Nervous System: Structure, Function, and Signal Transmission

Nervous System: Structure, Function, and Signal Transmission

Physiological Challenge: Information Processing in the Nervous System

The nervous system is responsible for detecting sensory information, transducing and transmitting signals, and enabling cognitive decision-making. This unit explores the cellular and molecular mechanisms underlying these processes.

Neurons and Glia

Neurons: Structure and Function

• Neurons are specialized cells that transmit electrical and chemical signals throughout the nervous system.

• Key structures include the axon (conducts impulses), dendrites (receive signals), and soma or cell body (contains the nucleus).

• Neurons are classified by function: sensory neurons (detect stimuli), motor neurons (control muscles), and interneurons (connect neurons within the CNS).

• Structural diversity among neurons (e.g., length of axons, branching of dendrites) relates to their specific roles in information processing.

Example: Motor neurons have long axons to transmit signals from the spinal cord to muscles, while interneurons often have highly branched dendrites for integrating information.

Glial Cells: Types and Functions

• Glia are non-neuronal cells that support, protect, and nourish neurons.

• Major types include:

  • Astrocytes (CNS): Maintain the blood-brain barrier, regulate ion balance.

  • Oligodendrocytes (CNS): Form myelin sheaths around axons.

  • Schwann cells (PNS): Myelinate axons in the peripheral nervous system.

  • Microglia (CNS): Immune defense cells.

  • Ependymal cells (CNS): Line ventricles, produce cerebrospinal fluid.

• Glial cells are essential for neuronal health, signal transmission, and repair after injury.

Example: Multiple sclerosis is a disease where oligodendrocytes are damaged, leading to loss of myelin in the CNS.

Organization of the Nervous System

CNS vs. PNS

• The central nervous system (CNS) consists of the brain and spinal cord; it integrates and processes information.

• The peripheral nervous system (PNS) includes all neural tissue outside the CNS and transmits signals to and from the CNS.

• White matter (myelinated axons) and gray matter (neuron cell bodies, dendrites) are key structural components.

Example: The spinal cord contains both white and gray matter, allowing for rapid communication and reflex integration.

PNS Organization

• The PNS is divided into:

  • Motor (efferent) division: Controls voluntary and involuntary muscle movements.

  • Autonomic nervous system: Regulates involuntary functions (e.g., heart rate, digestion).

  • Sympathetic and parasympathetic systems: Mediate 'fight or flight' and 'rest and digest' responses, respectively.

  • Afferent (sensory) division: Transmits sensory information to the CNS.

Brain Regions and Functions

• Major brain regions include the cerebrum, cerebellum, brainstem (midbrain, pons, medulla oblongata), thalamus, hypothalamus, and pituitary gland.

• Each region regulates specific functions, such as movement, sensory processing, hormone release, and homeostasis.

Example: The cerebellum coordinates voluntary movements and balance.

Membrane Potentials and Signal Transmission

Ions and Membrane Potentials

• Key ions involved: Na+, K+, Cl-, and Ca2+.

• The membrane potential is determined by the relative concentrations of these ions inside and outside the neuron.

• The equilibrium potential for an ion is calculated using the Nernst equation:

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

• The resting membrane potential (V_m) is typically around -70 mV in neurons.

Establishing the Resting Membrane Potential

• Leakage channels and Na+/K+ pumps maintain the resting potential.

• Equilibrium potentials for Na+ ($E_{Na}$) and K+ ($E_{K}$) influence the overall membrane potential.

Types of Membrane Potentials

• Equilibrium potential: The voltage at which there is no net movement of a particular ion.

• Resting potential: The steady-state voltage across the membrane of a resting neuron.

• Graded potential: Small, local changes in membrane potential that vary in size.

• Action potential: A rapid, all-or-none electrical signal that travels along the axon.

Action Potentials and Ion Channels

• Action potentials are generated by the sequential opening and closing of voltage-gated Na+ and K+ channels.

• Phases include depolarization, repolarization, and hyperpolarization.

• The refractory period ensures unidirectional propagation of the action potential.

Example: During depolarization, voltage-gated Na+ channels open, allowing Na+ influx and a rapid rise in membrane potential.

Types of Ion Channels

• Leakage channels: Always open, contribute to resting potential.

• Voltage-gated channels: Open in response to changes in membrane potential.

• Ligand-gated channels: Open in response to binding of a chemical messenger (e.g., neurotransmitter).

Propagation of Action Potentials

• Action potentials propagate along axons by local current flow.

• Myelination (by Schwann cells or oligodendrocytes) increases conduction speed via saltatory conduction.

• Larger axon diameter also increases conduction velocity.

Synaptic Transmission

Steps in Synaptic Transmission

• An action potential reaches the axon terminal, triggering opening of voltage-gated Ca2+ channels.

• Ca2+ influx causes synaptic vesicles to release neurotransmitter into the synaptic cleft.

• Neurotransmitter binds to receptors on the postsynaptic membrane, generating excitatory postsynaptic potentials (EPSPs) or inhibitory postsynaptic potentials (IPSPs).

• Summation of EPSPs and IPSPs (temporal and spatial) determines whether an action potential is generated in the postsynaptic neuron.

Key Terms and Concepts

• Threshold potential: The critical level to which a membrane potential must be depolarized to initiate an action potential.

• Synapse: The junction between two neurons where information is transmitted.

• Synaptic potential: The change in membrane potential of the postsynaptic cell.

Structures of the Nervous System

Major Structures to Know

• Gyri and sulci (surface features of the cerebrum)

• Lobes of the brain: frontal, parietal, temporal, occipital

• Corpus callosum, thalamus, hypothalamus, pituitary, pineal gland

• Brainstem: midbrain, pons, medulla oblongata

• Cerebellum, spinal cord, optic nerve/chiasm, olfactory bulbs

Essential Terms to Know

• axon, soma (cell body), motor neuron, glia, resting potential, action potential, hyperpolarization, gated channels, dendrite, neuron, sensory neuron, Schwann cell, graded potential, membrane potential, depolarization, refractory period, synapse, synaptic cleft, interneuron, myelin, threshold potential, synaptic potential, saltatory conduction

Additional Resources

• Crash Course videos on the nervous system (structure, action potential, synapses)