Nervous System Integration and Control

Nervous System Overview

The nervous system is a complex network responsible for receiving, integrating, and responding to information from both internal and external environments. It works closely with the endocrine system to maintain homeostasis and enables self-awareness and emotions.

• Receives information from multiple sources simultaneously.

• Integrates information to process and compile sensory input.

• Rapid response—can react in tenths of a second.

• Initiates specific responses such as muscle contraction, glandular secretion, and conscious movement.

• Maintains homeostasis in conjunction with the endocrine system.

Principal Parts of the Nervous System

The nervous system is divided into two main parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

• Central Nervous System (CNS): Consists of the brain and spinal cord. Functions to receive, process, and transfer information.

• Peripheral Nervous System (PNS): Composed of nerves outside the CNS. Includes:

  • Sensory division: Carries information toward the CNS.

  • Motor division: Carries information away from the CNS.

  • Somatic division: Controls skeletal muscles.

  • Autonomic division: Controls smooth muscle, cardiac muscle, and glands (subdivided into sympathetic and parasympathetic divisions).

Neurons: Communication Cells of the Nervous System

Neurons are specialized cells that generate and conduct electrical impulses, serving as the primary communication units of the nervous system.

• Sensory neurons: Found in the PNS, receive stimuli and transmit information to the CNS.

• Interneurons: Transmit information between components of the CNS.

• Motor neurons: Found in the PNS, transmit information away from the CNS.

Each neuron consists of three main parts:

• Cell body: Contains the nucleus and most cytoplasm and organelles.

• Dendrites: Receive incoming information.

• Axon: Conducts electrical impulses away from the cell body.

Action Potentials and Resting Potential

Neurons communicate via action potentials, which are electrical impulses generated and transmitted along their axons.

• Resting potential: The voltage difference across the cell membrane in a resting neuron; interior is negative relative to exterior.

• Sodium-potassium pump: Maintains resting potential by actively transporting three sodium ions (Na+) out and two potassium ions (K+) into the cell.

Equation:

Graded Potentials and Action Potential Initiation

Graded potentials are transient local changes in the resting potential that may depolarize or hyperpolarize the membrane. Summation of graded potentials can reach a threshold, triggering an action potential.

• Depolarization: Voltage-sensitive Na+ channels open, Na+ enters the axon, reversing membrane polarity.

• Repolarization: Na+ channels close, K+ channels open, K+ exits the axon, restoring polarity.

• Resting potential reestablishment: Normal activity of the sodium-potassium pump restores resting potential.

Refractory Periods

After an action potential, neurons experience refractory periods:

• Absolute refractory period: No new action potential can be generated.

• Relative refractory period: Membrane is hyperpolarized; harder to generate another action potential.

All-or-None and Self-Propagating Nature of Action Potentials

Action potentials are all-or-none events; once threshold is reached, the action potential is always the same in speed and voltage. They are self-propagating, moving like a wave down the axon.

• Stimulus strength: Encoded by the number of action potentials per unit time.

• Speed: Faster in larger diameter and myelinated axons.

Neuroglial Cells: Support and Protection

Neuroglial cells (glial cells) make up 80% of nervous system cells and provide support and protection for neurons.

• Schwann cells (PNS): Form myelin sheaths, speed up impulse transmission, aid in axon regeneration.

• Oligodendrocytes (CNS): Form myelin sheaths in CNS.

• Astrocytes (CNS): Support and nourish neurons.

Myelin sheath: Insulates axons, enables saltatory conduction (impulse jumps from node to node).

Disorders Associated with Myelin Sheath Degeneration

• Multiple sclerosis (MS): Autoimmune damage to myelin sheaths in CNS; symptoms include numbness, weakness, paralysis.

• Amyotrophic lateral sclerosis (ALS): Progressive damage to myelin sheaths in spinal cord; leads to muscle wasting and respiratory failure.

Synaptic Transmission

Information is transferred from a neuron to its target cell via synapses, involving the release and diffusion of neurotransmitters.

• Action potential arrives at axon terminus, causing Ca2+ influx.

• Neurotransmitter release: Neurotransmitter diffuses across synaptic cleft and binds to receptors on postsynaptic membrane.

• Graded potential: Results from ion diffusion through opened channels.

Neurotransmitters: Excitatory and Inhibitory Effects

Neurotransmitters can have excitatory or inhibitory effects on the postsynaptic cell, depending on their type, concentration, and receptor type.

• Excitatory: Depolarize the postsynaptic cell, approaching threshold.

• Inhibitory: Hyperpolarize the postsynaptic cell.

• Removal: Neurotransmitters are removed by reuptake, enzymatic destruction, or diffusion away.

Neural Information Processing: Convergence and Divergence

Postsynaptic neurons integrate and process information from multiple sources.

• Convergence: One neuron receives input from many others.

• Divergence: One neuron sends action potentials to multiple others.

Peripheral Nervous System: Relaying Information

The PNS relays information between tissues and the CNS via nerves composed of axons wrapped in protective sheaths.

• Cranial nerves: 12 pairs, connect directly to the brain.

• Spinal nerves: 31 pairs, connect to spinal cord; each carries both sensory and motor information.

Somatic Division: Control of Skeletal Muscles

The somatic division controls voluntary and involuntary (reflex) actions of skeletal muscles.

• Voluntary: Conscious control.

• Involuntary (reflexes): Mediated by spinal cord and nerves, e.g., flexor (withdrawal) reflex, crossed extensor reflex, stretch reflex.

Example: Stepping on a sharp object triggers a flexor reflex to withdraw the foot and a crossed extensor reflex to extend the other leg.

Autonomic Division: Automatic Body Functions

The autonomic division controls automatic functions of internal organs and consists of sympathetic and parasympathetic divisions.

• Sympathetic division: Arouses the body, prepares for emergencies (fight-or-flight), uses norepinephrine.

• Parasympathetic division: Relaxes the body, opposes sympathetic actions, uses acetylcholine.

• Both divisions: Innervate smooth muscle, cardiac muscle, glands, and internal organs; work antagonistically to maintain homeostasis.

Central Nervous System: Brain and Spinal Cord

The CNS is protected by bone, meninges, cerebrospinal fluid, and the blood-brain barrier.

• Bone: Skull and vertebrae.

• Meninges: Dura mater, arachnoid, pia mater.

• Cerebrospinal fluid: Shock absorber, produced in brain ventricles.

• Blood-brain barrier: Prevents entry of certain chemicals and pathogens.

Spinal Cord: Information Relay

The spinal cord acts as a superhighway for action potentials between the brain and body.

• White matter: Outer portion, myelinated axons (sensory and motor tracts).

• Gray matter: Center portion, cell bodies and dendrites.

Brain: Processing and Acting on Information

The brain is the command center, divided into hindbrain, midbrain, and forebrain.

• Hindbrain: Medulla oblongata (vital functions), cerebellum (movement coordination), pons (information flow).

• Midbrain: Coordinates vision, hearing, sleep/wakefulness.

• Forebrain: Hypothalamus (homeostasis), thalamus (sensory processing), limbic system (emotions, memory), cerebrum (higher functions).

Cerebrum and Cerebral Cortex

The cerebrum is divided into right and left hemispheres, connected by the corpus callosum. The cerebral cortex is the highly folded outer layer responsible for sensory integration, memory, abstract thought, and conscious control.

• Occipital lobe: Visual processing.

• Temporal lobe: Auditory processing, language comprehension.

• Parietal lobe: Sensory information from skin.

• Frontal lobe: Motor activity, speech, conscious thought.

Primary somatosensory area: Parietal lobe region receiving sensory input. Primary motor area: Frontal lobe region initiating motor activity.

Memory: Storage and Retrieval

• Short-term memory: Stored in limbic system, lasts hours.

• Long-term memory: Involves hippocampus and cerebral cortex, lasts days to years, involves permanent neuronal changes.

Disorders of the Nervous System

• Trauma: Concussion, spinal cord injury.

• Infections: Encephalitis, meningitis, rabies.

• Brain tumors: Abnormal growths, may be cancerous or noncancerous.

• Neural transmission disorders: Epilepsy, Alzheimer’s disease, Parkinson’s disease.

Concussion: Disruption of brain electrical activity, risk of subdural hematoma. Spinal cord injuries: Impair sensation and function below injury site. Encephalitis: Viral inflammation of the brain. Meningitis: Inflammation of meninges, bacterial or viral. Rabies: Infectious viral disease, transmitted by animal bites. Epilepsy: Recurring abnormal electrical activity, causes seizures. Alzheimer’s disease: Buildup of abnormal proteins, causes dementia. Parkinson’s disease: Loss of dopamine-releasing neurons, affects movement.