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

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