CRANIAL NERVES & PNS

CRANIAL NERVES 1. Twelve pairs of cranial nerves originate from the nose, eyes, inner ear, brain stem, and spinal cord. 2. They are named primarily based on their distribution and are numbered I – XII in order of attachment to the brain. Table 14.4 summarizes the types, locations, functions, and disorders of the cranial nerves.

DEVELOPMENT OF THE NERVOUS SYSTEM 1. The development of the nervous system begins with a thickening of a region of the ectoderm called the neural plate. 2. During embryological development, primary brain vesicles form from the neural tube and serve as forerunners of various parts of the brain. 3. The telencephalon forms the cerebrum, the diencephalons develops into the midbrain, the metencephalon develops into the pons and cerebellum, and the myelencephalon froms the medulla.

AGING AND THE NERVOUS SYSTEM 1. The brain grows rapidly during the first few years of life. 2. Age-related effects involve loss of brain mass and decreased capacity for sending nerve impulses.

AUTONOMIC NERVOUS SYSTEMS

OBJECTIVES Compare the structural and functional differences between the somatic and autonomic parts of the nervous system. Describe preganglionic and postganglionic neurons of the autonomic nervous system. Compare the anatomical components of the sympathetic and parasympathetic divisions of the autonomic nervous system.

OBJECTIVES Describe the neurotransmitters and receptors involved in autonomic responses. Describe the major responses of the body to stimulation by the sympathetic and parasympathetic divisions of the ANS. Describe the components of an autonomic reflex. Explain the relationship of the hypothalamus to the ANS.

 It is easy to forget that much of the human nervous system is concerned with routine, involuntary jobs, such as homeostasis, digestion, posture, breathing, etc. This is the job of the autonomic nervous system, and its motor functions are split into two divisions, with anatomically distinct neurones. Most body organs are innervated by two separate sets of motor neurones; one from the sympathetic system and one from the parasympathetic system. These neurones have opposite (or antagonistic) effects. In general the sympathetic system stimulates the “fight or flight” responses to threatening situations, while the parasympathetic system relaxes the body. The details are listed in this table:

Autonomic Nervous System One division of the autonomic nervous system, called the sympathetic nervous system, dominates in times of stress. It controls the "fight or flight" reaction, increasing blood pressure, heart rate, breathing rate, and blood flow to the muscles. Another division, called the parasympathetic nervous system, has the opposite effect. It conserves energy by slowing the heartbeat and breathing rate, and by promoting digestion and elimination (of waste). Most glands, smooth muscles, and cardiac muscles constantly get inputs from both the sympathetic and parasympathetic systems. The CNS controls the activity by varying the ratio of the signals. Depending on which motor neurons are selected by the CNS, the net effect of the arriving signals will either stimulate or inhibit the organ.

Motor fibers that govern involuntary responses, do not lead directly to the organs they innervate. Instead, they make their trips in two stages. The first set of fibers leads from the CNS to ganglia (which are collections of nerve cell bodies) that lie outside the CNS (the preganglionic fibers). At the ganglia the fibers form synaptic junctions with the dendrites of as many as twenty different cell bodies. The axons of these cell bodies form a second set of fibers, the postganglionic fibers. It is these postganglionic fibers that lead to the organs.

The chief ganglia involved in the autonomic nervous system form two lines running down either side of the spinal column. They are outside the bony vertebrae. These two lines of ganglia outside the column resemble a pair of long beaded cords. At the lower end, the two cords join and finish in a single central stretch. These lines of ganglia are sometimes called the sympathetic trunks (used by the sympathetic nervous system). Not all ganglia are located in the sympathetic trunks. Some are not; and it is possible for a preganglionic fiber to go right through, making no synaptic junction there at all, joining instead with ganglia located in front of the vertebrae. For the parasympathetic nervous system, some of the ganglia separating the preganglionic fibers from the postganglionic fibers are actually located within the organ the nerve is servicing. In that case, the preganglionic fiber runs almost the full length of the total track, whereas the postganglionic fiber is at most just a few millimeters long.

The splanchnic nerves, which originate from some of the thoracic nerves, have their preganglionic fibers ending in a mass of ganglia lying just behind the stomach. It represents the largest mass of nerve cells that is not within the CNS and is sometimes called the "abdominal brain". It is a vital spot to be protected during boxing.

COMPARISON OF SOMATIC AND AUTONOMIC NERVOUS SYSTEMS The somatic nervous system operates under conscious control; the ANS usually operates without conscious control. Sensory input for the somatic nervous system is mainly from the special senses and somatic senses; sensory input for the ANS is from interoceptors, in addition to special senses and somatic senses. The axons of somatic motor neurons extend from the CNS and synapse directly with an effector. Autonomic motor pathways consist of two motor neurons in series. The axon of the first motor neuron extends from the CNS and synapses in a ganglion with the second motor neuron; the second neuron synapses with an effector. The output (motor) portion of the ANS has two divisions; sympathetic and parasympathetic. Most body organs receive dual innervation; usually one ANS division causes excitation and the other causes inhibition. Somatic nervous system effectors are skeletal muscles; ANS effectors include cardiac muscle, smooth muscle, and glands.

ANATOMY OF AUTONOMIC MOTOR PATHWAYS Preganglionic neurons are myelinated; postganglionic neurons are unmyelinated. The cell bodies of sympathetic preganglionic neurons are in the lateral gray horns of the 12 thoracic and the first two or three lumbar segments of the spinal cord; the cell bodies of parasympathetic preganglionic neurons are in four cranial nerve nuclei (III, VII, IX, and X) in the brain stem and lateral gray horns of the second through fourth sacral segments of the spinal cord. Autonomic ganglia are classified as sympathetic trunk ganglia (on both sides of vertebral column), prevertebral ganglia (near or inside visceral effectors). Sympathetic preganglionic neurons synapse with postganglionic neurons in ganglia of the sympathetic trunk or in prevertebral ganglia; parasympathetic preganglionic neurons synapse with postganglionic neurons in terminal ganglia.

ANS NEUROTRANSMITTERS AND RECEPTORS Cholinergic neurons release acetylcholine, which binds to nicotinic or muscarinic cholinergic receptors. In the ANS, the cholinergic neurons include all sympathetic and parasympathetic preganglionic neurons, all parasympathetic postganglionic neurons, and sympathetic postganglionic neurons that innervate most sweat glands. In the ANS, adrenergic neurons release norepinephrine. Both epinephrine and norepinephrine bind to alpha and beta adrenergic receptors. Most sympathetic postganglionic neurons are adrenergic. Table 15.2 summarizes the types of cholinergic and adrenergic receptors. An agonist is a substance that binds to and activates a receptor, mimicking the effect of a natural neurotransmitter or hormone. An antagonist is a substance that binds to and blocks a receptor, thereby preventing a natural neurotransmitter or hormone from exerting its effect.

PHYSIOLOGICAL EFFECTS OF THE ANS The sympathetic division favors body functions that can support vigorous physical activity and rapid production of ATP (fight-or-flight response); the parasympathetic division regulates activities that conserve and restore body energy. The effects of sympathetic stimulation are longer lasting and more widespread than the effects of parasympathetic stimulation. Table 15.3 compares structural and functional features of the sympathetic and parasympathetic divisions.