Unit A - Neurons and Action Potential Notes

Neurons

Specialized cells that are functional units of the nervous system. They respond to physical/chemical stimuli, conduct electrochemical signals, and release chemicals that regulate various body processes.

Nerves

Neurons grouped into bundles of tissue, arranged in a similar way to muscles

Sensory (Afferent) Neurons

Part of the peripheral nervous system and transmit impulses from sensory receptors to the central nervous system, are unipolar.

unipolar

only have a single projection extending from their cell body.

Motor (Efferent) Neuron

Mostly multipolar neurons are part of the peripheral nervous system; they transmit information from the central nervous system to muscles, glands, and other organs (ie., effectors).

multipolar

multiple projections extending from their cell bod

Interneurons (Association Neurons)

Are found entirely within the central nervous system and act as a link between the sensory and motor neurons. Most are multipolar.

Glial cells

Outnumber neurons 10 to 1 and function to support them. They provide nourishment and repair, remove wastes, and defend against infection. They are support cells of the nervous system.

Schwann cells

Are a type of glial cell; each provides myelin for one neuron. A single axon will have many _________ on the axon, and the entire cell surrounds the axon. Its functions include

• Digesting dead and dying neurons

• Arrange as cylinders for paths of growth for new cells

  • Stumps of severed axons may regenerate if they find a cylinder; otherwise, they will die

Dendrites

Receive nerve impulses from other neurons and relay it to the body

Cell Body (Soma)

Contains major cell organelles, cytoplasm, and a nucleus. If the input received from the dendrites is large enough, it will relay it to the axon.

Axon

Conducts nerve impulses away from the cell body

Myelin sheath

Fatty, insulating protein layer that encloses the axons of some neurons, and is formed by Schwann cells (a type of glial cell)

myelinated

Axons with the myelin sheath

unmyelinated

Axons without the myelin sheath

Synaptic Terminal

The end of the neuron, and the site of release of the neurotransmitters

synaptic cleft

The space between it and the dendrites of the next neuron

stimulus

When a change in the environment is detected by a receptor

response

When your body reacts to a stimulus

reflex arcs

rapid and unconscious/involuntary response, and are facilitated by a simple connection of neurons

ASIME

• A - A sensory receptor/afferent (think fingertips) receives a stimulus

• S - Sensory (afferent) neurons travel through the PNS to relay the info to the CNS

• I - Interneurons (association neurons) process and integrate the info and relay commands to the motor neurons in the PNS

• M - Motor (efferent) neurons then relay the info from the CNS to a muscle/effector

• E - Effector then completes the reflex response to the initial stimulus

resting membrane potential

The difference in electrical charges between the intracellular fluid (ICF), which has a negative charge relative to the extracellular fluid (ECF) in a neuron. It is typically around -70 mV

polarized

A neuron that exhibits a resting potential

sodium-potassium pump

A cellular mechanism that actively transports sodium ions out of and potassium ions into a cell, maintaining the resting membrane potential. It uses ATP energy, and for every 3 Na⁺ ions out, 2 K⁺ ions go in.

action potential

When a stimulus triggers the neuron, and the charge in the neuron rapidly shifts from negative to positive,

Depolarization

Is achieved when the voltage-gated sodium channels open along the cell membrane and allow sodium ions from the outside to go in, which causes a change in the membrane potential to +35mV.

all-or-none law

If the membrane potential reaches approximately -55 mV, it reaches the threshold. If the threshold is reached, then an action potential fires; if the threshold is not reached, there is no action potential. There are no "small" and "big" action potentials in the same neuron; only a change in action potential frequency happens when there is a stronger stimulus.

Repolarization

Happens when, as a result of the change in membrane potential, the sodium channels close and potassium gates open. K+ ions travel along their concentration gradient to the outside of the neuron.

hyperpolarized

When too much potassium leaves because the potassium channels stay open slightly too long. The neuron becomes more negative than its normal resting potential. The potassium channels then close.

refractory period

A brief period that happens once the potassium channels close. Then the sodium-potassium pump helps reestablish the resting state; the ion concentrations gradually return to their resting levels.

Absolute refractory period

• Another action potential is impossible.

• The sodium channel's h gates are locked shut.

• Na⁺ cannot enter.

• Therefore, another action potential cannot occur.

Relative refractory period

• The H gates are open again.

• But the neuron is still hyperpolarized.

• It takes a stronger-than-normal stimulus to trigger another action potential.

saltatory conduction

The process by which action potentials jump between the nodes of Ranvier along myelinated axons increases the speed of signal transmission.

multiple sclerosis

An autoimmune disorder that greatly affects the ability of neurons to carry electrochemical signals. The breakdown of the myelin sheath surrounding the axons in the CNS.

presynaptic neuron

The neuron that sends neurotransmitters to transmit signals across a synapse.

postsynaptic neuron

The neuron that receives neurotransmitters across a synapse, resulting in the transmission of a signal.

summation

When the initiation of an action potential in a postsynaptic neuron requires more than one signal from a presynaptic neuron(s). Two types:

1. Temporal summation → rapid, repetitive excitation from a single persistent input.

2. Spatial summation → simultaneous activation of several excitatory inputs.

Electrical Synapses

Cell membranes of the presynaptic and postsynaptic cells are connected. Action potentials in the presynaptic cell can be passed directly to the postsynaptic cell. This facilitates rapid transmission of action potentials and works faster than chemical synapses.