The Nervous System: Tissues, Cells, & Molecules to Gross Anatomy

Functions and Fundamental Divisions of the Nervous System

  • The nervous system is responsible for detecting, responding, and adapting to changes occurring both inside and outside the body to maintain homeostasis.

  • It controls body movement and carries out higher mental functions, including memory, attention, decision-making, emotion, thought, and language.

  • There are two major divisions of the nervous system:

    • Central Nervous System (CNS): Composed of the brain and the spinal cord.

    • Peripheral Nervous System (PNS): Composed of the cranial nerves and the spinal nerves.

Cellular Components: Neurons and Glial Cells

  • Nervous tissue consists of two basic cell types: neurons and glia.

  • Neurons (nerve cells) are the basic functional units of the nervous system.

  • Glial cells (glia) provide nourishment and support for the neurons.

  • The anatomy of a neuron includes several specialized structures:

    • Cell Body (Soma): The main part of the cell containing the nucleus and most organelles.

    • Dendrites: Multiple short extensions that receive signals from other neurons or sensory receptors; they carry information toward the soma.

    • Axon: A single, long extension that carries information away from the soma toward other cells.

    • Axon Terminals: The ends of the axon where messages are passed to the next cell in the circuit.

Functional Classification of Neurons

  • Sensory Neurons (Afferent Path): These take impulses from sensory receptors to the CNS. Sensory receptors are specialized structures that detect environmental stimuli (changes).

  • Interneurons: Located entirely within the CNS (brain and spinal cord), these receive information and send it to other neurons, connecting sensory and motor neurons.

  • Motor Neurons (Efferent Path): These carry information away from the CNS to an effector, such as a muscle or a gland, to execute a response.

Neural Insulation and Signal Speed

  • Myelin Sheath: Axons are electrically insulated by a white lipid coating called the myelin sheath. In the PNS, this sheath is formed by Schwann cells.

  • Schwann Cells: A type of glial cell that wraps around axons to insulate one neuron from another and facilitate signal transmission.

  • Nodes of Ranvier: Uninsulated gaps in the myelin sheath between Schwann cells. These gaps allow nerve conduction to occur at a faster rate.

Information Transfer Within and Between Neurons

  • Information transfer within a single neuron is electrical. This involves changes in electrical charge across the axon's plasma membrane, known as an action potential.

  • Information transfer between neurons is chemical and occurs at specialized areas called synapses.

  • The general flow of information follows this sequence: Dendrite →\rightarrow cell body →\rightarrow axon →\rightarrow axon terminals →\rightarrow synaptic cleft →\rightarrow dendrite of the next neuron.

The Mechanics of Synaptic Transmission

  • Synapse: A specialized area where information transfer happens between cells.

  • Synaptic Cleft: The physical gap between the sending neuron and the receiving neuron.

  • Neurotransmitters: Signal chemicals (e.g., acetylcholine, dopamine, serotonin) stored in synaptic vesicles within the axon terminals. They transfer information across the synaptic cleft.

  • The step-by-step process of transmission at the synapse is as follows:

    1. An action potential arrives at the axon terminal.

    2. Voltage-gated Ca2+Ca^{2+} channels open.

    3. Calcium ions (Ca2+Ca^{2+}) enter the nerve terminal and activate synaptic vesicles.

    4. Neurotransmitters are released from the vesicles into the synaptic cleft via exocytosis.

    5. Neurotransmitters diffuse across the synapse.

    6. Neurotransmitters bind to specific receptors on the post-synaptic (receiving) cell.

    7. This binding opens further channels, allowing sodium ions (Na+Na^+) to enter, which initiates a signal or change in electrical activity in the post-synaptic cell.

Functional Map of the Peripheral Nervous System (PNS)

  • The PNS consists of nerves outside the CNS, which are organs made of bundles of axons, blood vessels, and connective tissue layers.

  • Nerve Divisions by Origin:

    • Cranial Nerves: 12 pairs that extend from the brain.

    • Spinal Nerves: 31 pairs that arise from the spinal cord.

  • Functional Divisions:

    • Sensory (Afferent) Path: Includes somatic afferent (signals from skin, muscles, joints, and special senses) and visceral afferent (signals from body organs).

    • Motor (Efferent) Path: Divided into the Somatic and Autonomic systems.

Somatic and Autonomic Nervous Systems

  • Somatic Nervous System: Governs voluntary movements and reflexes. It carries sensory information from the skin and skeletal muscles to the CNS and sends motor commands from the CNS to skeletal muscles.

  • Autonomic Nervous System: Regulates involuntary activities involving cardiac muscle, smooth muscle, and glands. Cell bodies for this system are located outside the CNS. It is further divided into two opposing systems:

    • Sympathetic Division: Coordinates the "fight or flight" response. It increases heart rate, blood pressure, and breathing while slowing down digestion to handle stress.

    • Parasympathetic Division: Coordinates the "rest and digest" response. It counters the sympathetic system by slowing heart rate and breathing and returning housekeeping functions to normal.

The Spinal Reflex Arc

  • Reflexes are automatic, predictable responses that occur quickly without conscious thought.

  • The Spinal Reflex Arc involves both the spinal cord and spinal nerves through these steps:

    1. A stimulus is detected by a sensory receptor.

    2. Information is relayed along the sensory (afferent) nerve pathway to the control center.

    3. The control center (in the spinal cord or brain) integrates the information via interneurons.

    4. A decision is sent along the motor (efferent) nerve pathway.

    5. The effector (skeletal muscle) receives the signal and responds (e.g., muscle contraction), leading to the removal of the stimulus.

Protection and Composition of the Central Nervous System (CNS)

  • The CNS is protected by four primary mechanisms:

    1. Bone: The skull protects the brain, and the vertebrae protect the spinal cord.

    2. Meninges: Three layers of protective membranes:

      • Dura Mater: The tough, thick, outermost layer.

      • Arachnoid Mater: The spider-like middle layer where cerebrospinal fluid circulates in the subarachnoid space.

      • Pia Mater: The delicate, translucent innermost layer attached directly to the surface of the brain and spinal cord.

    3. Ventricles and Cerebrospinal Fluid (CSF): CSF cushions and protects the CNS, circulating between meninges, in brain chambers (ventricles), and the central canal of the spinal cord.

    4. Blood-Brain Barrier: A highly selective semipermeable border made of epithelial cells linked by tight junctions. It allows water, glucose, amino acids, gases, and steroids to pass but protects the brain from circulating pathogens and many medications.

  • Tissue Types in the CNS:

    • Gray Matter: Contains cell bodies and non-myelinated axons.

    • White Matter: Contains myelinated axons.

Major Regions of the Brain

  • Cerebrum: The largest region, divided into two hemispheres and four lobes:

    • Frontal Lobe: Voluntary movement, thinking, and language.

    • Parietal Lobe: Touch, pain, and relation of body parts.

    • Temporal Lobe: Hearing.

    • Occipital Lobe: Vision.

  • Diencephalon: Contains three main structures:

    • Thalamus: A relay station that receives sensory input and sends it to the appropriate area of the cerebral cortex.

    • Hypothalamus: Maintains homeostasis (energy, sleep, thirst, temperature, water balance) and controls the pituitary gland.

    • Pineal Gland: Secretes melatonin to control daily (circadian) rhythms.

  • Cerebellum: Located under the occipital lobe; it features a white matter pattern called the arbor vitae. It maintains posture, balance, and coordination.

  • Brainstem: Consists of the midbrain, pons, and medulla oblongata (which merges with the spinal cord). It maintains vital functions like breathing, blood pressure, and heart rate.

Anatomy of the Spinal Cord

  • The spinal cord runs through the vertebral canal and carries information between the brain and peripheral nerves.

  • Central Canal: A small opening in the center containing CSF.

  • Gray Matter: Shaped like an "H" in the center of the cord.

  • White Matter: Surrounds the central gray matter.

Summary of Nervous System Functioning

  • Reception of Sensory Input: Receptors respond to stimuli and generate signals that travel from the PNS to the CNS.

  • Information Processing and Integration: The CNS reviews sensory information and handles the formation, storage, and use of memories.

  • Generation of Output:

    • Motor Output: CNS signals travel through the PNS to muscles and glands.

    • Thought: Signals remain within the CNS.