Comprehensive Study Guide: Nervous System Anatomy & Physiology

Nervous System Overview

Central Nervous System (CNS) vs. Peripheral Nervous System (PNS)

The nervous system is divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). Each has distinct structures and functions.

• CNS: Consists of the brain and spinal cord. Responsible for processing and integrating information.

• PNS: Composed of nerves and ganglia outside the CNS. Connects the CNS to limbs and organs.

• Basic differences: CNS is the control center; PNS transmits signals to and from the CNS.

Afferent vs. Efferent Divisions

The PNS is further divided into afferent (sensory)efferent (motor)

• Afferent division: Carries sensory information from receptors to the CNS.

• Efferent division: Transmits motor commands from the CNS to effectors (muscles and glands).

Neural Anatomy

Bundles of Axons: Nerves vs. Nerve Tracts

Axons are bundled differently in the CNS and PNS.

• Nerves: Bundles of axons in the PNS.

• Nerve tracts: Bundles of axons in the CNS.

Connective Tissue Layers Surrounding Neurons

Neurons are protected by several layers of connective tissue.

• Endoneurium: Surrounds individual axons.

• Perineurium: Encloses bundles of axons (fascicles).

• Epineurium: Encloses the entire nerve.

Neuroglial Cells and Their Roles

Neuroglia support and protect neurons.

• Astrocytes: Maintain blood-brain barrier, provide structural support.

• Oligodendrocytes: Form myelin in CNS.

• Schwann cells: Form myelin in PNS.

• Microglia: Act as immune cells in CNS.

• Ependymal cells: Line ventricles, produce cerebrospinal fluid (CSF).

Brain Anatomy & Function

Major Parts of the Brain and Their Functions

The brain consists of several major regions, each with specific functions.

• Thalamus: Relay station for sensory information.

• Reticular Activating System: Regulates wakefulness and sleep-wake transitions.

• Hypothalamus: Controls autonomic functions, hormone release, and homeostasis.

• Medulla Oblongata: Regulates vital functions (heart rate, breathing).

• Pons: Relays signals between cerebrum and cerebellum; involved in sleep and respiration.

• Limbic System: Involved in emotion, motivation, and memory.

• Cerebellum: Coordinates movement and balance.

Gyri and Sulci

Gyri are the raised folds of the brain, while sulci are the grooves. They increase the surface area for cortical neurons.

Brainstem Anatomy

The brainstem includes the midbrain, pons, and medulla oblongata. It controls basic life functions and acts as a conduit for information between the brain and spinal cord.

Diencephalon

The diencephalon includes the thalamus, hypothalamus, and epithalamus. It is involved in sensory and autonomic functions.

Visual Cortex Location

The primary visual cortex is located in the occipital lobe of the cerebrum.

Spinal Cord Anatomy

Basic Structure

The spinal cord is organized into regions and roots.

• Dorsal root: Carries sensory (afferent) information into the spinal cord.

• Ventral root: Carries motor (efferent) information out of the spinal cord.

• Gray matter horns: Dorsal, ventral, and lateral horns contain different types of neurons.

Dermatomal Maps

Dermatomes are areas of skin supplied by sensory fibers from a single spinal nerve. Dermatomal maps are used clinically to diagnose nerve injuries.

Spinal Nerve Plexuses and the Phrenic Nerve

Spinal nerves form plexuses (networks) that supply limbs and other body regions.

• Cervical plexus: Includes the phrenic nerve, which innervates the diaphragm.

• Brachial, lumbar, and sacral plexuses: Supply the upper and lower limbs.

Cranial and Spinal Nerves

Cranial vs. Spinal Nerves

There are 12 pairs of cranial nerves and 31 pairs of spinal nerves.

• Cranial nerves: Emerge from the brain; numbered with Roman numerals based on their position from anterior to posterior.

• Spinal nerves: Emerge from the spinal cord; named according to the vertebral level.

• Major cranial nerves: Vagus nerve (X) carries over 75% of parasympathetic output.

Neurotransmitters and Receptors

Types of Neurotransmitters

Neurons in the autonomic nervous system secrete different neurotransmitters.

• Cholinergic fibers: Release acetylcholine (ACh).

• Adrenergic fibers: Release norepinephrine (NE) or epinephrine (Epi).

Receptors

• Cholinergic receptors: Nicotinic and muscarinic receptors.

• Adrenergic receptors: Alpha (α) and beta (β) receptors.

• Receptors on skeletal muscle cells: Nicotinic acetylcholine receptors.

• Receptors on postganglionic neurons of autonomic ganglia: Nicotinic acetylcholine receptors.

Classes of Receptors

• Cholinergic: Nicotinic (found at neuromuscular junctions and autonomic ganglia) and muscarinic (found on target organs of parasympathetic system).

• Adrenergic: Alpha-1, alpha-2, beta-1, beta-2, beta-3 (found on target organs of sympathetic system).

Autonomic Nervous System (ANS)

Parasympathetic vs. Sympathetic Divisions

The ANS is divided into parasympathetic (PSNS) and sympathetic (SNS) divisions.

• PSNS: Rest and digest; uses acetylcholine as neurotransmitter; dual innervation with SNS.

• SNS: Fight or flight; uses norepinephrine and epinephrine; dual innervation with PSNS.

• Dual innervation: Most organs receive input from both divisions.

Pre- and Postganglionic Neurons

• Preganglionic neurons: Usually myelinated.

• Postganglionic neurons: Usually unmyelinated.

Reflexes

Reflex Arc and Types of Reflexes

A reflex arc is the neural pathway involved in a reflex action.

• Components: Receptor, sensory neuron, integration center, motor neuron, effector.

• Types: Developmental (innate) vs. acquired (learned); monosynaptic (single synapse, e.g., stretch reflex) vs. polysynaptic (multiple synapses, e.g., withdrawal reflex).

• Examples: Stretch reflex, Golgi tendon reflex, withdrawal reflex.

Functional Concepts

Contralateral Control

The left side of the cerebrum controls the right side of the body and vice versa.

Association vs. Perception vs. Projection

• Association: Integration of sensory and motor information.

• Perception: Conscious awareness of a sensation.

• Projection: Localization of a sensation to a specific body region.

Types of Memory

• Short-term memory: Temporary storage; limited capacity.

• Long-term memory: More permanent; involves changes in synaptic strength.

• Maintenance: Long-term potentiation and protein synthesis.

Action Potentials

Basic Action Potential Information

An action potential is a rapid change in membrane potential that propagates along a neuron.

• Depolarization: Na+ influx.

• Repolarization: K+ efflux.

• Absolute refractory period: No new action potential can be initiated.

• Relative refractory period: A stronger stimulus can initiate another action potential.

• Na+/K+ pump: Maintains resting membrane potential by moving 3 Na+ out and 2 K+ in.

Equation:

$\text{Na}^+/\text{K}^+ \text{ pump:} \, 3\, \text{Na}^+_{\text{out}} : 2\, \text{K}^+_{\text{in}}$

EPSP and IPSP

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

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

Role of Enzymes

• Acetylcholinesterase: Breaks down acetylcholine in the synaptic cleft.

• Monoamine oxidase: Degrades monoamine neurotransmitters (e.g., norepinephrine).

• Inhibitors: Prolong neurotransmitter action by preventing breakdown.

Cerebrospinal Fluid (CSF) and Meninges

CSF Production and Reabsorption

• Produced: By choroid plexus in ventricles.

• Reabsorbed: By arachnoid villi into venous sinuses.

Meningeal Layers (Order)

• Dura mater (outermost)

• Arachnoid mater (middle)

• Pia mater (innermost)

Spaces and Fluids

• Subdural space: Contains serous fluid.

• Subarachnoid space: Contains cerebrospinal fluid (CSF).

Connections Between Ventricles

• Interventricular foramina: Connect lateral ventricles to third ventricle.

• Cerebral aqueduct: Connects third ventricle to fourth ventricle.

Adrenal Medulla and Neurohormones

Release of Epinephrine and Norepinephrine

• Adrenal medulla: Releases 80% epinephrine and 20% norepinephrine as neurohormones.

• Longer-lasting effects: Neurohormones circulate in blood, affecting target organs for longer than neurotransmitters.

Summary Table: Cholinergic vs. Adrenergic Fibers and Receptors

Fiber Type	Neurotransmitter	Receptor Types	Location
Cholinergic	Acetylcholine (ACh)	Nicotinic, Muscarinic	All preganglionic neurons, PSNS postganglionic neurons, neuromuscular junction
Adrenergic	Norepinephrine (NE), Epinephrine (Epi)	Alpha (α), Beta (β)	SNS postganglionic neurons, target organs

Additional info:

• "Somesthetic cortex (picture of the 'little man')" refers to the sensory homunculus, a visual representation of the body mapped onto the primary somatosensory cortex.

• "Largest portion of diencephalon" is the thalamus.

• "Absolute vs. relative refractory" periods are critical for understanding nerve impulse propagation and frequency coding.