Nervous System: Sensory, Integration, and Motor Functions

• Nervous tissue consists of cells that detect, transmit, and store information in the form of electrical impulses, along with cells that aid in performing these tasks.
• Neurology studies the nervous system and its disorders.
• Nervous tissue is functionally organized into the Nervous System.
• The three basic functions of the nervous system are sensory input, integration, and motor output.
• Sensory function detects changes in the environment, either externally or internally.
• Integrative function analyzes sensory information, stores some of it, and decides on appropriate responses.
• Motor function responds to stimuli via muscle contraction and/or glandular secretion.
• The nervous system is structurally divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).
• The Central Nervous System (CNS) includes the brain and spinal cord.
• The Peripheral Nervous System (PNS) includes nerves and sensory receptors, divided into afferent and efferent components.
• The PNS has two functional components.
• The Afferent (sensory) Nervous System transmits sensory information from receptors to the CNS.
• Somatic sensory transmits information from skin, muscles, bones, and joints.
• Visceral sensory transmits information from internal organs and body cavities.
• The Efferent (motor) Nervous System transmits responses from the CNS to muscles and glands.
• Somatic motor controls skeletal muscles voluntarily.
• Visceral motor controls cardiac, smooth muscles, and glands involuntarily, and is also known as the Autonomic Nervous System.
• The Autonomic Nervous System has sympathetic and parasympathetic divisions.
• The sympathetic division is for emergency responses or "Fight or Flight".
• The parasympathetic division is for non-emergency situations, such as normal everyday body functions like "Rest & Digest".

Cells of the Nervous System

• Neurons are electrically excitable cells that conduct electrical impulses, known as action potentials.
• Neurons have a limited ability to repair themselves when damaged and cannot divide.
• Neurons are some of the largest and most complex cells in the body.
• Neurons have three parts: cell body, dendrites, and axon.
• The cell body (soma) contains a large nucleus with a prominent nucleolus.
• The cell body (soma) contains abundant rough ER and polyribosomes, viewed as Nissl bodies.
• Cell bodies (soma) contain many mitochondria.
• Cell bodies also contain cell inclusions called lipofuscin, which are products of enzymatic degradation.
• The cytoskeleton of the cell body is made of neurofibrils, including microfilaments, neurofilaments, and microtubules.
• Microfilaments are the smallest, made of actin.
• Neurofilaments are intermediate in size.
• Microtubules are the largest.
• Dendrites receive stimuli and conduct action potentials towards the cell body.
• Axons conduct action potentials away from the cell body; each neuron has only one axon.
• Axons begin at the axon hillock, where the action potential is initiated.
• Axons contain smooth ER and mitochondria, but no ribosomes.
• Axons may be myelinated (conducts impulses faster) or unmyelinated (conducts impulses slower).
• The end of an axon is called the axon terminal.
• Synaptic end bulbs at the tip of the axon terminal store a neurotransmitter; over 100 have been identified.
• When an action potential reaches the axon terminal, it causes the neurotransmitter to be released into a synapse between the neuron and an adjacent cell.
• The neurotransmitter diffuses across the synaptic space and binds to receptors on the adjacent cell, which could be another neuron, a muscle cell, or a gland cell.
• This may or may not generate another action potential in that cell, depending on the cell's receptors and/or the neurotransmitter.

Functional Classification of Neurons

• Sensory neurons transmit information to the CNS.
• Motor neurons transmit information from the CNS.
• Interneurons are neurons not specifically sensory or motor, forming connections between sensory and motor neurons or performing special functions (e.g., brain neurons).
• Most neurons in the body are functionally classified as interneurons.

Structural Classification of Neurons

• Multipolar neurons have many dendrites and one axon, including all motor neurons and most neurons in the brain and spinal cord.
• Most neurons in the body are structurally classified as multipolar.
• Bipolar neurons have one primary dendrite and one axon (e.g., olfactory neurons in the nose and some neurons in the retina of the eye); they are rare.
• Unipolar (pseudounipolar) neurons have a short dendrite and a long axon that have fused to form a continuous process that "bypasses" the cell body (e.g., most sensory neurons).
• Anaxonic neurons have many dendrites but no axon; they are rare and found in the retina and brain.

Neuroglial Cells

• Neuroglial cells physically and physiologically support neurons.
• They do not conduct action potentials.
• Neuroglial cells are small cells capable of cell division.

Types of Neuroglial Cells

• Astrocytes aid neurons with metabolism and form the blood-brain barrier around capillaries.
  • There are two types: protoplasmic (gray matter) and fibrous (white matter).
  • Astrocytes are the most abundant glial cell in gray matter.
• Microglial cells are CNS macrophages that phagocytize foreign pathogens and cellular debris.
• Ependymal cells are cuboidal to low columnar cells that line spaces inside the brain and spinal cord.
  • They secrete cerebrospinal fluid (CSF), which provides a stable chemical environment for neurons.
• Oligodendrocytes myelinate axons in the CNS and are the most abundant glial cell in white matter.
  • One oligodendrocyte can myelinate several axons.
• Schwann cells (neurilemmocytes) myelinate axons in the PNS and form the myelin sheath (neurilemma).
  • Each Schwann cell can only myelinate one axon at a time.
• Satellite cells support/protect clusters of neuron cell bodies in the PNS.
• Myelin is a lipid produced in the cytoplasm of oligodendrocytes and Schwann cells.
• These cells "wrap" themselves around the axons of neurons to form myelin sheaths.
• Myelinated neurons conduct action potentials much faster than non-myelinated neurons, accounting for the appearance of “white matter” in the brain and spinal cord.
• Gaps between adjacent myelin sheaths are called Nodes of Ranvier; the action potential “jumps” from node to node as it is conducted down the axon referred to as saltatory conduction.
• A nerve is a bundle of axons in the PNS.
• A tract is a bundle of axons in the CNS.
• A ganglion is a group of nerve cell bodies in the PNS.
• A nucleus is a group of nerve cell bodies in the CNS.
• Nerves have connective tissue components similar to that of skeletal muscle.
  • The epineurium is a dense irregular connective tissue that surrounds the entire nerve (holds axons together).
  • The perineurium is a dense irregular connective tissue that surrounds bundles of axons within a nerve.
  • The endoneurium is a loose connective tissue that surrounds an individual axon and its myelin sheath.

Central Nervous System

• The central nervous system consists of the brain and spinal cord.

Spinal Cord

• The spinal cord lies within the vertebral canal, where the vertebrae offer protection.
• The spinal cord is encircled by three connective tissue coverings called meninges: dura mater, arachnoid mater, and pia mater.
  • The dura mater is the "tough mother"; a dense connective tissue and the strongest of the three coverings, surrounded by the epidural space (between it and the vertebrae), which is mostly filled with fat.
  • The arachnoid mater is a thin, spider-web-like layer attached to the underside of the dura.
  • It sends out thin fibers that form connections with the pia mater (in the brain), and the space between the arachnoid and pia is called the subarachnoid space, which is filled with CSF.
  • The pia mater is the "delicate mother"; a very thin, translucent layer that lies directly on the spinal cord.
  • It contains lots of blood vessels.
  • The pia mater also forms denticulate ligaments, which attach to the dura mater and help to hold the spinal cord in place.

Internal Structure

• Gray matter is located internally and is in the shape of an "H" or butterfly, containing unmyelinated axons, neuronal cell bodies, and glial cells.
  • The "upper" arms of the "H" are called dorsal horns, containing axons of incoming sensory neurons.
  • The "lower" arms of the "H" are called ventral horns, containing cell bodies and axons of outgoing motor neurons.
  • The lateral horns (middle of the "H") contain autonomic cell bodies and axons.
• White matter is located externally, surrounding the gray matter, and contains myelinated axons with oligodendrocytes and Schwann cells.
  • It is divided into ascending (sensory) and descending (motor) tracts.
• The space in the center is called the central canal, lined with ependymal cells, but they do not make CSF.
• Axons of sensory neurons entering the cord are called dorsal roots; the cell bodies of these unipolar neurons are grouped together in “swellings” just outside the cord called dorsal root ganglia.
• Axons of motor neurons leaving the cord are called ventral roots.

Brain

• The brain consists of the brainstem, diencephalon, cerebrum, and cerebellum.
• It is divided into an outer cortex (gray matter) and a deeper medulla (white matter).
• Fluid-filled spaces inside the brain are called ventricles.
• The walls of these spaces are lined with a tissue network called the choroid plexus, which produces cerebrospinal fluid, a filtrate of blood plasma.
• Gray matter is located externally in the cerebrum and cerebellum, where it is called the cortex.
  • It is thrown into folds called gyri (cerebrum) or folia (cerebellum) and grooves (sulci & fissures).

Cerebral Cortex

• The cerebral cortex has six layers (from superficial to deep).
• The Molecular (or Plexiform) Layer contains neuronal axons, dendrites, and horizontal cells.
• The Outer (External) Granular Layer contains granular (stellate) cells.
• The Outer (External) Pyramidal Layer contains small pyramidal cells and is the thickest layer.
• The Inner (Internal) Granular Layer contains glial cells and stellate cells.
• The Inner Pyramidal (or Ganglionic) Layer contains large pyramidal cells, which are the main output cells of the cerebrum, and is the thinnest layer.
• The Multiform Layer contains a variety of cells.

Cerebellar Cortex

• The cerebellar cortex has three layers.
  • The Molecular Layer contains a few stellate cells and basket cells (deep).
  • The Purkinje Layer is a thin layer that contains Purkinje cells (very large cells), which are the main output cells of the cerebellum.
  • The Granular Layer contains many granule cells (dark) and Golgi cells (light).