Nervous System Functions & Anatomy

Nervous System Functions

• The nervous system receives information from receptors to monitor changes in both internal and external environments.
• It processes and integrates all incoming information.
• It elicits appropriate responses from cells and tissues to maintain homeostasis and behavior.

Anatomical Organization

• Central Nervous System (CNS): Includes the brain and spinal cord.
• Peripheral Nervous System (PNS): Contains all other nerves and ganglia.

Central and Peripheral Nervous System

• Structures of the PNS are called ganglia and nerves and are seen as distinct structures.
• The equivalent structures in the CNS, called nuclei and tracts, aren't easily seen in an overall view. The best way to study them is in prepared tissue under a microscope.

The Central Nervous System (CNS)

• The brain is divided into cerebrum, pons, and cerebellum.
• The spinal cord has cervical and lumbar enlargements and a conus medullaris.

Gray Matter and White Matter

• A brain section shows white matter surrounded by gray matter.
• Gray matter forms the outer cortex of the brain and contains various nuclei.
• White matter consists of tracts.

Gray and White Matter (Detail)

• Gray matter includes neuron cell bodies, dendrites, and synapses.
• It also includes cortex (surface layer) of the cerebrum and cerebellum.
• Nuclei in gray matter are masses of functionally related cell bodies.
• White matter contains axons and myelinated nerve tracts.
• Tracts are bundles of axons within the CNS.
• White matter includes inner layers of the cerebrum and cerebellum, and outer layers of the spinal cord.

Cross-section of Spinal Cord

• A cross-section of a thoracic spinal cord shows posterior, anterior, and lateral horns of gray matter, along with posterior, anterior, and lateral columns of white matter.
• The organization of gray and white matter is reversed compared to the brain.

Tracts and Nuclei in the CNS

• Nuclei are groups of cell bodies interconnected by tracts (axons) in the CNS.
• Nuclei are analogous to ganglia and tracts are analogous to nerves in the PNS.

Different Definitions of Nucleus

• An atomic nucleus comprises protons and neutrons.
• A cellular nucleus houses DNA.
• A nucleus in the CNS is a localized center for function and contains neuron cell bodies.

The PNS (Peripheral Nervous System)

• The PNS contains nerves that transmit information to and from the CNS. There are multiple plexus and nerves (musculocutaneous, radial, median, iliohypogastric, genitofemoral, obturator, ulnar, common peroneal, deep peroneal, superficial peroneal, and tibial nerves.) This section also includes an image identifying those nerves in reference to the body

Structure of a Nerve (PNS)

• Nerves are composed of connective tissue coverings (epineurium, perineurium, endoneurium), fascicles, blood vessels, axons of neurons, Schwann cells (myelinating), afferent fibers (approaching the CNS), and efferent fibers (exiting the CNS).

Structure of a Ganglion (PNS)

• Ganglia contain clusters of neuronal cell bodies with similar functions.
• Axons project to similar structures.
• Satellite cells provide homeostatic support for neurons in the ganglia

Somatic, Autonomic, and Enteric Structures of the Nervous System

• The CNS comprises the brain and spinal cord.
• The somatic system includes spinal nerves (sensory and motor), and sensory ganglia.
• The autonomic system includes sympathetic and parasympathetic ganglia.
• The enteric nervous system is part of the digestive tract.

Organization of the Nervous System

• The nervous system is organized into central and peripheral divisions.
• The peripheral system further divides into sensory and motor divisions.
• The motor division includes somatic and visceral divisions.
• Visceral divisions include sympathetic and parasympathetic divisions.

Functional Organization of the PNS

• Sensory division (afferent signaling) collects information from the body.
• Somatic sensory receives information from skin, muscles, bones, joints, senses.
• Visceral sensory receives information from internal organs (heart, lungs, etc.).

Divisions of the Visceral Motor Division (Autonomic Nervous System)

• Sympathetic division: "fight or flight" responses (increased heart rate, breathing, inhibits digestion)
• Parasympathetic division: calming effects (slows heart, breathing, stimulates digestion)

Sensory Input

• Receptor structures in the body sense environmental stimuli (temperature, touch, pain, stretch).
• Special senses (eyes, ears, nose, tongue) involve specialized receptors for stimuli.

Neuron Classification by Shape

• Unipolar neurons have one process that includes both the axon and dendrite.
• Bipolar neurons have two processes: an axon and a dendrite.
• Multipolar neurons have more than two processes: an axon and two or more dendrites.

Other Neuron Classifications

• Pyramidal cells in the cerebral cortex have a pyramidal shape.
• Purkinje cells in the cerebellum are named after the scientist who described them.
• Olfactory neurons are named for their function.

Parts of a Neuron

• Neurons have cell membranes, dendrites, cell bodies (soma), axons, oligodendrocytes, nodes of Ranvier, myelin sheaths, and synapses.

The Process of Myelination

• Myelinating glia wrap layers of cell membrane around the axon segment to insulate it.
• One Schwann cell insulates one segment of a peripheral nerve in the PNS
• An oligodendrocyte may insulate multiple axon segments in the CNS

Motor Neuron

• Dendrites collect signals, the axon passes signals to a neuromuscular junction, and myelin insulates the axon.

The Motor Response

• Motor responses result from sensory input and CNS integration

Receptor Neurons Classification by Cell Type

• Sensory neurons can have free nerve endings or encapsulated endings.
• Photoreceptors (rod cells) are specialized receptor cells in the eyes.
• Receptor cells generate potentials which trigger signals to the CNS for sensory information processing

Glial Cells of the CNS

• Astrocytes: support homeostasis.
• Microglia: support immunity.
• Oligodendrocytes: formation of myelin.
• Ependymal cells: line fluid-filled cavities in the CNS

Glial Cells of the PNS

• Satellite cells: surround ganglionic neuronal cell bodies.
• Schwann cells: surround axons within a nerve.

The Cell Membrane of a Neuron

• The neuron's cell membrane consists of a phospholipid bilayer and transmembrane proteins (including receptor proteins that work as ion channels).

When their Gates Are Opened, Ion Channels Can Change the Membrane Voltage of a Cell

• A recording electrode and a reference electrode are used to measure voltage across a cell membrane.
• Measuring the charge between these two electrodes determines the membrane potential.
• Most cells have a negative voltage (typically close to -70 mV) when resting.

Resting Membrane Potential

• The Na+/K+ ATPase creates a voltage across the cell membrane. This creates a resting membrane potential of -70 mV for cells.
• Ion concentration gradients are maintained.

Leakage Channels

• K+ and Na+ ions use channels without gating (leak channels) to equilibrate across the membrane, thereby prevent voltage gradients from becoming too extreme.

Mechanically Gated Channels

• Mechanical pressure/touch on surrounding tissues causes these channels to open.
• Ions diffuse across the membrane along their concentration gradients.
• The charge across the membrane changes based on the diffusion.

Voltage-Gated Channels

• Voltage gated channels open when the voltage at the membrane reaches the threshold.
• Amino acids within these proteins are sensitive to the charge which causes opening into the ion.

Ligand-Gated Channels

• A neurotransmitter binds to a specific location on the extracellular surface of the channel protein.
• The binding causes the gate to open allowing select ions to flow (e.g., sodium, calcium, potassium).

Neurotransmitter Receptor Types

• Ionotropic receptors: are channels that open when a neurotransmitter binds to them.
• Metabotropic receptors: are complex structures that initiate metabolic changes in the cell when a neurotransmitter binds.

Classes of Neurotransmitters

• Monoamines (biogenic amines): These include modified amino acids like epinephrine, norepinephrine, dopamine, histamine, and serotonin.
• Amino acids: These include glycine, glutamate, aspartate, GABA (γ-aminobutyric acid).
• Neuropeptides: These include chains of 2 to 24 amino acids such as β-endorphin and substance P.

The Synapse

• The synapse is the connection between a neuron and its target cell.
• The presynaptic neuron contains a synaptic bouton where Ca2+ entry triggers neurotransmitter release into the synaptic cleft.
• Neurotransmitters bind to receptors on the postsynaptic neuron.
• Neurotransmitters are cleared from the synapse via enzymatic degradation, neuronal uptake, or glial uptake.

Graded Potentials

• Graded potentials are temporary changes in membrane voltages caused by opening ion channels.
• Depolarization or hyperpolarization results depending on the stimulus
• Magnitude of the stimulus determines the size of the measured potential

Excitatory postsynaptic potentials (EPSPs)

• EPSPs are depolarizing graded potentials.
• This increases the likelihood of an action potential.

Inhibitory postsynaptic potentials (IPSPs)

• IPSPs are hyperpolarizing graded potentials.
• This decreases the likelihood of an action potential.

Action Potentials

• Action potentials are rapid, large changes in membrane voltage that travel along the neuron membrane.
• This change in voltage is due to the opening and closing of voltage-gated ion channels (primarily Na+ and K+).
• The spread of these changes is what creates the action potential wave.

Graph of Action Potential

• Depolarization is the initial upward part of the action potential curve.
• Repolarization follows depolarization, with the voltage returning to its resting value.
• Hyperpolarization surpasses the resting membrane potential and then voltage returns to the resting state.

Stages of an Action Potential

• Membrane potential at rest is -70 mV.
• A stimulus initiates the depolarization phase.
• The voltage rises to +30 mV due to Na+ channel opening and fast activation.
• The voltage falls due to K+ channel opening and delayed activation.
• After repolarization, a hyperpolarization phase can occur.
• The voltage returns to the resting state.

Saltatory Conduction

• Saltatory conduction describes the propagation of action potentials along myelinated axons.
• The action potential jumps between the nodes of Ranvier, accelerating signal transmission.

Summation

• Summation is the combined effect of multiple postsynaptic potentials on a neuron's dendritic tree.
• Temporal summation: occurs when high frequency stimuli from one presynaptic neuron lead to larger postsynaptic potentials.
• Spatial summation: occurs when simultaneous stimuli from multiple presynaptic neurons lead to larger postsynaptic potentials.

Postsynaptic Potential Summation

• The overall change in membrane potential is the sum of all excitatory and inhibitory postsynaptic potentials.
• Different combinations of potentials lead to various results for the overall membrane potential reading.

This quiz covers the essential functions of the nervous system, including information processing and maintaining homeostasis. It also explores the anatomical organization of the Central and Peripheral Nervous Systems, detailing their components and structures.