Neurons, Nervous System, and Action Potentials

Sensory Input

Gathers information from external and internal environments.

Integration

Processes and interprets sensory input.

Motor Output

Causes a response in effector organs (muscles or glands).

Central Nervous System (CNS)

Components: Brain and Spinal Cord; Function: Integrative and control centers.

Peripheral Nervous System (PNS)

Components: Cranial nerves and Spinal nerves; Function: Communication lines between the CNS and the rest of the body.

Sensory (Afferent) Division

Conducts impulses from receptors to the CNS.

Somatic sensory fibers

Fibers from skin, joints, and skeletal muscles.

Visceral sensory fibers

Fibers from visceral organs.

Motor (Efferent) Division

Conducts impulses from the CNS to effectors (muscles and glands).

Somatic Nervous System (SNS)

Voluntary control; conducts impulses to skeletal muscle.

Autonomic Nervous System (ANS)

Involuntary control; conducts impulses to cardiac muscle, smooth muscle, and glands.

Sympathetic Division

Mobilizes body systems during activity ('fight or flight').

Parasympathetic Division

Conserves energy and promotes housekeeping functions during rest ('rest and digest').

Neuron (Nerve Cell)

General Characteristics: Longevity, amitotic (do not divide), high metabolic rate.

Dendrites

Receptive regions; conduct electrical signals toward the cell body.

Cell Body (Soma)

Contains the nucleus and most organelles; integrating center.

Axon Hillock

Cone-shaped area where the action potential is initiated.

Axon

Conducting region; generates and transmits the action potential away from the cell body.

Axon Terminal (Synaptic Knob)

Secretory region; releases neurotransmitters.

Myelin Sheath

Insulating layer; increases transmission speed.

Node of Ranvier

Gaps in the myelin sheath along the axon, where the action potential is regenerated during saltatory conduction.

Multipolar Neurons

Three or more processes (usually one axon and multiple dendrites); 99% of neurons fall into this class.

Bipolar Neurons

Rare; has two processes—one axon and one dendrite—extending from the cell body.

Unipolar Neurons

A single, short process that divides T-like into proximal (central) and distal (peripheral) branches.

CNS Neuroglia

Includes Astrocytes, Microglial Cells, Ependymal Cells, and Oligodendrocytes.

PNS Neuroglia

Includes Satellite Cells and Schwann Cells.

Astrocytes

Most abundant; support neurons, control chemical environment (blood-brain barrier).

Microglial Cells

Immune cells; phagocytize microorganisms and debris.

Ependymal Cells

Line central cavities of the brain and spinal cord; produce cerebrospinal fluid (CSF).

Oligodendrocytes

Form myelin sheaths around CNS axons.

Satellite Cells

Surround neuron cell bodies in PNS ganglia; function similar to Astrocytes.

Schwann Cells

Form myelin sheaths around PNS axons; vital for peripheral nerve regeneration.

Resting Membrane Potential (RMP)

The voltage difference across the plasma membrane of a resting neuron (typically ).

Establishment of RMP

Maintained by the Sodium-Potassium Pump (Na-K pump), which actively transports out for every in, and by differential ion permeability.

Graded Potentials

Short-lived, localized changes in membrane potential (e.g., in dendrites or cell body).

Graded Potentials Strength

Strength decreases with distance.

Depolarizing Graded Potentials

Membrane becomes less negative

Hyperpolarizing Graded Potentials

Membrane becomes more negative.

Integration at Axon Hillock

The decision to fire an action potential is made at the axon hillock, where graded potentials (EPSPs and IPSPs) are summed.

Action Potential (AP)

A brief, massive change in membrane voltage that is self-propagating and does not diminish over distance.

Threshold for AP

The critical voltage (usually ) required to trigger the AP; follows the 'All-or-None' principle.

Stages of the Action Potential

Includes Depolarization, Repolarization, and Hyperpolarization (Undershoot).

Depolarization Stage

Na+ Voltage-gated channels open, and floods into the cell, making the membrane rapidly positive (RMP positive peak).

Repolarization Stage

Na+ channels inactivate; Voltage-gated K+ channels open, and flows out of the cell, restoring the negative charge inside.

Hyperpolarization (Undershoot) Stage

K+ channels close slowly, leading to a slight dip below the RMP before the Na-K pump restores the RMP.

Absolute Refractory Period

During depolarization, the nerve cell cannot initiate another action potential, ensuring one-way transmission.

Relative Refractory Period

During repolarization, the nerve cell can initiate another AP, but it is more difficult because the stimulus must be stronger than normal.

Continuous Conduction

Slow propagation in unmyelinated axons; action potential opens adjacent channels sequentially.

Saltatory Conduction

Fast propagation in myelinated axons; the action potential skips from node to node (Nodes of Ranvier), where ion channels are concentrated.

Synapses

Junctions that mediate the transfer of information from one neuron to another (or an effector cell).

Presynaptic Neuron

Houses vesicles filled with neurotransmitter in its synaptic knob.

Postsynaptic Neuron

Contains proteins that function as receptors and ion gates.

Chemical Synaptic Transmission

The process where an action potential (AP) arrives at the axon terminal of the presynaptic neuron, triggering the release of neurotransmitter by exocytosis into the synaptic cleft.

Postsynaptic Potential (PSP)

A change in the membrane potential of the postsynaptic neuron caused by the binding of neurotransmitters.

Excitatory Postsynaptic Potential (EPSP)

A local depolarization that increases the likelihood of the postsynaptic cell reaching threshold and generating an action potential (AP).

Inhibitory Postsynaptic Potential (IPSP)

A local hyperpolarization that makes the membrane potential more negative and decreases the likelihood of generating an action potential (AP).

Summation

The process by which postsynaptic potentials (PSPs) are added together.

Temporal Summation

Multiple EPSPs arrive in rapid succession from a single presynaptic neuron.

Spatial Summation

EPSPs arrive simultaneously from multiple different presynaptic neurons.

Neurotransmitter Effects Termination

Neurotransmitter effects are terminated by reuptake, degradation by enzymes, or diffusion away from the synapse.

Voltage-gated Channels

Channels that open in response to changes in membrane potential, allowing ions to enter the terminal.

Exocytosis

The process by which neurotransmitters are released from vesicles into the synaptic cleft.

Neurotransmitter Diffusion

The movement of neurotransmitters across the synaptic cleft to bind to receptors on the postsynaptic membrane.

Ion Channels

Channels that open in response to neurotransmitter binding, allowing ions to flow into or out of the postsynaptic neuron.