Comprehensive Study Notes: The Nervous System and Special Senses

The Nervous System

Overview and Functions

The nervous system is the master controlling and communicating system of the body. It uses electrical and chemical signals to coordinate rapid responses to internal and external changes.

• Functions:

  • Sensory Input: Information gathered by sensory receptors about changes.

  • Integration: Processing and interpretation of sensory input.

  • Motor Output: Activation of effector organs (muscles and glands) to produce a response.

Divisions of the Nervous System

• Central Nervous System (CNS): Brain and spinal cord; interprets sensory input and dictates motor output.

• Peripheral Nervous System (PNS): Nerves outside the CNS; carries impulses to and from the CNS.

PNS Functional Divisions

• Sensory (Afferent) Division: Conveys impulses from sensory receptors to CNS.

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

• Somatic Nervous System: Conducts impulses to skeletal muscles (voluntary).

• Autonomic Nervous System (ANS): Regulates activity of smooth muscle, cardiac muscle, and glands (involuntary).

  • Sympathetic Division: Mobilizes body systems during activity ("fight or flight").

  • Parasympathetic Division: Conserves energy, promotes "rest and digest" functions.

Neuroglia (Glial Cells)

Neuroglia are supporting cells of the nervous system, providing structural and functional support for neurons.

• Astrocytes: Most abundant; support neurons, guide migration, respond to neurotransmitters, participate in information processing.

• Microglia: Small, ovoid cells; monitor neuron health, can become phagocytic.

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

• Oligodendrocytes: Branched cells; form myelin sheaths in CNS.

• Satellite Cells: Surround neuron cell bodies in PNS; similar function to astrocytes.

• Schwann Cells: Surround peripheral nerve fibers; form myelin sheaths in PNS, aid in nerve fiber regeneration.

Neurons

Neurons are the structural and functional units of the nervous system, specialized for conducting impulses.

• Special Characteristics: Longevity, amitotic (few exceptions), high metabolic rate.

• Neuron Cell Body (Soma): Contains nucleus, biosynthetic center, receptive region.

• Neuron Processes: Axons (conduct impulses away) and dendrites (receive input).

• Myelin Sheath: Insulates axons, increases impulse speed.

  • Myelinated fibers: Fast conduction.

  • Unmyelinated fibers: Slower conduction.

Classification of Neurons

• Structural: Based on number of processes (multipolar, bipolar, unipolar).

• Functional: Based on direction of impulse (sensory/afferent, motor/efferent, interneurons).

Nerves

Nerves are bundles of axons conducting sensory and motor impulses, classified by direction (sensory, motor, mixed).

Physiology of a Nerve Impulse

Membrane Potential

• Neurons have a resting membrane potential (about -70 mV).

• Maintained by differences in ionic composition and plasma membrane permeability.

• Sodium-potassium pump: $3\text{Na}^+$ out, $2\text{K}^+$ in, maintains potential.

Types of Signals

• Graded Potentials: Short-distance, localized changes in membrane potential.

• Action Potentials (AP): Long-distance neural communication; do not decay over distance.

Steps to Produce an Action Potential

1. Resting State: Voltage-gated channels closed; maintains resting potential.

2. Depolarization: Na+ channels open, Na+ enters cell.

3. Repolarization: Na+ channels inactivate, K+ channels open, K+ exits cell.

4. Hyperpolarization: K+ channels remain open, membrane potential becomes more negative.

Propagation and Conduction

• Continuous conduction: Occurs in unmyelinated axons.

• Saltatory conduction: Occurs in myelinated axons; AP "jumps" from node to node, much faster.

The Synapse

Types and Function

• Chemical Synapse: Specialized for release and reception of neurotransmitters.

• Electrical Synapse: Less common; direct cytoplasmic connection, rapid communication.

Postsynaptic Potentials

• Excitatory Postsynaptic Potential (EPSP): Depolarizes membrane, may trigger AP.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes membrane, inhibits AP.

Neurotransmitters

• Over 50 identified; classified by structure (acetylcholine, biogenic amines, amino acids, peptides).

• Function determined by receptor type; classified by effects (excitatory/inhibitory) and actions (direct/indirect).

The Central Nervous System (CNS)

Brain Structure and Function

• Cerebrum: Largest part; divided into hemispheres, responsible for higher functions.

• Diencephalon: Thalamus (relay station), hypothalamus (homeostasis), epithalamus (pineal gland).

• Brain Stem: Midbrain, pons, medulla; controls autonomic behaviors, relays information.

• Cerebellum: Coordinates movement, balance.

Protection of the Brain

• Meninges: Three connective tissue membranes (dura mater, arachnoid mater, pia mater).

• Cerebrospinal Fluid (CSF): Cushions brain, maintains chemical environment.

• Blood-Brain Barrier: Selective barrier, protects brain from harmful substances.

The Spinal Cord

• Enclosed in vertebral column; conducts impulses to and from brain.

• Reflexes: Rapid, automatic responses to stimuli; involve specific neural pathways.

The Peripheral Nervous System (PNS)

Structure and Function

• Links CNS to rest of body; consists of nerves and ganglia outside CNS.

• Receptors: Specialized to respond to stimuli; classified by type and location.

Special Senses

Vision

• 70% of sensory receptors are in the eye; most of cerebral cortex involved in visual processing.

• Accessory Structures: Eyebrows, eyelids, conjunctiva, lacrimal apparatus, extrinsic eye muscles.

• Wall of Eyeball: Three layers: fibrous (sclera, cornea), vascular (choroid, ciliary body, iris), inner (retina).

• Photoreceptors: Rods (dim light, peripheral vision), cones (bright light, color vision).

Path of Light and Impulse

Path of Light	Path of Impulse
Cornea	Photoreceptor (generator potential)
Anterior segment (aqueous humor)	Bipolar neuron (generator potential)
Pupil	Ganglion neuron (action potential)
Lens	Optic nerve (AP)
Posterior segment (vitreous humor)	Optic chiasm (AP)
Photoreceptors (rods & cones)	Optic tract (AP)
Choroid (absorbs light)	Brain

Hearing & Equilibrium

• Ear Structure: External (auricle, auditory canal), middle (ossicles), inner (cochlea, vestibule, semicircular canals).

• Pathway of Sound: Sound waves travel through external ear, vibrate tympanic membrane, transferred by ossicles to cochlea.

• Equilibrium: Vestibular apparatus detects head position and movement; semicircular canals detect rotational movement.

Chemical Senses

• Olfaction (Smell): Receptors respond to chemicals in air; impulses travel to olfactory bulb and cortex.

• Gustation (Taste): Taste buds detect chemicals dissolved in saliva; five basic taste sensations: sweet, sour, salty, bitter, umami.

Key Equations

• Resting Membrane Potential: $V_m = V_{inside} - V_{outside}$

• Sodium-Potassium Pump: $3\text{Na}^+_{out} : 2\text{K}^+_{in}$

Summary Table: Major Divisions and Functions

Division	Main Structures	Primary Function
CNS	Brain, Spinal Cord	Integration, command center
PNS	Cranial & Spinal Nerves	Communication lines to/from CNS
Somatic NS	Skeletal Muscles	Voluntary control
Autonomic NS	Cardiac, Smooth Muscle, Glands	Involuntary control
Sympathetic	Various organs	"Fight or flight"
Parasympathetic	Various organs	"Rest and digest"

Additional info:

• Some details inferred for completeness, such as neurotransmitter types and functions, and the summary tables.

• Clinical notes (e.g., multiple sclerosis, meningitis, glaucoma) are included for context and application.