Nervous System Anatomy

• Chapter 13 covers the anatomy of the nervous system and its functions

Learning Objectives

• Identify brain landmarks and regional functions.
• Review other brain areas' functions and how neuronal circuits process data.
• Understand the gross and cross-sectional anatomy of the spinal cord.
• Differentiate between ascending and descending tracts in the spinal cord
• Identify and describe the protective structures of the brain, along with their functions.
• Identify the features of nerve structure and related connective tissues like epineurium, perineurium, endoneurium, fascicles, and the nerve itself.
• List the cranial nerves by name and number and their specific functions.
• Understand how spinal nerves are arranged and organized.
• List the four spinal nerve plexuses and name nerves emerging from each.

Central Nervous System Components

• The central nervous system includes the brain and the spinal cord.

Adult Brain Regions

• The adult brain is composed of the cerebrum, the diencephalon, the brain stem, and the cerebellum

Brain Surface Features

• The brain surface is marked by ridges (gyri) and valleys (sulci).
• A fissure is a deep sulcus that is sometimes branching.
• Folds increase the surface area, allowing more grey matter to fit.

Major Fissures and Sulci

• The longitudinal fissure separates the left and right cerebral hemispheres.
• The central sulcus separates the frontal and parietal lobes.
• The lateral sulcus separates the parietal and temporal lobes.
• The transverse fissure separates the cerebrum and cerebellum.

Lobes of the Cerebrum

• The lobes of the cerebrum include the frontal lobe, parietal lobe, occipital lobe, and temporal lobe.
• The insular lobe is deep to the central sulcus.

Cerebral Cortex Considerations

• The cerebral cortex has three functional areas: motor, sensory, and association.
• Each hemisphere controls the contralateral side of the body.
• Cortical function lateralization can occur in just one hemisphere.
• Conscious behavior involves the entire cortex.

Motor Areas of the Frontal Lobe

• The primary motor cortex is in the precentral gyrus.
  • It generates conscious, specific musculoskeletal movement.
• The premotor cortex is anterior to the precentral gyrus.
  • It coordinates complex motor tasks like playing the piano or typing.
• Broca's area controls motor movement for speech and is typically in the left hemisphere.
• The frontal eye field controls eye movement.

Somatosensory Areas

• The primary somatosensory cortex is in the postcentral gyrus.
  • It receives somatosensory information from skin and proprioceptors.
  • Proprioception includes sensations from skeletal muscle, joints, and tendons, giving body position information.
• The somatosensory association cortex is posterior to the postcentral gyrus.
  • It integrates sensory input from the primary somatosensory cortex.
• The visceral sensory area is in the insula lobe.

Special Sensation Areas

• The occipital lobe contains the primary visual cortex and visual association area.
• The temporal lobe has the primary auditory cortex, auditory association area, and primary olfaction cortex.
• The insula lobe includes the gustatory and vestibular cortices; the latter is also in the parietal lobe.

Anterior Association Area

• The anterior association area is in the prefrontal cortex.
  • It involves intellect, complex learning (cognition), recall, and personality.
  • Working memory, abstract ideas, judgment, reasoning, persistence, and planning are all involved.
  • This area matures slowly during development.

Posterior Association Area

• The posterior association area includes parts of the temporal, parietal, and occipital lobes.
  • It recognizes patterns and faces, locates us and our surroundings in space, and interprets multiple stimuli simultaneously.
  • Wernicke's area is responsible for interpreting written and spoken language.

Limbic Association Area

• The limbic association area is the emotional-visceral brain.
  • It responds to perceived threats and the sense of danger as part of the limbic system.
  • It influences emotion, motivation, memory, and learning.

Hemispheric Specialization

• The left hemisphere specializes in speech, language-based skills, and analytical skills.
• The right hemisphere specializes in intuition and analyzing the emotional context of a conversation.
• In 90% of individuals, the left hemisphere controls language, math, and logic.
• In 10% of people, the roles of hemispheres are switched or equally shared.

Diencephalon

• The thalamus, hypothalamus, and epithalamus are parts of the diencephalon.
• The thalamus makes up 80% of the diencephalon.
  • It is a relay station for sensory information coming into the cerebral cortex and routes incoming information to the right spot.
  • The thalamus is a gateway to the cortex, mediating sensation, motor activities, cortical arousal, learning, and memory.
• The hypothalamus is the main visceral control center of the body; it regulates autonomic nervous system and endocrine system
  • It is responsible for regulating body temperature, food intake, water balance, thirst, and sleep-wake cycles.
  • It initiates physical responses to emotions.
• The epithalamus containing the pineal gland which secretes melatonin.
  • It works together with the hypothalamus to control the sleep-wake cycle.

Brain Stem

• The brain stem includes the midbrain, pons, and medulla oblongata.
• The midbrain contains the corpora quadrigemina which are visual and auditory reflex centers.
• The pons contains ascending and descending tracts
  • It serves as a pathway between higher brain centers and the spinal cord.
  • It relays motor cortex and cerebellum information.
  • The pons regulates rhythmic breathing along with the medulla oblongata.
• The medulla oblongata is the autonomic reflex center.
  • The cardiovascular reflex center adjusts force and rate of hear contraction
    • The vasomotor center regulates blood pressure by changing vessel diameter
  • The respiratory reflex center controls respiratory rhythm, rate and depth of breathing.
  • It is related to other reflexes such as vomiting, hiccupping, swallowing, coughing, and sneezing

Cerebellum

• The cerebellum represents 11% of the total brain mass.
  • Input from the cerebral motor cortex, brain stem nuclei, and sensory receptors gets processed.
    • Timing and patterns of skeletal muscle contraction get provided for smooth, coordinated movements and agility.
  • The cerebellum is responsible for balance and posture, speech, driving, typing, and playing a musical instrument.

Spinal Cord Functions

• The spinal cord facilitates two-way communication between the brain and the body.
• It contains spinal reflex centers for integration.
• The spinal cord starts at the foramen magnum and ends at the conus medullaris (L1 vertebra).
  • The filum terminale anchors it.
• The cauda equina transmits information below L1
  • A collection of nerve roots found at the bottom of the vertebral canal.

Spinal Nerves

• There are 31 pairs of spinal nerves in PNS.
  • Each splits into dorsal and ventral rami.
• They are named and numbered based on the related vertebral region and vertebrae.
• Spinal nerves carry both sensory and motor information.
• Plexuses (interlacing nerve networks) are formed. -There are four plexuses.
  • T1 - T12 are not part of a plexus.

Dorsal and Ventral Rami

• The dorsal ramus innervates the skin and muscles of the back.
• The ventral ramus supplies the anterior body wall and limbs.
• The rami communicantes are part of the sympathetic division in the thoracic/lumbar regions.

Nerve Plexuses

• Only ventral rami form plexuses.
• The nerve plexus is a network of braided nerves
• The four plexuses are cervical, brachial, lumbar, and sacral.
• Each muscle in a limb receives its nerve supply from more than one spinal nerve.

Spinal Cord Structure

• Protection: Vertebrae (bone), Meninges, Cerebrospinal fluid, Epidural space
• Gray matter can be divided into dorsal horn, lateral horn, and ventral horn
• White matter can be divided into dorsal funiculus, lateral funiculus, and ventral funiculus

Information Transmission in Spinal Cord

• Nerves can carry both sensory and motor
• Dorsal root ganglion relates to unipolar sensory neurons
• Dorsal root contains axons of unipolar sensory
• Ventral contains exons of motor neurons

Nerve Structure

• A nerve is a cord-like organ of the PNS.

  • It contains myelinated and unmyelinated peripheral axons enclosed by connective tissue.
  • Blood vessels and nervous tissue are present within.

• Endoneurium

  • Loose connective tissue
  • Surrounds individual axons and their myelin sheaths.

• Perineurium

  • Coarse connective tissue.
  • Surrounds individual fascicles (bundles of axons).

• Epineurium

  • Tough fibrous sheath.
  • Surrounds a nerve
    • Nerve: bundles of fascicles.

Classifying Nerves

• Nerves carry sensory, motor, or and mixtures of both.
• Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic fibers where pure sensory and/or purely motor nerves are rare
• Types of fibers in mixed nerves: -Somatic afferent and somatic efferent -Visceral afferent and visceral efferent
• Peripheral nerves classified as cranial or spinal nerves: -Spinal nerves connect to the spinal cord -Cranial nerves connect to the brain or brain stem

Neuronal Pathways

• The neural pathways are characterized by:
  • Decussation (crossing over in the CNS).
  • Relaying information in 2-3 neuron chains.
  • Somatotopy (mapped to the body’s structure).
  • Symmetry (paired symmetrically).

Brain Pathways or Tracts

• Pathways connect different brain areas, brain and spinal cord, and CNS and PNS.
• There are many types and functions including:
  • Association fibers (connect different hemisphere parts).
  • Commissural fibers (connect gray areas of the two hemispheres).
  • Projection fibers (ascend from lower brain and/or cord centers and descend from the cerebral cortex).

Brain Protective Features

• Protective skull bones.
• Meninges -The meninges consist of 3 layers of tissue (membranes) that continuously surround the brain and spinal cord
• Functions: Covers, protects, and isolates the CNS; Protects blood vessels and encloses venous sinuses ; Contains cerebrospinal fluid; Forms partitions in the skull to limit brain movement
• Clinical relevance: Meningitis (bacterial/viral infection of meninges)
  • If it spreads to the brain it is know as encephalitis

Meninges- 3 Layers

• Arachnoid mater - Middle layer with web-like extensions -Separation from pia mater by subarachnoid space -Filled with CSF
• Dura mater -Most superficial layer -Strongest layer -Two layers of fibrous connective tissue: - Periosteal layer -Meningeal layer -Subdural space separates the dura and arachnoid mater
• Pia mater -Innermost layer -Most fragile layer -Delicate vascularized connective tissue that clings to the brain

Cerebrospinal Fluid

• Cerebrospinal fluid is produced by ependymal cells in the choroid plexus.
• The function are to provide the cushion of the brain, help float the brain, and helps circulates nutrients
• Choroid Plexus -Cluster of capillaries on the roof of each ventricle enclosed by pia mater -Surrounded by layer of ependymal cells -Ependymal cells circulate the CSF -Plasma filtrate: amount- 150 ml and replaced every 8 hours.

Ventricular System

• Ventricles are hollow cavities filled with CSF
• Ependymal cells line cavities that run continuously in both brain and spinal cord
• Lateral ventricle
• Third ventricle
• Fourth ventricle

Hydrocephalus

• Hydrocephalus is clinical relevance for ventricular system =Occurs when obstruction blocks CSF circulation or drainage and resulting in increased pressure - Newborns- increased pressure causes head to enlarge -Adults- rigidity skull keeps pressure within and could potential cause brain damage -In order to treat this- CSF is drained and ventricular shunt is taken into abdominal cavity

The Blood- Brain Barrier

• Astrocyte Feet
• Capillary - Tight junction between endothial -Key structure if blood brain barrier