Nervous System: Organization and Cells

• Nervous tissue is excitable and conductive, and possesses cells with specialized conductivity

Morphological Divisions of the Nervous System

• The central nervous system contains the brain and spinal cord
• Peripheral nervous system contains cranial nerves (12 pairs), spinal nerves (31 pairs) and the autonomic nervous system

Functional Divisions of the Nervous System

• Somatic efferent division innervates skeletal muscles and skin, relating to the body
• Visceral efferent division innervates smooth and cardiac muscles and glands and comprises the autonomic nervous system

Types of Cells in the Nervous System

• Neurons are active conducting elements and are the parenchymal functional cells
• Neuroglia are supporting elements and consist of astrocytes, oligodendrocytes, microglia, and ependymal cells

Types of Neroglial Cell

• Astrocytes involve the blood-brain barrier
• Oligodendrocytes produce myelin sheath in the central nervous system
• Microglia are phagocytic cells (protecting)
• Ependymal cells line the ventricles in the brain and spinal cord, facilitating flow of cerebrospinal fluid

Neurons and Nerve Impulses

• Neurons are the basic units of the nervous system which conduct electrical impulses
• Dendrites transmit impulses toward the cell body and are an afferent process
• Axons conduct impulses away from the cell body and are an efferent process
• Synapses are the point of connection between neurons
• Myelin Sheath is an insulating material covering axons in the central and peripheral nervous system
• Electrical impulses jump from node to node in the myelin sheath in saltatory conduction
• Node of Ranvier is the periodic gap in the myelin sheath, which facilitates rapid impulse conduction

The Central Nervous System

• Consists of the brain and spinal cord
• The brain is protected within the skull and is a vital part of the central nervous system
• The brain is the most complex and largest mass of nervous tissue, comprising billions of neurons
  • Male brain weighs about 1380 grams
  • Female brain weighs about 1250 grams
• The brain originates from a neural tube, initially forming three swellings

Central Nervous System

• Gray matter has nerve cell bodies and true dendrites, and is in the outer part of the brain
• White matter has myelinated nerve fibers, and is in the inner part of the brain
• Ganglia are clusters of neuron cell bodies outside the central nervous system

Embryological Divisions of the Brain

• The forebrain (prosencephalon) divides into:
  • Telencephalon (cerebrum)
  • Diencephalon (thalamus, hypothalamus, epithalamus, subthalamus)
• The midbrain (mesencephalon) divides into:
  • Corpora quadrigemina (tectum) with superior colliculi involved in visual reflexes
  • Inferior colliculi involved in auditory reflexes
  • Cerebral peduncles
  • Vertebral aqueduct of sylvius (iter)
• The hindbrain (rhombencephalon) divides into:
  • Metencephalon (cerebellum, pons)
  • Myelencephalon (medulla oblongata)

Forebrain (Prosencephalon)

• Cerebrum (telencephalon) is involved in higher functions:
  • Discriminatory identification
  • Integrating sensory information
  • Memory
  • Reasoning
  • Language usage
  • Emotional behavior
  • Initiating movement

Types of Fissures & Sulci of the Brain

• Fissures are deep depressions in the cerebral cortex
• Sulci are shallower grooves in the cerebral cortex

Types of Fissures

• Separating the two cerebral hemispheres is the Longitudinal fissure
• Dividing the frontal and parietal lobes from the temporal lobe is the Lateral (sylvian) fissure
  • Supramarginal gyrus located at the end
• Anterior median fissure is a deep groove along the spinal cord
• Dorsal median fissure is a shallower groove containing the middle cerebral artery
• Separating the frontal lobe from the parietal lobe is the Central sulcus (rolandic fissure)
• Separating the cerebellum from the cerebrum is the Transverse fissure
• Separating the occipital lobe from the parietal lobe is the Parieto-occipital fissure
• Locating the visual center (calcarine area) is the Calcarine fissure

Lobes of the Cerebral Cortex

• The frontal lobe is located anterior to the central sulcus and controls motor activity and personality
• The parietal lobe is located posterior to the central sulcus, and is responsible for sensory processing
• The temporal lobe is located beneath the lateral sulcus, involved in hearing and olfaction
• The occipital lobe occupies the posterior part of the cerebral hemisphere, and is the visual center
• The insula (island of reil) is exposed when the lateral sulcus is separated

Functional Areas of the Cerebrum

• Primary motor area (pre-central gyrus) located in the frontal lobe and controls voluntary movements
  • Also known as Brodmann's area 4
• Pre-motor area coordinates a series of movements, and is located anterior to the motor area
• Pre-frontal area is responsible for behavior, character, emotional state, foresight, good judgement, and abstract thinking
• Primary sensory/somesthetic area (post-central gyrus) processes sensations of pain, temperature, pressure, touch etc
  • Also known as Brodmann's areas 3, 1, 2
• Motor speech area controls muscles for expression and is located in the inferior frontal gyrus
  • Also known as Brodmann's areas 44 (pars opercularis), 45 (pars triangularis), or Broca's area (Brodmann's areas 44&45)

Diencephalon

• Thalamus relays sensory impulses (excluding olfactory)
• Hypothalamus regulates temperature, feeding, biorhythms and emotions
• Corpus Callosum links cerebral hemispheres
• Cingulate gyrus is located above the corpus callosum
• Cingulate sulcus is a depression above the cingulate gyrus

Cerebellum

• Structure is oval-shaped with a central constriction and expanded lateral portions
• Cerebellum lobes are the:
  • Anterior lobe for musclue tone
  • Posterior lobe for voluntary motor activity
  • Flocculonodular lobe for equilibrium, posture and balance

Midbrain (Rhombencephalon)

• The midbrain links the forebrain and hindbrain for motor coordination
• Midbrain components:
  • Cerebral peduncles are cylindrical nerve fiber tracts connecting the forebrain to the hindbrain
  • Corpora quadrigemina involved in visual and auditory reflexes

Hindbrain (Prosencephalon)

• Pons is anterior to the cerebellum, it is a bridge-like structure that links various parts of the brain
• Medulla oblongata is continuous with the spinal cord, and lies ventral to the cerebellum
• The medulla oblongata contains vital regulatory and reflex centers controlling the circulatory system, breathing, swallowing, vomiting, coughing, and sneezing

CSF

• Cerebrospinal Fluid circulates within the brain's ventricles, spinal cord, and subarachnoid space
• It is produced by the choroid plexus, and provides nutrition with immune functions
• It is similar in composition to lymph, and absorbed through arachnoid villi

Ventricles of the Brain

• Lateral ventricles communicate with the third ventricle via the interventricular foramen
• The third ventricle is positioned between thalamic nuclei and continuous with cerebral aqueduct
• The fourth ventricle communicates with the subarachnoid space through the foramen of Luschka and Magendie

Meninges

• The meninges protect the brain & spinal cord
  • Dura mater is the outermost layer
  • Arachnoid mater is the middle layer
  • Pia mater is the innermost layer
• The Spinal cord is continuous with the medulla oblongata and extends from the foramen magnum to the first lumbar vertebra, about 45cm

Spinal cord enlargements

• Cervical enlargement supplies nerves to the arms, C4 to T2
• Lumbar enlargement supplies nerves to the legs, widest at T12
• Conus medullaris is the tapering part of the spinal cord

Peripheral Nervous System (PNS)

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

Cranial Nerves

• There are 12 pairs of cranial nerves attached to the brain
• The types of cranial nerves are:
  • Olfactory (I) - (S) for smell, with damage causing anosmia (loss of smell)
  • Optic (II) - (S) for vision, with damage causing blindness
  • Oculomotor (III) - (M, PS) for eye movements and parasympathetic control of the iris
  • Trochlear (IV) - (M) thinnest nerve, for superior oblique muscle
  • Trigeminal (V) - (Mx) with ophthalmic, maxillary, mandibular branches; sensory to face, motor to mastication muscles
  • Abducens (VI) - (M) for the lateral rectus muscle, damage impairs eye movement
  • Facial (VII) - (Mx, PS) for facial expression, parasympathetic lacrimal and salivary glands (except parotid) damage effects taste and facial expression
  • Vestibulocochlear (VIII) - (S) for hearing and balance
  • Glossopharyngeal (IX) - (Mx, PS) for taste, pharynx muscles, and parasympathetic control of the parotid gland
  • Vagus (X) - (Mx, PS) longest nerve, for internal organs, laryngeal and pharyngeal muscles, and parasympathetic to smooth muscles in organs
  • Accessory (XI) - (M) innervates the sternocleidomastoid and trapezius muscles
  • Hypoglossal (XII) - (M) controls tongue muscles, damage impairs movement

Nerve Classifications

I, II, and VIII are sensory nerves III, IV, VI, XI and XII are motor nerves V, VII, IX, and X are mixed nerves The spinal cord produces 31 pairs of spinal nerves emerging from intervertebral foramina

Spinal Nerves

• Dorsal roots contain sensory neuron fibers
• Ventral roots contain motor neuron fibers
• Spinal nerve distribution
  • 8 cervical
  • 12 thoracic
  • 5 lumbar
  • 5 sacral
  • 1 coccygeal -Plexuses include nerve fiber networks from ventral rami of certain spinal nerves

General Vs Special Senses

• General: touch, pressure, proprioception, temperature, and pain
• Special: smell, taste, vision, hearing, and equilibrium

Sensory Receptors

• Mechano-, chemo-, and photoreceptors respond to mechanical, chemical, and light stimuli