Lecture Outline: The Nervous System
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- Nervous System Overview
- Control System
- One of two control systems (nervous and endocrine) out of 11 total organ systems.
- Controls the other nine organ systems.
- Each control system also controls the other.
- Involved in maintaining homeostasis.
- Signals
- Both nervous and endocrine systems operate via signals.
- Endocrine system uses only chemical signals (hormones), which are molecules broadcast throughout the body via the bloodstream.
- Nervous system uses two categories of signals:
- Electrochemical signals (action potentials).
- Changes in voltage carried by ions.
- Produced and transmitted only by excitable tissues (nervous and muscle tissue).
- Chemical signals (neurotransmitters).
- Released from axon terminals and act locally, not entering the bloodstream.
- Can be the same chemical as a hormone, but functional difference in operation.
- Require appropriate protein receptors for effect.
- Major Functions (in order of occurrence)
- Sensory input: Information coming into the nervous system, specifically toward the central nervous system (CNS).
- Via sensory neurons using action potentials.
- Allows CNS to read current values of variables for homeostasis comparison to a set point.
- Sense vs. Sensation:
- Sense/Sensory input: Any information coming toward the CNS, often unconsciously.
- Sensation: High-level processing in the brain, leading to conscious awareness.
- Integration: High-level processing of sensory information in the CNS.
- Can be conscious (sensation) or unconscious (e.g., controlling heart beat, digestion).
- Determines appropriate changes for homeostasis.
- Motor output: Information sent from the CNS to target tissues.
- "Motor" refers to movement, but not only skeletal muscles.
- Can target all three muscle types (skeletal, cardiac, smooth) and glands.
- Causes muscles to contract or glands to secrete.
- Hierarchical Breakdown
- Nervous System
- Central Nervous System (CNS): Brain and spinal cord.
- Peripheral Nervous System (PNS): Everything else, including cranial and spinal nerves (bundles of neurons).
- Sensory Division (Afferent): Carries incoming sensory information toward the CNS.
- Motor Division (Efferent): Carries outgoing motor information away from the CNS toward target tissues/effector organs.
- Somatic Division: Controls only skeletal muscles; under voluntary control. Single motor neuron from CNS to muscle.
- Autonomic Division: Controls involuntary actions (cardiac muscle, smooth muscle, glands); behind-the-scenes. Two neurons in series with a synapse: presynaptic and postsynaptic neurons.
- Parasympathetic Division: Antagonistic to sympathetic; often decreases activity.
- Sympathetic Division: Antagonistic to parasympathetic; often increases activity.
- Nervous System Cell Types
- Neurons
- Excitable cells: Able to produce and transmit action potentials and release neurotransmitters.
- "Business end" of the nervous system, doing the actual controlling.
- A neuron is an individual cell; a nerve is a bundle of many neurons.
- Neurons are either sensory or motor; they do not change direction of information.
- Neuroglia (Glial Cells)
- Non-excitable cells that support neurons.
- Do not produce action potentials or neurotransmitters themselves.
- Central Nervous System Neuroglia (found only in brain or spinal cord)
- Astrocytes:
- Star-shaped cells with elaborate plasma membranes.
- Wrap around capillaries to form and maintain the blood-brain barrier.
- Protects CNS by controlling what leaks from bloodstream.
- Microglial Cells:
- Smaller than most neuroglial cells.
- Act as phagocytes ("cell eaters") to engulf and destroy unwanted substances, protecting neurons.
- Ependymal Cells:
- Line interior surfaces of hollow parts of CNS (ventricles in brain, central canal of spinal cord).
- Produce and secrete cerebrospinal fluid (CSF) at choroid plexuses.
- Oligodendrocytes:
- Form myelin sheaths around axons of neurons in the CNS.
- Myelin makes white matter appear white.
- Speed up propagation of action potentials (insulation).
- Oligo means "more than one but not many"; one oligodendrocyte covers parts of multiple axons.
- Peripheral Nervous System Neuroglia (found only outside CNS)
- Schwann Cells (Neurolemmocytes):
- Form myelin sheaths around axons of neurons in the PNS.
- Unlike oligodendrocytes, one Schwann cell covers only part of one axon.
- Myelinated axons have gaps called Nodes of Ranvier, which are necessary for faster action potential propagation.
- Satellite Cells:
- Cover the cell bodies of neurons.
- Least understood function.
- Signal Transmission
- Resting Membrane Potential
- All living cells have a voltage difference across their plasma membrane.
- More positive charge outside, more negative charge inside.
- Caused by continuous operation of sodium-potassium exchange pumps, creating unequal distribution of ions (charge gradient).
- Action Potentials (Electrochemical Signals)
- Drastic, temporary change in membrane voltage.
- Generation:
- Triggered by opening of voltage-gated sodium ion channels in the neuron membrane.
- Sodium ions (positive charge) rush into the cell, making the interior less negative/more positive.
- Propagation:
- Chain reaction where one action potential causes the next one beside it.
- Travels down the axon without diminishing in strength.
- Myelination significantly increases propagation speed by allowing "jumping" between Nodes of Ranvier.
- Recovery:
- After action potential, sodium-potassium pumps re-establish resting membrane potential.
- Allows for subsequent action potentials (e.g., continuous muscle contraction).
- Synapses
- Site of communication between two excitable cells (neuron to neuron, or neuron to muscle/gland).
- Components: Presynaptic cell (upstream neuron), synaptic cleft (extracellular fluid space), postsynaptic cell (downstream neuron/muscle/gland).
- Neurotransmitter Release (Chemical Signal)
- Action potential reaches axon terminal of presynaptic neuron.
- Causes opening of voltage-gated calcium ion channels.
- Calcium ions (higher concentration outside) flow into the axon terminal.
- Incoming calcium ions directly trigger exocytosis of neurotransmitter-filled vesicles.
- Neurotransmitters are released into the synaptic cleft.
- Neurotransmitter Binding and Ion Channels
- Neurotransmitters diffuse across synaptic cleft and bind to specific protein receptors on the postsynaptic membrane.
- Binding causes a conformational (shape) change in the receptor protein.
- This shape change opens chemically-gated ion channels in the postsynaptic membrane.
- Ions (e.g., sodium) flow through these channels into the postsynaptic cell, causing a voltage change.
- In a postsynaptic neuron, if enough sodium ions enter, it generates a new action potential, continuing the signal.
- Neurotransmitter Removal
- Neurotransmitters must be removed from receptors/synaptic cleft to allow precise signaling and relaxation.
- Enzymes (proteins) degrade neurotransmitter molecules, preventing them from staying bound to receptors indefinitely.
- Once released, the receptor returns to its original shape, closing the channel and allowing the postsynaptic cell to recover to resting potential.
- Brain Anatomy
- Major Subdivisions
- Cerebrum: Largest, most conspicuous part; responsible for higher functions ("the brain" in common terms).
- Features gyri (ridges) and sulci (grooves/valleys).
- Divided into lobes (e.g., occipital, temporal, parietal, frontal) named after overlying cranial bones.
- Central sulcus: Major groove dividing anterior (motor) and posterior (sensory) processing areas.
- Precentral gyrus: Primary motor area (anterior to central sulcus).
- Postcentral gyrus: Primary somatic sensory area (posterior to central sulcus).
- Hemispheres: Right and left halves, divided by longitudinal fissure.
- Not divided by motor/sensory function; both hemispheres have both.
- Cerebellum: Second largest part; "little brain" due to similar wrinkled appearance.
- Crucial for "muscle memory" and fine motor control, smoothing out movements.
- Thalamus: "Switchboard" for almost all sensory information processing before reaching cortex.
- Hypothalamus: Located inferior ("hypo") to the thalamus.
- Overlaps anatomically/physiologically with nervous and endocrine systems.
- Contains neurons that control hormone release from the pituitary gland.
- Brain Stem: Inferior part of the brain.
- Includes Pons (larger bulge) and Medulla Oblongata (smaller, inferior bulge).
- Medulla Oblongata marks the official end of the brain, continuous with spinal cord.
- Cerebral Cortex
- Superficial part of the cerebrum.
- Different regions devoted to different functions (e.g., visual area, olfactory area).
- Somatotopy ("Homunculus"): Mapping of body parts onto the cerebral cortex.
- Distorted human representation reflects amount of brain power (neurons) devoted to a body part for sensation or motor control.
- Hands, face, and tongue have disproportionately large areas.
- Brain Fiber Types (Neurons)
- Association Fibers: Connect different parts within the same cerebral hemisphere; do not cross over.
- Commissural Fibers: Cross over (decussate) to connect areas in opposite hemispheres.
- Bundled commissural fibers form a commissure.
- Corpus Callosum: The largest and most conspicuous commissure, connecting the two hemispheres.
- Projection Fibers: Extend from the brain down into the spinal cord; can be very long.
- Ventricles and Cerebrospinal Fluid (CSF)
- Hollow, fluid-filled spaces within the brain and spinal cord (central canal).
- Provide shock absorption and cushioning for the CNS.
- Four ventricles in the brain (all interconnected):
- Two Lateral Ventricles (symmetrical, off-midline).
- Third Ventricle (on midline, superior to fourth, between thalamus halves).
- Fourth Ventricle (on midline, in brain stem).
- Central Canal of spinal cord (connected to fourth ventricle).
- Cerebrospinal Fluid (CSF):
- Produced by ependymal cells in choroid plexuses within ventricles.
- Circulates through internal cavities and also surrounds the entire brain and spinal cord externally.
- Drains into the bloodstream via arachnoid granulations to maintain constant pressure.
- Peripheral Nervous System
- Structure of Nerves
- Nerves are bundles of fascicles, and fascicles are bundles of nerve fibers (neurons, primarily axons).
- Connective tissue wrappings:
- Epineurium: Surrounds the entire nerve.
- Perineurium: Surrounds each fascicle.
- Endoneurium: Surrounds each individual axon (nerve fiber).
- Cranial Nerves
- 12 pairs (24 total) of nerves connected directly to the brain.
- Can be sensory-only (afferent), motor-only (efferent), or mixed (containing both sensory and motor neurons).
- Spinal Nerves
- Nerves connected to the spinal cord.
- Named by region of spinal cord (e.g., cervical, thoracic).
- All paired (right and left).
- Form plexuses (web-like networks) in certain regions.
- Each spinal nerve is a two-way street but composed of one-way neurons:
- Posterior (dorsal) root: Bundle of only sensory (afferent) neurons carrying incoming information to CNS.
- Anterior (ventral) root: Bundle of only motor (efferent) neurons carrying outgoing information from CNS.
- These roots join to form a mixed spinal nerve.
- Motor Divisions (Revisiting PNS Motor Division)
- Somatic Motor Nervous System:
- Targets: All skeletal muscles.
- Pathway: Single motor neuron from CNS to skeletal muscle.
- Autonomic Nervous System:
- Targets: Cardiac muscle, smooth muscle, glands.
- Pathway: Two neurons in series (presynaptic, postsynaptic) with a synapse between them.
- Subdivisions (Parasympathetic and Sympathetic) are antagonistic for fine control.