Chapter 8: The Nervous System – Structured Study Notes

The Nervous System

Divisions of the Nervous System

The nervous system is divided into central and peripheral components, each with specialized roles in processing and transmitting information throughout the body.

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

• Peripheral Nervous System (PNS): All nervous tissue outside the CNS, including sensory receptors and nerves. It detects stimuli and transmits information to and from the CNS.

  • Sensory (Afferent) Division: Brings information to the CNS from receptors, monitoring internal and external events.

  • Motor (Efferent) Division: Carries motor commands from the CNS to effectors (target organs).

    • Somatic Nervous System (SNS): Voluntary control; innervates skeletal muscles.

    • Autonomic Nervous System (ANS): Involuntary control; innervates cardiac muscle, smooth muscle, glands, and adipose tissue.

      • Sympathetic Division: Prepares the body for physical activity ("fight or flight").

      • Parasympathetic Division: Calms the body and regulates resting functions ("rest and digest").

Nervous Tissue

Nervous tissue is composed of neurons and neuroglia, each with distinct structures and functions.

• Neurons: The functional units of the nervous system; specialized for receiving stimuli and transmitting action potentials. They are amitotic and require high amounts of oxygen and glucose.

  • Dendrites: Short, branched processes that receive stimuli.

  • Cell Body (Soma): Contains the nucleus and organelles; integrates incoming signals.

  • Axon: Conducts impulses away from the cell body.

    • Axon Hillock: Site where the axon originates from the cell body.

    • Axon Collaterals: Branches of the axon.

    • Axon Terminal: End of the axon, adjacent to the synapse.

• Structural Types of Neurons:

  • Multipolar: Multiple dendrites, one axon; most common in CNS and all motor neurons.

  • Bipolar: One dendrite, one axon; rare, found in retina, inner ear, nasal cavity.

  • Unipolar: Single process that splits; most sensory neurons for touch and pain.

• Functional Types of Neurons:

  • Sensory (Afferent) Neurons: Transmit information from receptors to CNS; mostly unipolar.

  • Interneurons: Located within CNS; process information, responsible for memory, planning, learning.

  • Motor (Efferent) Neurons: Transmit commands from CNS to effectors; mostly multipolar.

Neuroglia (Glial Cells)

Neuroglia are supporting cells in the nervous system, more numerous than neurons, and essential for neuron function and health.

• CNS Neuroglia:

  • Astrocytes: Maintain blood-brain barrier, provide structural support, regulate interstitial fluid, recycle neurotransmitters, form scar tissue.

  • Oligodendrocytes: Form myelin sheaths in CNS, increasing action potential speed.

  • Microglia: Phagocytic cells that remove debris, waste, and pathogens.

  • Ependymal Cells: Line brain and spinal cord cavities; produce, monitor, and circulate cerebrospinal fluid (CSF).

• PNS Neuroglia:

  • Schwann Cells: Form myelin sheaths in PNS; aid in nerve fiber regeneration.

  • Satellite Cells: Surround and support neuron cell bodies in PNS.

Neuron Organization in CNS and PNS

Neurons are organized into clusters and bundles for efficient communication.

• CNS:

  • Nuclei: Clusters of neuron cell bodies (gray matter).

  • Tracts: Bundles of axons (white matter).

• PNS:

  • Ganglia: Clusters of neuron cell bodies.

  • Nerves: Bundles of axons.

Membrane Potential

Membrane potential is the voltage difference across a cell membrane, essential for nerve impulse transmission.

• Causes: Unequal distribution of ions due to selective permeability and active transport.

• Ion Concentrations:

  • ECF: High in Na+ and Cl-

  • Cytosol: High in K+

• Ion Movements:

  • Leak channels (always open), ligand-gated channels (open in response to neurotransmitters), voltage-gated channels (open/close with membrane potential changes).

  • Sodium-potassium pump: Ejects 3 Na+ out, brings 2 K+ in.

Equation (Sodium-Potassium Pump):

$3\ \mathrm{Na}^+_{\text{out}} \leftarrow \text{Pump} \rightarrow 2\ \mathrm{K}^+_{\text{in}}$

Neural Activity

Changes in membrane potential underlie all neural signaling, including muscle contraction and gland secretion.

• Resting Potential: The stable, negative charge of a neuron at rest.

• Graded Potential: Small, localized change in membrane potential due to a stimulus.

• Action Potential: Large, rapid change in membrane potential that propagates along the axon.

Synapse

The synapse is the junction where communication occurs between two cells, typically involving neurotransmitter release.

• Presynaptic Cell: Releases neurotransmitters into the synaptic cleft.

• Postsynaptic Cell: Has receptors for neurotransmitters, leading to changes in membrane permeability.

• Neurotransmitters can be excitatory or inhibitory; the net effect determines if an action potential is generated.

Action Potentials (Detailed)

Action potentials are the fundamental electrical signals of the nervous system, involving rapid depolarization and repolarization of the membrane.

• Phases:

  • Depolarization: Opening of sodium channels; Na+ influx makes the inside less negative.

  • Repolarization: Sodium channels close, potassium channels open; K+ efflux restores negativity.

• Development:

  1. Graded potential depolarizes membrane to threshold.

  2. Action potential triggered: Na+ channels open, then close as K+ channels open.

  3. Sodium-potassium pump restores original ion distribution.

• Propagation:

  • Continuous: Unmyelinated axons; action potential moves along every segment.

  • Saltatory: Myelinated axons; action potential jumps between nodes of Ranvier, increasing speed.

• Speed Factors: Larger diameter and myelination increase propagation speed; urgent signals use fastest fibers.

Functional Anatomy of the CNS

The CNS processes sensory information and coordinates responses, protected by specialized membranes and fluid.

• Meninges: Three connective tissue layers covering the brain and spinal cord.

  • Dura Mater: Outermost, tough membrane; contains dural sinuses in the brain.

  • Arachnoid Mater: Middle layer; subarachnoid space filled with CSF.

  • Pia Mater: Innermost, delicate layer attached to brain surface.

• Spinal Cord: Conducts information, coordinates locomotion, and mediates reflexes; gives rise to 31 pairs of spinal nerves.

• Brain: Contains 97% of neural tissue; responsible for sensory input, integration, motor output, and mental activity.

  • Cerebrum: Largest region; conscious thought, memory, intellect.

  • Diencephalon: Includes thalamus (relay, mood, movement), hypothalamus (ANS, endocrine), epithalamus (pineal gland, melatonin).

  • Brainstem: Midbrain (wakefulness), pons (relay), medulla oblongata (vital functions).

  • Cerebellum: Balance, coordination, motor learning.

  • Limbic System: Emotion and memory (hippocampus, amygdaloid body).

• Ventricles: Fluid-filled chambers producing and circulating CSF.

  • Left/right lateral ventricles, third ventricle (diencephalon), fourth ventricle (between brainstem and cerebellum).

• Cerebrospinal Fluid (CSF): Cushions CNS, transports nutrients and waste.

The Peripheral and Autonomic Nervous Systems

The PNS connects the CNS to limbs and organs, while the ANS regulates involuntary functions.

• Peripheral Nervous System:

  • Cranial Nerves: 12 pairs; connect directly to the brain.

  • Spinal Nerves: 31 pairs; connect to the spinal cord.

  • Connective Tissue Layers: Epineurium (outer), perineurium (bundles/fascicles), endoneurium (individual axons).

• Cranial Nerves and Functions:

Number	Name	Type	Main Function(s)
I	Olfactory	Sensory	Smell
II	Optic	Sensory	Vision
III	Oculomotor	Mixed (mainly motor)	Eye movement, pupil constriction
IV	Trochlear	Mixed (mainly motor)	Eye movement
V	Trigeminal	Mixed	Facial sensation, mastication
VI	Abducens	Mixed (mainly motor)	Lateral eye movement
VII	Facial	Mixed	Taste, facial expression, secretion
VIII	Vestibulocochlear	Mixed (mainly sensory)	Hearing, balance
IX	Glossopharyngeal	Mixed	Taste, swallowing, salivation
X	Vagus	Mixed	Visceral sensation, swallowing, speech
XI	Accessory	Mixed (mainly motor)	Swallowing, head/neck movement
XII	Hypoglossal	Mixed (mainly motor)	Tongue movement

• Spinal Nerve Distribution: 31 pairs (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal); each has dorsal and ventral rami.

• Nerve Plexuses:

  • Cervical: Neck and diaphragm

  • Brachial: Pectoral girdle and upper limbs

  • Lumbar and Sacral (Lumbosacral): Pelvic girdle and lower limbs

Reflexes

Reflexes are rapid, automatic responses to stimuli, essential for homeostasis and protection.

• Reflex Arc Components:

  1. Receptor

  2. Sensory neuron

  3. Interneuron (sometimes absent)

  4. Motor neuron

  5. Effector

• Types of Reflexes:

  • Monosynaptic: One synapse; fast (e.g., patellar reflex).

  • Polysynaptic: Multiple synapses; slower, more complex (e.g., withdrawal reflex).

The Autonomic Nervous System (ANS)

The ANS regulates involuntary physiological functions, with two main divisions exerting opposing effects.

• Sympathetic Division ("Fight or Flight"):

  • Increases alertness, metabolic rate, heart rate, blood pressure, respiratory rate

  • Reduces digestive and urinary functions

  • Activates sweat glands

• Parasympathetic Division ("Rest and Digest"):

  • Decreases metabolic rate, heart rate, blood pressure

  • Increases digestive gland secretion, motility, and blood flow

  • Stimulates urination and defecation

Example: Patellar Reflex (Monosynaptic)

• Tap patellar tendon → receptor in quadriceps muscle stretched

• Sensory neuron carries message to motor neuron

• Motor neuron stimulates quadriceps to contract

Example: Withdrawal Reflex (Polysynaptic)

• Pain receptor activated by tissue damage

• Sensory neuron stimulates interneuron, which activates motor neuron

• Effector muscle contracts to withdraw from stimulus

Additional info: The sodium-potassium pump is essential for restoring resting membrane potential after an action potential. Myelination greatly increases the speed of neural transmission, which is critical for rapid responses such as reflexes and urgent motor commands.