Hole’s Essentials of Human Anatomy & Physiology Chapter 09: Nervous System
Introduction to the Nervous System
The nervous system is a primary controller of body functions, helping to maintain homeostasis alongside the endocrine system via hormone regulation. It is responsible for thinking, movement, and the internal physiological processes that sustain life.
Major Aspects of Nervous System Function:
Sensory Input: Detecting internal and external changes through sensory receptors.
Integration and Processing: The decision-making phase where information is coordinated and analyzed.
Motor Output: The response phase involving the sending of impulses to effectors (muscles or glands).
Main Cell Types:
Neurons: Specialized cells designed to communicate via electrical impulses with other neurons or body cells.
Neuroglia (Glial Cells): Known as "nerve glue," these cells support, protect, nourish, and insulate neurons. They do not generate or conduct nerve impulses.
Neurotransmitters: Chemical messengers located in a synapse that convey an electrical impulse from a neuron to another cell.
Nervous System Organization
The organs of the nervous system are categorized into two primary divisions:
Central Nervous System (CNS):
Comprises the brain and the spinal cord.
Acts as the center for integration of information and decision-making.
Peripheral Nervous System (PNS):
Comprised of cranial and spinal nerves that connect the CNS to the rest of the body.
Contains sensory (afferent) and motor (efferent) divisions.
Motor Functions of the PNS:
Somatic Nervous System: Responsible for overseeing conscious activities and controlling voluntary skeletal muscles.
Autonomic Nervous System: Responsible for controlling involuntary effectors such as smooth muscle, cardiac muscle, and various glands.
General Functions of the Nervous System
Sensory Function:
Facilitated by sensory receptors that detect changes inside and outside the body.
Information is transported from these receptors via sensory neurons into the CNS.
Integrative Function:
Coordination of sensory data occurs within the CNS.
This processing forms the logical basis for decision-making.
Motor Function:
Nerve impulses produced in the CNS are conducted along motor neurons to effectors.
Effectors are muscles or glands that execute the response decided upon by the CNS.
Neuron Structure: Anatomy of a Nerve Cell
Basic Components: A neuron consists of a cell body, tubular cytoplasm-filled dendrites, and a tubular axon.
Cell Body (Soma): Contains essential organelles including mitochondria, lysosomes, a Golgi apparatus, chromatophilic substance (Nissl bodies — analogous to rough ER), neurofilaments (protein threads), and a large nucleus with a prominent nucleolus.
Dendrites: Multiple short, branching processes that provide a large receptive surface for communication with other neurons. They conduct impulses toward the cell body.
Axon: A single long process that conducts impulses away from the cell body. It arises from a specialized thickening on the soma called the axon hillock.
Neurilemma: The outer layer of the Schwann cell that surrounds the axon.
Myelin Sheath: A lipid-rich insulating layer. Larger axons in the PNS are enclosed by these sheaths (myelinated fibers), while narrow gaps between Schwann cells are called nodes of Ranvier. Myelin significantly increases the conduction speed of nerve impulses.
Synaptic Knob: The distal expansion at the end of an axon terminal used for communication with other cells.
Classification of Neurons
Structural Classification:
Multipolar Neurons: Feature many dendrites and one axon. Most neurons found in the CNS (both interneurons and motor neurons) are this type.
Bipolar Neurons: Have only two processes (one dendrite and one axon) extending from opposite ends of the cell body. These are specialized and found in sense organs like the eyes, nose, and ears.
Unipolar Neurons (Pseudo-unipolar): Possess a single process extending from the cell body that divides into two branches (peripheral and central process) acting as one axon. Cell bodies are typically grouped in ganglia outside the CNS.
Functional Classification:
Sensory (Afferent) Neurons: Mostly unipolar (some bipolar); they carry impulses from peripheral receptors into the CNS.
Interneurons (Association Neurons): Multipolar; located entirely within the CNS where they link other neurons and integrate data. Groups of these cell bodies are called nuclei.
Motor (Efferent) Neurons: Multipolar; they carry instructions from the CNS out to peripheral effectors (muscles/glands).
Neuroglia: Types and Functions
CNS Neuroglia:
Microglia: Small cells acting as phagocytes to clear bacteria and debris; they also produce scar tissue.
Oligodendrocytes: Form the myelin sheath around axons within the brain and spinal cord. They do not have a neurilemma.
Ependymal Cells: Line the cavities of the CNS and produce cerebrospinal fluid (CSF).
Astrocytes: Found between blood vessels and neurons. They provide structural support, regulate ion concentrations, and form the blood-brain barrier (BBB) to protect the brain from chemical fluctuations.
PNS Neuroglia:
Schwann Cells: Produce the myelin sheath for axons in the peripheral nervous system.
Satellite Cells: Provide a protective coating around the cell bodies of neurons located in PNS ganglia.
Regeneration of Neurons
PNS Axons: Can regenerate if damaged because the neurilemma of Schwann cells provides a guiding pathway for the regrowing axon toward its original connection.
CNS Axons: Generally do not regenerate upon injury because oligodendrocytes lack a neurilemma and the environment does not favor regrowth.
Cell Membrane Potentials and Charges
Polarity: Neuron membranes are polarized, meaning there is a different charge on the inside versus the outside.
Ions and Charge Distribution:
The inside of the cell is more negative than the outside.
Sodium ions () are in higher concentration outside the cell.
Potassium ions () are in higher concentration inside the cell.
Large negatively charged proteins and ions stay inside the cell.
Resting Membrane Potential: The charge of a neuron at rest, typically measured at .
Threshold Potential: The specific level of stimulation required to trigger an action potential, usually .
The Action Potential and Nerve Impulses
Depolarization: When threshold is reached, voltage-gated channels open. flows into the cell, shifting the charge from negative toward positive (up to approximately ).
Repolarization: channels close while voltage-gated channels open. flows out of the cell, restoring the negative internal charge.
Hyperpolarization: A brief period where the membrane potential dips below before resting potential is re-established by the sodium-potassium pump.
All-or-None Response: An action potential either fires completely (if it hits ) or not at all. All action potentials in a single neuron are of equal strength.
Refractory Period: A duration during and after an action potential when a neuron cannot be stimulated again. This ensures impulses move in only one direction and limits firing frequency.
Impulse Conduction Methods
Continuous Conduction: Occurs in unmyelinated axons; the impulse travels the entire length of the membrane sequentially.
Saltatory Conduction: Occurs in myelinated axons; the impulse "jumps" from one Node of Ranvier to the next. This is significantly faster.
Conduction Speed: Proportional to axon diameter. Thick, myelinated motor axons can conduct at , while thin, unmyelinated sensory axons may conduct as slowly as .
The Synapse and Synaptic Transmission
Synapse Components:
Presynaptic Neuron: The cell sending the impulse.
Postsynaptic Neuron: The cell receiving the impulse.
Synaptic Cleft: The physical gap between the two cells.
Process:
An action potential reaches the synaptic knob.
The knob membrane becomes permeable to calcium ions (), which diffuse inward.
Presence of calcium causes synaptic vesicles to fuse with the knob membrane.
Vesicles release neurotransmitters into the cleft via exocytosis.
Neurotransmitters bind to receptors on the postsynaptic membrane.
Actions:
Excitatory Neurotransmitters: Increase sodium permeability, bringing the membrane closer to threshold.
Inhibitory Neurotransmitters: Increase potassium or chloride permeability, making the inside more negative (hyperpolarized) and an action potential less likely.
Neurotransmitter Removal: To prevent continuous stimulation, neurotransmitters are decomposed by enzymes (e.g., acetylcholinesterase) or taken back into the presynaptic neuron (reuptake).
Selected Neurotransmitters and Actions
Acetylcholine (ACh): CNS (skeletal muscle control) and PNS (stimulates skeletal muscle contraction; autonomic effects).
Monoamines:
Norepinephrine: Creates a sense of "feeling good" in CNS; low levels linked to depression.
Dopamine: Sense of "feeling good"; deficiency linked to Parkinson disease.
Serotonin: Primarily inhibitory; leads to sleepiness.
Histamine: Released in hypothalamus; promotes alertness.
Amino Acids:
GABA: Generally inhibitory in the CNS.
Glutamic acid: Generally excitatory in the CNS.
Neuropeptides:
Substance P: Excitatory; involved in pain perception.
Endorphins/Enkephalins: Inhibitory; reduce pain by blocking Substance P.
Gases:
Nitric Oxide: Involved in vasodilation (PNS) and memory (CNS).
Impulse Processing and Logic
Neuronal Pools: Groups of CNS neurons that work together to process specific inputs. A neuron may receive both excitatory and inhibitory signals and will only fire if the net sum reaches threshold.
Facilitation: Repeated stimulation of an excitatory presynaptic neuron increases neurotransmitter release, making the postsynaptic neuron more likely to reach threshold.
Convergence: Impulses from multiple axons lead to a single neuron, allowing for summation of data from different sources.
Divergence: One neuron sends impulses to multiple output fibers, serving to amplify the signal.
Nerve Structure and Classification
A nerve is a bundle of axons in the PNS.
Connective Tissue Coverings:
Epineurium: Outer layer enclosing the whole nerve.
Perineurium: Covers bundles of fibers called fascicles.
Endoneurium: Covers individual axons.
Classification:
Sensory (Afferent) Nerves: Carry impulses to the CNS.
Motor (Efferent) Nerves: Carry impulses to effectors.
Mixed Nerves: Contain both sensory and motor fibers (most common type).
Neural Pathways and Reflexes
Reflex Arc Components:
Receptor: Senses internal or external change.
Sensory Neuron: Carries information toward the CNS.
Interneuron: Acts as a reflex center in the CNS.
Motor Neuron: Carries the command out of the CNS.
Effector: Muscle or gland that performs the action.
Reflexes: Automatic, involuntary responses that maintain homeostasis (e.g., heart rate, swallowing).
Patellar Reflex (Knee-jerk): A simple 2-neuron reflex (sensory and motor). Striking the patellar ligament stretches the quadriceps, leading to contraction and knee extension. Helps maintain posture.
Withdrawal Reflex: A protective 3-neuron reflex. Touching something painful triggers sensory neurons to send messages to interneurons, which command motor neurons to contract flexor muscles and inhibit extensors while notifying the brain of pain.
The Meninges and CSF
Meninges: Three protective membranes surrounding the CNS.
Dura Mater: Outermost, tough, dense connective tissue; contains blood vessels and forms the internal periosteum of the skull. Separation from vertebrae creates the epidural space.
Arachnoid Mater: Middle, thin, weblike membrane; lacks blood vessels.
Pia Mater: Innermost, very thin membrane; contains many blood vessels/nerves and is attached directly to the brain and spinal cord surfaces.
Subarachnoid Space: Located between the arachnoid and pia mater; contains Cerebrospinal Fluid (CSF).
Cerebrospinal Fluid (CSF): Produced by Choroid Plexuses (capillaries from pia mater) in the brain ventricles. It circulates through the subarachnoid space and central canal,Providing cushioning and nutrition.
Spinal Cord Anatomy
Extends from the foramen magnum to the level of the and lumbar vertebrae.
Structure:
31 segments, each giving rise to a pair of spinal nerves.
Cervical and Lumbar Enlargements: Thickened regions providing nerves to limbs.
Cauda Equina: A "horse's tail" of spinal nerves at the inferior end.
White Matter: Surrounds the gray matter; consists of myelinated nerve tracts.
Gray Matter: Butterfly-shaped core containing interneurons/cell bodies. Features posterior, anterior, and lateral horns.
Central Canal: Located in the center of the gray matter; contains CSF.
Tracts:
Ascending Tracts: Carry sensory data up to the brain (e.g., Spinothalamic tract for pain/temp).
Descending Tracts: Carry motor instructions down (e.g., Corticospinal/pyramidal tracts for voluntary muscle; Extrapyramidal for balance/posture).
The Brain: Major Regions and Functions
Contains approximately 100 billion multipolar neurons.
Structure: Gray matter on the outside (cortex), white matter on the inside (opposite of the spinal cord).
Four Main Parts:
Cerebrum: Associated with higher mental functions, interpretation, and voluntary movement.
Diencephalon: Processes sensory input and maintains homeostasis.
Brainstem: Connects the brain to the spinal cord; regulates visceral activities.
Cerebellum: Coordinates complex muscular activities and posture.
The Cerebrum
Cerebral Hemispheres: Two mirror-image halves connected by the Corpus Callosum (bundle of fibers).
Surface Features:
Gyri: Ridges.
Sulci: Grooves.
Fissures: Deep grooves (Longitudinal and Transverse).
Lobes: Frontal, Parietal, Temporal, Occipital, and the deep Insula.
Cerebral Cortex: Thin outer layer of gray matter containing of all neuron cell bodies in the nervous system.
Functional Areas:
Sensory Areas: Cutaneous (parietal), Visual (occipital), Auditory (temporal), Taste (insula/base of sulcus), Smell (temporal).
Association Areas: Analyze input; involve reasoning, judgment, emotions, and memory. Wernicke’s area (left temporal) is for language comprehension.
Motor Areas: Primary motor cortex (frontal) contains pyramidal cells (upper motor neurons). Broca’s area (left frontal) controls speech muscle movements.
Crossover: Sensory and motor fibers cross over in the brainstem or spinal cord; the left hemisphere controls the right side of the body and vice versa.
Hemisphere Dominance: Most people are left-hemisphere dominant for language and intellectual tasks. The nondominant side (usually right) handles nonverbal functions like orientation and intuition.
Basal Nuclei (Basal Ganglia): Deep gray matter masses (caudate, putamen, globus pallidus) that produce dopamine and relay motor impulses to facilitate voluntary movement.
The Diencephalon
Located between hemispheres, above the midbrain, surrounding the third ventricle.
Thalamus: The "relay station" that sorts and directs all sensory input (except smell) to the cortex. Provides general awareness of sensations.
Hypothalamus: The primary regulator of homeostasis. Controls heart rate, BP, temperature, water balance, hunger, digestive secretions, and sleep. Links nervous and endocrine systems via the pituitary gland.
Limbic System: Includes parts of the diencephalon and cortex; controls emotional expression. It generates feelings of fear, pleasure, and sorrow to guide survival behavior.
The Brainstem
Midbrain: Located between diencephalon and pons. Houses centers for visual and auditory reflexes and main motor pathways.
Pons: Between midbrain and medulla. Regulates rate and depth of breathing and transmits impulses between the cerebrum and cerebellum.
Medulla Oblongata: Extends from pons to foramen magnum. All ascending/descending tracts pass through here. It contains vital reflex centers:
Cardiac Center: Adjusts heart rate.
Vasomotor Center: Controls blood vessel diameter for BP regulation.
Respiratory Center: Regulates the rhythm of breathing.
Reticular Formation (RAS): A network of fibers throughout the brainstem. Increased activity causes wakefulness; decreased activity leads to sleep. Damage results in a coma.
The Cerebellum
Located posterior to the brainstem under the occipital lobes. Two hemispheres are connected by the vermis.
Structure: Gray matter cortex over white matter (Arbor Vitae). Connects to CNS via three pairs of cerebellar peduncles.
Function: Integrates sensory data regarding body position and coordinates voluntary movements to ensure they are smooth and accurate.
Cranial and Spinal Nerves
Cranial Nerves: 12 pairs labeled by Roman numerals (I-XII).
I: Olfactory (Sensory - Smell).
II: Optic (Sensory - Vision).
III: Oculomotor (Motor - Eye movement, eyelids, lens focus).
IV: Trochlear (Motor - Eye movement).
V: Trigeminal (Mixed - Facial sensation and chewing).
VI: Abducens (Motor - Eye movement).
VII: Facial (Mixed - Taste, facial expression, tears, saliva).
VIII: Vestibulocochlear (Sensory - Equilibrium and hearing).
IX: Glossopharyngeal (Mixed - Pharynx, tonsils, tongue, swallowing).
X: Vagus (Mixed - Speech, swallowing, heart/viscera regulation).
XI: Accessory (Motor - Soft palate, pharynx, neck, back).
XII: Hypoglossal (Motor - Tongue movement).
Spinal Nerves: 31 pairs (8 Cervical, 12 Thoracic, 5 Lumbar, 5 Sacral, 1 Coccygeal).
Arise from two roots: Sensory posterior root (with a ganglion) and Motor anterior root.
Plexuses: Networks of spinal nerves.
Cervical (C1-C4): Neck and Phrenic nerve (diaphragm).
Brachial (C5-T1): Arms and hands (Musculocutaneous, Ulnar, Median, Radial, Axillary).
Lumbosacral (L1-S4): Abdomen, genitalia, and legs (Sciatic, Femoral, Obturator).
Note: Thoracic nerves (T1-T11) do not form plexuses; they become intercostal nerves.
The Autonomic Nervous System (ANS)
Independent system controlling involuntary visceral functions (smooth/cardiac muscle and glands).
Sympathetic Division: Operates under stress/emergency ("Fight or Flight"). Preganglionic fibers arise from T1 to L2 and synapse in paravertebral ganglia.
Parasympathetic Division: Operates under normal, restful conditions ("Rest and Digest"). Fibers arise from the brainstem and sacral region, synapsing in terminal ganglia near effectors.
ANS Pathways: Consist of a preganglionic neuron (CNS to ganglion) and a postganglionic neuron (ganglion to effector).
Neurotransmitters in ANS:
Cholinergic Fibers: All preganglionic fibers and parasympathetic postganglionic fibers release Acetylcholine (ACh).
Adrenergic Fibers: Most sympathetic postganglionic fibers release Norepinephrine (and cause effects like increased HR and pupil dilation).
Questions & Discussion
Why do CNS axons not regenerate?
Unlike the PNS, where Schwann cells and their neurilemma guide regrowing axons, the CNS is myelinated by oligodendrocytes which lack a neurilemma. The absence of this guiding sheath prevents regeneration in the brain and spinal cord.
What defines an all-or-none response in a neuron?
This principle states that if a stimulus is strong enough to reach the threshold potential (), an action potential is triggered and will travel at full strength. Increasing the stimulus strength beyond threshold does not increase the action potential's intensity; it either happens at or .
The nervous system is a primary controller of multifaceted body functions, playing a crucial role in maintaining homeostasis alongside the endocrine system through the regulation of hormones, which are essential for various physiological processes. It governs not only thought processes and movement but also the internal physiological mechanisms that are vital for sustaining life and responding to environmental changes.
Major Aspects of Nervous System Function:
Sensory Input: This involves the detection and transmission of internal bodily changes (like temperature, pain, and pressure) as well as external environmental changes (such as light, sound, and touch) through specialized sensory receptors that convert stimuli into neural signals.
Integration and Processing: The processing stage occurs primarily in the central nervous system (CNS), where the brain analyzes sensory input, synthesizes this data with past experiences, and coordinates appropriate responses based on this information.
Motor Output: This is the response phase, where the CNS sends impulses to effectors (muscles or glands) to execute a specific motor command or physiological response following the integration process.
Main Cell Types:
Neurons: Highly specialized cells responsible for transmitting information throughout the nervous system via electrical impulses. Neurons are defined by their unique structure, which includes a cell body (soma), dendrites for receiving signals, and an axon for sending impulses.
Neuroglia (Glial Cells): Often referred to as the "nerve glue," neuroglia perform supportive functions such as protecting, nourishing, and insulating neurons. Unlike neurons, they do not generate or conduct nerve impulses themselves but are essential for maintaining neuronal health and efficiency.
Neurotransmitters: These are chemical messengers located in a synapse, which transmit signals from one neuron to another or from neurons to effector cells. They play a critical role in modulating various functions such as mood, cognition, and motor control, and different neurotransmitters are associated with different effects on the body.