Nervous System: Structure, Function & Classification

Nervous System Overview

• It controls and coordinates all bodily activities

Components of the Nervous System

• Nerve cells/neurons, neuroglia, and loose connective tissue

Functions of the Nervous System

• Gathers sensory input, integrates data, and forms motor output

Classification of the Nervous System

• Central Nervous System (CNS)
  • Brain and spinal cord
• Peripheral Nervous System (PNS)
  • Nerves outside the brain and spinal cord, including cranial nerves, spinal nerves, ganglia, and receptor organ parts

Central Nervous System

• Consists of the brain and spinal cord

Peripheral Nervous System

• Includes cranial nerves, spinal nerves, and the autonomic nervous system

Protection of the Central Nervous System

• Skull and vertebral column, meninges, and cerebrospinal fluid

Meninges

• They are membranes surrounding the brain and spinal cord
• Dura Mater (outer layer):
  • Strong layer composed of two fibrous connective tissue layers
  • These layers separate to form dural sinuses
• Arachnoid Mater (middle layer):
  • Loose membrane separated from the dura mater by the subdural space
  • It is separated from the pia mater by the subarachnoid space, filled with cerebrospinal fluid and large blood vessels
  • Arachnoid villi allow CSF absorption into venous blood
• Pia Mater (inner layer):
  • Fine fibrous connective tissue containing minute blood vessels
  • Adheres to the brain's surface, dipping into fissures

Meningeal Spaces

• Subdural Space: Lies between the arachnoid mater and dura mater
• Subarachnoid Space: Lies between the arachnoid mater and pia mater and contains CSF
• Epidural Space: Lies between the dura mater and vertebral wall, containing fat and small blood vessels

Cerebrospinal Fluid (CSF)

• Watery solution similar to blood plasma and formed by the choroid plexus
• Provides a watery cushion to protect the brain and spinal cord
• It circulates in the arachnoid space, ventricles, and central canal of the spinal cord
• Functions to give resistance to central nervous system organs, protects the CNS from trauma, and nourishes the brain, as well as transporting chemical signals

Brain Divisions

• Forebrain
  • Cerebrum and diencephalon
• Midbrain
• Hindbrain
  • Pons, medulla oblongata, and cerebellum

Cerebrum

• Largest part of the brain with outer gray matter and inner white matter
• It comprises two cerebral hemispheres connected by the corpus callosum
• Each hemisphere consists of four lobes: frontal, parietal, temporal, and occipital

Lobes of the Cerebrum

• Each lobe is separated from others by sulci
• The brain's surface includes sulci and gyri
• Main sulci: central, lateral, and parietooccipital

Function of the Lobes

• Frontal lobe: controls motor function, intellect, and personality
• Parietal lobe: controls sensation
• Temporal lobe: controls hearing and memory
• Occipital lobe: controls vision

Diencephalon

• Sits on top of the brainstem and is enclosed by the cerebral hemispheres
• It is subdivided into four parts: thalamus, hypothalamus, subthalamus, and epithalamus

Brain Stem

• Directly attached to the brain and spinal cord with nerve fibers and nerve cells
• It consists of the midbrain, pons, and medulla oblongata

Functions of the Brainstem

• Midbrain: Integral part of auditory pathways and controls involuntary functions
• Pons: Contains sleep and respiratory centers for breathing regulation, and controls both consciousness and level of concentration
• Medulla Oblongata: It regulates heart rate (cardiac center), blood vessel diameter (vasomotor center), coughing, and sneezing (respiration center), and controls swallowing and vomiting

Cerebellum

• It lies posterior to the pons and the medulla oblongata
• It consists of two cerebellar hemispheres connected by the vermis
• Functions include coordination of movement, control of posture, and equilibrium

Ventricles and Cerebrospinal Fluid (CSF)

• Four connecting cavities within the cerebrum and brain stem are continuous with the spinal cord's central canal
• Filled with CSF
• The Brain Ventricles:
  • Right and left lateral ventricles: within the cerebral hemisphere
  • Third ventricle: lies inside the diencephalon cavity
  • Fourth ventricle: lies inside the brainstem cavity

Brain Ventricles

• Lateral ventricles are in the cerebral hemispheres and horseshoe-shaped
• Third ventricle is in the diencephalon and connected to the lateral ventricles via the interventricular foramen
• Cerebral aqueduct connects the third and fourth ventricles
• Fourth ventricle lies in the hindbrain and connects to the spinal cord's central canal

Spinal Cord

• Long cylindrical part of the lower brain present in the vertebral canal
• Extends from the C1-L1 vertebra, passing through the foramen magnum
• It is the continuation of the medulla oblongata; the lower part is called the conus with a length of 42-45 cm
• There are two enlargements in the cervical and lumbar regions

Spinal Cord Anatomy

• Meninges cover the spinal cord
• There are 31 pairs of spinal nerves present in the intervertebral foramina
• The spinal nerves have dorsal and ventral roots

Spinal Cord Tissue

• Exterior white matter with conduction tracts
• Internal gray matter with cell bodies
  • The gray matter has dorsal (posterior) and anterior (ventral) horns

Spinal Cord Interior

• Central canal filled with cerebrospinal fluid

Peripheral Nervous System: Spinal Nerves

• Spinal nerves arise from the spinal cord and attach by motor anterior and sensory posterior roots, which combine to form a trunk
• These trunks then divide into two anterior and posterior rami
• There are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal

Origin of Cranial Nerves

• 12 cranial nerves in total
• The olfactory nerve (CN I) and optic nerve (CN II) originate from the Cerebrum
• Cranial nerves III – XII arise from the brainstem, from a specific section

Cranial Nerves

• Oculomotor (III) and Trochlear (IV) stem from midbrain
• Trigeminal (V), Abducens (VI), and Facial (VII) stem from the pons
• Vestibulocochlear (VIII) arise in pons
• Glossopharyngeal (IX), Vagus (X), Accessory (XI), and Hypoglossal (XII) are attached to the medulla oblongata

Function of Cranial Nerves

• I. Olfactory: Smell (sensory)
• II. Optic: Vision (sensory)
• III. Oculomotor: Eye muscles (motor)
• IV. Trochlear: Eye muscles (motor)
• V. Trigeminal: Facial sensation (sensory), Chewing (motor)
• VI. Abducens: Eye muscles (motor)
• VII. Facial: Taste (sensory), face muscles (motor)
• VIII. Vestibulocochlear: Balance and hearing (sensory)
• IX. Glossopharyngeal: Taste (sensory), swallowing (motor)
• X. Vagus: Affects sweating, peristalsis, heart rate, and speech and breathing
  • Contains branches in the ear canal
• XI. Accessory Nerve: Controls neck and upper back muscles (motor)
• XII. Hypoglossal: Tongue movement (motor)

Sympathetic Nervous System

• "Fight or flight" response: Increases heart rate, blood pressure, and respiratory rate, causes bronchodilation, and decreases intestinal motility

Parasympathetic Nervous System

• Operates at rest: Lowers heart rate and breathing rate, stimulates intestinal motility and gland secretion, and causes bronchoconstriction

Special Senses

• Taste, smell, sight, hearing, and touch

General Senses of Touch

• Include temperature, pressure, and pain

Taste Buds

• Tiny sensory organs on the tongue, sending taste messages to the brain
• Found in papillae of the tongue mucosa, specifically in fungiform and circumvallate papillae
• The adult human tongue contains between 2,000 and 8,000 taste buds

Sense of Smell

• Olfactory epithelium contains neuron receptors covering the superior nasal concha
• Olfactory receptor cells are bipolar neurons with olfactory cilia

Vision

• 70% of all sensory receptors are in the eye, which is protected by fat and the bony orbit
• Accessory structures: eyebrows, eyelids, conjunctiva, lacrimal apparatus, and extrinsic eye muscles

The Eyeball

• Eye Layers: The outermost coat is composed of the opaque sclera(posteriorly) and clear cornea (anteriorly)
• Intermediate Vascular Tunic (Uvea)
  • Choroid Region: supplies blood to the eye tunics
  • Ciliary Body: composed of smooth muscles that anchor the lens
  • Iris: colored part of the eye controlling pupil and regulating the amount of light that can enter
• Sensory Tunic: Retina: A two-layered membrane with photoreceptors that can transduce light energy
  • Rods: For dim light
  • Cones: For bright light

Composition of the Eyeball

• Anterior chamber: lies between the cornea and iris
• Posterior chamber: lies between the iris and lens
• Eye fluid helps with nutrition and maintaining the natural shape of the lens

The Ear: Hearing and Balance

• Three parts: the inner, outer, and middle ear
• The outer and middle ear are involved with hearing
• Inner ear functions include equilibrium

Outer Ear

• Auricle (pinna), as well as the external auditory canal (short, curved tube filled with ceruminous glands)
• Tympanic Membrane (eardrum): Thin connective tissue vibrating in response to sound
  • Transfers from the outer and middle ear as a boundary

Middle Ear (Tympanic Cavity)

• The air-filled, mucosa-lined cavity connects to the nasopharynx by the Eustachian tube which helps to equalize pressure

Ear Ossicles

• Tympanic cavity containing the malleus, incus, and stapes which transit vibratory motion of the eardrum to the oval window

Inner Ear

• Bony Labyrinth: Contains the vestibule, the cochlea, and the semicircular canals and filled with perilymph
• Membranous Labyrinth: Series of membranous sacs within the bony labyrinth and filled with potassium-rich fluid