Central and Peripheral Nervous System: Structure and Function

Central Nervous System (CNS)

Evolution of the Brain

The brain has evolved in complexity from simple vertebrates (rat, cat) to primates (chimpanzee, human). This evolution is marked by the expansion of motor, sensory, and association areas, allowing for advanced functions such as reasoning, memory, and voluntary movement.

• Motor areas: Control voluntary movements.

• Sensory areas: Process sensory input.

• Association areas: Integrate information for higher cognitive functions.

• Example: Human brains have the largest association areas, supporting complex thought and language.

Major Parts of the Brain

The human brain is the control center of the nervous system, weighing about 1.5 kg and protected by the cranium. It consists of four major parts:

1. Cerebral hemisphere

2. Cerebellum

3. Diencephalon

4. Brain Stem

Cerebral Hemisphere

The paired cerebral hemispheres, collectively called the cerebrum, are the largest and most superior part of the brain.

• Gyri: Elevated ridges of tissue.

• Sulci: Shallow grooves separating gyri.

• Fissures: Deep grooves separating large brain regions.

• Longitudinal fissure: Separates the two hemispheres.

• Lobes: Divided by fissures/sulci and named after overlying cranial bones.

The Lobes of the Cerebral Hemispheres

Each hemisphere is divided into lobes, each with distinct functions:

Lobe	Location	Characteristics
Frontal Lobe	Around the forehead	Emotions, reasoning, planning, movement, speech, creativity, judgment
Parietal Lobe	Behind frontal lobes, above temporal lobes	Processes sensory input (touch, pain, pressure, temperature), language functions
Temporal Lobe	Sides of the brain, above ears	Hearing, memory, language, emotional response, auditory processing
Occipital Lobe	Back of the brain	Visual processing, recognition

Regions of the Cerebral Hemisphere

• Cortex of gray matter: Involved in speech, memory, logical and emotional response.

• Internal white matter: Consists of fibers carrying impulses to and from or within cortex.

• Basal nuclei: Islands of gray matter deep within white matter, help regulate voluntary motor activities.

Ventricles

The brain contains four cavities called ventricles, filled with cerebrospinal fluid (CSF):

• Two lateral ventricles (one in each hemisphere)

• Third ventricle (connected to lateral ventricles)

• Fourth ventricle (connected to third ventricle and central canal of spinal cord)

Cerebellum

The cerebellum is located under the occipital lobe and consists of two hemispheres connected by the vermis. It has an outer cortex of gray matter and inner white matter.

• Anatomical lobes: Anterior, posterior, flocculonodular (divided by primary and posterolateral fissures)

• Zones: Vermis (midline), intermediate zone, lateral hemispheres

• Functional divisions:

  • Cerebrocerebellum: Planning movements, motor learning, visual guidance

  • Spinocerebellum: Regulates body movements, error correction, proprioception

  • Vestibulocerebellum: Controls balance, ocular reflexes

• Function: Precise timing of skeletal muscle activity, balance, equilibrium. Damage causes ataxia (clumsy, disorganized movement).

Diencephalon

The diencephalon sits atop the brain stem, enclosed by the cerebral hemispheres. Major structures:

• Thalamus: Sensory responses, integration, determines pleasant/unpleasant sensations

• Hypothalamus: Regulates body temperature, water balance, metabolism, emotions (thirst, appetite, pain, pleasure)

• Epithalamus: Contains pineal gland, sensitive to light, endocrine role

Brain Stem

The brain stem is about 3 inches long and includes:

• Midbrain: Controls eye movement, lens shape, pupil diameter

• Pons: Connects medulla with cerebellum, contains transverse nerve fibers

• Medulla oblongata: Lowest part, connects to spinal cord, controls vital functions

Spinal Cord

The spinal cord is a cylindrical structure (~42 cm long) that continues from the brain stem, protected by the vertebral column and surrounded by CSF.

• Regions: Cervical, thoracic, lumbar, sacral, coccygeal

• Enlargements: Cervical (upper limbs), lumbar (lower limbs)

• 31 pairs of spinal nerves: Arise from the cord, exit vertebral column

• Gray matter: Neuron cell bodies, dendrites, divided into horns (posterior, anterior, lateral)

• White matter: Myelinated axons, divided into columns (ventral, dorsal, lateral), each with sensory/motor tracts

Gray Matter Organization

• Somatic sensory (SS): Axons of somatic sensory neurons

• Visceral sensory (VS): Neurons of visceral sensory neurons

• Visceral motor (VM): Cell bodies of visceral motor neurons

• Somatic motor (SM): Cell bodies of somatic motor neurons

White Matter Pathways

• Ascending fibers: Sensory tracts

• Descending fibers: Motor tracts

• Commissural fibers: Connect opposite sides of cord

Peripheral Nervous System (PNS)

Overview

The PNS consists of nerves and ganglia outside the CNS, connecting the CNS to limbs and organs.

• Cranial nerves: 12 pairs, primarily serve head and neck

• Spinal nerves: 31 pairs, serve body regions

Structure of Nerve

• Endoneurium: Inner sheath

• Perineurium: Middle sheath

• Epineurium: Outermost sheath

• Mixed nerves: Carry both sensory and motor fibers (all spinal nerves)

• Sensory (afferent) nerves: Carry impulses toward CNS

• Motor (efferent) nerves: Carry impulses away from CNS

Additional info:

• These notes cover the structure and function of the nervous system, including the CNS and PNS, relevant to General Biology topics such as animal form and function, nervous system, and sensory systems.