Spinal Cord Anatomy and Meninges Review

Nervous System Overview

• The nervous system controls and adjusts bodily activities, producing rapid but short-lived responses.
• The system includes the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

Central Nervous System (CNS)

• The CNS (brain and spinal cord) integrates, processes, and coordinates sensory input with motor output.
• It's the center for intelligence, memory, learning, and emotions.
• The CNS processes and coordinates sensory input and motor output.

Peripheral Nervous System (PNS)

• The PNS consists of all peripheral nerves and nervous tissue outside the CNS.
• It delivers sensory information to and carries motor commands from the CNS.
• The afferent division brings sensory information to the CNS.
• The efferent division carries motor commands to muscles and glands.

Overview

• Information processing within the CNS leads to motor commands passing through the efferent division of PNS.
• The Somatic and Autonomic Systems are components of the nervous system.
• The autonomic system (efferent) is further subdivided, into voluntary and involuntary functions.
• Sensory receptors receive information relayed to the CNS.

Cellular Organization in Neural Tissue

• Neurons transmit electrical signals throughout the body.
• Neurons consist of a soma, axon, and dendrites.
• Neuroglia support and protect the neurons.

Neuron Structure

• Dendrites receive stimuli from other cells or environmental changes.
• The cell body contains the nucleus, ribosomes, mitochondria, etc.
• The axon transmits electrical signals.
• The axon terminal transmits signals.
• An action potential travels down the axon.

Nissl Bodies & Chromatophilic Substance

• Nissl bodies and Chromatophilic substance are clusters of RER and free ribosomes.

Structural Classification of Neurons

• Anaxonic neurons have multiple processes, but axons are indistinguishable.
• Bipolar neurons have two processes separated by the cell body.
• Pseudounipolar neurons have a single process that elongates.
• Multipolar neurons have one axon and multiple dendrites.

Synapses

• Synapses are intercellular communication sites.
• There are synapses between neurons, between neurons and muscles (neuromuscular), and between neurons and glands (neuroglandular)
• Synapses typically, involve a presynaptic cell, neurotransmitters, and a postsynaptic cell.

Structure of a Synapse

• Terminal arborization and terminal boutons are part of an axon's structure.
• Neurotransmitters are released from the axon terminal boutons to affect postsynaptic neurons.
• The synapse is an important structural feature that affects postsynaptic neurons.
• A synapse’s structure dictates its functionality.
• Vesicular Synapse is one type of synapse

Multiple Synapses

• Neurons can form multiple synapses.

Neuronal Circuits

• Divergence spreads signal to multiple neurons/pools in the CNS.
• Convergence brings input to a single neuron/pool from multiple sources.
• Serial processing works in a sequential manner.
• Parallel processing works in a simultaneous manner.
• Reverberation enables feedback mechanisms that can be either excitatory or inhibitory.

Neuroglia Functions

• They form the framework for neural tissue in the CNS.
• They support the cellular environment.
• They perform phagocytosis (immune cells).
• They can reproduce (mitosis division).

Neuroglia Cell Types in the CNS

• Astrocytes
• Oligodendrocytes
• Microglia
• Ependymal cells

Neuroglia Cell Types in the PNS

• Satellite cells
• Schwann cells (neurolemmocytes)

Classification of Neuroglia

• Satellite cells and Schwann cells are PNS support glial cells.
• Oligodendrocytes and Astrocytes are CNS support glial cells.
• Different kinds of glial cells support neurons in different roles.

Astrocytes

• They have many cytoplasmic processes.
• They control the chemical content of the interstitial environment.
• They help maintain the blood-brain barrier.
• They isolate neurons from general circulation.
• They provide structural framework.
• They help repair damaged neurons.
• They guide neurogenesis.

Blood-Brain Barrier

• Tight junctions and astrocytes facilitate the blood-brain barrier function.
• Tight junctions prevent the movement of solutes between endothelial cells.
• Astrocyte feet secrete paracrines promoting tight junctions.

Oligodendrocytes

• Similar to astrocytes in function.
• They have smaller cell bodies and fewer cytoplasmic processes.
• They produce Myelin to wrap axons effectively.

White Matter and Grey Matter

• White matter contains myelinated axons; grey matter contains neuron bodies, dendrites, and unmyelinated axons.

Myelination of the CNS (White matter)

• Myelinated axons are in white matter, unmyelinated are in grey matter
• Oligodendrocytes myelinate the axons of CNS (brain and spinal cord)
• White matter and grey matter each have distinct neuron compositions.

Neuroglia of the PNS

• Satellite cells regulate matter exchange, while Schwann cells form myelin sheaths.
• Schwann cells, also called neurolemmocytes, form the myelin sheath surrounding peripheral axons.

Peripheral Axons & Schwann Cells

• Every peripheral axon, myelinated or unmyelinated, is covered by Schwann cells (neurolemmocytes).

Anatomy of a Peripheral Nerve

• Epineurium surrounds the entire nerve.
• Perineurium surrounds each fascicle (bundle of axons).
• Endoneurium surrounds individual axons.

Spinal Cord

• The spinal cord conducts nerve impulses to and from the brain.
• The spinal cord processes sensory information for reflex/motor actions.
• It's about 45 cm long, passing from the brain to vertebra L1-L2.
• It has cervical, thoracic, lumbar, sacral, and coccygeal regions.

Gross Anatomy of the Spinal Cord

• 31 spinal segments. Each segment has: dorsal root, dorsal root ganglia, ventral root, and spinal nerves.
• The conus medullaris is the inferior tip of the spinal cord.
• The cauda equina resembles a horse's tail.

Spinal Meninges

• These specialized membranes protect, stabilize, and absorb shock.
• Continuous with cranial meninges.
• Denticulate ligaments anchor the spinal cord.
• Dura mater is the outermost, tough layer.
• Arachnoid mater lies between dura and pia mater; cerebrospinal fluid flows in the subarachnoid space.
• Pia mater is the deepest layer, tightly bound to the spinal cord.

Sectional Anatomy of the Spinal Cord

• Gray matter consists of the central canal (CSF filled), with neuron cell bodies and glial cells.
• White matter contains axons organized into tracts (columns) located outside the gray matter.
• Gray matter is organized into posterior, lateral, and anterior horns, and a gray commissure.
• White matter contains posterior, lateral, and anterior columns, conveying sensory and motor tracts.

Features of the Spinal Cord

• Structures observed in a transverse (cross-section) view of the spinal cord include white matter, gray matter, central canal, anterior median fissure, and posterior median sulcus

Organization of the Spinal Cord's Gray Matter

• Neuronal cell bodies are organized into groups (nuclei) within the spinal cord's gray matter; sensory and motor nuclei are important distinctions—sensory nuclei receive information, motor nuclei send information.
• Gray matter is organized into posterior, lateral, and anterior horns, in a transverse sectional view.
• A gray commissure connects the anterior horns.

Organization of the Spinal Cord's White Matter

• Columns of nerves (funiculi) form the white matter.
• Posterior, lateral, and anterior white columns exist.
• Columns contain sensory and motor tracts (ascending and descending, respectively).

Spinal Cord Sensory Information

• Sensory information from the back, body wall, and limbs enters through the dorsal root.
• Information enters sensory receptors and travels through the spinal cord.
• Interoceptors for visceral organs and exteroceptors for body wall and limbs relay sensory input.
• Rami communicantes are structures associated with sensory and visceral pathways

Spinal Nerves

• 31 pairs of nerves arise from the spinal cord. There are 8 cervical, 12 thoracic, 5 lumbar, and 5 sacral, and 1 coccygeal nerves.

Reflexes

• A reflex is an involuntary motor response.
• The reflex arc is the neural pathway involved in a reflex.
• A reflex arc starts at a sensory receptor and concludes at a peripheral receptor.
• Reflex arcs include receptor, sensory neuron, interneuron, motor neuron, and effector regions.

Pathway of a Reflex Arc

• The sequence of events begins with sensory receptor activation, followed by sensory information relay to the CNS.
• The CNS processes this information, triggering motor neuron response.
• The response is then signaled to effectors.

Spinal Reflexes (Stretch reflex)

• A reflex action initiated when a muscle is stretched.
• The stretch reflex is used for physical adjustments to maintain posture or balance.
• Sensory neurons, spinal cord processing, and motor neuron activation cause the muscle to contract to return to its original length.

Steps of a Stretch Reflex

• Stimulus stretches the muscle.
• Muscle spindle activation of sensory neurons
• Sensory neuron information processing in CNS
• Motor neuron activation occurs
• Muscle contraction occurs as a response.

Patellar Reflex

• The patellar reflex is a stretch reflex involving quadriceps muscle spindles.
• Striking the patellar tendon stretches the quadriceps muscle spindles, activating sensory neurons.
• Spinal cord processing activates motor neurons in the quadriceps.
• Resulting quadriceps contraction produces a reflexive kick.