Chapter 13: The Spinal Cord, Spinal Nerves, and Spinal Reflexes
Structural and Functional Organization of the Nervous System
Structural Organization of the Nervous System:
Central Nervous System (CNS): Composed of the brain and spinal cord. Serves as the primary processing centers for sensory data and motor commands.
Peripheral Nervous System (PNS): Composed of cranial nerves and spinal nerves. Serves as sensory input and motor response pathways connecting the CNS to peripheral tissues and organs.
Nature and Function of Reflexes:
Reflexes are quick, rapid, automatic, involuntary nerve responses triggered by specific internal or external stimuli.
Spinal Reflexes: Involuntary responses controlled exclusively by the spinal cord alone, functioning independently without sensory input or processing from the brain.
Practical Example: Dropping a hot pan—a spinal reflex triggers immediate motor commands to release the pan before the sensory information ascends to the brain and conscious pain is perceived.
Gross Anatomy and Structure of the Spinal Cord
Dimensions and Extent of the Adult Spinal Cord:
Length: Approximately ().
Width: Approximately ().
Extent: Extends from the base of the brainstem (foramen magnum) down to the level of vertebrae and .
Growth Timeline: The spinal cord stops lengthening around age , whereas the surrounding vertebral column continues to grow, causing the cord to end superior to the distal end of the vertebral canal.
Symmetry: Exhibits bilateral symmetry along its entire length.
Regions: Divided into four anatomical regions: cervical, thoracic, lumbar, and sacral.
Segments: Composed of individual spinal cord segments, each giving rise to a pair of lateral spinal nerves.
Superficial Landmarks:
Posterior Median Sulcus: A shallow longitudinal groove located on the posterior (dorsal) surface of the spinal cord.
Anterior Median Fissure: A deeper longitudinal groove located along the anterior (ventral) surface of the spinal cord.
Central Canal: A narrow internal passageway filled with cerebrospinal fluid (CSF) running down the longitudinal axis of the cord.
Spinal Cord Enlargements:
Regions of the cord supplying nerve innervation to the limbs feature expanded gray matter and are visibly wider:
Cervical Enlargement: Supplies nerves to the shoulder girdle and upper limbs.
Lumbosacral Enlargement: Supplies nerves to the pelvic girdle and lower limbs.
Distal End Anatomy:
Conus Medullaris: The tapered, conical terminal region of the spinal cord located inferior to the lumbosacral enlargement.
Cauda Equina: The collection of posterior and anterior nerve roots extending inferiorly beyond the conus medullaris through the lower vertebral canal, named for its structural resemblance to a horse's tail.
Filum Terminale: A thin strand of fibrous connective tissue extending from the tip of the conus medullaris down to the coccyx, where it integrates into the coccygeal ligament to anchor the spinal cord vertically.
Spinal Nerve Roots, Ganglia, and Naming Conventions
Spinal Nerve Roots:
Anterior Root (Ventral Root): Contains the axons of motor neurons that convey efferent motor commands away from the CNS to peripheral effectors.
Posterior Root (Dorsal Root): Contains the axons of sensory neurons that convey afferent sensory information from peripheral receptors into the CNS.
Rootlets: Smaller bundles of axon fibers formed as posterior and anterior roots fan out before entering or leaving the surface of the spinal cord.
Spinal Ganglia (Dorsal Root Ganglia):
Nodular enlargements containing the cell bodies of sensory neurons that form the posterior root.
Positioned anatomically within the intervertebral foramina between the pedicles of adjacent vertebrae.
Formation and Distribution of Spinal Nerves:
Emergence: A pair of spinal nerves emerges laterally at each spinal segment formed by the distal union of the anterior root and posterior root.
Total Number: pairs of spinal nerves in total.
Mixed Nerves: All spinal nerves are classified as mixed nerves because they contain both afferent (sensory) and efferent (motor) fibers.
Functional Rami Branches:
White Ramus Communicans and Gray Ramus Communicans: Autonomic nerve tracts carrying fibers that innervate smooth muscle, internal organs, and visceral glands.
Posterior Ramus: Branch that innervates the deep muscles and skin of the back.
Anterior Ramus: Larger branch that innervates the anterior and lateral body wall, trunk, and upper and lower limbs.
System for Naming Spinal Nerves:
Designated by their corresponding vertebral region and numerical level.
Cervical Region Rule: Spinal nerve exits superior to the first cervical vertebra (). Spinal nerve exits inferior to the seventh cervical vertebra ().
All Other Regions Rule: All non-cervical spinal nerves are named for the vertebra located immediately superior to their point of exit (e.g., spinal nerve exits inferior to vertebra ).
Meninges and Protection of the Central Nervous System
General Characteristics of Spinal Meninges:
Three specialized, layered membranes wrapping around the spinal cord to supply physical protection, shock absorption, and structural support for blood vessels.
Continuous superiorly with the cranial meninges.
Meningitis: Inflammation of the meningeal membranes caused by bacterial or viral infection.
Dura Mater (Outermost Layer):
Tough, fibrous layer composed of dense collagen fibers.
Continuous superiorly with the cranial dura mater, fusing with the periosteum of the occipital bone around the foramen magnum.
Tapers distally into a dense cord of collagen fibers that joins the filum terminale within the coccygeal ligament.
Epidural Space: A protective space positioned between the dura mater and the bony inner wall of the vertebral canal, containing loose connective tissue and protective adipose tissue.
Subdural Space: A narrow potential space deep to the dura mater separating it from the arachnoid mater.
Arachnoid Mater (Middle Layer):
Arachnoid Membrane: A web-like outer membrane lined with simple squamous epithelia.
Arachnoid Trabeculae: An inner delicate network of collagen and elastic fibers spanning the subarachnoid space to anchor into the pia mater.
Subarachnoid Space: The fluid-filled space containing arachnoid trabeculae located between the arachnoid mater and pia mater. Filled with cerebrospinal fluid (CSF), which distributes dissolved gases, nutrients, and waste products.
Lumbar Puncture (Spinal Tap): Clinical procedure used to withdraw cerebrospinal fluid (CSF) from the subarachnoid space in the lower lumbar region.
Pia Mater (Innermost Layer):
A meshwork of elastic and collagen fibers firmly bound to the underlying neural tissue of the spinal cord.
Superficial blood vessels supplying the spinal cord course along the outer surface of the pia mater within the subarachnoid space.
Denticulate Ligaments: Paired lateral extensions of the pia mater extending across the subarachnoid space to anchor into the dura mater, preventing lateral movement of the spinal cord.
Internal Sectional Anatomy: Gray Matter and White Matter
Gray Matter (Structural and Functional Organization):
Composed primarily of neuron cell bodies, neuroglia, and unmyelinated axons surrounding the central canal.
Form masses called nuclei, organized structurally into horns:
Posterior Gray Horns: Contain somatic sensory and visceral sensory nuclei that process incoming signals from peripheral receptors.
Anterior Gray Horns: Contain somatic motor nuclei that issue outgoing motor commands to skeletal muscles.
Lateral Gray Horns: Located exclusively in the thoracic and upper lumbar segments (--); contain visceral motor nuclei governing autonomic effectors.
Gray Commissures( anterior and posterior): Narrow bands of gray matter superior and inferior to the central canal containing decussating axons crossing between the left and right sides of the spinal cord.
White Matter (Structural and Functional Organization):
Composed of organized bundles of myelinated and unmyelinated axons.
Divided structurally into three pairs of regions termed columns (funiculi):
Posterior White Columns: Situated between the posterior gray horns and the posterior median sulcus.
Anterior White Columns: Situated between the anterior gray horns and the anterior median fissure. Axons cross from the sides of spine
Anterior White Commissure: Transverse white matter bridge where nerve fibers cross from one side of the spinal cord to the other.
Lateral White Columns: Located on each side of the spinal cord between the anterior and posterior white columns.
Tracts (Fasciculi):
Bundles of CNS axons that transmit uniform types of sensory or motor information in a single direction.
Ascending Tracts: Transport sensory information superiorly toward processing centers in the brain.
Descending Tracts: Transport motor commands inferiorly down the spinal cord to effector motor neurons.
Anterior horn is handling sensory signals go out through posterior
Peripheral Nerves, Connective Tissue Layers, and Dermatomes
Connective Tissue Wrappings of Nerves:
Epineurium: Outermost dense network of collagen fibers enclosing the entire peripheral nerve bundle.
Perineurium: Middle connective tissue layer that partitions axons into distinct internal bundles called fascicles. Axon bundles
Endoneurium: Delicate innermost layer of connective tissue surrounding each individual axon.
Peripheral Nerves and Dermatomes:
Peripheral nerves form from the branching, merging, and resortment of spinal nerve roots, sharing continuous connective tissue sheaths.
Dermatome: A specific bilateral region of skin surface innervated by sensory fibers originating from a single pair of spinal nerves.
Clinical Associations: numbness of dermatome determines spinal nerve disorder
Peripheral Neuropathies: Functional sensory or motor losses affecting specific dermatomes resulting from physical nerve trauma, structural compression, or illness.
Shingles: Viral reactivation of the varicella-zoster virus within sensory ganglia, manifesting as painful skin rashes and vesicular eruptions strictly localized along single dermatomes. One side, dormant chickenpox causes shingles
Enteroreceptors: not as precise as exteroreceptors
Major Nerve Plexuses
Definition and Significance of Nerve Plexuses:
All mixed nerves
Formed by anterior and posterior roots
Complex, interwoven networks of nerve fibers formed by the interconnecting anterior rami of adjacent spinal nerves.
Enables axons from multiple spinal segments to blend together to form individual peripheral nerves, allowing redundant innervation to limbs.
Cervical plexus: neck and diaphragm c1-c5
Brachial plexus; arm shoulder
Lumbar; lower back
Sacral plexus l4-s4
Cervical Plexus (--):
Composed of anterior rami of spinal nerves through .
Innervates the neck, muscles of the throat, skin of the scalp behind the ear, and the primary respiratory muscle (diaphragm).
Major Cervical Nerves:
Phrenic Nerve (--): Formed by inputs from , , and ; supplies motor and sensory innervation to the diaphragm. Injury can effect ones breathing ability
Lesser Occipital Nerve (): Innervates skin of the neck and scalp superior and posterior to the ear.
Great Auricular Nerve (--): Innervates skin over the posterior ear and neck region.
Transverse Cervical Nerve (--): Innervates skin of the anterior neck triangle.
Ansa Cervicalis (--): Innervates extrinsic laryngeal muscles (geniohyoid, thyrohyoid, omohyoid, sternothyroid, sternohyoid) via cranial nerve XII (hypoglossal nerve).
Supraclavicular Nerves (--): Innervate skin of the shoulder and lower neck.
Brachial Plexus (--):
Composed of anterior rami of spinal nerves through .
Innervates the pectoral girdle, upper back, and upper limbs.
Structural Organization:
Anterior rami merge to form three large Trunks: Superior, Middle, and Inferior trunks.
Trunks split and resort their fibers into three Cords: Lateral, Posterior, and Medial cords (named relative to their anatomical relationship to the axillary artery).
Major Brachial Nerves:
Musculocutaneous Nerve: Arises from the lateral cord; innervates flexor muscles of the anterior arm.
Median Nerve: Arises from both lateral and medial cords; innervates anterior forearm flexors and digital flexors.
Ulnar Nerve: Arises from the medial cord; innervates anteromedial forearm flexors and intrinsic hand muscles.
Axillary Nerve: Arises from the posterior cord; innervates the deltoid and teres minor muscles.
Radial Nerve: Arises from the posterior cord; innervates posterior extensor muscles of the arm and forearm.
Know what the innervates
Carpal Tunnel Syndrome:
The median nerve passes through the carpal tunnel, a narrow passageway on the anterior wrist bounded by carpal bones and flexor retinaculum, alongside muscle tendons.
Repetitive stress causes inflammation and swelling of adjacent flexor tendons, compressing the median nerve and resulting in pain, tingling, or sensory loss across areas of the hand innervated by the median nerve.
Lumbar Plexus (--):
Composed of anterior rami of spinal nerves through .
Innervates the lower abdominal wall, pelvic girdle, and lower limbs.
Major Lumbar Nerves:
Iliohypogastric Nerve
Ilio-inguinal Nerve
Genitofemoral Nerve: compression can effect genital sensation
Femoral Nerve
Obturator Nerve
Lateral Femoral Cutaneous Nerve
Sacral Plexus (--):
Composed of anterior rami of spinal nerves through .
Innervates the pelvic girdle, gluteal region, perineum, and lower limbs.
Major Sacral Nerves:
Superior Gluteal Nerve
Inferior Gluteal Nerve
Pudendal Nerve sitting on bicycle can injury this nerve
Sciatic Nerve: The longest and largest nerve in the body 1in.Passes deep to the gluteus maximus and divides into two main branches:
Common Fibular Nerve (Fibular Nerve)
Tibial Nerve
Sensory Innervation of the Ankle and Foot:
Distinct peripheral nerve branches supply sensory coverage to specific regions of the ankle and foot: Saphenous nerve, Sural nerve, Fibular nerve, and Tibial nerve.
Mapping localized touch and pain perception, combined with tests of muscle motor function, allows clinicians to pinpoint damage to specific peripheral nerves.
Functional Organization of Neurons and Neuronal Pools
Population Breakdown of CNS and PNS Neurons:
Sensory Neurons: Approximately neurons; convey afferent sensory input into the CNS.
Motor Neurons: Approximately neurons; send efferent motor commands out to peripheral effectors.
Interneurons: Approximately neurons; situated entirely within the CNS to process incoming information, plan motor responses, and coordinate complex neural activity.
Concept of Neuronal Pools:
Functional groupings of interconnected interneurons in the CNS with defined input pathways and designated output destinations.
Capable of exerting excitatory or inhibitory control over distinct regions of the brain or spinal cord.
Five Patterns of Neural Circuits in Neuronal Pools:
Divergence: Spread of nerve impulses from a single presynaptic neuron or pool to multiple postsynaptic neurons or pools. Common in sensory pathways distributing information to multiple processing areas. Distributing signal
Convergence: Input from several presynaptic neurons synapses onto a single postsynaptic neuron. Example: Simultaneous voluntary and involuntary control of the diaphragm during respiration—conscious motor paths from the cerebral cortex and subcortical respiratory centers converge onto the same phrenic motor neurons. Subconscious control of diaphragm
Serial Processing: Step-by-step transmission of sensory signals sequentially along a single neural path from one neuron or pool to the next. Example: Pain signals passing sequentially through chained interneuron pools to reach conscious brain centers.
Parallel Processing: Simultaneous processing of identical sensory information across multiple distinct neuronal pools at the same time. Example: Stepping on a sharp object causes simultaneous activation of pools that flex the leg, shift balance to the opposite leg, and trigger a vocal response.
Reverberation: Axon collaterals extend backward to re-stimulate presynaptic neurons in the circuit, establishing a self-sustaining positive feedback loop. Continues until synaptic fatigue occurs or inhibitory signals break the loop. Functions to maintain continuous actions such as breathing, muscle tone, and states of consciousness. Positive feedback loop.
Neural Reflexes and Classification Schemes
Properties of Neural Reflexes:
Rapid, involuntary, automatic motor responses to specific sensory stimuli.
Act as fundamental functional units of neural behavior, producing identical motor outputs each time the stimulus occurs.
Five Core Components of a Reflex Arc:
Sensory Receptor: Detects internal or external changes.
Sensory Neuron: Conducts action potentials into the spinal cord or brainstem.
Information Processing Center: CNS interneurons process and integrate sensory input.
Motor Neuron: Transmits action potentials out of the CNS to the periphery.
Effector: Muscle fiber or gland cell that executes the physical response.
Five Sequential Steps in a Spinal Reflex Arc:
Step 1: A physical or chemical stimulus activates a sensory receptor.
Step 2: Action potential is generated in the sensory neuron when threshold is reached; axon enters the spinal cord via the posterior root.
Step 3: Information processing takes place within the spinal cord gray matter, usually involving one or more interneurons.
Step 4: Motor neuron activation occurs, firing action potentials whose axons exit the spinal cord via the anterior root.
Step 5: Effector (skeletal muscle, smooth muscle, cardiac muscle, or gland) responds to neurotransmitter release, performing the reflexive motor action.
Receptors for nerves
Four Major Schemes for Classifying Reflexes:
By Development:
Innate Reflexes: Inborn, genetically programmed neural reflex pathways established prior to birth, part of genetics (e.g., withdrawal reflex, chewing, visual tracking).
Acquired Reflexes: Rapid, automatic motor patterns learned and refined through repetition and experience (e.g., applying pressure to a vehicle brake pedal during an emergency). NASCAR drivers unlearns reflexes
By Motor Response:
Somatic Reflexes: Direct involuntary contractions of skeletal muscles (includes superficial reflexes and stretch/deep tendon reflexes).
Visceral Reflexes (Autonomic Reflexes): Control internal visceral effectors such as smooth muscle, cardiac muscle, or glandular secretions.( adrenaline release)
By Circuit Complexity:
Monosynaptic Reflex: Simplest reflex circuit involving a direct synapse between one sensory neuron and one motor neuron (1 synapse), yielding minimal latency and rapid execution.(protective in nature)
Polysynaptic Reflex: Reflex circuit incorporating at least one interneuron between the sensory and motor neurons (2 to several hundred synapses); processing latency increases proportionally with the number of synapses involved. Intersegmental reflex arcs involve interactions across multiple spinal cord segments.(most common) the more synapses the slower it will be
By Information Processing Site:
Spinal Reflexes: Integrated and processed exclusively within the gray matter of the spinal cord.🩻
Cranial Reflexes: Integrated and processed within structural centers of the brain.🧠
Reflex Pathways and Physiological Mechanisms
Monosynaptic Stretch Reflexes:
Regulates skeletal muscle length throughout the body to prevent overstretching (e.g., the patellar or "knee-jerk" reflex). Large diameter and myelin; faster signal
Features high conduction velocity due to innervation by large-diameter myelinated sensory fibers.
Sequential Steps of a Stretch Reflex:
Mechanical stretching of a skeletal muscle occurs.
Distortion of embedded muscle spindle receptors triggers action potentials in sensory neurons.
Sensory axon enters the spinal cord and synapses directly onto anterior horn motor neurons.
Somatic motor neurons send excitatory commands to extrafusal muscle fibers, causing reflexive contraction of the stretched muscle to restore baseline length.
Muscle Spindle Structure and Function:
Sensory receptors responsible for monitoring muscle length in stretch reflexes.
Consists of small bundles of specialized intrafusal muscle fibers surrounded by standard extrafusal muscle fibers.
Extrafusal fibers are responsible for main muscle tone and gross contraction.
VIntrafusal fibers are innervated by sensory and motor neurons (whose dendrites wrap around the central region) and gamma motor neurons (whose axons are termed gamma efferents).
Gamma efferents adjust intrafusal fiber tension, preserving receptor sensitivity during muscle contraction. Gamma effernt axons
Postural Reflexes:
Complex reflex systems incorporating both monosynaptic stretch reflexes and polysynaptic intersegmental reflexes.
Functions to maintain upright body position against gravity. Extremely sensitive
Operates via continuous, fine-tuned micro-adjustments of opposing trunk and limb muscle groups.
Polysynaptic Reflex Mechanisms: more complicated control multiple muscle groups
Tendon Reflex:
Prevents excessive tension development in contracting skeletal muscles to protect muscles and tendons from tearing.
Sensory receptors are Golgi tendon organs stimulated when collagen bundles are overstretched.
Stimulates inhibitory interneurons in the spinal cord, reducing motor neuron firing; increased muscle tension yields proportional muscle relaxation.
Withdrawal Reflexes:
Automatic motor patterns that pull body parts away from painful or dangerous stimuli (e.g., flexor reflex pulling a hand off a hot pan).
The strength, speed, and spatial distribution of the response scale directly with stimulus location and intensity.
Reciprocal Inhibition:
Necessary circuit mechanics in withdrawal reflexes: when flexor muscles are stimulated to contract, interneurons in the spinal cord simultaneously send inhibitory post-synaptic potentials (IPSPs) to suppress the motor neurons of antagonistic extensor muscles (and vice versa).
Ipsilateral vs. Contralateral Reflex Arcs:
Ipsilateral Reflex Arc: Sensory input and motor output occur on the exact same side of the body (e.g., stretch, tendon, and flexor reflexes).
Contralateral Reflex Arc: Sensory input triggers a motor output on the opposite side of the body.
Crossed Extensor Reflex:
Operates in tandem with withdrawal reflexes in weight-bearing limbs (e.g., stepping on a sharp tack).
Flexor reflex lifts the injured foot while the contralateral crossed extensor reflex extends the opposite leg to balance full body weight.
Maintained over time by reverberating neural circuits.
Five Summary Characteristics of Polysynaptic Reflexes:
Involve pools of interneurons that process excitatory or inhibitory signals.
Involve multiple spinal cord segments to coordinate multi-joint motor responses. Activates muscles in many areas.
Utilize reciprocal inhibition to coordinate agonist and antagonist muscle pairs. One contracts and one relaxes
Feature reverberating circuits to prolong the motor response past the initial stimulus.
Cooperate in complex networks to generate smooth, coordinated physical behaviors.
Central Processing: Brain Control and Modification of Reflexes
Integration and Control of Spinal Reflexes:
Although spinal reflexes function automatically, higher centers in the brain exert modulation, facilitating or inhibiting spinal reflex responses via descending motor pathways.
Voluntary movements utilize pre-programmed spinal reflex motor patterns (e.g., running, walking, jumping).
Reinforcement of Spinal Reflexes:
Higher centers enhance reflex sensitivity by firing descending signals to excite interneurons or motor neurons in the spinal cord.
Continuous excitatory input produces general facilitation, lowering the firing threshold of motor neurons and reinforcing reflex execution.
Inhibition of Spinal Reflexes:
Higher centers suppress reflex responses by descending stimulation of inhibitory interneurons or producing IPSPs directly on reflex motor neurons. Suppress post synaptic neurons
Clinical Reflex Testing - Plantar Reflex vs. Babinski Reflex:
Plantar Reflex:
Normal adult reflex response elicited by stroking the lateral outer margin of the sole of the foot.
Produces curling of the toes (plantar flexion).
Babinski Reflex:
Abnormal adult response produced when stroking the sole of the foot results in dorsiflexion of the hallux and fanning of the toes.
Normal in infants prior to age because descending corticospinal motor tracts are not yet fully myelinated.
In adults, the presence of the Babinski reflex indicates structural damage to descending CNS motor pathways due to loss of normal descending inhibition.