The Nervous System

The nervous system is the most complex system of the body, yet it is very conservative in terms of change
The nervous system determines responses of the body to changes in internal and external environments
It also acts as a messenger and coordination system for the body
The primary components of the nervous system are:

• the central nervous system - the brain and spinal cord
• the peripheral nervous system - the cranial and spinal nerves
All parts of the nervous system are composed of a common cellular subunit - the neuron

Neurons
Neurons are derived from ectoderm from the neural tube, neural crest cells, or ectodermal placodes.
The parts of the neuron are (Fig. 16.2, p. 590):

  • Cell body (trophic) - contains the nucleus and metabolic machinery of the cell
  • Dendrites (receptive) - form extensions into tissues that may synapse to one or many other neurons
  • Axon (conductive) - a long cytoplasmic process also called the nerve fiber; can transmit nerve impulses over a long distance (up to 1 m) without diminution of the amplitude of the signal
  • Telodendria (transmissive) - the terminal branches of an axon; make contact with other neurons at synapses
  • Ganglia - groups of neuron cell bodies that lie peripheral to the central nervous system in vertebrates
  • The types of neurons in the body include:
  • Bipolar neurons - cell body located near the middle of the axon; characteristic of retinal neurons
  • Unipolar neurons - cell body lies off to one side of axon; characteristic of sensory neurons
  • Multipolar neurons - cell body lies very close to dendrites; characteristic of motor neurons
  • Transmission of an action potential
    The nerve impulse is an electrical phenomenon that passes as a wave along the surface membrane of a nerve fiber
    Normally the nerve cell is at resting potential, based on the concentration of sodium and potassium ions inside and outside of the cell
    During an action potential, the neuron goes through several steps: 1. High concentration of sodium ions outside; negative charge inside cell compared to outside
    2. Axon stimulated, ion channels open causing action potential; sodium ions rush into cell, membrane potential reversed and membrane is depolarized
    3. Nerve impulse travels down the axon as a wave of depolarization
    4. Sodium pumped outside of cell and resting potential restored
    Between neurons, nerve impulse must travel across a space or synapse - the telodendria contains synaptic vesicles that contain neurotransmitters (e.g., acetylcholine, noradrenaline, serotonin, dopamine)
    Neurotransmitters are released when the nerve impulse reaches the telodendria, and then cross the synaptic cleft to reach the dendrites of the next neuron in line causing the impulse to be sent by the postsynaptic nerve cell

    Other cells of the nervous system:

    Neuronal circuitry
    The general parts of the neuronal circuitry include three basic types of neurons: Primary sensory neurons, or afferent neurons: carry impulses from free nerve endings or receptor cells into the central nervous system
    Motor, or efferent neurons: carry impulses from the central nervous system to effectors, such as muscles or glands
    Interneurons: receive signals from sensory neurons, integrate information and send signals to motor neurons
    Somatic fibers relate to the skin and its derivatives, and to voluntary muscles
    Visceral fibers related to involuntary muscles and glands of the organ systems
    Spinal cord and spinal nerves
    In the simplest reflex arc, messages from receptor organs are transferred within the spinal cord directly from afferent fibers to efferent fibers, which then send appropriate messages to effector organs
    The function of the spinal cord is to receive incoming impulses, integrate and coordinate them, transmit them to wherever they should go within the central nervous system, and send responses to the peripheral nervous system as appropriate
    The general structure of the spinal cord is best exemplified by a cross-section of the spinal cord of an amniote (Fig. 16.7, p. 593): • the grey matter lies on the interior of the cord while white matter lies on the exterior
    • the grey matter resembles the letter H, with the upper arms called the dorsal columns or horns, and the lower arms called the ventral columns or horns
    • the grey commissure makes up the cross arm of the H and transmits fibers from one side of the cord to the other
    • the external white matter is divided into right and left sides by the dorsomedian sulcus and the ventromedian fissure
    • the dorsal horn of the cord receives terminations of primary sensory neurons
    • the ventral horn contains the dendrites and cell bodies of motor neurons
    Three types of neuronal pathways are common to all vertebrates: Reflexes Involve only sensory, motor, and interneurons of the spinal cord which constitutes a three-neuron reflex arc - helps the body to perform a rapid integrated movements that requires that certain muscles contact with the appropriate force at the appropriate time
    Other reflexes include intersegmental reflexes that involve neurons that decussate on the other side of the body - decussation is the crossing of neuronal tracts in the midline of the central nervous system - intersegmental reflexes are responsible for maintaining coordinated movements such as swimming or walking.
    Conditioned reflexes are innate reflexes that become elaborated as a result of an animal’s repetitive experiences
    Pathways from lower to higher brain centers Utilize ascending tracts in the spinal cord - most ascending impulses decussate along the way Pathways from the brain to lower centers Impulses utilize descending tracts in the spinal cord, which may decussate in the brain prior to traveling to the appropriate muscle
    The central pattern generators, groups of neurons in the spinal cord and in the brain, whose activity is responsible for innate cyclical movements of body parts - central pattern generators do not require continued sensory input in order to cause a response
    Spinal nerves usually attach by roots to the spinal cord
    In more primitive species, the dorsal and ventral roots form separate dorsal and ventral spinal nerves
    In all other vertebrates, dorsal and ventral nerves unite to form single spinal nerve, with sensory fibers entering through the dorsal root (ramus) and motor fibers leaving through the ventral root (ramus)
    Spinal nerves are defined by their location and include cervical, thoracic, lumbar, sacral and caudal nerves - the more caudal spinal nerves form a bundle known as the cauda equina

    Plexuses are networks of nerves or blood vessels formed before nerves are distributed to the muscles

    • the cervical plexus supplies the ventral neck muscles
    • the brachial plexus supplies the pectoral appendage
    • the lumbosacral plexus supplies the pelvic appendage
    • the coccygeal plexus supplies some of the other pelvic muscles
    Cranial nerves
    Whereas spinal nerves are uniform in occurrence, configuration of roots and branches, nerve fiber components and relation to the central nervous system, cranial nerves are not
    Cranial nerves can be present in some vertebrates and missing in others, may split inn the course of evolution to become two, or fuse to become one
    Serial homology (e.g., segmentation or metamerism) is less evident in cranial nerves than in spinal nerves
    Cranial nerves are classified in one of three general categories: 1) In series with dorsal roots of spinal nerves • join the brainstem at a lateral (not ventral) level
    • include mixed nerves that contain a combination of sensory and motor neurons
    • includes nerves V, VII, IX, X, XI
    2) In series with ventral spinal nerves • join the brainstem at the ventral level
    • contain somatic motor fibers, and supply branchiometric muscles, so sometimes are called branchiometric nerves
    • includes nerves III, IV, VI, XII
    3) No counterpart in spinal series because its nerves serve structures that are peculiar to the head (nose, eye, ear, lateral line system) • sensory fibers
    • includes nerves 0, I, II, VIII
    The cranial nerves are as follows:
  • 0 - Terminal nerve: part of chemosensory system, such as for responding to olfactory pheromones. Absent in cyclostomes, birds, and humans.
  • I - Olfactory: runs from the olfactory epithelium to the olfactory bulb of the brain.
  • II - Optic: runs from the eye to the brain. Ganglion cells in the retina may cross over under the brain at the optic chiasma.
  • III - Oculomotor: supplies external ocular muscles (dorsal rectus, medial rectus, ventral rectus, ventral oblique). Has ciliary branch that passes to muscles of the iris and ciliary muscles.
  • IV - Trochlear: supplies the dorsal oblique muscle of the eye.
  • V - Trigeminal: has three branches: opthalmic (serves the head region), maxillary (serves the upper jaw) and mandibular (serves the lower jaw). Where the branches intersect and cell bodies are found is called the semilunar ganglion.
  • VI - Abducens: supplies the lateral oblique muscle of the eye.
  • VII - Facial: associated with the spiracle and derivatives of the hyoid arch. Serves muscles responsible for facial expression. Component also serves tear glands and salivary glands. Ganglion called geniculate ganglion where palatine, hyomandibular, opthalmic and buccal branches meet.
  • VIII - Statoacoustic or vestibulocochlear or auditory: serves the inner ear. The anterior branch serves the organ of equilibrium, while the posterior branch is responsible for equilibrium and hearing.
  • IX - Glossopharyngeal: associated with pharynx, taste buds and salivary gland. The visceral sensory fibers of the glossopharyngeal have the petrosal ganglion, while the somatic sensory fibers have the superior ganglion.
  • X - Vagus: contains four branches that supply the branchiometric muscles of the 4 - 7 visceral arches (or their derivatives)
  • XI - Accessory
  • XII - Hypoglossal: serves hypobranchial muscles of the throat and tongue
  • Nerves XI and XII are called occipital nerves because they are only considered to be distinct cranial nerves in tetrapods (Figure 12-15, p. 478 in text)

    Autonomic nervous system
    The autonomic system is not isolated structurally or functionally from the central or peripheral nervous system. However, it relates exclusively to involuntary functions of the body. Includes only visceral fibers.

    Distinctive features of the autonomic nervous system are:

    1. Every pathway includes a neuron having its cell body inside the CNS and a neuron cell body outside CNS.
    Fibers between ganglia and CNS are preganglionic and myelinated. Fibers between ganglia and end organs are postganglionic and unmyelinated
    2. Divisible into several sets of fibers Sympathetic - part of autonomic nervous system that leaves CNS from parts of spinal cord. Activity of the sympathetic nervous system helps an animal adjust to stress by promoting physiological processes that increase energy available to body tissues. Also called thoracolumbar outflow.
    Parasympathetic - part of the autonomic nervous system that leaves CNS from cranial and sacral nerves. Promotes metabolic processes that produce and store energy. Also called craniosacral outflow.
    Enteric - Complex net formed by neurons located within the wall of the gut. Activated directly by local physical and chemical stimuli and mediate local reflexes.
    In mammals, postganglionic parasympathetic fibers secrete acetylcholine and are called cholinergic fibers. Postganglionic sympathetic fibers (which elicit the "fight or flight" response) secrete noradrenaline (norepinephrine) and are called adrenergic fibers. The effects of stimulation of the two systems are shown in Table 12-3, p. 481 in the text, and in the figure of the mammalian autonomic nervous system in the handout.

    Brain
    The brain is the most complicated organ in the body, and a complex, fully-formed brain is one of the derived characteristics of vertebrates.

    Development
    By the time the neural folds close over the neurocoel, the anterior portion of the neural tube has begun to increase in diameter and become distinguished from the remainder of the neural tube
    The developing brain expands at three levels as vesicles separated from each other by constrictions:

    prosencephalon - forebrain
    mesencephalon - midbrain
    rhombencephalon - hindbrain
    These three regions then separate into several additional regions
    The anterior part of the prosencephalon develops into The prosencephalon is also the location of the formation of the optic vesicles, and the infundibulum, which will form part of the pituitary gland or neurohypophysis
    The telencephalon will also differentiate anteriorly to form the olfactory bulbs
    The rhombencephalon forms an anterior metencephalon (which will form the adult cerebellum) and a posterior myelencephalon

    Within the division of the brain are cavities called vesicles, which later grow to form expansions or ventricles.

    Although most brains have nearly straight axes, the brain of embryos of birds and mammals acquires three flexures For the adult brain of vertebrates we use three primary subdivisions: the cerebrum, the cerebellum and the brainstem.

    The brainstem is the first region to form in development, is the least variable, and receives all the cranial nerves (except for the terminal and olfactory nerves). Part of the adult metencephalon and all of the diencephalon , mesencephalon and myelencephalon are included in the brainstem. The brainstem controls most of the vegetative functions of the body, and is thus vital for life.

    The cerebellum and pons (the ventral part of the metencephalon of birds and mammals that has a band of transverse fibers) are the principal adult derivatives of the metencephalon. The cerebellum and pons contribute to coordination of motor function.

    The cerebrum is the adult derivative of the telencephalon, and dominates the brain in both size and control.

    Surrounding the adult brain are layers of mesodermally-derived connective tissue called meninges (singular: meninx). Whereas cyclostomes and fishes only have a single envelope called the primitive meninx, amphibians have two layers, consisting of an outer dura mater which is extremely dense and protective, and a pia-arachnoid or secondary meninx which is more delicate and vascular. Mammals have three meninges: pia mater (which follows all the convolutions of the brain and is the most interior), the arachnoid layer (which is delicate and sends strands to the pia mater), and the dura mater (the outer, more protective meninx). The area between the dura mater and the arachnoid layer is called the subdural space; the area between the arachnoid layer is called the subarachnoid space. An additional layer of tissue lies between the two hemispheres of the cerebrum and is called the falx cerebri.

    Posterior brainstem: medulla through midbrain
    The posterior brainstem is the site for connection of cranial nerves into the central nervous system. Each cranial nerve has a nucleus in the posterior brainstem for each type of fiber it carries (e.g., somatic sensory, visceral sensory, somatic motor, visceral motor).

    The reticular formation is found in all vertebrates, and is a network of short interneurons in the brainstem that forms a primitive integrating system. It projects into the cerebrum, cerebellum, cranial nuclei, and the spinal cord, and is essential for consciousness as well as control of the cardiovascular and respiratory systems.

    The ruber nucleus and substantia nigra are two other important parts of the brain that are located in the posterior brainstem. The ruber nucleus plays a role in the coordination of motor functions. The substantia nigra is involved in the memory of learned tasks, and death of its cells is associated with Parkinson’s disease.

    The roof of the midbrain is called the tectum. The tectum of non-mammalian vertebrates is the site for the optic lobes, which are the primary center for the perception of vision. In mammals, vision is perceived in the cerebrum. However, while the mammalian cerebrum tells the animal what an object is, the tectum tells the mammal where in space a visual object is. In the tectum the optic lobes are called anterior colliculi. Behind them are the posterior colliculi which may be important in coordination of auditory reflexes. Together the colliculi form four bumps called the corpora quadrigemina.

    Other features of the posterior brainstem include are the pyramidal tracts, which are tracts of motor fibers that run from the cerebral cortex to the spinal cord without interruption. The cerebra peduncles are also important, as they are the sites where the cerebellum joins the brainstem.

    Anterior brainstem: diencephalon
    The anterior brainstem differs from the posterior part in having no nuclei for cranial nerves and no reticular formation, and for relating to functions that are more highly evolved.
    The dorsal part of the diencephalon is the epithalamus, most of which is nonnervous in function. It includes two evaginations: the parietal organ and the pineal body, which function as an endocrine gland and a sense organ.
    The lateral parts of the diencephalon are called the thalamus. The thalamus is a relay center to the cerebrum, and functions in awareness, as well as in the perception of pain and pleasure.
    The ventral part of the diencephalon is the hypothalamus. It controls most of the autonomic functions of the body, including water balance, temperature regulation, appetite and digestion, blood pressure, sleep and waking, sexual behavior, and emotions. On the ventral surface of the hypothalamus is the optic chiasma, where the optic nerves converge and cross. The hypophysis (pituitary gland) also lies on the ventral side, and functions as an endocrine gland

    Cerebellum and pons
    The cerebellum develops from the dorsal part of the metencephalon

    • highly convoluted in mammals and birds into convex folds or gyri (singular: gyrus) and concave grooves or sulci (singular: sulcus)
    • in cross section, the cerebellum is composed of mostly white matter on the cortex and branching white matter in the interior, giving a tree-like appearance called the arbor vitae
    • functions to control motor coordination and to maintain equilibration
    Cerebrum
    The cerebrum is the largest part of the brain, and develops from the telencephalon.
    Each half of the cerebrum termed a cerebral hemisphere • the olfactory bulb is at the anterior end of each hemisphere
    • the corpus striatum - a group of basal nuclei in the base of the cerebrum through which white fibers pass and functions in the movement of muscle masses, and some visual perception
    • the cortex - forms the roof and side walls of the cerebrum; the hippocampus, which is important in spatial memory, may be damaged in individuals with Alzheimer’s disease.
    Definitions: Astrocytes - star-shaped nutritive and supportive glia cells of the central nervous system Central pattern generators - groups of neurons in the spinal cord and in the brain whose activity is responsible for innate cyclical movements of body parts

    Ganglion - group of neuron cell bodies that lie peripheral to the central nervous system in vertebrates

    Neuroglia - cells in the central nervous system that help to support, protect and maintain the neurons

    Microglia - small neuroglial cells of mesodermal origin, some of which are phagocytic

    Node of Ranvier - regions of the axon that lie between the Schwann cells, where the plasma membrane of a myelinated axon is close to the extracellular fluid

    Oligodendrocytes - neuroglial cells of ectodermal origin that myelinate axons in the central nervous system, and forms the white matter of the central nervous system (unmyelinated axons are grey matter)

    Plexus - networks of nerves or blood vessels formed before nerves are distributed to the muscles

    Schwann cells - also called neurilemma. Cells of neural crest origin that form a thin sheath that surrounds an unmyelinated axon, or, after having myelinated an axon, lies on the surface of the myelin sheath

    Telodendria - the terminal branches of an axon