The Nervous System

Overview of the Nervous System

The nervous system is the master controlling and communicating system of the body. It is responsible for detecting stimuli, processing information, and coordinating responses to maintain homeostasis and enable complex behaviors.

• Sensory input: Detects external and internal stimuli through specialized receptors.

• Integration: Processes and interprets sensory input, making decisions about appropriate responses.

• Control of Muscles and Glands: Directs the activity of muscles and glands to produce movement and regulate bodily functions.

• Homeostasis: Maintains stable internal conditions by regulating other organ systems.

• Mental Activities: Responsible for cognition, memory, emotion, and consciousness.

Divisions of the Nervous System

The nervous system is divided into two main anatomical and functional components:

• Central Nervous System (CNS): Consists of the brain and spinal cord, enclosed in bony coverings. It is the primary site for integration and processing of information.

• Peripheral Nervous System (PNS): Composed of nerves (bundles of axons in connective tissue) and ganglia (clusters of neuron cell bodies). It connects the CNS to limbs and organs.

The PNS is further subdivided into:

• Sensory (Afferent) Division: Transmits action potentials from sensory receptors to the CNS.

• Motor (Efferent) Division: Carries action potentials away from the CNS to effectors (muscles and glands).

The Motor Division includes:

• Somatic Nervous System: Innervates skeletal muscle for voluntary movements.

• Autonomic Nervous System (ANS): Innervates cardiac muscle, smooth muscle, and glands. The ANS has three subdivisions:

  • Sympathetic Division: Most active during physical activity ("fight or flight").

  • Parasympathetic Division: Regulates resting functions ("rest and digest").

  • Enteric Division: Controls the digestive system.

Cells of the Nervous System

Neurons

Neurons are excitable cells that transmit electrical signals. They are the functional units of the nervous system and are specialized for rapid communication.

• Cell Body (Soma): Contains the nucleus and nucleolus; site of protein synthesis (Nissl bodies).

• Dendrites: Receive signals from other neurons.

• Axon: Conducts action potentials away from the cell body. The trigger zone is where the axon originates and action potentials are generated.

Types of neurons based on structure:

• Multipolar Neurons: Several dendrites and a single axon (e.g., motor neurons).

• Bipolar Neurons: One dendrite and one axon (e.g., sensory neurons in the retina).

• Unipolar Neurons: Single axon (e.g., sensory neurons).

Neuroglial (Glial) Cells

Glial cells are non-neuronal cells that support, protect, and nourish neurons. They account for over half of the brain's weight and maintain the environment for neuronal function.

• Astrocytes: Most abundant; form the framework of the CNS, regulate tissue fluid composition, convert glucose to lactate, secrete nerve growth factors, and influence synaptic signaling. Involved in sclerosis (formation of scar tissue after injury).

• Microglia: Macrophages derived from monocytes; remove debris and respond to infection, trauma, or stroke.

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

• Oligodendrocytes: Form myelin sheaths in the CNS by wrapping around nerve fibers.

• Schwann Cells: Myelinate axons in the PNS.

Myelin and Nerve Fiber Classification

Myelin Sheath

Myelin is an insulating layer around nerve fibers, formed by oligodendrocytes in the CNS and Schwann cells in the PNS. It consists of 20% protein and 80% lipid, giving it a white appearance. Myelination increases the speed of nerve impulse conduction and is completed by late adolescence.

• In the CNS, myelin consists of multiple layers of oligodendrocyte membrane.

• In the PNS, the outermost coil is the Schwann cell's neurilemma.

Nerve Fiber Classification

Nerve fibers are classified based on diameter, myelination, and conduction velocity:

Electrophysiology of Neurons

Membrane Potentials

Neurons are highly excitable and respond to stimuli by generating action potentials. The difference in electrical charge across the plasma membrane is called the membrane potential.

• Resting Membrane Potential (RMP): Typically -70 mV; maintained by differences in ionic composition and permeability of the membrane.

• ICF (intracellular fluid) has higher K+ and negatively charged proteins; ECF (extracellular fluid) has higher Na+ and Cl-.

• Membrane is more permeable to K+ than Na+; K+ diffuses out, Na+ diffuses in.

• Sodium-potassium pump: Maintains gradients by pumping 3 Na+ out and 2 K+ in.

Equation for Resting Membrane Potential:

Additional info: The actual equation is the Goldman-Hodgkin-Katz equation, which considers permeability and concentration of multiple ions.

Action Potentials

An action potential (AP) is a rapid, reversible change in membrane potential that propagates along the axon. It is the principal means of long-distance neural communication.

• Occurs only in excitable cells (neurons and muscle cells).

• All-or-none law: APs either occur fully or not at all.

• Do not decrease in strength with distance.

• Phases: Depolarization (Na+ influx), Repolarization (K+ efflux), Hyperpolarization (K+ continues to leave or Cl- enters).

Action Potential Equation:

Conduction Velocity and Myelination

Myelin sheaths increase conduction velocity by insulating axons and enabling saltatory conduction, where the action potential "jumps" from node to node (Nodes of Ranvier).

• Myelinated axons conduct impulses up to 30 times faster than unmyelinated axons.

• Voltage-gated Na+ channels are concentrated at the nodes.

Synapses and Neural Integration

The Synapse

A synapse is the junction between two cells where communication occurs. The presynaptic cell sends the signal; the postsynaptic cell receives it.

• Electrical synapse: Direct flow of ions through gap junctions; rapid transmission.

• Chemical synapse: Neurotransmitters released from presynaptic neuron bind to receptors on postsynaptic cell.

Postsynaptic Potentials

• Excitatory Postsynaptic Potential (EPSP): Neurotransmitter binding opens channels for Na+ and K+; net depolarization brings membrane closer to threshold for AP.

• Inhibitory Postsynaptic Potential (IPSP): Neurotransmitter binding opens K+ or Cl- channels; hyperpolarization moves membrane further from threshold, reducing likelihood of AP.

Neurotransmitters

Classification by Chemical Structure

Neurotransmitters are the chemical messengers of the nervous system. Most neurons produce two or more neurotransmitters, which can have different effects depending on the receptor and location.

Additional info: Neurotransmitters can produce fast or slow responses and may induce Ca2+ influx in astrocytes.