Nervous System Notes

• Coordinates cellular functions in three basic steps:

  • Senses information and sends messages to the central nervous system (CNS).

  • Processes and determines appropriate responses based on the received information.

  • Issues commands to muscles and organs to carry out the determined responses.

Nervous System Subdivisions

Central Nervous System (CNS)

• Composed of the brain and spinal cord, which are enclosed in the skull and vertebral column.

• Responsible for processing and integrating sensory inputs, as well as coordinating responses.

Peripheral Nervous System (PNS)

• Composed of nerves leading to and from the CNS, providing pathways for sensory input and motor output.

• Divided into two functional divisions:

  • Sensory Division: Carries signals from receptors to the CNS regarding environmental changes.

  • Motor Division: Carries signals from the CNS to muscles and glands; divided into:

  • Somatic Motor Division: Controls voluntary movements of skeletal muscles.

  • Autonomic Nervous System (ANS): Controls involuntary responses (smooth muscles, cardiac muscles, glands), further divided into:

    • Sympathetic Division: Prepares the body for action (fight or flight response).

    • Parasympathetic Division: Calms the body, promoting rest and digestion.

Neurons Structure and Function

• Neurons are the main communication cells of the nervous system, with approximately 1 trillion neurons present.

• Basic structure includes:

  • Neurosoma (cell body): Contains the nucleus and organelles; lacks centrioles, does not undergo mitosis.

  • Dendrites: Receive input from neighboring neurons, featuring a branching structure that allows for increased connectivity and information processing.

  • Axon: A single extension from the neurosoma that transmits signals to other cells, forming a synapse at the axon terminal.

Classes of Neurons

Structural Classification

• Multipolar Neurons: Characterized by one axon and multiple dendrites; they are the most common type of neuron.

• Bipolar Neurons: Consist of one axon and one dendrite; commonly associated with sense organs.

• Unipolar Neurons: Feature one process that leads away from the soma and branches into a T-shape; primarily involved in sensory functions.

Functional Classification

• Sensory (Afferent) Neurons: Relay sensory information to the CNS; usually classified as unipolar or bipolar.

• Interneurons: Process, store, and retrieve information within the CNS; typically multipolar.

• Motor (Efferent) Neurons: Conduct signals from the CNS to effectors (muscles and glands); generally multipolar.

Neuroglial Cells

• Neuroglial cells outnumber neurons at least 10 to 1 and play vital roles in supporting and protecting neurons.

In CNS

• Oligodendrocytes: Form the myelin sheath around CNS axons.

• Ependymal Cells: Line brain cavities and produce cerebrospinal fluid (CSF).

• Microglia: Function as immune cells that clear debris and pathogens from the CNS.

• Astrocytes: Support neurons, maintain the blood-brain barrier (BBB), and regulate blood flow.

In PNS

• Satellite Cells: Provide support and protection for neuron cell bodies.

• Schwann Cells: Form the myelin sheath for PNS axons and facilitate regeneration after injury.

Myelin Sheath

• Insulates axons, significantly increasing the speed of signal conduction.

• Formed by oligodendrocytes in the CNS and Schwann cells in the PNS.

• Features a segmented structure with nodes of Ranvier that allows for rapid signal propagation.

White Matter vs Gray Matter

• White Matter: Composed of myelinated axons; primarily involved in signal transmission.

• Gray Matter: Contains neuron cell bodies, dendrites, and unmyelinated axons; largely involved in processing information.

Resting Membrane Potential (RMP)

• Refers to the charge difference across the neuronal membrane when not stimulated, with the inside of the cell being negative and the outside positive.

• Maintained by the sodium-potassium pump, which pumps 3 Na+ ions out and 2 K+ ions in per cycle.

Nerve Signaling

• Involves two types of ion channels:

  • Ligand-Gated Channels: Open in response to the binding of a specific molecule.

  • Voltage-Gated Channels: Open in response to changes in membrane potential.

Local vs Action Potential

• Local Potential: Represents an initial change in membrane potential that can initiate an action potential if strong enough.

• Action Potential: Characterized by a rapid, all-or-nothing response that propagates along the axon, involving depolarization and repolarization phases.

Conducting a Nerve Signal

• Action potentials propagate along the axon and can occur via:

  • Continuous Conduction: Typically occurs in unmyelinated axons.

  • Saltatory Conduction: Takes place in myelinated axons, facilitating quicker signal transmission.

Transmitting Nerve Signals

• Action potentials trigger the release of neurotransmitters from presynaptic neurons into the synaptic cleft, influencing postsynaptic neurons' activity.

Disease Example: Parkinson's Disease

• Characterized by progressive loss of motor control due to degeneration of dopamine-releasing neurons, adversely affecting both movement and mental health.