Neural Wiring and Reflexes, Biological Signaling

main components of central nervous system

brain and spinal cord

main function of central nervous system

center for integration and control

main components of peripheral nervous system

1. all neurons completely or partially outside of the central nervous system

2. sensory neurons

3. motor neurons

main function of sensory neurons

receive and relay signals from external and internal environment 

main function of motor neurons

relay motor signals to target muscles, organs, and glands

what is a nuclei in the central nervous system?

group of cell bodies

what is a tract in the central nervous system?

groups of axons

what is a ganglion in the peripheral nervous system?

group of cell bodies

what is a nerve in the peripheral nervous system?

groups of axons

components of gray matter

1. neuronal cell bodies

2. dendrites

3. terminals 

4. unmyelinated axons

component of white matter

neuronal myelinated axons

afferent neuron

conveys information toward the CNS

what type of neuron is a sensory nerve that relays a signal towards the spinal cord?

afferent neuron

efferent neuron

conveys information away from the CNS

what type of neuron is a motor nerve that excites a skeletal muscle fiber?

efferent neuron

interneuron

1. processes signals within a structure

2. restricted to CNS

3. often inhibitory

what type of neuron relays signals between a sensory and motor neuron?

interneuron

divergent neural pathways

1. one presynaptic neuron branches to affect a large number of postsynaptic neurons

2. typically all excitatory or inhibitory 

is the neuromuscular junction an example of divergence or convergence?

divergence

convergent neural pathways

1. many presynaptic neurons provide input to a small number of postsynaptic cells

2. can be both excitatory and inhibitory

is the purkinje cell of the cerebellum an example of divergence or convergence?

convergence 

different ways of coding a sensory stimulus

1. perceptual threshold

2. specific receptors

3. population coding

4. frequency coding

perceptual threshold

level of stimulus needed to be aware of a sensation

specific receptors in coding of a sensory stimulus

distinguishes the quality of a signal

population coding

the number of receptors activated correlates to the strength of the stimulus

frequency coding

the greater the frequency of action potentials, the more intense the signal is preceived 

specific receptors that interpret the quality of sensor signals

1. thermoreceptors

2. photoreceptors

3. chemoreceptors

4. mechanoreceptors

5. osmoreceptors

6. nocireceptors

7. proprioceptors

receptive field of sensory neurons

the area of which a sensory neuron can detect stimuli

where are small receptive fields found?

in more sensitive areas

convergence of receptive fields

1. overlapping of fields causes different primary sensory neurons to converge to a secondary sensory neuron

2. a single signal goes to brain

non-convergence of two receptive fields

1. two stimuli can activate separate primary and secondary neurons

2. two-point discrimination 

types of receptor adaptation

1. tonic receptors

2. phasic receptors

tonic receptors

slowly adapting receptors that respond for the duration of a stimulus

phasic receptors

rapidly adapt to a constant stimulus and turn off

monosynaptic reflex

has a single synapse between the afferent and efferent neurons

polysynaptic reflex

has two or more synapses, involves an interneuron

antagonistic pairing of skeletal muscle examples

1. bicep contracting while tricep lengthens

2. quadricep contracting while hamstring lengthens

flexors of skeletal muscle

moves appendage toward body

examples of flexors

1. bicep

2. hamstring

extensors

move appendage away from body

examples of extensors

1. tricep

2. quadricep

extrafusal fibers of skeletal muscle

1. muscle fibers that receive stimuli from a NMJ and contract to generate force 

2. stimulated through alpha motor neurons

intrafusal fibers (spindles) of skeletal muscle

1. sensory elements embedded inside the muscle 

2. sense stretch

3. gives sensory input to CNS about muscle position and movement 

what type of afferents do intrafusal fibers use to relay signals to the spinal cord?

type Ia and II

do intrafusal fibers produce tension or contain sarcomeres?

no

extrafusal fibers at rest maintain a certain level of ____

tension 

the stretch reflex steps

1. extrafusal muscle stretch occurs

2. intrafusal muscle increases afferent signals to spinal cord

3. increased efferent output through alpha motor neurons

4. extrafusal muscle contracts 

type of feedback loop associated with stretch reflex

negative feedback

what type of reflex and movement does the patellar tendon (knee jerk) demonstrate?

monosynaptic stretch reflex and reciprocal inhibition of the antagonistic muscle 

what does tapping the patellar tendon do to the quadricep?

stretches the muscle

what do intrafusal fibers do when quadricep is stretched?

they sense stretch and fire action potentials through an afferent sensory neuron

what happens when the spinal cord receives the input from intrafusal fibers during a knee jerk reflex?

two efferent paths are fired 

efferent path 1 of the knee jerk reflex

1. somatic motor neuron

2. acts on the quadricep to contract

3. leg swings forward 

inhibitory efferent path of knee jerk reflex

1. an interneuron inhibits the somatic motor neuron 

2. acts on the hamstring to stay relaxed 

3. allows leg extension 

when can a withdrawal and crossed extensor reflex occur?

in response to a painful stimulus 

what type of receptor is activated by a painful stimulus?

nocireceptor

what occurs when a nocireceptor is activated?

primary sensory neuron enters the spinal cord and diverge 

ascending pathways to brain (when stepping on something painful)

one collateral activates ascending pathways for pain sensation and postural adjustment to the brain

withdrawal reflex in response to stepping on something painful 

1. alpha motor neurons are activated

2. pulls foot away from stimulus

3. extensors inhibited 

4. flexors contract 

crossed extensor reflex in response to stepping on something painful 

1. alpha motor neurons are activated

2. supports body as weight shifts

3. extensors contract

4. flexors inhibited 

two types of intrafusal fibers

1. nuclear bag fibers

2. nuclear chain fibers 

nuclear bag fibers

use Ia afferents to relay signals of muscle velocity

nuclear chain fibers

use Ia and II afferents to relay signals of muscle length and velocity 

what type of motor neurons innervate intrafusal fibers?

gamma motor neurons

where are golgi tendon organs found?

small structures that link the muscle and the tendon 

what do golgi tendon organs sense?

direct muscle tension because they are arranged in series with muscle

what type of fibers do golgi tendon organs use?

type Ib fibers to relay signals to CNS

how are golgi tendon organs wired compared to spindles?

1. wired opposite to spindles

2. when force increases too much, Ib fibers cause the motor neuron to be inhibited to stop contraction

spindle reflex overview

the addition of a load stretches the muscle and the spindles, creating a reflex contraction

how does the golgi tendon organ reflex protect muscles?

protects the muscle from excessively heavy loads by causing the muscle to relax and drop the load 

paracrine/juxtacrine chemical signaling

a signal acts on neighboring regions

autocrine signaling

1. the cell releases contents that bind back to the same cell

2. can also affect neighboring cells

a target response is ____ dependent

receptor

alpha-receptor on intestinal blood vessel in response to epinephrine

causes vessel constriction 

beta-receptor on skeletal muscle blood vessel in response to epinephrine

causes vessel dilation 

examples of membrane bound receptor ligands that are hydrophilic

1. glucagon

2. insulin

3. growth hormone

4. serotonin

5. acetylcholine 

examples of intracellular receptor ligands that are small and hydrophobic

1. thyroid hormone

2. estrogen

3. testosterone

4. cortisol 

genomic and non-genomic effects of intracellular/nuclear signaling

1. genomic effects: activate or inactivate DNA transcription

2. non-genomic effects: opening/closing of ion channels 

types of membrane bound receptors

1. channel linked

2. enzyme linked

3. g protein coupled

4. integrin

channel linked receptor steps

1. signal binds

2. channel opens

3. ions flow across membrane

examples of channel linked receptors

1. GABAa

2. nicotinic acetylcholine receptor

3. ionotropic glutamate

4. 5-HT3 (serotonin)

enzyme linked receptor steps

1. signal binds

2. enzyme activated

3. enzyme generates product 

examples of enzyme linked receptors

1. tropomyosin receptor kinase B

2. insulin receptor

3. EGF, TBF (growth factors)

tyrosine kinase receptor steps

1. ligands binds to extracellular domain

2. receptor dimerization

3. autophosphorylation on tyrosines using ATP 

4. activated tyrosines in cytosolic domain can activate relay proteins

what protein does the insulin receptor directly phosphorylate after autophosphorylation?

the insulin receptor substrate IRS

what does phosphorylation of the IRS protein trigger?

second messenger pathways that activate transcription factors or other enzymes to change metabolism 

how do insulin receptors affect glucose movement into the cell?

when insulin binds, GLUT4 glucose transporters get embedded in the membrane to allow glucose to enter the cell 

structure of g protein coupled receptors GPCRs

seven transmembrane alpha helices

subunits of g protein

1. alpha

2. beta

3. gamma

alpha subunit of g protein

binds and hydrolyzes GTP

what happens when the alpha subunit of the g protein is bound to GTP?

GTP-alpha dissociates from the beta-gamma subunit

what happens after the dissociation of alpha and beta-gamma g protein subunits?

both subunits activate their respective effectors

when do the g protein subunits reassemble and become inactive?

following hydrolysis of GTP to GDP

different types of alpha subunits of g proteins

i, s, or q

i alpha subunit of g protein function

1. ion channels

2. inhibition of cAMP

3. phospholipases 

s alpha subunit of g protein function

increase cAMP

q alpha subunit of g protein function

1. increase DAG

2. IP3

types of amplifier enzymes present in g protein coupled receptor pathways

1. adenylyl cyclase

2. guanylyl cyclase

3. phospholipase C

what do g proteins activate?

amplifier enzymes

what do amplifier enzymes activate?

second messenger molecules