MCAT Biology - The Nervous System

tubocurarine

parlytic, blowdarts, used for anaesthesia, still could sense

neurons

specialized cells capable of transmitting electrical impulses and then translat, ing those electrical impulses into chemical signals

cell body/soma.

contains neuron’s nucleus, endoplasmic reticulum and ribosomes

dendrites

appendages which receive incoming messages from other cells

axon hillock

integrates the incoming signals

action potentials

transmission of electrical impulses down the axon; excitatory or inhibitory; all or nothing; cause the release of neurotransmitters into the synaptic cleft

axon

long appendage that terminates in close proximity to a target structure (a muscle, a gland, or another neuron)

myelin

a fatty membrane that provides insulation to prevent signal loss or crossing of signals and increase the speed of conduction in the axon

myelin sheath

maintains the electrical signal within one neuron

oligodendrocytes

produce mylein in CNS

Schwann cells

produce mylein in PNS

nodes of Ranvier

small breaks in the myelin sheath with exposed areas of axon membrane used for saltatory conduction

nerve terminal/synaptic bouton (knob)

nlarged and flattened structure at end of the axon to maximize transmission of the signal to the next neuron and ensure proper release of neurotransmitters

neurotransmitters

the chemicals that transmit information between neurons

demyelination

the body mounts an immune response against its own myelin, leading to the destruction of this insulating substance; slows down information transfer

multiple sclerosis (MS)

common demyelinating disorder; the myelin of the brain and spinal cord is selectively targeted; many different kinds of neurons are demyelinated → wide variety of symptoms including weakness, lack of balance, vision problems, and incontinence

synaptic cleft

space between the terminal portion of the axon rof the pre-synaptic neuron and the dendrites of the adjacent, post-synaptic neuron

synapse

the nerve terminal, synaptic cleft, and postsynaptic membrane together

nerve

Multiple neurons bundled together in the PNS; may be sensory, motor, or mixed, which refers to the type(s) of information they carry

ganglia

clusters of cell bodies of neurons of the same type

tracts

axons bundled together in the CNS; only carry one type of information.

nuclei

the cell bodies of neurons in the same tract

glial cells/neuroglia

cells in the nervous system that are not neurons but provide structure and support for them

Astrocytes

nourish neurons and form the blood–brain barrier

Ependymal cells

line the ventricles of the brain and produce cerebrospinal fluid

blood–brain barrier

controls the transmission of solutes from the bloodstream into nervous tissue

cerebrospinal fluid

physically supports the brain and serves as a shock absorber

Microglia

phagocytic cells that ingest and break down waste products and pathogens in the central nervous system

resting membrane potential

the net electric potential difference that exists across the cell membrane, created by movement of potassium and sodium ions across that membrane; maintained by membrane proteins

ex. neurons -70 mV

potassium leak channels

allow the slow leak of potassium out of the cell, where the concentration is lower

inside concentration: 140 mM

outside concentration: 4 mM

equilibrium potential of potassium

no more net movement of the ion, as the cell is in equilibrium with respect to potassium; around -90 mV; negative sign is because a positive ion (potassium) is leaving the cell

sodium leak channels

allow the slow leak of potassium into the cell, where the concentration is lower

inside concentration: 12 mM

outside concentration: 145 mM

equilibrium potential of sodium

no more net movement of the ion, as the cell is in equilibrium with respect to sodium; around 60 mV; positive because sodium is moving into the cell

Na⁺/K⁺ ATPase

pumps sodium and potassium back to where they started: potassium into the cell and sodium out of the cell, to maintain their respective gradients

more ATP is spent by this transport protein to maintain these gradients than for any other single purpose.

depolarization

Excitatory input; raising the membrane potential, V_m, from its resting potential; neuron more likely to fire an action potential

hyperpolarization

Inhibitory input; lowering the membrane potential from its resting potential; makes the neuron less likely to fire an action potential;

efflux of potassium ions, overshooting the membrane potential; makes neuron refractory to more action potentials

threshold value

membrane potential of -55mV to -40 mV, above which an action potential will be triggered

summation

additive effect of multiple signals, excitatory and inhibitory

temporal summation

multiple signals are integrated during a relatively short period of time

spatial summation

additive effects are based on the number and location of the incoming signals; a large number of inhibitory signals firing directly on the soma will cause more profound hyperpolarization of the axon hillock than the depolarization caused by a few excitatory signals firing on the dendrites of a neuron

electrochemical gradient

the interior of the cell is more negative than the exterior of the cell; there is a higher concentration of sodium outside the cell than inside; both favors the movement of positively charged sodium cations into the cell

inactivation

proteins chennales have a conformational stage other than open and closed; functionally same as closed

ex. sodium channels when Vm is from +35 mV to resting potential

repolarization

restoration of the negative membrane potential from potassium ions being driven out of the cell

absolute refractory period

no amount of stimulation can cause another action potential to occur

relative refractory period

there must be greater than normal stimulation to cause an action potential because the membrane is starting from a potential that is more negative than its resting value

impulse propagation

action potential travel down the axon; Action potentials are propagated down the axon when proximal sodium channels open and depolarize the membrane, inducing distal sodium channels to open as well; because of the refractory character of these channels, the action potential can move in only one direction.

tetrodoxin (TTX)

pufferfish toxin, blocks voltage-gated Na+ channels, blocking neuronal transmission; causes death by preventing nerve signalling to diaphragm, leading to paralysis of the muscle and cessation of breathing

effector

the postsynaptic cell when it is not a neuron but a gland or muscle

chemical synapse

uses neurotransmitters to send messages from one cell to the next

neurotransmitters

small messenger molecules in neurons; stored in membrane-bound vesicles in the nerve terminal; bind to receptors on postsynaptic membrane; must be removed to prevent constant signalling via enzymes, reuptake, diffusion

reuptake carriers

absorb neurotransmitters back into the presynaptic neurons

ex. serotonin (5-HT), dopamine (DA) and norepinephrine (NE)

cocaine blocks reuptake

enzymatic breakdown of neurotransmitters

acetylcholine (ACh) by acetylcholinesterase (AChE)

drugs for Alzheimer’s, glaucoma, and myasthenia gravis can block the enzyme — also nerve gases

diffusion of neurotransmitters

Nitric oxide (NO)

homeostasis

dynamic equilibrium; keeps organisms alive

Sensory/afferent neurons

transmit sensory information from sensory receptors to the spinal cord and brain

Motor/efferent neurons

transmit motor information from the brain and spinal cord to muscles and glands

Interneurons

found between other neurons; most numerous of the three types; located predominantly in the brain and spinal cord; often linked to reflexive behavior

central nervous system (CNS)

composed of the brain and spinal cord

white matter

part of brain consists of axons encased in myelin sheaths; deeper in brain; superficial in spine

grey matter

unmyelinated cell bodies and dendrites; superficial in brain; deep in spine

brainstem,

responsible for basic life functions such as breathing

spinal cord

four regions: cervical, thoracic, lumbar, and sacral

Almost all of the structures below the neck receive sensory and motor innervation

vertebral column

backbone; transmits nerves at the space between adjacent vertebrae.

dorsal root ganglia

cell bodies of sensory neurons

spinal pathway

Sensory neurons bring information in from the periphery and enter on the dorsal (back) side

Motor neurons exit the spinal cord ventrally, or on the side closest to the front of the body

peripheral nervous system (PNS)

made up of nerve tissue and fibers outside the brain and spinal cord

ex. all 31 pairs of spinal nerves and 10 of the 12 pairs of cranial nerves (- olfactory and optic (CNS))

somatic nervous system

sensory and motor neurons distributed throughout the skin, joints, and muscles

a motor neuron goes directly from the spinal cord to the muscle without synapsing

autonomic nervous system (ANS)

regulates heartbeat, respiration, digestion, and glandular secretions and other involuntary muscles; regulates body temperature by activating sweating or piloerection

peripheral component contains two neurons - preganglionic neuron and postganglionic neuron

parasympathetic nervous system

conserve energy; resting and sleeping states; reduce heart rate and constrict the bronchi; manage digestion; manged by acetylcholine

vagus nerve (cranial nerve X)

responsible for much of the parasympathetic innervation of the thoracic and

abdominal cavity.

sympathetic nervous system

activated by stress; “fight-or-flight”; preganglionic use acetylcholine, postganglionic use norepinepherine

reflex arcs

control reflexive behavior; interneurons relay information to higher structure and can send signals to muscles immediately.

monosynaptic reflex arc

single synapse between the sensory neuron that receives the stimulus and the motor neuron that responds to it

ex. knee-jerk - pattelar tendon stretched → afferent neuron → spinal cord → efferent neuron → contraction of the quadriceps muscles → leg extension → less tension on patellar tendon

polysynaptic reflex arc

at least one interneuron between the sensory and motor neurons

ex. withdrawal - stepping off a nail and balancing weight on other leg