part 3 nervous tissue

Stage 1 — Resting State

The neuron is near −70 mV. Voltage-gated Na⁺ and K⁺ channels are closed; leak channels remain open.

Stage 2 — Depolarization

A strong enough local depolarization reaches threshold and opens voltage-gated Na⁺ channels. Na⁺ rushes into the cell, making the inside less negative and then positive. This opens more Na⁺ channels in a positive-feedback cycle. The membrane potential may rise to about +30 mV.

Stage 3 — Repolarization

Na⁺ channels become inactivated, reducing Na⁺ entry. Voltage-gated K⁺ channels open; K⁺ leaves the cell, making the inside negative again.

Stage 4 — Hyperpolarization

K⁺ channels close slowly, so extra K⁺ leaves and the membrane briefly becomes more negative than its resting level. Channels reset, and the membrane returns to resting conditions.

The action potential triggers

nearby membrane regions, so the signal moves along the axon.

an unmyelinated axons,

each neighboring segment must depolarize. In myelinated axons, saltatory conduction carries the signal node to node and is faster.

Absolute refractory period:

another action potential cannot occur because Na⁺ channels are open or inactivated and have not reset.

Relative refractory period

a second action potential is possible, but a stronger-than-usual stimulus is needed because some K⁺ channels remain open and the membrane is hyperpolarized.

Action potentials

are alike in size. The nervous system represents stimulus strength mainly by firing frequency: stronger stimulus = more action potentials per second.

A synapse

is a communication point between a neuron and another cell. In a chemical synapse, the cells are separated by a tiny synaptic cleft.

Presynaptic cell

=sends the message. Postsynaptic cell = receives the message.

step number 1 of Synaptic Transmission

An action potential reaches the presynaptic axon terminal.

step number 2 of Synaptic Transmission

Voltage-gated Ca²⁺ channels open and Ca²⁺ enters the terminal.

step number 3 of Synaptic Transmission

Ca²⁺ causes synaptic vesicles to fuse with the membrane and release neurotransmitter by exocytosis.

step number 4 of Synaptic Transmission

Neurotransmitter diffuses across the synaptic cleft and binds to matching receptors on the postsynaptic membrane.

step number 5 of Synaptic Transmission

Receptor activation changes ion-channel activity and creates a graded postsynaptic potential.

step number 6 of Synaptic Transmission

The neurotransmitter effect ends through reuptake, enzymatic breakdown, or diffusion away from the cleft.

EPSP (excitatory postsynaptic potential):

depolarizes the postsynaptic membrane and makes an action potential more likely.

IPSP (inhibitory postsynaptic potential):

usually hyperpolarizes the membrane or otherwise makes an action potential less likely.

The effect depends on

the receptor and the ion channels it controls—not just on the neurotransmitter name alone.

Summation:

the neuron adds together incoming signals. Temporal summation occurs when signals arrive rapidly from one source; spatial summation occurs when signals arrive from multiple locations.

Synaptic potentiation:

repeated activity can make a synapse more effective, often by increasing Ca²⁺ and neurotransmitter release or increasing the postsynaptic response.

Long-term potentiation (LTP)

is a lasting increase in synaptic strength and is important in learning and memory.

Presynaptic inhibition:

another neuron reduces neurotransmitter release from a presynaptic terminal, making the effect on the next cell smaller.

Neurotransmitters

are chemical messengers released by neurons. A neuron may use more than one type.

Acetylcholine (ACh):

used at neuromuscular junctions and by many autonomic and CNS neurons. Acetylcholinesterase (AChE) breaks it down.

Biogenic amines:

dopamine, norepinephrine (NE), epinephrine, serotonin, and histamine. They help regulate brain activity, mood/emotional behavior, and biological rhythms; NE is also used by autonomic neurons.

Amino-acid neurotransmitters:

glutamate and aspartate are generally excitatory; GABA and glycine are generally inhibitory in many pathways.

Neuropeptides:

Substance P helps transmit pain signals. Endorphins and related peptides can reduce pain perception. Other peptides help regulate functions such as digestion.

Substance P

helps transmit pain signals. Endorphins and related peptides can reduce pain perception. Other peptides help regulate functions such as digestion.