Lecture 24: Endocrine System I

what are the only retinal cells that produce action potentials?

ganglion cells

photoreceptor cells

convert light into electrical signals

what is night vision called?

scotopic vision

which neuroepithelial cell is responsible for night/scotopic vision?

rod cells

which neuroepithelial cell is responsible for photopic (day) vision and color vision?

cone cells

T or F: cone and rod cells are both a specialized type of neuroepithelial cell

true

T or F: cone and rod cells do not continually renew their discs

F: these cells continually renew their discs

how many discs are inside the outer segment of rod cells?

approx 1,000

which visual pigment molecule is found inside a a rod's discs?

rhodopsin molecules

what structure of neuroepithelial cells is specialized to absorb light?

cilium

what are the 3 segments of rod cells?

-outer segment
-inner segment
-cell body

what is the visual pigment of cones?

photopsin (also contains a retinal moiety)

what makes up a rhodopsin molecule?

-opsin (protein)
-retinal (vitamin a derivative)

what are the two conformations of retinal?

cis and trans

structures inside cones and rods

how many types of cones do we have? what are the types?

3; S cones (blue), M cones (green), L cones (red)

what is our perception of color based on?

a mixture of nerve signals representing cones with different absorption peaks

what is normal color vision called?

trichromatic vision (we can see all three colors)

wavelengths of the different cones

the rods contain ______________ which is a visual pigment specialized in ___________ vision.

rhodopsin; scotopic

the cones contain ______________ which is a visual pigment specialized in ___________ vision.

photopsin; photopic/trichromatic

In the dark, photoreceptors are depolarized/hyperpolarized and decrease/increase their release of glutamate neurotransmitter

depolarized; increase

Light causes these photoreceptors to depolarize/hyperpolarize and decrease/increase their release of glutamate neurotransmitter

hyperpolarize; decrease

T or F: Biochemical reactions return the cell to its 'dark state' and the visual cycle converts all-trans retinal back to 11-cis retinal for rhodopsin regeneration

T

phototransduction

conversion of light energy into action potentials (in retina)

steps in phototransduction in the rods (dark) - step 1

11-cis retinal is bound to rhodopsin

steps in phototransduction in the rods (dark) - step 2

The G-protein transducin (T) is inactive

steps in phototransduction in the rods (dark) - step 3

phosphodiestherase (PDE) is inactive

steps in phototransduction in the rods (dark) - step 4

cGMP triggers the opening of cation channels: NA+ influx

steps in phototransduction in the rods (dark) - step 5

This depolarizes the mb potential of the inner segment (IS) and triggers glutamate (Glu) release

steps in phototransduction in the rods (light) - step 1

upon light absorption, 11-cis is converted to all-trans retinal (bleaching) and dissociates from rhodopsin

steps in phototransduction in the rods (light) - step 2

this activates T, which in turn activated PDE

steps in phototransduction in the rods (light) - step 3

PDE degrades cGMP

steps in phototransduction in the rods (light) - step 4

decrease in cGMP causes a closure of Na channels

steps in phototransduction in the rods (light) - step 5

the inner segment (IS) is hyperpolarized and the Glu exocytosis is stopped

what is the sequence of the light path from the photoreceptor cells to the optic nerve?

1. photoreceptors (cones, rods)

2. bipolar cells

3. ganglion cells

4. optic nerve

amacrine cell

interneuron that modifies the rate of electrical firing in bipolar cells

horizontal cell

interneuron that modifies the rate of electrical firing in bipolar cells

function of photoreceptors

captures light and converts it into electrical signals (phototransduction)

bipolar cells

1st-order neurons of the visual pathway

ganglion cell

2nd-order neurons (some act as photoreceptors that express the photopigment melanospin)

the axons of the retinal ganglion cells form the ___________ ___________.

optic nerve

stereoscopic vision

the ability to see our environment in 3D

fixation point

area of overlap (stereoscopic vision) between the left and right visual fields

what happens in the optic chiasm?

half of the fibers from each optic nerve cross over to the opposite side of the brain (hemidecussation)

the visual projection pathway

1. eyes

2. optic nerve

3. optic chiasm

4. optic tract

5. optic radiation

6. occipital lobe

where does the optic nerve arrive?

axons of the retinal ganglion cells (CN II)

where route do most fibers in the optic tract follow?

the neurons in the optic tract go through the thalamus (where the lateral geniculate nucleus) is located and then to the cortex; 3rd order neurons arise in the LGN

where does conscious visual sensation occur?

occipital lobe (visual cortex)

pretectal nucleus

involved in photopupillary and accommodation reflexes

superior colliculus

control visual reflexes of the extrinsic eye muscles

optic nerve fibers from the melanopsin-containing ganglion cells

pretectal nucleus and superior colliculus

endocrine system

made up of all the glands, tissues, and cells that secrete hormones

hormones

chemical messengers that are transported by the bloodstream and stimulate physiological responses in cells of another, tissue or organ

organs of the endocrine system

-hypothalamus
-pineal gland
-parathyroid glands
-pituitary gland
-thyroid gland
-adrenal gland
-pancreas

organs containing tissues that secrete hormones (organs that have secondary endocrine functions)

-heart
-thymus
-adipose tissue
-digestive tract
-kidneys
-gonads
-ovaries

endocrine glands

have intracellular effects (alter cell metabolism); secrete hormones directly into the blood (ductless)

exocrine glands

extracellular effects (e.g. digestion of food); release substances into a duct or opening to the inside or outside of the body

what is a gland?

an organ that makes one or more substances such as hormones, digestive juices, sweat, tears, saliva, or milk

the pancreas as an exocrine gland

acinar cells are organized in clusters and upon stimulation, discharge digestive enzymes into a system of intercalated ducts which empty into the proximal duodenum

the pancreas as an endocrine gland

hormones such as glucagon, insulin and somatostatin are secreted into the blood by alpha, beta and delta cells of the islets of Langerhans

neuroendocrine (neurosecretory) cells

they act like neurons in many aspects, but like endocrine cells, they release hormones into the bloodstream

example of neuroendocrine (neurosecretory) cells - magnocellular neurosecretory cells

magnocellular neurosecretory cells within the supraoptic nucleus and paraventricular nucleus of the hypothalamus; synthesize oxytocin and vasopressin (ADH) and release them into bloodstream; these cells have neurons that have axons that travel towards the posterior pituitary gland

example of neuroendocrine (neurosecretory) cells - neurosecretory neurons in hypothalamic nuclei

neurosecretory neurons in hypothalamic nuclei synthesize adenohypophyseal hormone-releasing factors and secrete them into the hypophyseal portal vein in order to control the secretion of adenohypophyseal hormone from the anterior pituitary gland

similarities between the NS and endocrine systems - communication

-both serve for internal communication: they complement each other
-some neurons trigger H secretion, and some H stimulate or inhibit neurons

similarities between the NS and endocrine systems - function

-several chemicals function as both NT and hormones: noepinephrine, dopamine, and antidiuretic hormone (ADH)
-ex: dopamine can be considered a hormone when secreted by an endocrine cell but a NT when secreted by a nerve cell

similarities between the NS and endocrine systems - effects

-some hormones and NT produce identical effects on the same organ
-ex: both NA and glucagon stimulate the liver to break down glycogen and release glucose

similarities between the NS and endocrine systems - target cells

-only certain target organs or target cells respond to NT or H (receptors)
-In the case of hormones, it can also occur that the circulating hormone is inactive and only the target cells have the enzyme needed to convert it to active form

hypophyseal portal system

system of blood vessels that links the hypothalamus and the anterior pituitary

steps in hypophyseal portal system

what mechanism does the hypophyseal portal system provide?

it provides the mechanism for hypothalamic neuroendocrine regulation of pituitary function

anterior pituitary = ?

adenohypophysis

what is the hypophyseal portal system known as?

"blood connection"

components of the hypophyseal portal system

1. superior hypophyseal artery

2. primary plexus (primary capillaries)

3. long portal veins

4. secondary plexus (second capillary network)

5. hypophyseal veins

plexus

network of capillaries

where is the secondary plexus?

in the anterior pituitary

what are the primary and secondary plexus connected by?

long portal veins (AKA venules); these travel down the stalk

what is the posterior pituitary controlled by?

the neuroendocrine reflexes (the release of hormone in response to nerve signals)

where are hormones stored?

hormones are stored in the nerve endings until a nerve signal coming down the same axons triggers their release

what is the hypothalamo-hypophyseal tract known as?

"nerve connection"

neurohypophysis = ?

posterior pituitary gland

where does the hypothalamo-hypophyseal tract run?

The axons (nerves) run down the stalk as a bundle from the hypothalamic nuclei to the posterior pituitary (neurohypophysis)

which hypothalamic nuclei are involved in the hypothalamo-hypophyseal tract?

paraventricular nucleus and the supraoptic nucleus

where is the supraoptic nucleus located?

above the optic chiasm

where is the paraventricular nucleus located?

walls of the 3rd ventricle

hypothalamic hormones - CRH

corticotropin-releasing hormone; this is a releasing regulatory hormone

what are the hypothalamic hormones released by?

released by hypophyseal portal system for the delivery to the anterior pituitary gland

hypothalamic hormones - TRH

Thyrotropin releasing hormone; this is a releasing regulatory hormone

hypothalamic hormones - GH-RH

growth hormone releasing hormone; this is a releasing regulatory hormone

what do releasing hormones do?

stimulate the pituitary to release hormones

what do inhibiting hormones do?

suppress pituitary secretion

hypothalamic hormones - GH-IH

growth hormone inhibiting hormone; this is an inhibiting regulatory hormone

what is GH-RH AKA?

somatostatin

hypothalamic hormones - PIH

prolactin-inhibiting hormone; this is an inhibiting regulatory hormone

hypothalamic hormones - GnRH

gonadotropin-releasing hormone; this is a releasing regulatory hormone

which hormones are released by the posterior pituitary?

ADH and oxytocin; this is a direct release of hormones from sensory stimulation and osmoreceptor stimulation

which hormones are released by the adrenal medulla

epinephrine and noepinephrine; this is under direct control by the nervous system

what does the ovulation test detect?

a rise in luteinizing hormone (LH) in the urine; a rise in this hormone signals the ovary to release the egg; this test is used by women to predict egg release (this is when pregnancy is most likely to occur)

what stimulates the release of prolactin?

TSH (thyrotropin)

abbreviation for prolactin

PRL