INTRODUCTION
The nervous and endocrine systems are the regulatory & control systems of the body. The NERVOUS SYSTEM has 3 functions:
(1) Sensory input: The nervous system receives sensory information from millions of SENSORY RECEPTORS that detect changes (stimuli) inside and outside of the body. The sensory stimulus is converted to a nerve impulse by a sensory neuron, which transmits the nerve impulse to the central nervous system.
(2) Integration: The central nervous system (brain and spinal cord) processes and interprets the sensory input and determines the appropriate motor response.
(3) Motor output: The central nervous system responds to the sensory stimulus by sending a nerve impulse down a motor neuron, which will stimulate muscle tissue to contract or a gland to secrete a product.
Information travels through the nervous system very quickly by electrical nerve impulses, which are called nerve impulses or action potentials.
ORGANIZATION OF THE NERVOUS SYSTEM: FIGS. 17.1 & 17.2
The nervous system is divided into two principal parts: the CENTRAL NERVOUS SYSTEM and the PERIPHERAL NERVOUS SYSTEM.
The CENTRAL NERVOUS SYSTEM (CNS) is the control center for the entire nervous system. The CNS consists of the BRAIN and the SPINAL CORD and performs the function of integration (see #2 above).
The PERIPHERAL NERVOUS SYSTEM (PNS) lies outside of the CNS. It consists mainly of nerves that connect the brain and spinal cord to sensory receptors, muscles, and glands. SPINAL NERVES carry impulses to and from the spinal cord. CRANIAL NERVES carry impulses to and from the brain. There are 2 parts to the PNS: the SOMATIC NERVOUS SYSTEM and the AUTONOMIC NERVOUS SYSTEM. The SOMATIC NERVOUS SYSTEM deals with sensations that you are conscious of and controls skeletal muscle (voluntary) contractions. The AUTONOMIC NERVOUS SYSTEM (ANS) deals with visceral (internal) sensations and involuntary control of smooth muscle, cardiac muscle and glands.
The SOMATIC NERVOUS SYSTEM of the PNS consists of somatic sensory neurons and somatic motor neurons. SOMATIC SENSORY (AFFERENT) NEURONS convey nerve impulses from SENSORY RECEPTORS to the CNS. These sensory receptors include TOUCH, PRESSURE, PAIN, & TEMPERATURE RECEPTORS in the SKIN; PROPRIOCEPTORS in the SKELETAL MUSCLES, TENDONS & JOINTS (informing the CNS of the position of the body & body parts, and movement of limbs); & SPECIAL SENSE ORGANS (responsible for sensations of taste, smell, vision, hearing, and equilibrium).
SOMATIC MOTOR (EFFERENT) NEURONS conduct nerve impulses from the CNS to skeletal muscle, stimulating the muscle to contract.
The AUTONOMIC NERVOUS SYSTEM of the PNS consists of autonomic sensory and motor neurons. AUTONOMIC SENSORY (AFFERENT) NEURONS conduct nerve impulses from sensory receptors of the internal organs & blood vessels to the CNS, keeping the CNS informed about events inside the body. Autonomic sensory receptors include chemoreceptors that detect changes in CO2 levels in the blood, and stretch receptors in organs such as the stomach, intestines, urinary bladder, and uterus.
AUTONOMIC MOTOR (EFFERENT) NEURONS conduct nerve impulses from the CNS to smooth muscle, cardiac muscle and glands. Autonomic motor neurons control involuntary contractions of smooth muscle and cardiac muscle, as well as secretions of both exocrine and endocrine glands. The autonomic system may stimulate smooth muscle contractions, cardiac muscle contraction and glandular secretions, or it may inhibit those processes.
The MOTOR portion of the AUTONOMIC NERVOUS SYSTEM is further subdivided into two divisions: the PARASYMPATHETIC DIVISION and the SYMPATHETIC DIVISION (Chapter 20). The PARASYMPATHETIC DIVISION is the "housekeeping division", controlling involuntary activities while the body is at rest. It keeps ventilation (breathing) rate and heart rate low, and stimulates digestion and kidney function. The SYMPATHETIC DIVISION controls the "fight-or-flight response", which is the body's response to stress, emergencies, and exercise. The sympathetic division increases ventilation (breathing) and heart rate, increases blood flow to the brain and skeletal muscles, causes the pupils of the eyes to dilate, and slows down digestive and urinary function. Notice that the fight-or-flight response is also stimulated by the hormones epinephrine and norepinephrine from the adrenal medulla.
HISTOLOGY OF NERVOUS TISSUE
Nervous tissue consists of 2 types of cells: neurons and neuroglia. See FIG. 17.3(b). NEURONS are nerve cells that rapidly conduct nerve impulses from one part of the body to another. NEUROGLIA are cells that support and protect the neurons. See FIG. 17.6. Some examples of neuroglia include microglia, ependymal cells, oligodendrocytes, and Schwann cells. MICROGLIA are a special type of FIXED MACROPHAGE in the brain and spinal cord nervous tissue that protect the CNS by engulfing and destroying pathogens and dead nerve tissue. EPENDYMAL CELLS secrete cerebrospinal fluid (CSF) in the brain. CSF cushions and protects the brain and spinal cord. OLIGODENDROCYTES produce insulated coverings called MYELIN SHEATHS around the nerve fibers in the CNS. SCHWANN CELLS produce MYELIN SHEATHS around the nerve fibers in the PNS. We will discuss the concept of the myelin sheath a little later.
NEURONS
3 characteristics of neurons:
1. Neurons are extremely long-lived. As long as they are not damaged by disease or trauma, they can last a lifetime.
2. Neurons have a high metabolic rate, requiring a continuous and abundant supply of oxygen and glucose. 3. Neurons have limited powers of regeneration. If the cell body of a neuron is destroyed, that neuron can not be replaced. Mature neurons are incapable of cell division. Extensions of the neuron (nerve fibers) can sometimes be repaired.
STRUCTURE OF NEURONS: FIG. 17.3
Neurons have a main CELL BODY, which is 5-140 micrometers in diameter. The cell body contains the nucleus, mitochondria and other cellular organelles. The cell bodies are USUALLY located within the CNS, where they are protected by the bones of the cranium & vertebral column. Inside the CNS, the cell bodies are clustered into groups called NUCLEI (singular, NUCLEUS). Some neuron cell bodies are found in the PNS, where they occur in clusters called GANGLIA (singular, GANGLION).
The DENDRITES are extensions of the cell body that conduct electrical signals TOWARD the cell body. Dendrites are usually short, thick & highly branched.
The AXON is a single long, thin process that conducts nerve impulses AWAY from the cell body to another neuron, muscle cell or gland cell. Axons vary in length from a few millimeters to a meter or more. The TRIGGER ZONE (FIG. 17.4) where nerve impulses are generated is near the AXON HILLOCK of the axon. Side branches off the axon are called AXON COLLATERALS. Each axon and axon collateral terminates by branching extensively into fine filaments called AXON TERMINALS. There can be 10,000 or more axon terminals per neuron. The ends of the axon terminals are expanded into bulblike structures called SYNAPTIC END BULBS, which are important in the transmission of nerve impulses from one neuron to another neuron, or from a neuron to a muscle or gland cell.
The term "NERVE FIBER" usually refers to the axon. Many nerve fibers are surrounded by a multilayered, white, fatty, segmented covering called the MYELIN SHEATH. Nerve fibers with a myelin sheath are MYELINATED; nerve fibers without a myelin sheath are UNMYELINATED. The myelin sheath protects and insulates the nerve fibers from one another, and it increases the speed of transmission of nerve impulses. The myelin sheath has gaps at intervals along the nerve fiber called the NODES OF RANVIER. The nerve impulse that travels down a myelinated axon jumps from NODE OF RANVIER to NODE OF RANVIER. Because the nerve impulse jumps from node to node, myelinated fibers conduct nerve impulses more rapidly than unmyelinated fibers.
Myelin sheaths are produced by specialized neuroglia. In the PNS, the SCHWANN CELL winds repeatedly around the nerve fiber as shown in FIG. 17.7. The myelin sheath consists of several Schwann cells wrapped around the nerve fiber, with a node of Ranvier between each Schwann cell. FIG. 17.6 shows the OLIGODENDROCYTE, which forms the myelin sheath around nerve fibers in the CNS.
The functional classification of neurons is based on the direction in which the neuron transmits nerve impulses. See your reflex arc hand-out. SENSORY (AFFERENT) NEURONS transmit impulses from sensory receptors to the CNS. MOTOR (EFFERENT) NEURONS carry impulses away from the CNS to the EFFECTOR ORGAN (skeletal muscle, smooth muscle, cardiac muscle, or gland). INTERNEURONS (association neurons) carry nerve impulses from sensory neurons to motor neurons and are located in the brain or spinal cord. Interneurons are the neurons that integrate incoming information from the sensory neurons and relay the response to the motor neurons. Most neurons in the nervous system are interneurons.
The structural classification is based on the number of axons and dendrites that extend from the cell body. There are 3 types of neurons according to the structural classification: MULTIPOLAR, BIPOLAR and UNIPOLAR. See FIG. 17.4. Note the location of the TRIGGER ZONE, where nerve impulses are first generated in the axon.
1. MULTIPOLAR NEURONS (FIGS. 17.3 & 17.4) have several branched dendrites and one long axon. The AXON is often referred to as a "NERVE FIBER". Multipolar neurons are the most common type of neuron in the body. MOTOR NEURONS of the PNS and ASSOCIATION NEURONS of the CNS are MULTIPOLAR neurons.
2. BIPOLAR NEURONS (Fig. 16.5) have one axon and one dendrite that extend from opposite sides of the cell body. Bipolar neurons are rare; they act as SENSORY RECEPTOR CELLS in some of the SPECIAL SENSE ORGANS (the visual receptors of the RETINA of the EYEBALL, FIG. 22.7, page 681; & the OLFACTORY RECEPTORS of the OLFACTORY MUCOSA of the nasal cavities, FIG. 22.1, page 673).
3. UNIPOLAR NEURONS (FIGS. 17.4) have a single process that emerges from the cell body. The single process divides into an axon and a dendrite. Unipolar neurons are found chiefly in the PNS, where they function as SENSORY NEURONS. Most sensory neurons (with the exception of the retina of the eyeball and the olfactory mucosa; see #2 above) are unipolar. SENSORY RECEPTORS at the end of the dendrites detect the stimulus, which is converted into an electrical nerve impulse at the "trigger zone". The trigger zone is at the junction between the dendrite and axon. The axon delivers the impulse to the CNS (brain or spinal cord). The cell bodies of sensory neurons in SPINAL NERVES are located outside the CNS in POSTERIOR (DORSAL) ROOT GANGLIA (see the reflex arc hand-out and FIG. 18.3, page 559).
See the reflex arc hand-out. A REFLEX is a fast response to an internal or external change (stimulus). A REFLEX ARC is the simplest pathway for a nerve impulse to travel, from SENSORY NEURON to INTERNEURON to MOTOR NEURON to the EFFECTOR ORGAN (muscle tissue or gland). Your hand-out shows you a somatic sensory reflex arc with the spinal cord at the center. Note that the sensory neuron in this example is unipolar and the association & motor neurons are multipolar. The central nervous system is the center of the reflex arc. You can also see an example of a reflex (the patellar reflex) in FIG. 18.5, page 563.
ARRANGEMENT OF NERVE FIBERS
A NERVE FIBER refers to a long neuron process. It usually refers to an axon, but sometimes may be a dendrite. In the PNS, nerve fibers are arranged into bundles called NERVES. See the reflex arc hand-out and FIG. 18.6, page 564. (Note how the nerve fibers are bundled together into fascicles by connective tissue. What does this resemble?) Most nerves contain both sensory and motor nerve fibers and are called MIXED NERVES. Mixed nerves contain the following types of nerve fibers:
1. SOMATIC AFFERENT (SENSORY) NERVE FIBERS conduct nerve impulses from the somatic sensory receptors in the skin, skeletal muscles and joints to the CNS.
2. AUTONOMIC (VISCERAL) AFFERENT NERVE FIBERS conduct nerve impulses from sensory receptors in the internal organs & blood vessels to the CNS.
3. SOMATIC EFFERENT (MOTOR) NERVE FIBERS conduct nerve impulses from the CNS to skeletal muscles.
4. AUTONOMIC (VISCERAL) EFFERENT NERVE FIBERS conduct nerve impulses from the CNS to smooth muscle, cardiac muscle and glands.
NERVE IMPULSE TRANSMISSION
Nerve impulses travel from neuron to neuron across a junction between the two neurons called a SYNAPSE. Within each synapse is a fluid-filled space called the SYNAPTIC CLEFT. The neuron conducting impulses toward the synapse is called the PRESYNAPTIC NEURON. The presynaptic neuron is the INFORMATION SENDER. The neuron that receives the nerve impulse and transmits the impulse away from the synapse is called the POSTSYNAPTIC NEURON. The postsynaptic neuron is the INFORMATION RECIPIENT.
SYNAPTIC END BULBS of the PRESYNAPTIC NEURON synapse with the DENDRITES or the CELL BODY of the POSTSYNAPTIC NEURON. See FIG. 17.10.
In the PNS, MOTOR NEURONS transmit nerve impulses to MUSCLE CELLS or to GLAND CELLS. A neuron transmits nerve impulses to a muscle fiber across a synapse called a NEUROMUSCULAR JUNCTION (FIG. 10.6, page 273). A neuron transmits nerve impulses to glandular cells across a synapse called a NEUROGLANDULAR JUNCTION.