MCAT: Biology: Neurobiology

dendrites

a neuron's bushy, branching extensions that receive messages and conduct impulses toward the cell body

soma

cell body of a neuron

axon hillock

The conical region of a neuron's axon where it joins the cell body; typically, the region where nerve signals are generated.

axon

the extension of a neuron, ending in branching terminal fibers, through which messages pass to other neurons or to muscles or glands

nodes of ranvier

Gaps in the myelin sheath to which voltage-gated sodium channels are confined.

myelin

A layer of fatty tissue segmentally encasing the fibers of many neurons enables vastly greater transmission speed of neural impulses as the impulse hops from one node to the next.

axon terminus

End of axon where neurotransmitters are released.

saltatory conduction

Rapid transmission of a nerve impulse along an axon, resulting from the action potential jumping from one node of Ranvier to another, skipping the myelin-sheathed regions of membrane.

Schwann cells

Supporting cells of the peripheral nervous system are responsible for the formation of myelin.

oligodendrocyte

a type of glial cell that forms myelin in the central nervous system

unipolar neuron

A neuron with one axon attached to its soma; the axon divides, with one branch receiving sensory information and the other sending the information into the central nervous system.

bipolar neuron

a nerve cell that has a single dendrite at one end and a single axon at the other end found in the retina

resting membrane potential

An electrical potential is established across the plasma membrane of all cells by the Na+/K+ ATPase and the K+ leak channels. -70 mV in most cells

Na+/K+ ATPase

A protein found in the plasma membrane of all cells in the body that uses the energy of an ATP (hydrolyzes ATP) to move three Na+ ions out of the cell and two K+ ions into the cell, thus establishing concentration gradients for these ions across the cell membrane. Pumps out one net positive ion

K+ leak channels

allow K+ to leak out across the membrane

charge on the inside of a cell

-70mV

depolarization

movement away from resting potential in the positive direction

hyperpolarization

move away from the rest potential in the negative direction

repolarization

return to rest potential

equilibrium potential

potential at which there is no driving force on an ion, equal concentration on either side of the membrane

what is happening during depolarization

Input pushes the potential past the threshold, and voltage-gated Na+ and K+ channels open. The voltage-gated Na+ channel opens quickly, and Na+ influx depolarizes the cell

What is happening during the peak of an action potential

The voltage-gated Na+ channel is inactivated, and the voltage-gated K+ channel is fully open

What is happening during repolarization

influx of K+ repolarizes the cell

what is happening during hyperpolarization

The voltage-gated K+ channels close slowly, and extra K+ leaves to hyperpolarize the cell

absolute refractory period

time during which another action potential is impossible; Na+ channels are inactivated and the cell is too positive, near the Na+ equilibrium potential

relative refractory period

The period of time following an action potential, when it is possible, but difficult, for the neuron to fire a second action potential, Na+ channels are closed, and the cell is too negative. It is further from the threshold and near the K+ equilibrium potential

how long does an action potential take

2-3 ms

presynaptic neuron

conducts impulses toward the synapse

postsynaptic neuron

the neuron on the receiving end of the synapse

synapse

the junction between the axon tip of the sending neuron and the dendrite or cell body of the receiving neuron

electrical synapses

ions flow directly from one cell to the next in gap junctions, always an excitatory signal that causes an action potential in the postsynaptic cell, bidirectional flow of ions, and the process is unregulated. seen in CARDIAC MUSCLE CELLS

chemical synapse

a type of synapse at which a chemical (a neurotransmitter) is released from the axon of a neuron into the synaptic cleft, where it binds to receptors on the next structure (either another neuron or an organ)

steps of chemical synapses

1. The action potential arrives at the voltage-gated Ca2+ channel, and the entering Ca2+ breaks the synapse

2. Neurotransmitter vesicles migrate to the membrane, fuse with it, and release neurotransmitters into the synapse

3. Neurotransmitters diffuse across the synapse and bind to receptors on postsynaptic cells

4. Neurotransmitter-gated ion channels open, and ions flow according to the gradient.

5. The flow continues until the neurotransmitters are removed. Acetylcholine is broken down by acetylcholinesterase. NT are reuptake into the presynaptic cell

flow of ions in chemical synapses

Salty C surrounds our cells: Na+, Cl-, and Ca2+ are higher outside the cell and flow in when ion channels open. Therefore, K+ flows out of the cell.

neurons only make

one type of neurotransmitter but can respond to many

leftover neurotransmitters in synaptic cleft

can be recycled or broken down

how medication affects neurotransmitters

change the amount of time neurotransmitters spends in the cleft to adjust responses

the response of the postsynaptic cells depends on

ions and receptors, not the neurotransmitters

synapsin

tethers neurotransmitter vesicles to cytoskeleton

excitatory post synaptic potential

input that pushes potential towards threshold, like Ca2+ and Na+ influx

inhibitory post synaptic potential

input pushes potential away from threshold, influx of K+ or efflux of Cl-

summation

adding EPSP and IPSP at axon hillock, if enough input to push past threshold and trigger and action potential

spatial summation

add up inputs from multiple sources

temporal summation

add up frequent impulses from a single source

greater effect of summation occurs

closer the synapse is to the axon hillock

afferent neurons

Carry sensory input, Approaching the CNS from the PNS

interneurons

CNS neurons that internally communicate and intervene between the sensory inputs and motor outputs; integration

efferent neurons

carry out commands sent out to the body

reflex

rapid integration to avoid potential injury

patellar tendon stretch reflex

aka knee jerk reflex; patellar ligament stretched, quads contract/knee extends and hamstring stays relaxed

hindbrain

medulla, pons, cerebellum

pons

balance

medulla

3 Bs: Breathing, Blood pressure, and Barfing (gag reflex)

spinal cord

reflexes, walking, urination, sex organs

cerebellum

hand-eye coordination

midbrain

relay of vision and hearing, wakefulness

limbic system

emotions

diencephalon

thalamus, hypothalamus, epithalamus

epithalamus

controls the pineal gland which makes melatonin which regulates circadian rhythms

thalamus

relay for conscious sensation except for smell

hypothalamus

controls the pituitary gland for homeostasis

white matter

myelinated axons

tract white matter

myelinated axon bundle in brain/CNS

nerve

bundle of myelinated axons in PNS

gray matter

cell bodies, dendrites, and unmyelinated axons

cortex gray matter

outer layer of cell bodies, dendrites and short axons in brain

frontal lobe

complex decision making, voluntary motion

parietal lobe

general sensation, taste

occipital lobe

visual processing

temporal lobe

smells, hearing, memory, emotions

ganglion

collection of unmyelinated nerve cell bodies in the peripheral nervous system

central nervous system

brain and spinal cord

peripheral nervous system

the sensory and motor neurons that connect the CNS to the rest of the body

somatic nervous system

The division of the peripheral nervous system that controls the body's skeletal muscles/voluntary movement. One neuron from the CNS connects to muscles and uses acetylcholine. Always excititory.

autonomic nervous system

The division of the PNS that controls the involuntary control of glands and smooth muscle. 2 neurons from the CNS connect to the organ, may be excitatory or inhibitory

sympathetic nervous system

The division of the autonomic nervous system that arouses the body, fight or flight. Uses norepinephrine

parasympathetic nervous system

The division of the autonomic nervous system that calms the body, rest, and digest. uses acetylcholine

chain ganglion

Ganglia from the sympathetic nervous system are linked together in the spine and fire at the same time to different organs.

affect of the parasympathetic nervous system

increase blood flow and activity to the GI tract and kidneys. Decrease heart rate, blood pressure, and respiratory rate

affect of the sympathetic nervous system

decrease in blood flow and activity of the GI tract and kidneys. Increase in heart rate, blood pressure, and respiratory rate. Increase in blood flow to the brain, skeletal muscles, and liver. Directly stimulates the adrenal medulla to release epinephrine to prolong the effects

mechanoreceptors

stimulated by physical shape changes; touch receptors

chemoreceptors

respond to chemicals

thermoreceptors

respond to changes in temperature

nociceptors

pain receptors

photoreceptors/electromagnetic receptors

stimulated by light

absolute threshold

the minimum stimulation needed to detect a particular stimulus 50 percent of the time

difference threshold

The minimum amount of difference that can be detected between two stimuli

sensory adaptation

ignore unchanging stimuli, can be retriggered if stimulus changes

bottom up processing

sensory receptors register info, sensory neurons send info to the brain, brain identifies the information

top-down processing

brain applies prior knowledge and experience and forms a holistic view of whats going on

iris

colored part of the eye. regulates the diameter of the pupil

lens

biconvex structure that focuses light on the retina

cornea

external transparent layer of the eye

pupil

black opening in the middle of the eye

ciliary muscles

regulate the curvature of the lense

fovea

The central focal point in the retina, around which the eye's cones cluster. responsible for extreme visual activity

retina

layer at the back of the eye sensitive to light

optic disc

blind spot, place on retina where optic nerve forms

optic nerve

bundle of axons leaving the eye towards the brain

lens at rest

pulled flat by ligaments allowing for far vision