Module 5

Module 5: Organization of the Nervous System

Nervous System as a Control System

The nervous system is a complex network that coordinates the body's activities by transmitting signals to and from different parts of the body. It functions as a control system with several key components:

• Sensory Receptors: Specialized cells or structures that detect changes in the environment (stimuli).

• Afferent Pathways: Nerve fibers that carry sensory information from receptors to the central nervous system (CNS).

• Control (Integrating) Center: The CNS (brain and spinal cord), which processes sensory input and determines the appropriate response.

• Efferent Pathways: Nerve fibers that transmit commands from the CNS to effector organs.

• Effector (Target) Organs: Muscles or glands that carry out the response.

Example: Touching a hot surface activates sensory receptors in the skin, which send signals via afferent pathways to the CNS. The CNS processes the information and sends commands via efferent pathways to muscles, causing you to withdraw your hand.

Motor (Efferent) vs. Sensory (Afferent) Components

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

• Motor (Efferent) Division: Transmits commands from the CNS to effector organs (muscles and glands).

Anatomical and Functional Organization of the CNS

• Central Nervous System (CNS): Consists of the brain and spinal cord. It is responsible for integrating sensory information and coordinating bodily functions.

• Functional Organization: The CNS processes incoming sensory data, stores information, and initiates motor output.

Anatomical and Functional Organization of the PNS

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS (cranial and spinal nerves, ganglia, sensory receptors).

• Functional Organization: The PNS connects the CNS to limbs and organs, serving as a communication relay.

Nucleus vs. Ganglion; Nerve vs. Tract

• Nucleus: Cluster of neuron cell bodies within the CNS.

• Ganglion: Cluster of neuron cell bodies in the PNS.

• Nerve: Bundle of axons in the PNS.

• Tract: Bundle of axons in the CNS.

Central Nervous System: Cerebrum

Gray and White Matter in the CNS

• Gray Matter: Contains neuron cell bodies, dendrites, and unmyelinated axons; found in the cerebral cortex and nuclei.

• White Matter: Composed of myelinated axons; found beneath the cortex and forms tracts connecting different brain regions.

Cerebral Hemispheres and Lobes

• The cerebrum is divided into two hemispheres, each with five lobes:

  • Frontal Lobe

  • Parietal Lobe

  • Temporal Lobe

  • Occipital Lobe

  • Insula

Major Functions of Each Lobe

• Frontal Lobe: Voluntary movement, planning, reasoning, problem-solving, speech production (Broca's area).

• Parietal Lobe: Sensory perception, spatial awareness, language processing.

• Temporal Lobe: Auditory processing, memory, language comprehension (Wernicke's area).

• Occipital Lobe: Visual processing.

• Insula: Taste, visceral sensation, emotional responses.

Broca Area vs. Wernicke Area

• Broca Area: Located in the frontal lobe; responsible for speech production.

• Wernicke Area: Located in the temporal lobe; responsible for language comprehension.

Damage to Motor Speech Areas

• Broca's Aphasia: Difficulty producing speech; comprehension remains intact.

• Wernicke's Aphasia: Fluent but nonsensical speech; impaired comprehension.

Cerebral White Matter Tracts

• Association Tracts: Connect regions within the same hemisphere.

• Commissural Tracts: Connect corresponding areas between hemispheres (e.g., corpus callosum).

• Projection Tracts: Connect the cerebrum with lower brain regions and the spinal cord.

Central Nervous System: Diencephalon and Cerebellum

Major Structures and Functions

• Thalamus: Relay station for sensory information.

• Hypothalamus: Regulates homeostasis, endocrine functions, autonomic control.

• Epithalamus: Contains the pineal gland; involved in circadian rhythms.

• Cerebellum: Coordinates voluntary movements, balance, and posture.

• Pons: Relays signals between cerebrum and cerebellum; regulates breathing.

• Medulla Oblongata: Controls vital autonomic functions (heart rate, respiration).

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

Central Nervous System: Brainstem

Subdivisions of the Brainstem

• Midbrain (Mesencephalon)

• Pons

• Medulla Oblongata

Midbrain (Mesencephalon)

• Location: Uppermost part of the brainstem.

• Major Structures: Cerebral peduncles (motor tracts), superior colliculi (visual reflexes), inferior colliculi (auditory reflexes).

• Functions: Visual and auditory processing, motor control.

Pons

• Location: Middle portion of the brainstem.

• Functions: Relays information between cerebrum and cerebellum; regulates breathing.

Medulla Oblongata

• Location: Lowest part of the brainstem, continuous with the spinal cord.

• Functions: Controls autonomic functions such as heart rate, blood pressure, and respiration.

Central Nervous System: Spinal Cord

Gross Anatomy of the Spinal Cord

• Cervical and Lumbar Enlargements: Regions with increased neural input/output for limbs.

• Conus Medullaris: Tapered end of the spinal cord.

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

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

Spinal Roots and Ganglia

• Anterior (Ventral) Roots: Carry motor (efferent) signals from the spinal cord.

• Posterior (Dorsal) Roots: Carry sensory (afferent) signals to the spinal cord.

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

Cross-Sectional Anatomy of the Spinal Cord

• Anterior Horn: Contains motor neuron cell bodies.

• Lateral Horn: Contains autonomic motor neurons (thoracic and upper lumbar regions).

• Posterior Horn: Contains interneurons and sensory neuron terminals.

• Gray Commissure: Connects the two sides of gray matter.

• Central Canal: Contains cerebrospinal fluid.

• Funiculi (Anterior, Lateral, Posterior): Columns of white matter containing ascending and descending tracts.

Ascending and Descending Tracts

• Ascending Tracts: Carry sensory information to the brain.

• Descending Tracts: Carry motor commands from the brain to the body.

CNS Protection: Meninges, CSF, and Blood-Brain Barrier

Layers of the Meninges

• Dura Mater: Tough, outermost layer.

• Arachnoid Mater: Middle, web-like layer.

• Pia Mater: Delicate, innermost layer adhering to the CNS surface.

Protection of the CNS

• Meninges: Provide structural support and protection.

• Cerebrospinal Fluid (CSF): Cushions the brain and spinal cord, removes waste, and circulates nutrients.

• Blood-Brain Barrier (BBB): Selectively restricts passage of substances from the blood into the CNS, maintaining a stable environment.

CSF: Composition, Function, and Location

• Composition: Clear, colorless fluid containing water, ions, glucose, and minimal proteins.

• Function: Mechanical protection, chemical stability, waste removal.

• Location: Ventricles of the brain, central canal of the spinal cord, subarachnoid space.

Blood-Brain Barrier (BBB)

• Structure: Formed by tight junctions between endothelial cells of CNS capillaries.

• Significance: Protects the brain from toxins and pathogens, but can limit drug delivery.

Peripheral Nervous System: Cranial and Spinal Nerves

Cranial Nerves

• There are 12 pairs of cranial nerves, each with a specific name and number (I-XII).

Number	Name	Function	Type
I	Olfactory	Smell	Sensory
II	Optic	Vision	Sensory
III	Oculomotor	Eye movement, pupil constriction	Motor
IV	Trochlear	Eye movement	Motor
V	Trigeminal	Facial sensation, chewing	Mixed
VI	Abducens	Eye movement	Motor
VII	Facial	Facial expression, taste	Mixed
VIII	Vestibulocochlear	Hearing, balance	Sensory
IX	Glossopharyngeal	Taste, swallowing	Mixed
X	Vagus	Autonomic control of viscera	Mixed
XI	Accessory	Head and shoulder movement	Motor
XII	Hypoglossal	Tongue movement	Motor

Spinal Nerves and Plexuses

• Spinal Nerve: Formed by the union of dorsal and ventral roots; carries both sensory and motor fibers.

• Plexus: Network of intersecting nerves (e.g., brachial plexus).

• Roots: Dorsal (sensory) and ventral (motor) roots join to form a spinal nerve.

• Rami: Branches of spinal nerves that serve different body regions.

Neural Pathways and Reflexes

Reflexes and Innervation

• Reflex: Rapid, automatic response to a stimulus.

• Innervation: The supply of nerves to a specific body part.

Reflex Arc Components

• Receptor

• Sensory Neuron

• Integration Center

• Motor Neuron

• Effector

Autonomic Reflexes: Involve smooth muscle, cardiac muscle, or glands. Somatic Reflexes: Involve skeletal muscle.

Steps of a Physiological Reflex

1. Stimulus detected by receptor

2. Sensory input via afferent pathway

3. Integration in CNS

4. Motor output via efferent pathway

5. Response by effector organ

Autonomic Nervous System (ANS)

Definition and Relationship to PNS

• ANS: Subdivision of the PNS that controls involuntary functions (e.g., heart rate, digestion).

Parasympathetic vs. Sympathetic Divisions

Feature	Sympathetic	Parasympathetic
Origin	Thoracolumbar, T1-L2 (Thoracic + Lumbar)	Craniosacral, S2-S4 (Brain stem + Sacral)
Ganglia Location	Near spinal cord	Near or in target organs
Neurotransmitter at Target	Norepinephrine	Acetylcholine
Receptor Types	Adrenergic	Cholinergic
General Function	"Fight or flight"	"Rest and digest"

Coordination of Physiological Functions

• Both divisions often have opposing effects to maintain homeostasis (e.g., heart rate, pupil size).

Cholinergic vs. Adrenergic Receptors

Receptor	Neurotransmitter	Location	Response	Examples
Cholinergic (Nicotinic, Muscarinic)	Acetylcholine	All ANS ganglia, parasympathetic targets	Excitatory or inhibitory	Muscarine, nicotine
Adrenergic (Alpha, Beta)	Norepinephrine, epinephrine	Sympathetic targets	Excitatory or inhibitory	Albuterol, propranolol

Acetylcholine vs. Norepinephrine

• Acetylcholine: Released by cholinergic neurons; acts on nicotinic and muscarinic receptors.

• Norepinephrine: Released by adrenergic neurons; acts on alpha and beta receptors.

Abnormalities of the Nervous System

Predicting Effects of Nervous System Changes

• Demyelinating Diseases (e.g., Multiple Sclerosis): Loss of myelin slows or blocks nerve conduction, leading to muscle weakness, sensory disturbances.

Pathophysiology of Selected Conditions

• Multiple Sclerosis (MS): Autoimmune destruction of CNS myelin; symptoms include weakness, vision problems, and coordination loss.

• Stroke (Cerebrovascular Accident, CVA): Disruption of blood flow to the brain; causes loss of function in affected area.

• Concussion (Mild Traumatic Brain Injury): Temporary disruption of brain function due to trauma; symptoms include confusion, headache, and memory loss.

Overview of Senses

Sensory Receptors

• Sensory Receptor: Specialized cell or structure that detects a specific type of stimulus.

Key Terms

• Transduction: Conversion of stimulus energy into electrical signals.

• Perception: Conscious awareness of a sensation.

• Sensation: The process of detecting a stimulus.

• Adaptation: Decreased response to a constant stimulus over time.

Types of Sensory Receptors

• Thermoreceptor: Detects temperature changes.

• Photoreceptor: Detects light (e.g., rods and cones in the eye).

• Chemoreceptor: Detects chemical stimuli (e.g., taste, smell).

• Baroreceptor: Detects pressure changes.

• Nociceptor: Detects pain.

• Mechanoreceptor: Detects mechanical forces (e.g., touch, vibration).

Key Senses and Responsible Cells

• Vision: Photoreceptors (rods and cones)

• Auditory (Hearing): Hair cells in the cochlea

• Tactile (Touch): Mechanoreceptors in the skin

• Olfaction (Smell): Olfactory receptor neurons

• Gustation (Taste): Taste receptor cells

• Equilibrium (Balance): Hair cells in the vestibular apparatus

Gross and Microscopic Anatomy of the Eye

Tunics of the Eye

• Fibrous Tunic: Outer layer; includes the cornea (transparent, refracts light) and sclera (white, protective).

• Vascular Tunic (Uvea): Middle layer; includes the iris (controls pupil size), ciliary body (lens shape), and choroid (blood supply).

• Neural Tunic (Retina): Inner layer; contains photoreceptors (rods and cones).

Anterior and Posterior Cavities

• Anterior Cavity: In front of the lens; contains aqueous humor.

• Posterior Cavity: Behind the lens; contains vitreous humor.

Lens and Vision

• Lens: Transparent, biconvex structure that focuses light onto the retina.

Extrinsic Eye Muscles

• Six muscles control eye movement, allowing precise tracking and positioning.

Visual Senses

Path of Light Through the Eye

1. Cornea

2. Aqueous humor

3. Pupil

4. Lens

5. Vitreous humor

6. Retina

Light is refracted (bent) by the cornea and lens to focus images on the retina.

Signal Pathway from Retina to Brain

1. Photoreceptors (rods and cones) in the retina

2. Optic nerve

3. Optic chiasm

4. Optic tract

5. Thalamus (lateral geniculate nucleus)

6. Primary visual cortex in the occipital lobe

Image Formation on the Retina

• The optical system of the eye focuses light to form a real, inverted image on the retina.

Phototransduction and Light/Dark Adaptation

• Phototransduction: Process by which photoreceptors convert light into electrical signals.

• Light Adaptation: Adjustment of the eye to bright light (decreased sensitivity).

• Dark Adaptation: Adjustment to low light (increased sensitivity).

Rods vs. Cones

Feature	Rods	Cones
Location	Peripheral retina	Central retina (fovea)
Function	Low-light vision, no color	Color vision, high acuity
Number	More numerous	Fewer

Changes in Eye Anatomy and Vision

• Nearsightedness (Myopia): Image focuses in front of the retina; distant objects are blurry.

• Farsightedness (Hyperopia): Image focuses behind the retina; close objects are blurry.

• Color-Blindness: Deficiency or absence of certain cone types.