bio 225 exam 2

the vertebrate central nervous system consists of:

the brain and spinal cord

brain

integrating center

spinal cord

mediates information flow between brain and body (and vice versa)

nuclei

groups of neuronal cell bodies within the CNS

tracts

bundles of many axons within the CNS

ascending vs descending tracts

• ascending: carries info up to brain

• descending: carries info from brain down along spinal cord

vertebrate peripheral nervous system consists of what?

• all the neurons outside of the CNS

• nerves and ganglia

nerves

• bundles of many axons outside of the CNS (leaving CNS)

• consists of myelinated and unmyelinated axons enclosed in connective tissue

• may contain sensory or motor or both

• cranial vs spinal - classified based on what CNS they’re coming off of

ganglia

groups of neuronal cell bodies outside of the CNS

white matter

• shiny, bright appearance

• primarily composed of myelinated axons

• don’t typically see cell bodies + dendrites

grey matter

• dulled appearance

• contains dendrites, cell bodies, and unmyelinated axons

• short axons

organization of white and grey matter in brain vs spinal cord

brain: grey matter surrounds white

spinal cord: white matter surrounds grey in a butterfly design

structure and function of spinal cord

• mediates spinal reflexes

• pathway for impulses to and from the brain

• receives afferent/sensory signals via dorsal root

  • somatosensory neuron cell bodies in the dorsal root ganglion

• sends out efferent/motor signals via ventral root

  • motor neuron cell bodies in the ventral horn

pic: blue is afferent/sensory going in, red is efferent/motor going out

how does the withdrawal reflex achieve a coordinated movement?

It simultaneously activates required motor neurons and inhibits opposing ones to ensure smooth movement.

what are reflex arcs and how does divergence work in them?

• reflex arc: control many involuntary behaviors, most of which are mediated by multiple neurons and employ divergence

• divergence: 1 receptor spreads its signal to many effectors

What is the pathway of a reflex arc, and what is the role of interneurons?

• pathway: sensory neuron → interneuron → motor neuron

• interneurons: have short, branching axons that synapse onto multiple targets (motor neurons) to spread signal via divergence

simple reflex arcs

• rare

• contains only a single synapase/location of synapses

• requires only 2 neurons (cluster of neurons)

• 1 sensory signal in, 1 motor signal out

• ex: patellar reflex

contralateral - def and ascending vs. descending tracts

• opposite side

• most signals between the body and the brain (and vice versa) are contralateral

• ascending tracts carry afferent/sensory signals within the CNS

  • in from lefthand side, crossing over to righthand side of spinal cord, up to the right somatosensory cortex

• descending tracts carry efferent/motor signals within the CNS

  • right hand side of brain, carried down via descending tract, then some of the signals cross over to left side

• left side of the brain moves right side of the body

ipsilateral

• same side

• signals stay/exit on the same side of the CNS

what does the spinal cord directly connect to?

the hindbrain

hindbrain main components

• medulla oblongata

• pons

• cerebellum

medulla oblongata

regulates breathing, HR, blood pressure

pons

communicates between medulla, cerebellum, and forebrain

cerebellum

coordinates motor behaviors

forebrain/cerebrum

• controls “higher” processes

cerebrum structure

• frontal lobe

• temporal loba

• parietal lobe

• occipital lob

corpus callosum

• white matter tract

• connects the left and right cerebral hemispheres

cerebral cortex

• surface gray matter

• can be divided into functional areas

motor areas in the frontal lobe

• premotor cortex

• primary motor cortex

premotor cortex

coordinates movements of groups of muscles

primary motor cortex

• controls discrete skeletal muscles

• stimulation results in specific movements on opposite side of body

what happens when the premotor cortex is damaged?

loss of skill that can often be relearned

what happens when the primary motor cortex is damaged?

• paralysis of voluntary movements that are rarely regained

• reflexes remain intact

somatosensory areas in the parietal lobe

• primary somatosensory cortex

• somatosensory association area

primary somatosensory cortex

primary input from touch, temperature, and pain receptors in skin & muscle on opposite side of body

what happens when the primary somatosensory cortex is damaged?

loss sensation that rarely returns

somatosensory association area

• interpretation of sensation

• integration with memory

• ex: reach into pocket

what happens when the somatosensory association area is damaged?

loss of identification that can be relearned

ex of damaged somatosensory cortex vs somatosensory association area

normal: reach into pocket w/ key → recognize the ridges in denim material and coldness from key; recognize that it is a key

damaged: reach into pocket w/ key → cannot feel the coldness from key and cannot recognize that it is a key BUT can learn to recognize that it’s a key

visual areas of the CNS

• primary visual cortex

• visual association areas

• far more ipsilateral signals than in most other systems

primary visual cortex

• primary input from optic tracts

• activated by visual stimuli

visual association areas

integration and perception

limbic system and 5 functions (actually 7)

• many functionally important structures are situated below the cerebral cortex

• functions: smell, homeostasis, emotions, fear & aggression, learning & memory, relay station

diencephalon key structures

the thalamus and hypothalamus

thalamus

• filters and sorts sensory inputs

• routes information

hypothalamus

• diverse set of homeostatic functions

• regulates pituitary gland

• plays a key role in homeostatic regulation

hypothalamus: homeostatic regulation

• invovled in the stress response

  • HPA (hypothalamus, pituitary, adrenal) axis

• regulates food intake

• regulates body temperature

• helps maintain ion and water balance

• regulates circadian rhythms

homeostasis requires what?

the balancing of sympathetic and parasympathetic nervous systems

sympathetic nervous system

• most active during stress or physical activity

• “fight or flight” response

• increases heart rate & breathing, directs blood to working muscles during exerise

parasympathetic nervous system

• most active during periods of rest

• “resting and digesting”

• redirects energy toward maintenance activities, such as digestion

sympathetic and parasympathetic neurons differ structurally and functionally

sympathetic:

• preganglionic neurons have cell bodies in the CNS and axons that extend to their ganglion

• postganglionic neurons release entirely different NT (neuroepinepherine)

• ganglion near the CNS and long postganglionic axon

parasympathetic:

• postganglionic neurons have cell bodies in the ganglion and axons that extend from the ganglion to their target

• long preganglionic axon and ganglion near target organ

• receptors that respond to ACh are different for each

acetycholine (ACh)

• first NT to be discovered

• synthesized at axon terminals of “cholinergic” neurons

  • making and releasing primarily ACh

• NT released at neuromuscular junction and parts of central and peripheral nervous systems

• ACh has 2 types of receptors (AChR): nicotinic and muscarinic

acetylcholine receptors

nicotinic AChR and muscarinic AChR

nicotinic AChR

• ionotropic

  • when ACh binds, the channel will directly open and allow Na+ to enter

• excitatory

• produce EPSP in postsynaptic neuron

• found in autonomic and motor neurons

• non-specific cation channel (most often conduct Na+)

muscarinic AChR

• metabotropic

  • G-protein coupled receptors (GPCRs)

  • linked to “second” messenger systems

• found in smooth and cardiac muscle, glands, and CNS

• excitatory or inhibitory (depending on receptor type and G-protein pair)

norepinephrine/noradrenaline

• part of catecholamine: family of signaling chemicals made in your body that includes dopamine, norepinephrine, and epinephrine

• like epinephrine/adrenaline, but functions mostly as a sympathetic neurotransmitter

  • triggers immediate, targeted responses vs epinephrine is released by adrenal glands directly into bloodstream to circulate everywhere and produce broad, body-wide emergency effects

changes in action potential frequency of sympathetic neurons alter what?

the target response

• increase in sympathetic neurons → increase in AP firing rate → blood vessel constricts (in given example)

• decrease sympathetic neurons → decreased AP firing rate → blood vesssel dilates

adrenergic receptors

• receptor that norepinephrine/noradrenaline binds to

• also respond to epinephrine from the adrenal gland

• metabotropic

  • GPCRs linked to ion channel function via G-proteins

• found throughout the body

• diverse effects based on receptor subtype

adrenergic receptors subtype alpha1

• location: smooth muscle of blood vessels to skin and gut

• pathway: Gq → PLC (phospho-lipase C)

  • breaking down of PIP2 into IP3 and DAG

  • further consequences related to Ca2+ (activation of Ca2+ channels)

• effect: vasoconstriction

  • smooth muscles will contract and make diameter of blood vessels smaller

adrenergic receptors subtype alpha2

• location: adrenergic axon terminals

• pathway: Gi (inhibitory G protein) → ↓cAMP

  • when noroepinephrine binds, the G protein is Gi

  • inactive adenocyclase which is normally converting ATP to cAMP

  • decreases cAMP

  • inactives Ca2+, which is needed for noroepinephrine release

• effect: inhibit NE release

adrenergic receptors subtype beta1&2

• location:

  • beta1 - heart

  • beta2 - smooth muscle of blood vessels to heart & skeletal muscle

• pathway: Gs (stimulatory G protein) → ↑cAMP’

  • Gs active adenocyclcase which will convert more ATP into cAMP

  • activates Ca2+ channels, increasing Ca2+, increasing rate and strength of contraction

• effect:

  • increase heart rate and contraction strength

  • vasodilation

the adrenal medulla is innverated by what?

the sympathetic nervous system only

• preganglionic sympathetic neuron release ACh → binds to receptor in chromaffin cells → release epinephrine → released into blood stream as a hormone

adrenal medulla innveration

• the chromaffin cells release epinephrine into the blood stream in response to acetycholine from preganglionic sympathetic neurons

• the developmental and evolutionary origins suggest chromaffin cells are modified post-ganglionic neurons

differences in receptor subtypes among effector organs

• effector organs express different metabotropic adrenergic receptors

• differences explain the diverse effects of epinephrine/norepinephrine in various tissues

what systems regulate homeostatic processes?

the endocrine, parasympathetic, and sympathetic nervous systems

sensory systems

• detect stimuli and send info to an integrating center

• receives a stimulus → convert into neuronal signal → carried via afferent neurons into CNS

sensory stimuli

• detected in neurons or accessory cells

• generator and receptor potential

generator potential

• a graded potential that occurs in a sensory neuron as a direct result of the stimulus

• dendrite of sensory neuron has receptor for stimulus embedded in plasma membrane→ direct depolarziation of primary afferent neuron, creating a generator potential

• if this depolarization reaches the neuron’s trigger zone with enough strength, it initiates an AP that travels along axon to CNS

• **the cell detecting the stimulus and the neuron sending the signal to the brain are the same single cell - direct pathway

receptor potential

• a graded potential that occurs in a specialized receptor cell (accessory cell) due to a sensory stimulus and then must be transmitted to the neuron

• stimulus triggers a receptor protein that is localized in an accessory cell, causing depolarization called a receptor potential

• this depolarization triggers an influx of Ca2+ → vesicles release NT via exocytosis

• the NT bind to the receptors on the dendrite of an adjacent afferent neuron, inducing a secondary graded potential in that neuron

  • does not actually fire AP

• the detector cell is a separate accessory cell that must communicate via chemical neurotransmitters to activate the sensory neuron - indirect pathway

sensory receptors can be classified by their stimulus modality (6)

1. chemoreceptors: detect the presence of chemicals in the environment

2. mechanoreceptors: detect pressure and movement, including proprioception (detection of body and space)

3. photoreceptors: detect light

4. thermoreceptors: detect temperature

5. electroreceptors: detect electric fields

6. magnetoreceptors: detect magnetic fields

stimulus encoding and 4 impt features

• sensory receptors convert info about the stimulus into changes in membrane potential and eventually action potentials

• encode 4 important features:

  • stimulus modality - signal pathways

  • stimulus location - receptive field

  • stimulus intensity - frequency/strength

  • stimulus duration - tonic & phasic receptors

sensory pathways can encode stimulus modality

• a particular afferent neuron is associated with one modality of receptor and stimulus (adequate stimulus)

• each afferent neuron stimulates activity within a particular neural pathway

• perception based on pathway activated, not the stimulus identity

  • ex: close eyes, rub, open - what do u see? speckled light pattern

    • this is a response to pressure stimulus - perceieve light even tho stimulus was pressure

      
      

receptive field

• region of the sensory surface that causes a response when stimulated

• varies in size and therefore acuity

  • smaller receptive fields allow more precise localization of the stimulus (i.e. greater spatial acuity)

receptive field and localization of stimulus

receptive fields encode stimulus location based on the signal pathway

acuity

ability to resolve fine detail of a stimulus

lateral inhibition

• signals from neurons at the center of the receptive field inhibit surrounding neurons

• improves acuity

• enhances contrast: magnifies the difference in firing rates between the central point of stimulus and surrounding area

• decreases “noise”: mutes weak, redundant background signals from surrounding neurons

• improves edge detection/boundaries: allows the CNS to clearly identify where a stimulus begins and ends

action potential frequency encodes signal intensity

• sensory neurons code stimulus intensity by changes in action potential frequency

• ex: increased stimulus → increased frequency of action potentials

sensory receptors

• encode a limited range of stimulus intensities

• sensory neurons code stimulus intensity by changes in action potential frequency

  • ex: strong stimuli → high frequency

• dynamic range

• increase in magnitude of graded potential or frequency of AP

dynamic range

• range of stimulus intensities over which a receptor exhibits changing responses

• threshold: weakest stimulus that produces a response in a receptor

• saturation: top of the dynamic range (maximal response - won’t change even if stimulus intensity increases)

dynamic range and discrimination

• large dynamic range = poor sensory discrimination

  • large change in stimulus intensity causes a small change in AP frequency

• narrow dynamic range = good sensory discrimination

  • small changes in stimulus intensity causes a large change in AP frequency

sensory discrimination

• the ability to detect differences between stimuli

• improves by distributing responses amongst different receptors

• range fractionation: different receptors cover different parts of the stimulus intensity spectrum

  • expands the range of intensities the sensory intensities the sensory surface or system responds to

range fractionoism

different receptors cover different parts of the stimulus intensity spectrum

logarithmic encoding

• some receptors use logarithmic encoding

• logarithmic encoding of intensity allows for increased dynamic range without completely sacrificing discrimination

• better discrimination in lower range

tonic and phasic receptors

• encode stimulus duration

• tonic receptors respond for the entire duration of a stimulus

• phasic receptors respond to changes in a stimulus

phasic receptors

• respond to changes in a stimulus

• can encode stimulus changes in several ways

• pic: on set (first column), combo, off

sensory adaptation

• decreased response to a stimulus as duration increases

• tonic receptors are typically slow adapting

• phasic receptors adapt so quickly that they produce APs only when the stimulus changes

• for prolonged stimuli, sensory adaptation can continue until the stimulus is tuned out

AP frequency and prolonged stimulus

AP frequency often decreases

photoreceptors

convert light energy into changes in membrane potential

photoreceptive opsins

• seven-membrane-spanning GPCRs

• opsins localize to membranes in the outer segments of vertebrate photoreceptors

phototransduction

occurs thorugh chromophore isomerization (chemical goes from cis to trans conformation)

summary of light-induced events

• opsins covalently bind vitamin-A derived choromophores

• photons cause isomerization of the choromophore

• isomerization of chromophore leads to changes in the opsin

  • conformational change in opsin

  • dissociation of choromophore from obsin (“bleaching”)

• G-protein singnaling events cause changes in membrane potential

2 classes of photoreceptor are found throughout the animal kingdom

1. rhabdomeric receptors

   1. rhabdomeric PRs were recently discovered in vertebrates too → these PRs help entrain our internal clocks to environmental light/dark cycles

2. cilary receptors

   1. rods and cone are ciliary receptors (vision)

both have evolved multiple times (not clustered when looking at evolutionary tree)

rhabdomeric photoreceptors signal thorugh Gq

1. 11-cis 3-hydroxy retinal absorbs light and isomerizes into all trans 3-hydroxy retinal

2. all trans 3-hydroxy retinal dissociates from opsin

3. activated opsin activates a Gq protein

4. activated Gq activates PLC (phospholipase C), converting PIP2 to DAG and IP3

5. DAG activates a TRP cation channel (non-selective)

   1. gated by IP3 and DAG

6. Ca2+ and Na+ enter the cell, depolarizing it

ciliary photoreceptors signal through G¡/transducin

1. 11-cis Retinal absorbs light and isomerizes into all-trans retinal

2. all-trans retinal dissociates from opsin

3. activated opsin activates the G¡ protein (inhibitory G protein) transducin

4. transducin actives PDE (phosphodiesterose), which converts cGMP to GMP

   1. signaling molecule that can open CNG (cyclic nucleotide-gated) channel

5. the decreased cGMP closes a Na+ channel

6. Na+ entry decreases, hyperpolarizing the cell

mammalian eye

• allows formation of a bright, focused image

• light enters through front of eye and hits cornea → travels through pupil (hole in iris) → hits the lens, which is held in place by ciliary body → lens does fine tuning so light gets targeted to fovea (specialized part in retina where rods/cones located) → activation of photoreceptors → light passes through and hits choroid (absorbs any remaining light energy) → exit eye via optic disk through optic nerve

• muscle fibers of the iris change the pupil diameter and control the amount of light entering the eye

• the cornea refracts light and focuses it onto the lens, which is responsible for fine-tuning the focus onto the retina

  • the lens changes shape depending on the distance of the light source in a process called accommodation

• light passes through the retina and excites the photoreceptors

• the choroid is a pigmented layer that absorbs the light

muscle fibers of the iris

• change the pupil diameter in response to light

  • controled by SNS and PNS

• radial muscle layer (like the rays of sun, spreads out)

  • dilates pupil via SNS

• circular layer of muscles

  • contracts via PNS

choroids of nocturnal animals

• contain a reflective layer

• the tapetum lucidum reflects and amplifies dim light in nocturnal animals and make their eyes “glow”

cornea and lens

• convex lenses that direct light onto the retina

• when parallel light waves hit the lens, it will cause refraction that converges the waves onto a single point

• most refraction occurs at the cornea

• the lens is responsible for fine-tuning

• most of the focusing is done by the cornea

accommodation

• referes to the ability of the eye to focus light from different distances

• during accommodation, the cillary muscle contracts, the ligaments slcak, and the lens bulges (to change focal length)

• distant object

  • enters eye as parallel rays

  • short focal length

• nearby object

  • enters eye as nonparallel rays

  • increased focal length (point of convergence is beyond the retina → dispersed light hitting retina → not focused)

• nearby object w/ accommodation

  • focal length restored

rods and cones

• located at the back of the retina

• few layers of cells light must pass through in retina to hit the photoreceptors

  • activate G protein coupled receptor → cause activation of PDE → cause hyperpolarization

• decrease inhibitory NT (glutamate) release

  • activation of bipolar cells → activation of retinal ganglion cells → carry signal out via optic disk and optic nerve