Central and Peripheral Nervous System, Autonomic Nervous System, and Special Senses: Study Guide

Central Nervous System (CNS)

Brainstem: Parts and Functions

The brainstem is a critical structure connecting the brain and spinal cord, responsible for many automatic functions necessary for survival.

• Midbrain: Controls visual and auditory reflexes, and coordinates movement.

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

• Medulla oblongata: Controls vital autonomic functions such as heart rate, blood pressure, and respiration.

Example: Damage to the medulla can result in loss of respiratory function.

Cerebellum: Anatomy and Function

The cerebellum is located at the back of the brain and is involved in coordination, precision, and accurate timing of movements.

• Receives input from sensory systems and other parts of the brain and spinal cord.

• Regulates motor movements and helps maintain posture and balance.

Example: Cerebellar damage can cause ataxia, a lack of muscle coordination.

Protection of the Brain

The brain is protected by several structures:

• Meninges: Three connective tissue layers:

  • Dura mater: Tough, outermost layer.

  • Arachnoid mater: Middle, web-like layer.

  • Pia mater: Delicate, innermost layer adhering to the brain surface.

• Cerebrospinal fluid (CSF): Cushions the brain and removes waste.

• Blood-brain barrier: Selectively allows substances to pass from blood to brain tissue, protecting against toxins and pathogens.

Brain Injuries and Disorders

• Concussion: Temporary brain dysfunction due to trauma; symptoms include headache, confusion, dizziness.

• Contusion: Bruising of brain tissue; may cause permanent damage.

• Cerebrovascular accident (CVA or stroke): Disruption of blood flow to the brain, leading to tissue death; symptoms depend on affected area.

Spinal Cord: Anatomy and Function

The spinal cord is a long, thin bundle of nervous tissue that extends from the brainstem down the vertebral column.

• Gross anatomy: Consists of cervical, thoracic, lumbar, and sacral regions.

• Microscopic anatomy: Contains gray matter (cell bodies) and white matter (myelinated axons).

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

• Ganglia: Clusters of sensory neuron cell bodies outside the CNS.

• Function: Transmits sensory and motor information; mediates reflexes.

Spinal Cord Diseases and Trauma

• Paralysis: Loss of motor function due to injury.

• Paresthesia: Loss of sensory function.

• Spinal cord transection: Complete severing leads to loss of function below the injury.

Key Vocabulary

• Fissure: Deep groove in the brain.

• Gyrus: Raised ridge on the brain surface.

• Sulcus: Shallow groove between gyri.

• Lateralization: Specialization of function in one hemisphere.

• Contralateral: Opposite side control (e.g., left brain controls right body).

• Homunculus: Map of body regions on the cortex.

• Motor cortex: Controls voluntary movements.

• Somatosensory cortex: Processes sensory input.

• Meninges, concussion, contusion, cardiovascular accident: See above.

Peripheral Nervous System (PNS)

Divisions of the PNS

The PNS connects the CNS to limbs and organs. It is divided into:

• Somatic nervous system: Controls voluntary movements.

• Autonomic nervous system: Controls involuntary functions.

Classification of Receptors

• By function:

  • Mechanoreceptors: Detect mechanical force (touch, pressure).

  • Thermoreceptors: Detect temperature changes.

  • Photoreceptors: Detect light (in the eye).

  • Chemoreceptors: Detect chemicals (taste, smell).

  • Nociceptors: Detect pain.

• By location:

  • Exteroceptors: Respond to stimuli outside the body.

  • Interoceptors: Respond to internal stimuli.

  • Proprioceptors: Detect body position and movement.

Structure and Classification of Nerves

• Nerve: Bundle of axons in the PNS.

• Classification:

  • Mixed nerves: Contain both sensory and motor fibers.

  • Sensory (afferent) nerves: Carry impulses to the CNS.

  • Motor (efferent) nerves: Carry impulses from the CNS.

Connective Tissue Layers of Nerves

• Epineurium: Outermost layer, surrounds the entire nerve.

• Perineurium: Surrounds bundles (fascicles) of axons.

• Endoneurium: Surrounds individual axons.

Key Vocabulary

• Sensation: Awareness of changes in the environment.

• Perception: Interpretation of sensory input.

• Pain: Unpleasant sensory experience.

• Pain tolerance: Amount of pain an individual can endure.

• Epineurium, perineurium, endoneurium: See above.

• Somatic afferent: Sensory from skin, muscles to CNS.

• Somatic efferent: Motor from CNS to skeletal muscles.

• Visceral afferent: Sensory from organs to CNS.

• Visceral efferent: Motor from CNS to organs.

Autonomic Nervous System (ANS)

Definition and Comparison to Somatic Nervous System

The autonomic nervous system regulates involuntary functions such as heart rate, digestion, and respiratory rate. It differs from the somatic nervous system, which controls voluntary movements.

• Somatic: One neuron from CNS to muscle; neurotransmitter is acetylcholine; effect is always excitatory.

• Autonomic: Two-neuron chain (preganglionic and postganglionic); neurotransmitters include acetylcholine and norepinephrine; effect can be excitatory or inhibitory.

Efferent Pathways

• Preganglionic neuron: Cell body in CNS; axon extends to autonomic ganglion.

• Postganglionic neuron: Cell body in ganglion; axon extends to effector organ.

Neurotransmitters in the ANS

• Acetylcholine (ACh): Released by all preganglionic neurons and parasympathetic postganglionic neurons.

• Norepinephrine (NE): Released by most sympathetic postganglionic neurons.

Sympathetic vs. Parasympathetic Divisions

Feature	Sympathetic	Parasympathetic
Origin	Thoracolumbar (T1-L2)	Craniosacral (brainstem, S2-S4)
Function	"Fight or flight" (increases HR, dilates pupils)	"Rest and digest" (decreases HR, stimulates digestion)
Neurons	Short preganglionic, long postganglionic	Long preganglionic, short postganglionic
Neurotransmitter	Mostly norepinephrine	Acetylcholine

• Dual innervation: Most organs receive input from both divisions, allowing dynamic antagonism (balance).

Key Vocabulary

• Involuntary nervous system: Another name for ANS.

• Effectors: Tissues/organs acted on by the ANS (e.g., cardiac muscle, glands).

• Dual innervation, dynamic antagonism: See above.

• Craniosacral, thoracolumbar: Refer to origins of parasympathetic and sympathetic divisions, respectively.

Special Senses

Overview of Special Senses

The body has five special senses: vision, hearing, equilibrium (balance), taste, and smell.

• Vision: Detection of light by the eyes.

• Hearing: Detection of sound by the ears.

• Equilibrium: Maintenance of balance by the vestibular system.

• Taste: Detection of chemicals by taste buds.

• Smell: Detection of chemicals by olfactory receptors.

Sound Conduction Pathway

• Sound waves enter the external ear and vibrate the tympanic membrane.

• Vibrations are transmitted through the ossicles (malleus, incus, stapes) to the oval window.

• Vibrations enter the cochlea, moving the fluids and stimulating hair cells.

Anatomy of the Cochlea

• Cochlea: Spiral-shaped organ containing the organ of Corti (hearing receptor cells).

• Divided into three chambers: scala vestibuli, scala media, scala tympani.

Auditory Pathway

• Hair cells in the cochlea generate nerve impulses.

• Impulses travel via the cochlear nerve to the brainstem, then to the auditory cortex in the temporal lobe.

Balance and Equilibrium

• Semicircular canals: Detect rotational movement.

• Vestibule (saccule and utricle): Detect linear acceleration and head position.

• Structures like the maculae and crista ampullaris contain hair cells that respond to movement.

Key Vocabulary

• Sound detection: Process of converting sound waves to nerve impulses.

• Equilibrium: Sense of balance.

• Vestibular apparatus: Includes semicircular canals and vestibule.

• Saccule, utricle, ampulla, maculae, crista ampullaris: Structures involved in balance.

• Nystagmus: Involuntary eye movement, often due to vestibular dysfunction.

• Benign paroxysmal positional vertigo (BPPV): Common cause of vertigo due to displaced otoliths.

• Motion sickness: Caused by conflicting sensory input regarding movement.