has 3 main functions processed in 3 general areas of cerebrum:
• Receive sensory input (sensory cortex)
• Integrate sensory information (association cortex)
• Generate motor responses (motor cortex)
– “Consciousness resides in the cerebral cortex”
– (See Figure 11.15, 11.16, and 11.17)
• Einstein’s Brain – Not unusually large overall
• Actually slightly smaller than the average male brain
– Part of brain associated with mathematical thought (parietal lobe) was 15% wider than the average brain
– The groove extending from front to back of brain (sulcus) didn’t extend all the way back • Might have allowed more neurons to
connect across sides of the brain
– May also have had more glial cells (cells which support neurons’ activity)
– Cerebellum • Second-largest
structure of brain
• Located below cerebrum
• Controls many unconscious body functions (movement)
• Coordinates contraction of muscles, allowing smooth motion
– Thalamus
• Located beneath cerebrum
• “Relay center” – receives sensory information (except smell) and relays the information to sensory and association cortex
– Hypothalamus
• Beneath the thalamus
• Control many automatic responses, such as appetite, body temperature, and blood pressure
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– Limbic System
• Controls instinctive behavior (e.g. “fight-or-flight” response, territoriality, etc.)
• Also important in emotions (fear, anger, etc.) – Stimulating different parts of the limbic system with
electrodes can make a person feel rage, calmness, joy, or other emotions
– Brain stem
• Connects brain to spinal cord
• Controls many automatic body functions, such as heart rate, blood pressure, breathing, and swallowing
• Spinal Cord
– A ropelike aggregation of nervous tissue
– Nerves extending from spinal cord connect to skin, muscles, bones and organs of body
• YouTube video: “Neurons– How They Work– Human Brain”
• http://www.youtube.com/watch?v=o9p2ou1IyC0
• Disorders/Diseases of the Nervous System – Multiple Sclerosis
• Cause: damage to Myelin sheath
• Thought to be an autoimmune disorder (body attacking its own cells)
• Demyelination of axons reduces efficiency of signal transmission
• Eventually, the axons themselves may be destroyed
• Symptoms: mild weakness; tingling/numb feeling in part of the body; blurred vision; slurred speech
• Repeated attacks may continue to damage nervous system
• Eventually, loss of vision and increased weakness
– Stroke • Cause: lack of blood flow to part of brain
• Can result from an artery breaking inside brain, or when an artery supplying brain with blood is blocked
• Parts of brain die
• As a result, often lose muscle control in part of body
• Other parts of brain may take over this function and partial or complete recovery is possible
– Video: brain plasticity
– https://www.youtube.com/watch?v=2MKNsI5CWoU
– Alzheimer’s Disease
• Progressive loss of mental function
• Generally in people over 65
• Early symptoms: forgetfulness and irritability
• Causes not known – May be related to previous brain injury
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– Parkinson’s Disease • Deterioration of parts of brain that control movement
• Symptoms: shaking hands or head (tremors)
• Speaking may eventually become difficult
• Falls become more frequent
• Memory and thinking eventually deteriorate
• Cause: lack of dopamine – Can be treated with levadopa
• Ultimate cause unknown – Chemical pollutants may contribute
» Pesticides, herbicides, PCBs, etc.
• Fetal cell transplants may be helpful in restoring dopamine production
– Brain Tumors • Benign or malignant
• Benign: do not grow out of control, but can put pressure on parts of brain – Can be removed surgically
• Malignant tumors – Grow rapidly
– Can spread to other parts of body
• May be caused by exposure to carcinogenic chemicals – Oil refining, drug manufacturing, rubber manufacturing
industries
• Viruses and genetics may also play a role
• A brain tumor virus? • A 2002 study by neurosurgeon
Charles Cobbs found a common virus in nearly all brain tumors he studied
• CMV – cytomegalovirus – a form of the herpes virus – Found in 80% of the population – Usually harmless; fatigue – Was active in the brain tumors, but
dormant in other tissues – Glioblastoma Multiforme – a deadly
form of brain cancer – Senator Ted Kennedy died from this
disease August, 2009 – Senator John McCain was diagnosed
with this type of brain cancer in 2017 – Potential for a vaccine
• Can we improve the function of our brains through exercise?
• Video: exercise makes you smarter:
• http://www.youtube.com/watch?v=4v6OLCF5Qcg&feature=related
Bio 1102, Lec. 7, Part B: Chapter 12 -- Sensory Mechanisms
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Receptors Receive & Convert Stimuli
• Receptors: a structure specialized to receive certain stimuli
• Types of receptors: – Mechanoreceptors: respond to mechanical energy
such as sound, changes in fluid pressure, touch or pressure, stretching, or forces generated by gravity or acceleration
– Thermoreceptors: respond to heat or cold
– Pain receptors: respond to tissue damage or excessive temperature or pressure
– Chemoreceptors: respond to chemicals
– Photoreceptors: respond to light
Some receptors adapt to continuing stimuli
• Receptor adaptation: the ability for some receptors to “ignore” inputs – Stops sending signal to CNS, even though stimulus is still
present
– Example: sensation of wearing a ring
• Skin receptors for light touch and olfactory receptors adapt rapidly
• Receptors for pain, joint and muscle receptors, and “silent” receptors associated with homeostasis, adapt slowly or not at all
Somatic and Special Senses
• Somatic sensations: originate from receptors present at more than one location in the body
– Examples: temperature, touch, vibration, pressure, pain, and awareness of position or movement
• Special sensations: originate from receptors that are restricted to a particular part of the body
– Examples: taste, smell, vision, hearing, and balance
• The Eye
– Located in eye sockets in skull
– 6 small muscles attach eye and control movement
– Three layers of eye
• Outer protective layer (white sclera, but clear in front forming cornea)
• Middle layer (choroid) contains melanin – In front of eye, forms the colored iris
– Smooth muscle of iris controls pupil dilation
– Opening in iris – pupil (where melanin of middle layer is seen, as well as pigmented part of retina)
• Innermost layer -- retina
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– The Retina
• Consists of an outer pigmented layer and inner layer of photoreceptors – Specialized nerve cells that detect light
• 2 Types of Photoreceptors – Rods: sensitive to low light conditions
» Dim, grayish images
» Black-and-white
– Cones: operate only in brighter light
» Sharp images
» Color vision
» Red, blue, and green cones
• Different types of animals have the ability to see different parts of the electromagnetic spectrum – Dogs and cats mostly see greys, with some blues
and yellows
– Birds, monkeys and humans can see a wide range of colors. Why would we evolve the ability to see this diversity of colors, when animals like cats and dogs can’t?
– Vitamin A (which we derive from Beta Carotene, as found in carrots and other orange fruits & vegetables) necessary for vision • Vitamin A needed to produce retinal • A form of retinal is bound to rods and cones • Light strikes the retinal, causing it to dissociate
from the photoreceptor • This triggers a signal to the optic nerve • Your mom was right! Eat carrots to improve
your vision! (to a certain extent)
– Transmission of visual nerve impulses
• Rods/cones other neurons ganglion cells (a type of nerve cell), the axons of which unite to form optic nerve visual cortex of brain
• Focusing of Light
– Function of the cornea and lens
• Lens located behind iris
• Lens held in place and adjusted by thin ligaments that attach to smooth muscle in the ciliary body
• Changing shape of the lens focuses light
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– Cataracts
• A clouding of the lens
• Occurs with age, and also with damage from ultraviolet radiation
• Risk highest for dark-eyed people
• Surgery involves removing the lens and replacing it with a plastic one
• Preventative measures: wear sunglasses that exclude UV radiation
– Glaucoma
• Build up of fluid in anterior portion of eye
• Pressure can damage retina and optic nerve
• Can lead to blindness
• Can be treated with eye drops and help drain fluid from the anterior chamber of the eye
– Nearsightedness (Myopia) • Cause: eye too long or lens too strong
• Result: images far away are fuzzy, while nearby images are in focus (see Fig. 12.17)
• Common (20% of Americans)
• Can be corrected with glasses, contacts, or surgery
– Farsightedness (Hyperopia) • Cause: eye too short or lens too weak
• Result: images of far away objects are in focus, but nearby objects are fuzzy
• Also corrected with glasses, contacts, or surgery
– Astigmatism • Cause: surface of cornea or lens disfigured
• Result: fuzzy images
• Corrected with glasses, contacts, or surgery
• Taste
– Receptors for taste = taste buds
– Upper surface of tongue
– Respond to chemicals in food
• Dissolved in saliva
• Enter openings leading to interior of taste bud
– 5 basic flavors: sweet, sour, bitter, salty, and “umami” (meaty taste associated with MSG)
• Smell
– Receptors located in roof of nasal cavity
• Olfactory membrane
– Olfactory receptor cells have 6-8 projections called olfactory hairs
• Bind to chemicals in air
• The Ear: Hearing and Balance
– Outer Ear, Middle Ear, and Inner Ear
– Outer Ear
• Auricle
• Ear lobe
• External auditory canal
• Transmits sound waves to middle ear
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– Middle Ear • Located within temporal bone of skull
• Eardrum separates middle ear from external auditory canal
• Eardrum vibrates when sound waves strike it
• 3 bones in middle ear: hammer, anvil, and stirrup
– Hammer: attached to ear drum; vibrates when eardrum vibrates
– Anvil: attached to hammer; rocks back and forth when hammer vibrates
– Stirrup: attached to anvil; moves when anvil moves; attaches to a membrane in inner ear (oval window)
– Inner Ear
• Large cavity in temporal bone
• Contains the cochlea – a snail-shaped boney structure – Contains auditory receptors (hair cells) and fluid
– Vibrations in fluid stimulate hair cells
– Hair cells send impulses to auditory cortex of brain
– Hearing Loss
• Conduction Deafness: occurs when sound waves cannot be “conducted” to inner ear – Example: as a result of ear infections, scar tissue may build
up, causing bones of middle ear to fuse together
– Treated by hearing aids (transmit sound waves through skull to inner ear)
• Nerve deafness: results from damage to hair cells – May result from very loud noises which damage the hair cells
– May also result from damage to the nerve leading from the cochlea to the brain
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– Vestibular Apparatus
• Located near the cochlea
• 2 main parts: semicircular canal and the vestibule (saccule and utricle)
• Receptors here detect body position and movement
• Semicircular canals – Tubes of bone filled with fluid
– Base of each canal contains mechanoreceptors
– Detect movement of the fluid
– Detect rotational movement of head
• Vestibule – Consists of utricle and saccule; two fluid-filled chambers
– Otoliths, hard crystals of bone-like material, float in gel
– Also contain mechanoreceptors
– Provide information about gravity
– Example: acceleration, or your angle with respect to gravity (static body position)
Activity Quiz #7
• Log on to Carmen Canvas and complete Activity Quiz #7
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