Exam II Neuroscience

Glycine- inhibitory Neurotransmitter

• more localized than GABA

• about 50% of inhibitory snapses ise glycine

Synthesis

• From serine (Enzyme: serine hydroxymeythyltransferase)

Inactivation

• reuptake into presynaptic membrane by glycine transporters

Receptors

• ligand gated Cl- channels

Biogenic Amines - regulate brain functions and are active in PNS

Catecholamine synthesis

• All derives from tyrosine

• tyrosine turns into DOPA by adding hydroxyl group ( tyrosine hydroxylase)

• DOPA dearboxylase removes CO2 creating DOPAMINE

• dopamine hydroxylase adds OH creating NOREPINEPHRINE

• Phenylethanolamine N-methyltransferase (CH3) creating EPINEPHRINE

Dopamine(DA)

• coordination of movement

• motivation/ reward pathways

• motor control

• cognition

• impulse control

• risk of addicton ( loss of D2 receptors)

  Inactivation

  • uptake into glial cells or Presynaptic neuron by sopamine transporter (DAT)

  • enzymatic degradation (MAO) (COMT)

  DA participates in almost all centrally controlled movement

  Dopamine receptor action

• metabotropic receptors ONLY

• location and subtype determine action

• ALL work through G- proteins to stimulate/ inhibit adenylyl cyclase

• Adenylyl Cyclase changes ATP→ cAMP

Norepinnephrine (noradrenaline)

• brainstem projections to forebrain structures

• made in Locus Coeruleus

• responsible for attention, wakefulness, feeding behavior

• uses sympathetic ganglion as a peripheral transmitter

inactivation

• reuptake by norepinephrine transporter (NET)

• Enzymatic degradation ( MAO) (COMT)

Act at metatropic receptors

• alpha and beta adrenergic receptors

• Alpha1 recep. slows depolarization due to inhibiton of K+ channels

• Alpa2 recep. slows hyperpolarization to due activation of K+ channels

• 3 subtypes of beta receptors ( 2 found in neurons)

Epinephrine (adrenaline)

• found in brain at lower levels than other catecholamines

• metabolism similar to norepinephrine

• bind to alpha and beta adrenergic receptors

• made in axon terminal

inactivation

• uses NET for reuptake

• enzymatic degradation (MAO & COMT)

Histamine

• found in hypothalamus neurons that project to brain and spinal cord

• wake- promoting and attention

• controls reactivity of vestibular ( balance) system

• act through metabotropic receptor

Synthesis

• Histidine → Histidine decarboxylase (CO2) → Histamine

Inactivation

• MAO and histamine methyltransferase

Serotonin

• AKA 5- hydroxytryptamine ( 5-HT)

• derived from tryptophan

• found in the pons and upper brainstem region

• regulates sleep and wakefulness

• most receptors are metabotropic

• ONE ionotropic (5-HT3)

Inactivation

• reuptake by serotonin transporter (SERT)

• MAO degradation

ATP and other purines

ATP is an excitatory NT

• Motor neurons of spinal cord

• action in CNS

• metabolized to adenosine (NOt a NT)

3 classes of receptors

• P2x ionotropic receptors (faster)

  • allowing exit/ entrance of non-selectove cations

• Two classes of purigenic metabotropic receptors

Peptide neurotransmitters

• synthesis is similar to protein production on non-neuronal cells

• generally bind to Metatrobic receptors

• ex: Substance P, endorphins, enkephalins, etc)

• five categories based on amina acid sequence

  • Brain- gut peptides (CCK, substance P, )

  • opioid peptides ( enkephalins, endorphins)

  • pituitary peptides (ADH, TSH)

  • hypothalamic- releasing peptides (CRH, GHRH)

  • miscellaneous peptides

Pre-propeptides: inactive form of final NT

• synthesized in ER

• signal sequence to let cell know it will be secretes

pro-peptide: polypeptide after signal sequence is removed

• protein can be modified and cleaved multiple times before reaching final NT form to make different products

• pro-peptide makes multiple active peptides

inactivation

• degradaded by peptidases

Somatic sensory system

• afferent fibers reside in ganglia alongsidete spinal cord and brainstem

• afferent= PNS→ CNS (travels TO CNS)

• efferent = signals leaving CNS

  • dorsal root ganglia (body info)

  • cranial nerve ganglia ( head info)

• supplys sensory information from periphery→ CNS

• Trigeminal ganglia- sensory ganglia for facial sensation (HSV hides here)

• sensory afferent neurons are pseudounipolar

• Have cell body, one projection that splits

pseudounipolar: continuous fiber that has a single attachment to the cell body

• Action potentials are generated by changes in the skin or muscle

  • goes to synaptic terminal to communicate with CNS

Sensory transduction: converting exernal stimuli into electrical signals that the nervous system can use ( touch, feel,hear)

• similar in all somatic afferents

• generates a depolarization recepor potential

Sensory receptors

• mechanoreceptors (touch, pressure)

• Merkel cells- found in tips of epidermal ridges

  • slow adapting

  • highest spatial resolution

  • fine touch and pressure

• meissner’s corpuscle- dermal papillae

  • rapidly adapting

  • detect low frequency vibration while object in motion

  • important for grip control

• pancinian corpuscle

  • rapidly adapting

  • deep in dermis and subcut- layer

  • detect high vibration of objects that we are gripping ( writing )

• Ruffini afferent

  • slow adapting

  • located in ligaments and tendons ; parallel to skin

  • detect how the skin stretches

• specialized receptor tht encapsulates afferent fibers

• lower threshold than unencapsulated afferents

Free nerve endings

• afferent fibers that lack specialized receptor cells ( merkel, pacinian, etc.)

• important in pain sensation

• unencapsulated

How to tell them apart:

• axon diameter: determine action potential speed

  • Larger=faster conduction

• receptive field size : area of skin surface that results in a lot of action potentials

  • more branching = larger field

• temporal dynamics ( response timing)

  • Rapidly adapting afferents: rapid fire when stimulated but become muted over time ( dynamic properties)

  • Slowly adapting afferents: continue to fire with long term stimulation ( static properties)

• ability to adapt quickly or slowly

Group 1a- supply sensory receptors in muscle

• Receptor type- muscle spindle

• largest and send fastest signal

• instructs body how to respond

AB afferents - touch

AD and C afferents- pain and temperature

Golgi tendon organ- specialized for proprioception

• force of muscle conraction acts on tendon

  • increases tension on collagen

  • compress intertwined sensory receptor ( activates Action potential)

• Afferent 1b axons → local circut neurons → alpha motor neurons

• acts as feedback to regulate tension ( so muscles dont fall off of bone)

• More 1b =less alpha neurons

Sensory Pathways

• afferent mechanoreceptor info is transmitted through connective neurons

1st order neurons: located in dorsal root ganglia and cranial nerve ganglia

• afferent MCR that go from source → brainstem

• cutaneous mechanoreceptora end info via neurons in trigeminal ganglion

• central process from sensory roots to trigeminal BS

• terminate in neurons on trigeminal brainstem

Trigmenial complex includes: Principal nucleus and spinal nucleus

Principal nucleus- info from low threshols tactile senses

spinal nucleus- Pain, Temperature, Coarse touch

2nd order neurons- located in brainstem nuclei ( Brainstem → Thalamus)

• trigeminal BS axons cross midline

  • ascend to VPM via Trigeminal leminiscal tract

3rd order neurons: project from thalamus → cerebal cortex

• VPM of thalamus projects to SI in cerebral cortex

Dorsal column-medial leminiscal system: conveys tactile info from body

Trigeminothalamic system- tactile info from face

Proprioception- mechanoreceptors tht provide information about mechanical forces withing the body

• ex: muscle spindle, golgi tendon organ, joint receptor

types of lower motor neurons- any neuron that will innerviate muscle

• Gamma (y) motor neurons-efferent

  • innerviate muscle spindle

  • sense stretch of muscle

• Alpha (a) motor neurons- efferent

  • innerviate extrafusal muscle fibers

  • contraction of skeletal muscles

Muscle stretch reflex- stretch on muscle stimulate affterent neurons

• signals are transmitted to alpha motor neuron

• efferent signals travel back to muscle ( no cerebral cortex)

• reflex circuit is responsible for steady level of muscle tension ( muscle tone)

• spindle stimulation→ activation of sensory neuron → processing at motor neuron→activation of motor neuron → contraction of the muscle

• Muscle spindles

  • sensory receptors ebedded in muscle

  • detects stretch fo muscle

  • not involved in muscle force

  • made of 4-8 intrafusal fibers

  • run parallel to extrafusal fibers

types of intrafusal fibers:

• nuclear bag fibers

  • detect velocity of stretch ( dynamic) and sustained fiber stretch (static)

• nuclear chain fibers

  • detect static length of muscle

• typicle spindle is 1 static and 4-6 chain fibers

Sensory afferent neurons on muscle spindle:

• Group 1a(primary)

  • coil around all 3 intrafusal fibers ( bags and chains)

  • send info to CNS about length and velocity of stretch

• Group II (secondary)

  • innverviate nuclear chain and static nuclear bag fiber

  • send info about length only

  • fire tonically (steadily) proportional to stretch

• these neurons form a monosynaptic excitatiry connection with alpha motor neurons that innerviate the same muscle (homonymous)

  • interneurons inhibitory connections with alpha motor neurons that innerviate the antagonsit muscle ( heteronymous)

Reciprocal innervation- rapid contaction of stretched muscle and simultaneous relaxation of antagonist muscle

• gamma (y) motor neurons contol intrafusal fibers

  • keep intrafusal fibers taut and sensitive to stretch

• contraction of intrafusal muscle must math the extrafusal muscle

  • stretch in parallel

  • during contraction pull on fibers is removed and spindle goes slack

  • no more firing of afferent neurons

• y motor neurons cause spindle fiber to contract preventing insensitivity to stretch

• co- activation of y and aplha motor neurons