Human Physiology Unit 2 Exam

**In Unit 2 you will learn about the major control system of the body – the nervous system. You will learn about the particular cells of the system, the different branches of the system and how and when they work. You will also investigate different types of communication signals, electrical and chemical. Chemical signals will be addressed in Chapter 17 The Endocrine System. 

Keep thinking on a cellular level..**

– Nervous System 

– Somatic Nervous System 

– Autonomic Nervous System 

–  Endocrine System 

You will have 100 minutes to answer these 50 multiple choice questions. Many have more than one correct answer. You must choice the most correct answer. Once you open the exam you must finish it. If you go over the allotted time, the exam will close and your answers will be automatically submitted for grading. If you finish before time is up, remember to submit your exam.

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PHYSIOLOGY

Chapter 17 THE ENDOCRINE SYSTEM

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Long-distance signaling

Nervous system – uses electrical or chemical signals that result in quick response and short-term actions. External stimulus.

Endocrine system – chemical signals only = hormones; transported through blood, bind to target cells to effect a response.

May be quick such as epinephrine, or slow such as reproductive hormones.

Many hormones work at different receptor sites to produce different reactions at each site.

Response to internal stimuli to regulate homeostasis.

Endocrine gland
Endocrine glands are ductless glands that secrete hormones into surrounding interstitial fluid. These are then picked up by the blood and transported throughout body.
Examples: Pituitary, adrenal, thyroid, parathyroid, pancreas.
Many other organs and tissues have some endocrine function.
Endocrine = hormone enters blood or lymph and acts far away
Autocrine = some hormones act on the cells that secrete them
Paracrine = hormones that mainly stay local but act on cells other than themselves
Can only affect their target cells due to the specificity of receptors.

Basic types of hormones
Amine – modification of amino acid tyrosine or tryptophan eg. Melatonin
Peptide and Protein – chains of amino acids eg. Catecholamines, thyroid hormones, ADH, Growth hormone
Steroid Hormone – derived from cholesterol, insoluble in water so must travel with a protein carrier eg. Aldosterone, reproductive hormones, cortisol

Pathways of hormone action
Binding of Lipid-Soluble Hormones
A steroid hormone directly initiates the production of proteins within a target cell.
Steroid hormones diffuse through the cell membrane.
The hormone binds to its receptor in the cytosol, forming a receptor–hormone complex.
The receptor–hormone complex then enters the nucleus and binds to the target gene on the DNA.
Transcription of the gene creates a messenger RNA that is translated into the desired protein within the cytoplasm.

Pathways of hormone action
Binding of Water-Soluble Hormones
Water-soluble hormones cannot diffuse through the cell membrane.
Bind to a surface cell-membrane receptor. The receptor then initiates a cell-signaling pathway within the cell involving G proteins, adenylyl cyclase, the secondary messenger cyclic AMP (cAMP), and protein kinases.
Protein kinases phosphorylate proteins in the cytoplasm. This activates proteins in the cell that carry out the changes specified by the hormone.

Target cell response and regulation
Down Regulation – If there is too much hormone, cells decrease production of receptors to maintain normal levels of activity.
Up Regulation – If there is too little hormone, cells will make more receptors to trap all the hormone they can.
Permissive – cell must have more than one hormone present to allow activity of hormone(s). Eg. Thyroid and reproductive hormones
Synergistic – two with similar actions combine to amplify the response eg. FSH and estrogen
Antagonistic – hormones with opposing effect eg. Insulin and glucagon
Positive feedback loop – more = more
Negative feedback loop – more = less

Command center complex
Hypothalamus–Pituitary Complex
Secretes several hormones itself
Secretes hormones that regulate other hormones
Coordinates messages between endocrine and nervous system

Pituitary gland
Anterior Pituitary
Glandular in origin
Secretes 7 hormones
Posterior Pituitary
Neural in origin. An extension of the hypothalamus
Stores and releases 2 hormones produced by the hypothalamus

Posterior pituitary hormones
Oxytocin
Stimulates uterine contractions through positive-feedback mechanism
After childbirth, responsible for the milk-ejection reflex
Parent-newborn bonding
Sexual response
ADH or vasopressin (due to vasoconstriction)
An increase in blood osmolarity, primarily Na+, releases ADH. Acts on collecting ducts in kidney to open aquaporin channels and to resorb water back into the bloodstream.
Negative-feedback system
Alcohol inhibits ADH, thus allowing kidney to dump water. Creates dehydration and a hangover.
Diabetes Insipidus – underproduction of ADH.

Anterior pituitary hormones
Anterior pituitary actually makes the hormone but the hypothalamus controls the release through a stimulating hormone or an inhibiting hormone.
Growth hormone/ somatotropin
Promotes protein synthesis and tissue building
Controlled by GHRH and GHIH from the hypothalamus
Stimulates lipolysis in cells for ATP production and stimulates liver to break down glycogen. Both increase blood glucose.
Stimulates insulin-like growth factor from liver, increasing cell proliferation and protein synthesis especially in skeletal muscle and cartilage.
Gigantism = too much GH in children
Acromegaly = too much in adults, causes bones of face, feet and hands to grow.
GH deficiency/pituitary dwarfism = too little GH

Anterior pituitary hormones
Thyroid Stimulating Hormone (TSH)
Regulated by TRH from the hypothalamus
Regulates thyroid gland production of thyroid hormones through negative-feedback loop.
Synthesis of T3 and T4
TSH binds to cells of thyroid gland causing cell to take up iodide from the blood.
Inside the cell, iodide is oxidized to iodine and moved into colloid, a matrix surrounded by and produced by follicle cells.
In colloid, iodine is attached to the amino acid tyrosine in the protein thyroglobulin. Thyroglobulin may have one iodine or two. Thyroglobulins are then paired so the whole unit has either 3 iodines, T3, or 4 iodines, T4.

Anterior pituitary hormones
TSH cont’d.
T3, T4 are released into the blood. Most are bound to protein carriers. Free T3,T4 can move into cells to
Influence basal metabolic rate
Bind to mitochondria to increase ATP production. This increased production is inefficient and therefore much heat is released
Increase cells’ sensitivity to catecholamines – increases HR and BP
Hypothyroidism – Usually due to insufficient iodine in the diet. TSH increases, results in accumulation of thyroglobulin and colloid thus increasing the size of the thyroid gland = goiter.
Hyperthyroidism – Increase in T3,T4 usually as a result of a pituitary or thyroid tumor. Grave’s disease is autoimmune stimulation resulting in a goiter.

Hypothyroidism with goiter
Goiter
(credit: “Almazi”/Wikimedia Commons)

Anterior pituitary hormones
Thyroid gland also produces Calcitonin.
Increased Ca++ in the blood causes thyroid gland to release calcitonin. This results in
Decreased osteoclast activity
Increased osteoblast activity
Increase loss of Ca++ through urine
Parathyroid glands produce parathyroid hormone (PTH)
Released when Ca++ blood levels fall
Increases osteoclast activity, Ca++ resorption from the kidney, and absorption of dietary Ca++ from the gut.
Hyperparathyroidism – too much PTH, results in decreased bone density, may result in spontaneous fractures and/or Ca++ deposits in soft tissue
Hypoparathyroidism – affects Na+ balance thus muscles

Anterior pituitary hormones
Adrenocorticotropic Hormone (ACTH)
The hypothalamus releases Corticotropin-releasing hormone (CRH) that causes the pituitary to release ACTH.
ACTH is derived from a precursor molecule Pro-opiomelanotropin (POMC).
POMC also is a precursor to melanocyte-stimulating hormone and the endorphins, endogenous opioids that suppress pain.
ACTH acts on the Adrenal Gland in response to stress.
Cortex of the adrenal gland made up of three layers
Produces mineralocorticoids – aldosterone
Produces glucocorticoids – cortisol
Produces androgens
Medulla is an extension of the Autonomic Nervous System and produces catecholamines.

Anterior pituitary hormones
Adrenal gland, cont’d.
Cortex is a component of the Hypothalamus-pituitary-adrenal axis (HPA)
Function is to respond to stress through the HPA axis by regulating BP, BV, Fluid and electrolyte balance and long-term stress.
Mineralocorticoids – affect minerals especially Na+ and K+ to control fluid and electrolyte balance
Aldosterone – major mineralocorticoid. A decrease in Na+ in the blood causes the release of aldosterone to reabsorb Na+ in the kidney and ultimately water to increase BP and BV. Also, part of the Renin-Angiotensin-Aldosterone system. Angiotensin II signals the cortex to release aldosterone
Glucocorticoids – named thus due to their role in glucose metabolism
Cortisol – released as a result of long-term stress. Inhibits tissue building while increasing the breakdown of glycogen, triglycerides, and muscle protein into fuel. Also decreases the immune response to inflammation.

Anterior pituitary hormones
Adrenal gland cont’d.
Androgens – Supplemental sex/reproductive hormones. In the tissue they are converted to testosterone and estrogens.
The medullary tissue is composed of SNS neurons and is responsible for the release of catecholamines in response to short-term stress – Fight or Flight
General Adaptation Syndrome – how the body responds to stress; acute, chronic or both.
Alarm reaction – short-term, Fight or flight, catecholamines released from the medulla
Stage of resistance – when short-term is not relieved, body tries to compensate for the stress. Starving? Increase more nutrients from the gut…
Stage of exhaustion – decompensation or can’t compensate any longer. Physical and psychological results….

Anterior pituitary hormones
Disorders of ACTH
Cushing’s disease – too much cortisol, increased blood glucose, localized accumulation of fat, rapid weight gain. Result of pituitary tumor?
Addison’s disease – not enough cortisol. Decreased Na+ and BG. Symptoms are vague and so is cause.
FSH and LH – follicle stimulating hormone and luteinizing hormone. Reproductive hormones released during puberty in response to GnRH from hypothalamus.
FSH stimulates the production and maturation of gametes (ova and sperm)
LH stimulates ovulation, estrogens and progesterone production in females and testosterone in males.
Prolactin – promotes development of breast tissue and milk production

Gonadal and placental hormones
Testes produce testosterone – responsible for development of male reproductive tract and secondary sex characteristics. Ovary produces it also.
From ovary : Estrogens – responsible for development of female reproductive tract and secondary sex characteristics, regulates menstrual cycle, maintains pregnancy
Progesterone – regulates menstrual cycle, preps for and maintains pregnancy.
Placental hormones –
Human chorionic gonadotropin (hCG) – promotes progesterone synthesis to maintain pregnancy,
Placental lactogen – prepares for lactation
Relaxin – softens ligaments and widens pelvis

Intermediate pituitary hormone
Melanocyte-stimulating hormone
Derived from POMC
Local production in the skin to increase melanin production in response to UV light
MSH from hypothalamus decreases appetite and contributes to sexual arousal.

Pineal Gland
Cells secrete melatonin to regulate circadian rhythm.
Secretion varies according to light levels from the environment.
Light stimulates non-photoreceptor retinal cells. More light = less melatonin, Less light = more melatonin.

Pancreas
Exocrine portion secretes digestive enzymes into small intestine.
Endocrine portion secretes hormones into bloodstream.
Pancreatic islets – cells that secrete glucagon, insulin, somatostatin, and pancreatic polypeptide (PP)
α cells – produce glucagon in response to low blood glucose levels
1. Stimulates liver to breakdown glycogen and release glucose into blood.
2. Stimulates liver to convert amino acids into glucose = gluconeogenesis.
3. Stimulates lipolysis to release glucose for energy = gluconeogenesis

Pancreas
β cells – produce insulin when BG is high
Activates a receptor on cell membrane that moves glucose transporters to the cell membrane surface allowing cell to take in glucose for ATP production.
Stimulates the liver to turn extra glucose into glycogen for storage.
Δ cells – release somatostatin that inhibits both glucagon and insulin.
PP cells – secrete PP hormone to control appetite.

Disorders of the Pancreas
Diabetes mellitus –
Type I – autoimmune, affects β cells, no insulin produced
Type II – cells of the body are resistant to insulin, β cells make more and more insulin and finally become exhausted. At this point patient may need injectable insulin.

This OpenStax ancillary resource is © Rice University under a CC-BY 4.0 International license; it may be reproduced or modified but must be attributed to OpenStax, Rice University and any changes must be noted.

physiology

Chapter 15 THE AUTONOMIC NERVOUS SYSTEM

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Autonomic Nervous system

Fight or Flight? Rest and Digest?

Though the threats that modern humans face are not large predators, the modern world presents stimuli that trigger the same response.

The autonomic nervous system is adapted to this type of stimulus by controlling cardiac and smooth muscle and glandular tissue through involuntary responses.

(credit: Vernon Swanepoel)

Divisions of the Autonomic NS
Sympathetic Division of the Autonomic Nervous System
Sensory information arrives at the central nervous system. This information is processed and integrated. Then efferent responses are sent to target effectors throughout the body. These responses exit the CNS along the thoracic and lumbar spine.
This division is responsible for the Fight or Flight response.

Divisions of the Autonomic NS
Parasympathetic Division of the Autonomic Nervous System
Sensations arrive at the central nervous system, are processed and integrated and efferent responses exit the system at the brainstem through cranial nerves or at the sacral spinal cord. These responses terminate near or within the various organs of the body.
This division is responsible for the Rest and Digest response.

Function of the Autonomic Nervous system
Homeostasis is the balance between the two divisions of this system.
Target effectors will have dual innervation: synapses for the sympathetic division and synapses for the parasympathetic division.
Stimulation of the synapses creates opposite responses.
Sympathetic stimulation creates increased heart rate
Parasympathetic stimulation creates decreased heart rate.

Atypical synapses
Autonomic Varicosities
The connection between autonomic fibers and target effectors is not the same as the typical synapse, such as the neuromuscular junction. Instead of a synaptic end bulb, a neurotransmitter is released from swellings along the length of a fiber that makes an extended network of connections in the target effector.

Chemical signaling
Chemical signals can be neurotransmitters that are released across a synapse to bind to the effector cell or hormones that are released into the bloodstream to bind to effectors some distance away.
Synapse systems:
cholinergic – primary neurotransmitter is acetylcholine
adrenergic – neurotransmitter is norepinephrine
Both systems have several receptor types that respond differently to various neurotransmitters and chemicals. This becomes important when researchers are developing drugs.

Homeostasis and reflexes
Somatic reflexes are basically input sensory neuron to spinal cord then output neuron to muscle.
Autonomic reflexes are input sensory neuron to spinal cord or cranial nerve and output neuron from spinal cord or cranial nerve to a ganglion then another neuron to effector target.
Visceral reflexes do not need a central component therefore do not result in conscious perception i.e. you do not “feel” high blood pressure or high blood sugar.
However, sometimes you do “feel” visceral sensation.
This is referred pain.
Visceral sensory fibers enter the spinal cord at the same level as the somatosensory fibers and the brain misinterprets the signal.

Referred Pain
Conscious perception of visceral sensations map to specific regions of the body, as shown in this chart. Some sensations are felt locally, whereas others are perceived as affecting areas that are quite distant from the involved organ.

Autonomic reflexes
Short and Long Reflexes
Sensory input can stimulate either a short or a long reflex.
Long reflexes involve a sensory neuron that projects to the CNS then the efferent branch goes to the ganglion and effector.
The short reflex involves the direct stimulation of the ganglion and back out to the effector. E.g. Enteric Nervous System: Stretch of the stomach wall after eating afferent to ganglion, efferent back to stomach to produce contraction and peristalsis.

Balance of the systems
Dual innervation to effector sites by sympathetic and parasympathetic systems.
Target cells have receptors for both adrenergic and cholinergic neurotransmitters.
When sympathetic is stimulated, norepinephrine is released and depolarization occurs.
When parasympathetic is stimulated, acetylcholine is released and hyperpolarization occurs.
Example:
See a lion, sympathetic stimulated, norepinephrine released, heart rate increases.
See a kitten, parasympathetic is stimulated, acetylcholine is released, heart rate slows.
In blood vessels within skeletal muscle and sweat glands, no dual innervation. Sympathetic releases acetylcholine, dilates vessels and produces sweat.

Balance of the systems
Another example: Pupillary Reflex Pathways
The pupil is under competing autonomic control in response to light levels hitting the retina. The sympathetic system will dilate the pupil when the retina is not receiving enough light, and the parasympathetic system will constrict the pupil when too much light hits the retina.

Balance of the systems
Tone – organ systems tend to be under more influence from the sympathetic or the parasympathetic at resting state.
E.g. resting heart rate is under parasympathetic tone to keep the rate from rushing off at high rates.
Stress (from anything) increases sympathetic tone and has been shown to result in the release of inflammatory chemicals putting the body at risk for disease.
Doing things like exercise, yoga and meditating can increase parasympathetic tone and decrease that risk.

Central control
The hypothalamus is the source of most of the central control of autonomic function. It receives sensory input from the body and other parts of the brain. It integrates this information and then determines which division, sympathetic and parasympathetic, is activated.

Central control
The amygdala, part of the limbic lobe, is involved in memory and emotion. It connects to the hypothalamus to integrate sensory information to activate the sympathetic or parasympathetic division.
The medulla is continuous with the spinal cord and processes information from the cranial nerves; mostly to the parasympathetic division.

Effect of drugs on the system
Drugs that are able to bind at the effector sites (norepinephrine, acetylcholine, and epinephrine sites) can override homeostasis.
Sympathomimetic – simulate norepinephrine, increase production and release of norepinephrine, or block the removal or reuptake of norepinephrine. Result is increase depolarization and increase of sympathetic tone. E.g. caffeine
Sympatholytic – block the binding of epinephrine and norepinephrine thus deceasing sympathetic tone. E.g. β blocker heart medications. Block the adrenergic sites on the cardiac muscle and blood vessels resulting in decreased hear rate. Antianxiety drugs are also sympatholytic.
Parasympathomimetic – enhance cholinergic effects. E.g. pilocarpine constricts the iris. Used to treat some forms of glaucoma
Anticholinergic – decrease the effect of acetylcholine. E.g. atropine – dilates pupil. Scopolamine used to prevent motion sickness.

Effect of drugs on the system
Mydriasis
The sympathetic system causes pupillary dilation when norepinephrine binds to an adrenergic receptor in the radial fibers of the iris smooth muscle. Phenylephrine mimics this action by binding to the same receptor when drops are applied onto the surface of the eye in a doctor’s office. (credit: Corey Theiss)

Effect of drugs on the system
Belladonna Plant
The plant from the genus Atropa, which is known as belladonna or deadly nightshade, was used cosmetically to dilate pupils, but can be fatal when ingested. The berries on the plant may seem attractive as a fruit, but they contain the same anticholinergic compounds as the rest of the plant.

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physiology

Chapter 14 The

Somatic Nervous System

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Somatic Nervous System
Responsible for conscious perception of the environment and voluntary response to that sensory input.
Composed of :
Simple Reflex Arcs – touch a hot stove, pull hand away
Complex functions – reading and understanding what you read

Sensory perception
Sensation is the activation of sensory receptors at the stimulus level
Perception is the central processing and integration of that stimulus.
Sensory receptors are transmembrane proteins on the receptor/target cell. Usually activated by a protein that opens a ligand-gated ion channel, starting an action potential that will ultimately arrive at a control/integration/processing center. Often are activated by mechanical/pressure or thermal changes in the environment.

Types of receptors
Free nerve endings that receive stimulus e.g. pain receptors in the skin
Encapsulated endings embedded in connective tissue e.g. pressure receptors.
Specialized receptor cells that only interpret a specific type of signal. E.g. the receptors in the retina only react to photons.

Locations of receptors
Located near the source of a stimulus from the external environment e.g. skin receptors. Exteroceptor
Interoceptors receive signals from internal organs e.g. a sense in change in blood pressure.
Proprioceptors adjacent to moving parts. I.e. You know where you are in space.

Types of stimuli
Each type of receptor requires a certain type of stimulus to generate an action potential. This stimulus can be anything but will ultimately be transduced into an electrical signal.
Chemoreceptors – chemicals for taste and smell
Osmoreceptors – react to solute concentration changes in body fluids e.g. blood pressure and vasodilators
Nociceptors – react to pain through chemicals released during inflammation
Mechanorecptors – pressure and balance
Thermoreceptors – temperature changes

Types of sensation – Somatosensory
Somatosensation – the sense of touch with many types of receptors found all over the body in many types of tissue.

Name Historical (eponymous) name Location(s) Stimuli
Free nerve endings * Dermis, cornea, tongue, joint capsules, visceral organs Pain, temperature, mechanical deformation
Mechanoreceptors Merkel’s discs Epidermal–dermal junction, mucosal membranes Low frequency vibration (5–15 Hz)
Bulbous corpuscle Ruffini’s corpuscle Dermis, joint capsules Stretch
Tactile corpuscle Meissner’s corpuscle Papillary dermis, especially in the fingertips and lips Light touch, vibrations below 50 Hz
Lamellated corpuscle Pacinian corpuscle Deep dermis, subcutaneous tissue Deep pressure, high-frequency vibration (around 250 Hz)
Hair follicle plexus * Wrapped around hair follicles in the dermis Movement of hair
Muscle spindle * In line with skeletal muscle fibers Muscle contraction and stretch
Tendon stretch organ Golgi tendon organ In line with tendons Stretch of tendons

Types of sensation – gustation
Gustation – the sense of taste Associated with the tongue but receptors are found throughout the mouth and nasal passages.
Taste buds or gustatory receptor cells on the tongue are sensitive to chemicals dissolved in saliva. It is a Graded Response. Stimulus must reach threshold but the greater the concentration of chemical, the greater the response.
Salty – Na+
Sour – H+ or acids
Sweet – glucose
Bitter – alkaloids
Umami or Savory – L-glutamate an amino acid

Types of sensation – Olfaction
Olfaction or Smell
Bipolar neurons in the olfactory epithelium
Chemicals dissolve in the mucus on the epithelium.
Creates a graded potential that goes directly to the brain.
Goes to the temporal lobe for the smell part and the limbic system and hypothalamus where it is integrated with memory and emotion.

(Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Types of sensation – audition
Audition or Hearing
The external ear contains the auricle, ear canal, and tympanic membrane. The middle ear contains the ossicles and is connected to the pharynx by the Eustachian tube. The inner ear contains the cochlea and vestibule, which are responsible for audition and equilibrium, respectively. The Spiral ganglia of the cochlea transduce sound waves into a neural signal.

Audition or hearing
Transmission of Sound Waves to Cochlea
A sound wave causes the tympanic membrane to vibrate. This vibration is amplified as it moves across the ossicles. The amplified vibration is picked up by the oval window causing pressure waves in the fluid of the scala vestibuli and scala tympani.

Audition or hearing
The fluid movement cause bending of the basilar membrane. That in turn moves the hair cell and the stereocilia emerging from its apical surface. If the stereocilia are bent one way the tension opens ion channels and the cell depolarizes. If they bend the opposite direction, the ion channels close.

Audition or hearing
Different hair cells along the basilar membrane react to different frequencies of vibration. Therefore, hair cells at the base of the cochlea are activated only by high frequencies, whereas those at the apex of the cochlea are activated only by low frequencies.

Types of sensation – Equilibrium
Equilibrium or Balance
Another part of the inner ear, the vestibule, contains hair cells that sense head position, head movement and body motion. Stereocilia extend into a gel, the otolithic membrane, that contains calcium carbonate crystals, otoliths. As the head moves, the otoliths move. This causes the stereocilia to bend producing depolarization or hyperpolarization. Brain interprets the pattern and determines the position of the head.
Semicircular canals – respond to rotational movement in the same manner.

Types of sensation – Vision
Transduction of light waves into electrical stimuli. Light passes through the anterior chamber and is focused on the retina. The retina is composed of several layers of photoreceptors, rods and cones, and various supporting cells. The center of the retina has a small indentation known as the fovea where these supporting cells are absent and therefore the rods and cones can absorb all the light traveling into that area.

Vision
Light travels past the Retinal ganglion cells, past the bipolar cells past the bodies of the rods and cones and reacts with the photopigments in the outer segments of the rods and cones.
Light stimulation produces a graded potential in the photoreceptor cells.
Signal is transmitted to bipolar cells and then to retinal ganglion cells.
Retinal ganglion cells join together to form the optic nerve that passes through the optic disc and into the brain as the optic tract.
At the fovea these other neurons are absent and each photoreceptor cell attaches directly to one retinal ganglion cell so vision is more acute and efficient.

(Micrograph provided by the Regents of University of Michigan Medical School © 2012)

vision
Rods contain a photosensitive pigment – rhodopsin
Cones have three kinds of photosensitive pigments – opsins. Each is sensitive to a particular wavelength of light; red, blue or green.
Photosensitive pigments are bound to a cofactor – retinal, a form of vitamin A.
When light activates the pigment, retinal undergoes a conformational change called bleaching. This opens ion channels and starts the action potential. Bleaching creates a refractory period. It is a graded potential. Therefore, small amounts of light keep the action potential going but large amounts of light create an absolute refractory period.
Enzymes will return retinal to its normal conformation but in the meantime you see a “negative” after image.
Rods are sensitive to low light so bright light creates constant bleaching.
Cones are sensitive to bright light so in low light you see no color.

Vision
Comparison of Color Sensitivity of Photopigments
Comparing the peak sensitivity and absorbance spectra of the four photopigments suggests that they are most sensitive to particular wavelengths.

Sensory nerves
Stimulus → control center → response
Sensations of the head and neck go directly to the brain through the trigeminal pathway and are ipsilateral.
Peripheral sensations go to the spinal cord and are contralateral. The dorsal portion of the spinal cord is mainly sensory and the ventral portion is motor.
Simple reflex arc = sensation → sensory nerve → dorsal spinal column → ventral spinal column → motor neuron → muscle.
But if the sensation needs processing it takes a turn at the dorsal root and heads to the brain through an ascending pathway; spinothalamic tract for pain and/or temperature and the dorsal column system for touch and proprioception.

Sensory nerves
Trigeminal pathway – somatosensory from face, head, nasal cavity and mouth.
Gustation goes through facial and glossopharyngeal nerves.
Audition travels through vestibulocochlear nerve. When input is received from both ears, brain can localize sound.
Equilibrium and proprioception goes through the vestibular system.
While vision goes through the optic tract, some retinal ganglion cells are not image producing. They connect to the hypothalamus and establish circadian rhythm.

Sensory pathways
The Sensory Homunculus
Sensory axons group together as they travel to the brain and end up in one general area rather than scattered throughout.
This cartoon shows the general areas where sensory neurons land for integration in the brain.
The size of the drawing is relative to the numbers of axons arriving in the area.
Even though there are specific areas for perception, all sensation is integrated within the brain so we experience the world as a continuous whole.

Motor response pathways
Somatic nervous system controls skeletal muscle response to sensory input. Sensory control is occipital, temporal and parietal lobe dominated. Motor control is frontal lobe dominated. (Prefrontal lobe is for higher order functioning like working memory and executive functions)

Motor response pathways
Descending Pathway – frontal cortex → brain stem → spinal cord → musculature During this path impulses are being sent back to let the brain know how things are going and to revise instructions accordingly.
Cerebellum coordinates these two messages; descending from the brain and ascending from proprioceptor feedback.
Motor neurons connect at multiple synapses with the muscle fiber sarcolemma (cell membrane of the muscle cell)
Acetylcholine is the neurotransmitter. Opens ligand-gated ion channels to create depolarization and muscle contraction. No graded potential. Strength of contraction is determined by the frequency of the nerve impulse.

Motor reflexes
Simple reflex arc – sensation to spinal cord to motor neuron to muscle.
Complex reflex arc – has pathway through the brain.
Withdrawal reflex – requires control of an antagonistic reaction. I.e. one muscle must contract while the other relaxes. Interneuron acts to hyperpolarize one muscle while the other is depolarizing.
Stretch reflex – homeostasis says muscles shouldn’t be stretched. Once stretched, excites a receptor to contract muscle back into normal tension with hyperpolarization of the antagonistic partner. E.g. knee jerk reaction
Corneal reflex – to protect the eyeball, if the cornea is touched or there is too much light stimulation, the descending pathway stimulates the orbicularis oculi muscle to contract and the eye “blinks”

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PHYSIOLOGY

Chapter 12 THE NERVOUS SYSTEM AND NERVOUS TISSUE

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Structure of the nervous system

Structure of the Nervous system
Neuron is the communicator.
Glial cells are the associated cells. Astrocytes, microglia, oligodendrocytes

Structure of the nervous system
Gray Matter and White Matter
Gray matter consists of a large concentration of cell bodies, while white matter consists of a large concentration of axons. Called “white” because of the myelin around the axon.
(credit: modification of work by “Suseno”/Wikimedia Commons)

Structure of the nervous system
Central versus Peripheral
Collections of cell bodies that function together; CNS – nucleus PNS – ganglion
Collections of axons; CNS – tract PNS – nerve
Optic nerve v. optic tract

Function of the nervous system
Sensation, Integration and Response
Sensation – Registering a change from homeostasis known as a stimulus – taste, touch, sight, smell, hearing These are external stimuli. Internal stimuli can be the stretch of the stomach after eating, a change in blood ions.
Integration – Stimuli are presented to the part of the nervous system that processes that type of information. E.g. Sight goes to the eyes then to the brain. Some information is integrated with previous known information such as memories and classified accordingly through higher brain function.
Response – After integration a response occurs and can be voluntary or involuntary. E.g. Someone you are attracted to walks into the room
Sensation is sight
Integration is I see and recognize this person, I like them, I want to talk to them, I want to make a good impression
Voluntary response is making eye contact, walking to them, saying “hello”
Involuntary response is increased heart rate, sweating.

Function of the nervous system
Somatic nervous system controls voluntary responses.
Voluntary = move intentionally
Reflex = move unintentionally
Automatic = habitual movement (like driving)
Autonomic nervous system controls involuntary responses.
Regulates organ systems e.g. Heart rate, blood sugar
Enteric nervous system controls the smooth muscle and glandular tissue of the digestive system.
Can operate independently from the CNS

Structure of a neuron
Parts of a Neuron
The major parts of the neuron are labeled on a multipolar neuron from the CNS.

Types of neurons
Neuron Classification by Shape
Unipolar cells have one process that includes both the axon and dendrite and are typically sensory. Bipolar cells have two processes, the axon and a dendrite. Bipolar are “one-way” systems found in the retina and olfactory system. All other neurons are multipolar cells that have more than two processes, the axon and two or more dendrites.

Glial cells of the CNS
Glial Cells of the CNS
The CNS has astrocytes for support, oligodendrocytes for myelination, microglia act as phagocytes, and ependymal cells that create CSF.

Glial cells of the PNS
Glial Cells of the PNS
The PNS has satellite cells for support and Schwann cells for myelination.

Sensation, Integration and Response
Testing the Water
(1) The sensory neuron has endings in the skin that sense a stimulus such as water temperature. The strength of the signal that starts here is dependent on the strength of the stimulus. (2) Stimulus from the sensory endings, if strong enough, will initiate an action potential on a sensory neuron.(3) The axon of the peripheral sensory neuron enters the spinal cord and contacts another neuron in the gray matter. (4) An action potential at this neuron and travels up the sensory pathway to a region of the brain called the thalamus. (5) The sensory pathway ends when the signal reaches the cerebral cortex. (6) After integration with neurons in other parts of the cerebral cortex, a motor command is sent (7) An action potential is sent down the spinal cord. (8) The axon of the neuron emerges from the spinal cord in a nerve and connects to a muscle through a neuromuscular junction to cause contraction of the target muscle.

Function of a neuron
The function of a neuron is based on changes in the concentration of intra- and extracellular ions. Membrane Potential is the difference in the concentrations of these charges.
Ions move across the cell membrane through protein channels. Some channels require energy to open and close.

Function of a neuron
Ligand-Gated Channels
Some channels need a “ligand” to bind to the protein to activate. When the ligand, the neurotransmitter acetylcholine, binds to the extracellular surface of the channel protein, the pore opens to allow select ions through. The ions, in this case, are sodium, calcium, and potassium.

Function of a neuron
Mechanically Gated Channels
When a mechanical change occurs in the surrounding tissue, such as pressure or touch, the channel is physically opened. Thermoreceptors work on a similar principle. When the local tissue temperature changes, the protein reacts by physically opening the channel.

Function of a neuron
Voltage-Gated Channels
Voltage-gated channels open when the transmembrane voltage changes around them. Amino acids in the structure of the protein are sensitive to charge and cause the pore to open to the selected ion.

Function of neuron
Leakage Channels
In certain situations, the channel opens and closes randomly allowing ions to move across the membrane.

Membrane Potential
Measuring Charge across a Membrane with a Voltmeter
The difference between the charge intracellularly and extracellularly is measured in millivolts (mV). Membrane Potential assumes the extracellular space is 0mV therefore the Resting Membrane Potential for the cell is -70mV.
It is more negative inside than outside because there are 10x more Na+ ions outside than K+ inside.

Action Potential
Stages of an Action Potential
(1) At rest, the membrane voltage is -70 mV. (2) External stimulus is applied and a Na+ channel opens. If enough Na+ moves in to raise voltage from -70mV to -55mV voltage-gated channels open and more Na+ comes in. (3) As Na+ rushes in the cell begins to depolarize and the membrane voltage begins a rapid rise toward+30 mV. (4) Once it reaches +30mV, voltage-gated K+ channels open and K+ moves outside the cell. The membrane voltage starts to return to a negative value. (5) K+ channels take longer to close so repolarization continues past the resting membrane voltage, resulting in hyperpolarization. (6) Through the action of non-gated channels and the Na+/K+ pump the membrane voltage returns to the resting value.

Action potential
The whole thing takes about 2 milliseconds.
Once started can’t start another until the membrane potential reaches < -55mV. Absolute refractory if membrane potential is > -55mV
Refractory Period once membrane potential reaches < -55mV. At this point need a stronger stimulus that the first one. (Why we grab onto and put pressure on boo-boos) Action potential Starts at the dendritic end of the neuron. Na+ moves into the cell and travels along just inside the cell membrane. Raises voltage to above threshold level (-55mV) thus opening voltage-gated channels down the line….. Absolute refractory period keeps it from going backwards so only travels in one direction. Saltatory conduction – voltage-gated channels are located at the Nodes of Ranvier. Change in voltage starts to decrease between the nodes but gets a “boost” at each node when the channels open and more Na+ comes in. Neuron communication Changes in the membrane potential are determined by the size of the stimulus. (warm water versus hot water) Graded potentials are temporary changes in the membrane voltage that determine if an action potential occurs. Some types of stimuli cause depolarization of the membrane, whereas others cause hyperpolarization. It depends on the specific ion channels that are activated in the cell membrane. Depolarizing is usually excitatory while hyperpolarization is often inhibitory. Neuron communication Synapse – connection between electrically active cells Electrical synapse – AP passes from one cell to another as if it was one cell Chemical synapse – messenger is a neurotransmitter Chemical synapse AP reaches the end of the axon where voltage-gated Ca++ channels open. Ca2+ enters the bulb helping vesicle-bound neurotransmitter to fuse with the cell membrane and be released through exocytosis. The neurotransmitter diffuses across the synaptic cleft to bind to its receptor as a ligand activating ligand-gated Na+ channels. Na+ comes in, AP is generated. The neurotransmitter is cleared from the synapse either by enzymatic degradation, neuronal reuptake, or glial reuptake. Neurotransmitter systems Cholinergic system – neurotransmitter is Acetylcholine. 1. Nicotinic receptors – nicotine can bind to this as well as acetylcholine 2. Muscarinic receptors – muscarine is product of certain mushrooms Amino Acids – glutamate, GABA gamma-aminobutyric acid, glycine Biogenic amine – made from amino acids; Serotonin from tryptophan; dopamine, norepinephrine and epinephrine from tyrosine Neuropeptides – short chains of amino acids; Met-enkephalin and beta-endorphin Effects of neurotransmitters An ionotropic receptor is a channel that opens when the neurotransmitter binds to it. A metabotropic receptor is a complex that causes metabolic changes in the cell when the neurotransmitter binds to it (1). After binding, the G protein binds to the effector protein (2). When the G protein contacts the effector protein, a second messenger is generated (3). The second messenger can then go on to cause changes in the neuron, such as opening or closing ion channels, metabolic changes, and changes in gene transcription. This OpenStax ancillary resource is © Rice University under a CC-BY 4.0 International license; it may be reproduced or modified but must be attributed to OpenStax, Rice University and any changes must be noted.