Nervous System: Structure, Function, and Physiology Study Notes

Nervous System Organization

Central and Peripheral Nervous Systems

The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord, while the PNS includes all neural tissue outside the CNS.

• CNS: Brain and spinal cord; responsible for processing and integrating information.

• PNS: Nerves and ganglia outside the CNS; transmits signals between the CNS and the rest of the body.

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

• Efferent (motor) division: Carries motor commands from the CNS to effectors (muscles and glands).

• Somatic nervous system: Controls voluntary movements (skeletal muscles).

• Autonomic nervous system: Regulates involuntary functions (smooth muscle, cardiac muscle, glands).

Example: Touching a hot surface activates sensory neurons (afferent), which send signals to the CNS; the CNS processes the information and sends motor commands (efferent) to withdraw the hand.

Neuroglia and Neuron Structure

Types and Functions of Neuroglial Cells

Neuroglia (glial cells) support and protect neurons. They are essential for maintaining homeostasis, forming myelin, and providing support and protection for neurons in both the CNS and PNS.

• Astrocytes: Maintain the blood-brain barrier, provide structural support, regulate ion and nutrient concentrations.

• Oligodendrocytes: Form myelin sheaths in the CNS.

• Microglia: Act as phagocytes, removing debris and pathogens.

• Ependymal cells: Line ventricles of the brain and central canal of the spinal cord; produce and circulate cerebrospinal fluid (CSF).

• Schwann cells: Form myelin sheaths in the PNS.

• Satellite cells: Surround neuron cell bodies in ganglia; regulate environment.

Neuron Structure: Neurons consist of a cell body (soma), dendrites (receive signals), axon (transmits signals), axon hillock (initiates action potential), myelin sheath (insulates axon), nodes of Ranvier (gaps in myelin), and axon terminals (synaptic end bulbs).

Neuron Classification and Structure

Types of Neurons

Neurons are classified based on their structure and function.

• Multipolar neurons: Many dendrites, one axon; most common type in CNS.

• Bipolar neurons: One dendrite, one axon; found in sensory organs (e.g., retina).

• Unipolar neurons: Single process that splits into two branches; sensory neurons in PNS.

Functional classification: Sensory (afferent), motor (efferent), and interneurons (association).

Resting Membrane Potential and Action Potentials

Membrane Potential and Its Changes

The resting membrane potential is the electrical potential difference across the neuron's plasma membrane when the cell is not transmitting a signal, typically around -70 mV.

• Resting state: Inside of the neuron is negatively charged relative to the outside.

• Action potential: Rapid change in membrane potential that travels along the axon.

• Depolarization: Na+ channels open, Na+ enters the cell, membrane potential becomes less negative.

• Repolarization: K+ channels open, K+ leaves the cell, membrane potential returns to negative.

• Hyperpolarization: Membrane potential becomes more negative than resting.

• Refractory period: Time during which a neuron cannot fire another action potential.

Equation:

$V_m = V_{inside} - V_{outside}$

Example: During an action potential, the membrane potential rapidly shifts from -70 mV to +30 mV and back.

Synaptic Transmission

Types of Synapses and Synaptic Potentials

Synapses are junctions where neurons communicate with other neurons, muscles, or glands.

• Synapse types: Axosomatic (axon to soma), axodendritic (axon to dendrite), axoaxonic (axon to axon).

• Neuromuscular junction: Synapse between a motor neuron and a muscle fiber.

• Neurotransmitters: Chemical messengers released from presynaptic neurons.

• Excitatory postsynaptic potential (EPSP): Depolarizes the postsynaptic membrane, increasing likelihood of action potential.

• Inhibitory postsynaptic potential (IPSP): Hyperpolarizes the postsynaptic membrane, decreasing likelihood of action potential.

Example: Acetylcholine at the neuromuscular junction causes muscle contraction via EPSP.

Neurotransmitter Release and Inactivation

Mechanisms and Effects

Neurotransmitters are released from synaptic vesicles in response to an action potential and bind to receptors on the postsynaptic cell.

• Release: Triggered by Ca2+ influx into the presynaptic terminal.

• Inactivation: By enzymatic degradation (e.g., acetylcholinesterase for acetylcholine), reuptake, or diffusion away from the synapse.

• Cholinergic effect: Acetylcholine stimulates muscle contraction.

• Adrenergic effect: Norepinephrine and epinephrine affect heart rate, blood pressure.

Example: Acetylcholine is broken down by acetylcholinesterase, terminating its action at the synapse.

Nervous System Pathways and Structures

White Matter, Gray Matter, and Neural Pathways

The nervous system contains distinct regions and pathways for signal transmission.

• Gray matter: Contains neuron cell bodies, dendrites, and unmyelinated axons; found in the cerebral cortex and central spinal cord.

• White matter: Composed of myelinated axons; responsible for rapid signal transmission.

• Ganglion: Cluster of neuron cell bodies in the PNS.

• Nucleus: Cluster of neuron cell bodies in the CNS.

• Tracts: Bundles of axons in the CNS; ascending tracts carry sensory information to the brain, descending tracts carry motor commands from the brain.

Example: The corticospinal tract is a descending tract that transmits motor signals from the brain to the spinal cord.