Neural Tissue, Neurophysiology, and Spinal Cord: Study Guide for Exam #3

Neural Tissue

Organization of the Nervous System

The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS), each with distinct components and functions.

• CNS (Central Nervous System): Consists of the brain and spinal cord; responsible for integrating, processing, and coordinating sensory data and motor commands.

• PNS (Peripheral Nervous System): Includes all neural tissue outside the CNS; connects the CNS to limbs and organs.

• Afferent (Sensory) Division: Transmits sensory information from receptors to the CNS.

• Role of Receptors: Detect changes in the environment (stimuli) and initiate sensory pathways.

• Efferent (Motor) Division: Carries motor commands from the CNS to effectors (muscles and glands).

• Somatic Nervous System (SNS): Controls voluntary movements by innervating skeletal muscles.

• Autonomic Nervous System (ANS): Regulates involuntary functions (e.g., heart rate, digestion) by innervating smooth muscle, cardiac muscle, and glands.

• Role of Effectors: Effectors are target organs or tissues that respond to neural commands.

Composition of Neural Tissue

Neural tissue consists of neurons and neuroglia (glial cells), each with specialized roles.

• Neurons: The primary signaling cells; transmit electrical impulses.

• Neuroglia (Glial Cells): Support, protect, and nourish neurons.

Neuroglia in the CNS

• Astrocytes: Maintain the blood-brain barrier, provide structural support, regulate ion and nutrient concentrations.

• Oligodendrocytes: Form myelin sheaths around CNS axons, increasing conduction speed.

• Microglia: Act as phagocytes, removing debris and pathogens.

• Ependymal Cells: Line ventricles and central canal; produce and circulate cerebrospinal fluid (CSF).

Neuroglia in the PNS

• Schwann Cells: Form myelin sheaths around PNS axons; assist in axon regeneration.

• Satellite Cells: Surround neuron cell bodies in ganglia; regulate the environment around neurons.

Myelin in CNS vs. PNS

• CNS Myelin: Produced by oligodendrocytes; one cell myelinates multiple axons.

• PNS Myelin: Produced by Schwann cells; one cell myelinates a single axon segment.

• Axon Regeneration: Possible in PNS due to Schwann cell guidance; limited in CNS due to inhibitory environment and lack of supportive structures.

Structure and Function of Neurons

Neurons are specialized for communication and consist of several key structures.

• Soma (Cell Body): Contains the nucleus and organelles; metabolic center.

• Nissl Substance: Rough endoplasmic reticulum; involved in protein synthesis.

• Dendrites: Receive incoming signals from other neurons.

• Axon: Conducts action potentials away from the soma.

• Axon Collateral: Branches of the axon; allow communication with multiple targets.

• Telodendria: Terminal branches of the axon.

• Axon Terminal: Releases neurotransmitters to communicate with other cells.

• Structures Sending Action Potentials: Axons and axon collaterals.

• Structures Receiving Action Potentials: Dendrites and soma.

Structural Categories of Neurons

• Bipolar: One dendrite, one axon; found in sensory organs (e.g., retina).

• Unipolar: Single process splits into two branches; most sensory neurons in PNS.

• Multipolar: Many dendrites, one axon; most common type in CNS.

• Anaxonic: No obvious axon; found in brain and special sense organs.

Functional Categories of Neurons

• Sensory Neurons: Transmit sensory information to the CNS.

• Motor Neurons: Carry commands from CNS to effectors.

• Interneurons: Connect neurons within the CNS; involved in processing and integration.

Gray Matter vs. White Matter in the CNS

• Gray Matter: Contains neuron cell bodies, dendrites, and unmyelinated axons; site of synaptic integration.

• White Matter: Composed of myelinated axons; responsible for transmission of signals over distances.

Neurophysiology

Resting Membrane Potential (RMP)

The RMP is the electrical potential difference across the neuron's membrane at rest, typically around -70 mV.

• Maintained by the sodium-potassium pump and differential permeability of the membrane to ions.

• Equation: $E_{m} = \frac{RT}{zF} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$ (Nernst equation for potassium)

Graded Potentials

• Depolarization: Membrane potential becomes less negative (closer to zero).

• Hyperpolarization: Membrane potential becomes more negative.

• Graded potentials are local changes in membrane potential; their magnitude depends on stimulus strength.

Action Potential Generation

• All-or-none electrical impulse triggered when threshold is reached.

• Involves rapid depolarization (Na+ influx), repolarization (K+ efflux), and return to RMP.

• Equation: $I = g(V - E)$ (Ohm's law for ionic currents)

Action Potential Propagation

• Continuous Conduction: Occurs in unmyelinated axons; action potential moves along every segment of the membrane.

• Saltatory Conduction: Occurs in myelinated axons; action potential jumps between nodes of Ranvier, increasing speed.

Chemical Synapses

• Communication occurs via neurotransmitter release from presynaptic neuron to postsynaptic neuron.

• EPSP (Excitatory Postsynaptic Potential): Depolarizes postsynaptic membrane, increasing likelihood of action potential.

• IPSP (Inhibitory Postsynaptic Potential): Hyperpolarizes postsynaptic membrane, decreasing likelihood of action potential.

• Temporal Summation: Multiple signals from one synapse over time.

• Spatial Summation: Multiple signals from different synapses at the same time.

• Axoaxonic Synapse: Modifies neurotransmitter release; can facilitate (increase) or inhibit (decrease) presynaptic activity.

• Neurotransmitter: Chemical messenger released at synapse; directly affects postsynaptic cell.

• Neuromodulator: Modifies the effect of neurotransmitters, often with longer-lasting effects.

Ion Channels in Neurons

• Chemically-Gated Channels: Open in response to neurotransmitter binding; found on dendrites and soma.

• Voltage-Gated Channels: Open in response to changes in membrane potential; found on axons and axon terminals.

• Voltage-Gated Sodium and Potassium Channels: Essential for action potential initiation and propagation.

Spinal Cord

General Structure of the Spinal Cord

• Cervical Enlargement: Supplies nerves to upper limbs.

• Lumbosacral Enlargement: Supplies nerves to lower limbs.

• Conus Medullaris: Tapered end of the spinal cord.

• Filum Terminale: Fibrous extension anchoring the spinal cord to the coccyx.

• Cauda Equina: Bundle of spinal nerve roots below the conus medullaris.

Protection of the Spinal Cord

• Dura Mater: Tough outermost meningeal layer.

• Arachnoid Mater: Middle meningeal layer; web-like structure.

• Pia Mater: Delicate innermost layer; adheres to spinal cord surface.

• Cerebrospinal Fluid (CSF): Cushions and nourishes the spinal cord.

• Epidural Space: Space between dura mater and vertebral wall; contains fat and blood vessels.

• Denticulate Ligaments: Extensions of pia mater; stabilize the spinal cord laterally.

Functional Anatomy of a Spinal Cord Segment

• White Matter: Contains ascending (sensory) and descending (motor) tracts; transmits information.

• Gray Matter: Contains neuron cell bodies; site of synaptic processing.

• Posterior Gray Horn: Sensory nuclei.

• Lateral Gray Horn: Autonomic motor nuclei (thoracic and upper lumbar regions).

• Anterior Gray Horn: Somatic motor nuclei.

• Gray Commissure: Connects left and right sides of gray matter.

• Somatic and Visceral Nuclei: Somatic nuclei control skeletal muscle; visceral nuclei control smooth/cardiac muscle and glands.

Structure and Function of Spinal Nerves

• Spinal nerves are mixed nerves (contain both sensory and motor fibers).

• Posterior Root: Contains sensory axons entering the spinal cord.

• Anterior Root: Contains motor axons exiting the spinal cord.

• Posterior Root Ganglion: Contains cell bodies of sensory neurons.

Plexuses

Plexuses are networks of intersecting nerves that serve specific body regions.

• Cervical Plexus: Innervates neck and diaphragm; phrenic nerve controls the diaphragm (essential for breathing).

• Brachial Plexus: Innervates shoulder and upper limb.

• Lumbar Plexus: Innervates lower abdomen, anterior and medial thigh.

• Sacral Plexus: Innervates pelvis, posterior thigh, leg, and foot.

Spinal Reflexes

• Spinal reflexes are processed in the spinal cord, allowing rapid, automatic responses to stimuli.

• Reflex Arc Components:

  1. Receptor

  2. Sensory Neuron

  3. Integration Center (may include interneurons)

  4. Motor Neuron

  5. Effector

• Stretch Reflex: Example: Patellar reflex; helps maintain muscle tone and posture.

• Muscle Spindle: Sensory receptor within muscle; detects stretch and initiates reflex contraction.

• Withdrawal Reflex: Moves body part away from painful stimulus.

• Crossed Extensor Reflex: Complements withdrawal reflex by extending opposite limb for balance.

Key Terms in Spinal Reflexes

• Receptor: Detects stimulus.

• Sensory Neuron: Transmits afferent impulse to CNS.

• Motor Neuron: Transmits efferent impulse to effector.

• Interneuron: Connects sensory and motor neurons (in polysynaptic reflexes).

• Gray Commissure: Site of cross-communication in spinal cord.

• Anterior Gray Horn: Contains somatic motor neuron cell bodies.

• Monosynaptic Reflex: Single synapse between sensory and motor neuron (e.g., stretch reflex).

• Polysynaptic Reflex: Involves one or more interneurons.

• Reciprocal Inhibition: Inhibition of antagonist muscles during reflex action.

Diagnostic Significance of Reflexes

• Plantar Reflex: Normal response is toe flexion; abnormal (Babinski sign) indicates CNS damage in adults.

• Babinski Sign: Extension of big toe and fanning of other toes; normal in infants, pathological in adults.

• Spinal reflexes are useful diagnostic tools for assessing nervous system function and integrity of specific pathways.

Reflex Type	Number of Synapses	Example	Function
Monosynaptic	1	Patellar (knee-jerk)	Maintains posture, muscle tone
Polysynaptic	2 or more	Withdrawal reflex	Protects from injury

Example: The patellar reflex is a monosynaptic stretch reflex that helps maintain upright posture by causing the quadriceps muscle to contract in response to stretching.

Additional info: The Babinski sign is used clinically to assess the integrity of the corticospinal tract. In infants, the sign is normal due to incomplete myelination of descending motor pathways.