A&P Test 4

Sensory receptors

ending of neurons or separate, specialized cells that detect such things as temperature, pain, touch, pressure, light, sound, odors

Nerve

a bundle of axons and their sheaths that connects CNS to sensory receptors, muscles, and glands

Cranial nerves

originate from the brain; 12 pairs

Spinal nerves

originate from spinal cord; 31 pairs

Ganglion

collection of neuron cell bodies outside CNS

Plexus

extensive network of axons, and sometimes neuron cell bodies, located outside CNS

Sensory (afferent) division of PNS

transmits action potentials from receptors to CNS

Motor (efferent) division of PNS

transmits action potentials from CNS to effectors (muscles, glands)

Somatic NS

from CNS to skeletal muscles

Autonomic NS

from CNS to smooth muscle, cardiac muscle and certain glands

2 neuron system

first from CNS to ganglion; second from ganglion to effector

Sympathetic (Divisions of ANS)

Prepares body for physical activity (fight or flight)

Parasympathetic (Divisions of ANS)

Regulates resting or vegetative functions such as digesting food or emptying of the urinary bladder

Enteric (Divisions of ANS)

plexuses within the wall of the digestive tract

Neuroglia (Glia or Glial cells)

Support and protect neurons

Dendrites

short, often highly branched

Dendritic spines

little protuberance where axons synapse with dendrite

Initial segment

beginning of axon

Trigger zone

site where action potentials are generated

Axoplasm

cytoplasm in axon

Axolemma

cell membrane around axon

Presynaptic terminals

terminal boutons

Synapse

junction of a nerve cell with another cell

• Site where action potentials in one cell cause action potentials in another cell

Interneurons or association neurons

within CNS from one neuron to another

Multipolar

most neurons in CNS; motor neurons

Bipolar

sensory in retina of the eye and nose

Unipolar

single process that divides into two branches

Blood-brain barrier

protects neurons from toxic substances, allows the exchange of nutrients and waste products between neurons and blood, prevents fluctuations in the composition of the blood from affecting the functions of the brain

Astrocytes

• Support neurons

• Uptake neurotransmitters from synapse

• Produce chemicals that promote tight junctions to form blood-brain barrier

• Regulate extracellular brain fluid composition

Ependymal Cells

Line brain ventricles and spinal cord central canal

Choroid plexus

Secrete cerebrospinal fluid

Cilia

help move fluid thru the cavities of the brain

Microglia

Respond to inflammation, phagocytize necrotic tissue, microorganisms, and foreign substances that invade the CNS

Oligodendrocytes

form myelin sheaths if surrounding axon

Neural stem cells

replace olfactory neurons

Schwann cells

wrap around portion of only one axon to form myelin sheath

Satellite cells

• surround neuron cell bodies in sensory ganglia

• provide support and nutrients

• protects neuron cell body from toxins

Myelin

• Protects and insulates axons from one another

• Speeds transmission

• Functions in repair of axons

Nodes of Ranvier

gaps in myelination

Gray matter

unmyelinated axons

White matter

myelinated axons

In brain

gray is outer cortex as well as inner nuclei; white is deeper

In spinal cord

white is outer, gray is deeper

PNS gray matter

groups of cell bodies called ganglia

action potential

when cells produce electrical signals

Electrical Signals

Transfer of information from one part of body to another

Proteins

• Synthesized inside cell

• Large, don't cross the phospholipid bilayer of membrane

• Proteins are negatively charged

Leak Channels

Channels that allow certain ions to move freely across the plasma membrane in either direction at anytime (always open)

Gated ion channels

open and close because of some sort of stimulus

Ligand-gated ion channels

open or close in response to ligand such as acetylcholine binding to receptor protein

Voltage-gated ion channels

open or close in response to small voltage changes across the cell membrane

Touch receptors

respond to mechanical stimulation of the skin

Temperature receptors

respond to temperature changes in the skin

Potential difference

unequal distribution of charge exists between the immediate inside and immediate outside of the plasma membrane

-70 to -90 mV

resting membrane potential

Depolarization

Potential difference becomes smaller or less polar

• more positive

Hyperpolarization

Potential difference becomes greater or more polar

less gradient between inside and outside (depolarization)

If extracellular concentration of K+ increases

steeper gradient between inside and outside (hyperpolarization)

if extracellular ion concentration decreases

Na+ gates open and membrane depolarizes

If extracellular Ca2+ concentration decreases

gates close and membrane repolarizes or becomes hyperpolarized

If extracellular concentration of Ca2+ increases

Graded Potentials

• Result from

– Ligands binding to receptors

– Changes in charge across membrane

– Mechanical stimulation

– Temperature changes

– Spontaneous change in permeability

• Can summate or add onto each other

• Spread (are conducted) over the plasma membrane in a decremental fashion: rapidly decrease in magnitude as they spread over the surface of the plasma membrane

• Can cause generation of action potentials

• Occurs in cell body

Graded

Magnitude varies from small to large depending on stimulus strength or frequency

All-or-none principle

No matter how strong the stimulus, as long as it is *greater than threshold, then action potential will occur

Repolarization

more negative (may get afterpotential [slight hyperpolarization])

Refractory Period

Sensitivity of area to further stimulation decreases for a time

Absolute sensitivity

Complete insensitivity exists to another stimulus

Relative sensitivity

A stronger-than-threshold stimulus can initiate another action potential

Action Potential Frequency

Number of potentials produced per unit of time to a stimulus

Threshold stimulus

causes a graded potential that is great enough to initiate an action potential

Subthreshold stimulus

does not cause a graded potential that is great enough to initiate an action potential.

Maximal stimulus

just strong enough to produce a maximum frequency of action potentials

Submaximal stimulus

all stimuli between threshold and the maximal stimulus strength

Supramaximal stimulus

any stimulus stronger than a maximal stimulus

• Cannot produce a greater frequency of action potentials than a maximal stimulus.

Nerve fiber Type A

• Large-diameter

• Myelinated

• Conduct at 15-120 m/s

• Motor neurons supplying skeletal and most sensory neurons

Nerve fiber Type B

• Medium-diameter

• Lightly myelinated

• Conduct at 3-15 m/s

• Part of ANS

Nerve fiber Type C

• Small-diameter

• Unmyelinated

• Conduct at 2 m/s or less

• Part of ANS

Electrical Synapses

Gap junctions that allow graded current to flow between adjacent cells

Connexons

protein tubes in cell membrane.

Electrical Synapse

intercalated disk =

chemical synapses

Neurotransmitters released by action potentials in presynaptic terminal

norepinephrine

Recycled within presynaptic neuron or diffuses away from synapse

neuromodulators

Chemicals produced by neurons that facilitate action potentials

MAOIs

prevent the inactivation of serotonin, dopamine, norepinephrin, etc.

cocaine & amphetamines

increase the release and block the reuptake of norepinephrin and dopamine

Axoaxonic synapses

axon of one neuron synapses with the presynaptic terminal (axon) of another

Presynaptic inhibition

reduction in amount of neurotransmitter released from presynaptic terminal

Presynaptic facilitation

amount of neurotransmitter released from presynaptic terminal increases

Excitatory postsynaptic potential (EPSP)

Depolarization occurs and response stimulatory

Inhibitory postsynaptic potential (IPSP)

Hyperpolarization and response inhibitory

Spacial summation

Action potentials 1 & 2 cause the production of graded potential @ 2 different dendrites. These graded potentials summation @ the trigger zone to produce a graded potential that exceeds threshold, resulting in an action potential

Temporal summation

2 action potentials arrive in close succession @ the presynaptic membrane. The 1st action potential causes the production of a graded potential that doesn’t reach threshold @ the trigger zone. The 2nd action potential results in the production of a 2nd graded potential that summates w/ the 1st to reach threshold, resulting in the production of an action potential.

combined summation

An action potential is produced @ the trigger zone when the graded potentials produced as a result of the EPSPs & IPSPs summate to reach threshold

Neuronal Pathways and Circuits

Organization of neurons in CNS varies in complexity

Convergent pathways

many converge and synapse with smaller number of neurons

Divergent pathways

small number of presynaptic neurons synapse with large number of postsynaptic neurons

Oscillating circuit

outputs cause reciprocal activation

Spinal Cord

• Extends from foramen magnum to second lumbar vertebra

• Gives rise to 31 pairs of spinal nerves

• Not uniform in diameter throughout length

Cervical enlargement

supplies upper limbs

Lumbar enlargement

supplies lower limbs