Nervous System: Neurons, CNS, and PNS

• A Neuron is a specialized cell for transmitting electrical signals

Functions of the Nervous System

• Sensory function detects internal and external stimuli through receptors
• Integrative function processes and interprets sensory input to determine responses
• Motor function initiates responses by activating effectors, including muscles and glands

Central vs. Peripheral Nervous System

• The central nervous system (CNS) includes the brain and spinal cord and processes information and coordinates responses
• The peripheral nervous system (PNS) consists of nerves and ganglia outside the CNS, connecting the CNS to the body

Neuron Types

• Sensory (afferent) neurons transmit signals from receptors to the CNS
• Motor (efferent) neurons transmit signals from the CNS to effectors
• Interneurons connect sensory and motor neurons and process information

PNS Divisions

• The somatic nervous system (SNS) controls voluntary movements and innervates skeletal muscles
• The autonomic nervous system (ANS) controls involuntary functions, innervates smooth and cardiac muscle, and glands, and contains sympathetic and parasympathetic systems
• The enteric nervous system (ENS) governs the gastrointestinal tract and can interact with both the CNS and ANS

Neurons vs. Neuroglia

• Neurons are excitable cells that transmit electrical signals
• Neuroglia are supportive cells that protect, nourish, and insulate neurons

Functions of Neuroglia

• Astrocytes help maintain the blood-brain barrier and regulate ion and nutrient levels
• Microglia act as phagocytes to remove debris and pathogens
• Oligodendrocytes produce myelin in the CNS
• Schwann cells produce myelin in the PNS
• Ependymal cells line brain ventricles and produce/circulate cerebrospinal fluid
• Satellite cells provide support and nutrients to neurons in the PNS

Neuron Structure

• The cell body contains the nucleus and organelles
• Dendrites receive input from other neurons
• The axon transmits electrical impulses
• Axon terminals and synaptic end bulbs release neurotransmitters into the synaptic cleft

Axonal Transport

• Axonal transport moves substances along the axon
• Anterograde transport moves substances from the cell body to the axon terminals
• Retrograde transport moves substances from axon terminals to the cell body

Neuron Types

• Multipolar neurons have one axon and multiple dendrites, common in the CNS
• Bipolar neurons have one axon and one dendrite and are found in special senses
• Unipolar neurons have a single process splitting into two branches, common in sensory neurons

Definitions

• A nerve is a bundle of axons in the PNS
• Cranial nerves originate from the brain
• Spinal nerves originate from the spinal cord
• Ganglia are clusters of neuron cell bodies in the PNS

White vs. Gray Matter

• White matter is composed of myelinated axons
• Gray matter is composed of neuron cell bodies, dendrites, and unmyelinated axons

Ion Channels

• Voltage-gated channels open/close in response to voltage changes
• Ligand-gated channels open/close when specific chemicals bind
• Mechanically-gated channels open/close in response to physical deformation
• Leak channels allow constant ion flow

Graded vs. Action Potential

• Graded potentials are small, localized changes in membrane potential
• Action potentials are large, uniform depolarizations propagated along axons
• Hyperpolarization occurs when the membrane potential becomes more negative
• Hypopolarization/Depolarization occurs when the membrane potential becomes less negative

Neuron Action Potential

• Resting potential is -70 mV, maintained by Na+/K+ pumps
• Threshold is the membrane potential at which an action potential is triggered
• Depolarization happens from rapid Na+ influx
• Repolarization happens from K+ efflux restoring resting potential

All-or-None Principle

• An action potential occurs fully if the threshold is reached; otherwise, it does not occur

Refractory Period

• During the absolute refractory period, no new action potential can occur
• During the relative refractory period, an action potential can occur with a stronger stimulus

Summation

• Temporal summation involves multiple signals from one source over time
• Spatial summation involves signals from multiple sources

Nerve Fiber Classification

• A fibers are large, myelinated, and fast
• B fibers are medium, lightly myelinated
• C fibers are small, unmyelinated, and slow

Conduction

• Continuous conduction is slow in unmyelinated fibers
• Saltatory conduction is fast in myelinated fibers

Chemical vs. Electrical Synapses

• Chemical synapses use neurotransmitters and are slower
• Electrical synapses use gap junctions and are faster

Ionotropic vs. Metabotropic Receptors

• Ionotropic receptors open ion channels directly
• Metabotropic receptors trigger secondary messengers

Chemical Synapse Events

• Action potential triggers Ca2+ influx
• Neurotransmitter release into the synaptic cleft
• Binding to post-synaptic receptors

Definitions

• The pre-synaptic cell releases neurotransmitters
• The post-synaptic cell receives signals
• The synaptic gap is the space between cells
• A neurotransmitter is a chemical messenger
• Receptor sites bind neurotransmitters

Neurotransmitter Removal

• Neurotransmitters can be removed by diffusion away from the synaptic cleft
• Reuptake involves reabsorption into the presynaptic cell
• Enzymatic degradation involves breakdown by enzymes

Neurotransmitters

• Excitatory neurotransmitters depolarize the post-synaptic cell
• Inhibitory neurotransmitters hyperpolarize the post-synaptic cell

Post-Synaptic Potential

• IPSP is an inhibitory post-synaptic potential
• EPSP is an excitatory post-synaptic potential

Neurotransmitter Modifications

• Neurotransmitters are modified by altering release, receptors, or reuptake

Neural Circuits

• Simple series circuits have a one-to-one pathway
• Diverging circuits have one input and multiple outputs
• Converging circuits have multiple inputs and one output
• Reverberating circuits create a feedback loop
• Parallel after-discharge circuits have multiple pathways converging on one output

Plasticity and Neurogenesis

• Plasticity refers to neural adaptability
• Neurogenesis refers to the formation of new neurons

Regeneration and Repair

• Regeneration and repair is a process where damaged tissue regrows/renews, leading to nervous system restoration

Explore the nervous system's functions: sensory, integrative, and motor. Understand the central (CNS) and peripheral (PNS) nervous systems, neuron types, and PNS divisions. Learn about sensory, motor, and somatic functions.