Organization of the Nervous System

Overview

The nervous system is a complex network responsible for rapid communication and control throughout the body. It consists of specialized cells called neurons and supporting cells known as glial cells. Its main function is to transmit information quickly via action potentials (APs).

• Neurons: Primary signaling cells of the nervous system.

• Glial cells: Support, protect, and nourish neurons.

• Action potentials (APs): Electrical impulses that carry information.

General Functions of the Nervous System

• Sensory function: Detects changes in the environment (stimuli).

• Integrative function: Processes and interprets sensory input, deciding on responses.

• Motor function: Initiates responses by activating muscles or glands.

Functional Organization

• Sensory receptors: Convert environmental stimuli (light, sound, chemicals) into action potentials and/or neurotransmitter signals.

• Sensory (afferent) neurons: Carry sensory information from receptors to the CNS.

• Interneurons: Located in the CNS; integrate and process sensory information, then relay instructions to motor neurons.

• Motor (efferent) neurons: Transmit instructions from the CNS to effectors (muscles/glands).

Example:

Touching a hot surface activates sensory receptors in the skin, which send signals via sensory neurons to the spinal cord. Interneurons process the information and motor neurons trigger muscle contraction to withdraw the hand.

Central and Peripheral Nervous Systems

Central Nervous System (CNS)

The CNS consists of the brain and spinal cord. It is the main integration and control center for the body.

• Structure: Brain and spinal cord; contains interneurons and cell bodies of motor neurons.

• Function: Integration and generation of motor output signals.

Peripheral Nervous System (PNS)

The PNS includes all nerves outside the CNS. It connects the CNS to limbs and organs.

• Structure: Nerves (sensory, motor, or mixed) and ganglia; contains cell bodies of sensory neurons and axons of sensory/motor neurons.

• Function: Carries sensory input and motor output signals to/from the CNS.

Tissues of the Nervous System

White Matter

• Mainly axons, giving a whitish color due to the myelin sheath.

• Located in the outer layer of the spinal cord and inner layer of the brain.

• PNS: Bundles of myelinated axons (nerves).

Grey Matter

• Mainly dendrites and cell bodies, giving a beige/grey color.

• Located in the inner layer of the spinal cord and outer layer of the brain.

• PNS: Ganglia (small pockets of sensory or motor neuron cell bodies).

Peripheral Nervous System Divisions

Somatic Nervous System (SNS)

• Voluntary motor control: Controls skeletal muscles and reflexes.

• Responds mainly to external stimuli.

Autonomic Nervous System (ANS)

• Involuntary motor control: Regulates smooth muscle, cardiac muscle, and glands.

• Divided into parasympathetic and sympathetic sub-divisions, which have opposite effects on target organs.

PNS Structure

• Cranial nerves: Enter/exit CNS at the base of the brain.

• Spinal nerves: Enter/exit CNS along the spinal cord (cervical, thoracic, lumbar, sacral regions).

• Ganglia: Contain cell bodies of PNS neurons.

Autonomic Nervous System (ANS) Functions

Parasympathetic Division

• "Rest and digest": Prepares the body for intake, conservation, and storage of energy.

• Decreases heart rate, increases digestive activity, increases bladder contraction.

Sympathetic Division

• "Fight or flight": Prepares the body for energy-consuming activities and stressful situations.

• Increases heart rate, decreases digestive activity, decreases bladder contraction.

Comparison Table: Parasympathetic vs. Sympathetic Effects

Protecting the CNS

Cerebrospinal Fluid (CSF)

• Cushions and protects the spinal cord and brain.

• Bathes neurons, supplies nutrients, gases, hormones, water, and fluid for transporting defense cells.

Meninges

• Layers of connective tissue that protect and support the CNS.

• Cover the outside of the brain and spinal cord, allow CSF to circulate underneath.

Spinal Cord

The spinal cord carries signals from the brain to the rest of the body through millions of nerve fibers and neuron cell bodies. It also integrates and processes some reflexes.

• White matter: Tracts (bundles of axons).

• Grey matter: Horns (cell bodies and dendrites).

• Ganglion: Part of the PNS, contains neuron cell bodies.

Structural Regions of the Brain

• Cerebrum: Largest part, divided into left and right hemispheres.

• Thalamus: Categorizes, filters, and enhances sensory information.

• Hypothalamus: Controls pituitary gland and homeostasis (temperature, hunger, thirst, etc.).

• Pineal gland: Regulates biological clock and sleep cycles.

• Midbrain, Pons, Medulla oblongata: Form the brainstem, control basic life functions.

• Cerebellum: Planning center for body movements, learning and remembering motor skills.

Forebrain

• Includes thalamus, hypothalamus, cerebrum.

• Responsible for higher-level thinking and processing.

Brainstem

• Relays and filters information to/from all other brain regions.

• Controls basic life functions and consciousness.

• Includes midbrain, pons, and medulla oblongata.

Cerebellum

• Coordinates voluntary movements and balance.

• Processes sensory input related to movement.

Cerebrum

• Most sophisticated part of the brain.

• Divided into four lobes: frontal, parietal, temporal, occipital.

Example:

The cerebellum helps you learn and remember how to ride a bicycle, while the cerebrum allows you to think about the route and make decisions.

Key Terms and Definitions

• Neuron: A nerve cell specialized for transmitting electrical signals.

• Glial cell: A supporting cell in the nervous system.

• Action potential: A rapid change in membrane potential that transmits signals along neurons.

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

• Effector: A muscle or gland that carries out the response to a neural signal.

Equations and Scientific Notation

• Action Potential Propagation: where is membrane potential, is resting potential, and AP amplitude is the change during an action potential.

Additional info: Some context and definitions have been expanded for clarity and completeness.