NERVOUS SYSTEMS The simplest animal nervous systems, found in coelenterates (cnidarians), are net-like arrangements (NERVE NETS) of very fine axons with synapses at points of contact. More advanced invertebrates have one or more clusters of neuron cell bodies (= GANGLIA), and many (e.g. annelids and arthropods) have a large cluster (superganglion) at the anterior end of their ventral nerve cord, forming what is essentially a BRAIN. The octopus (a cephalopod mollusk) has the largest known brain (estimated to contain 100 million neurons) among invertebrates. VERTEBRATE NERVOUS SYSTEMS The neuron is the fundamental unit of the nervous system; however, there are other important components. GLIAL CELLS actually outnumber neurons by at least 10 to 1. These are support cells. They provide physical support and they direct the growth of neurons during development and repair. There are several kinds of glial cells within the CNS. Schwann cells are the primary glial cells within the PNS.
[see Figure]
CENTRAL NERVOUS SYSTEM
The central nervous system of chordates originates as a hollow dorsal tube. This, in vertebrates, subsequently develops into a spinal cord with a brain at the anterior end.
The CNS is enclosed in membranous coverings called MENINGES. Within them (and within the cavities (ventricles) ofd the brain and the central canal of the spinal cord) we find CEREBROSPINAL FLUID. This acts as a "shock absorber" and helps to protect the delicate brain tissues.
The CNS is enclosed in and perfused by the cerebrospinal fluid. Nevertheless, the CNS is also richly supplied with capillaries and has a constant high demand for glucose and oxygen, both of which must be supplied by the circulating blood. Although it makes up only about 2% of the body mass, the brain receives, at rest, almost 15% of the circulating blood.
A complex BLOOD-BRAIN BARRIER controls the passage of substances between the blood stream and the ECF. Some drugs and other substances (especially lipid soluble substances) can cross this barrier easily, but others are effectively blocked. The exchange between the blood stream and the cerebrospinal fluid is also selective.
[Fig. 6-41] The GRAY MATTER consists of the cell bodies of the efferent neurons, interneurons, and a variety of non-myelinated fibers.
The WHITE MATTER consists largely of ascending and descending myelinated axons of interneurons.
Ascending and descending fiber tracts cross over to the opposite side of the brain/body. Some (e.g. afferents for sensations of pain, heat, and cold) cross over near where they enter the spinal cord. Others (e.g. motor efferents, afferents for conscious proprioception and light touch) cross over in or near the brain. As a result, hemisection of the spinal cord will result in the loss of the latter on the ipsilateral side, but of the former on the contralateral side. [see Figure]
Clinically, this is known as Brown - Séquard's Syndrome.
[see Figure] The vertebrate brain develops from ECTODERM as a hollow dorsal tube. At the anterior end, the tube differentiates into a FOREBRAIN, MIDBRAIN, and HINDBRAIN.
[FIG. 6-38] The forebrain includes the cerebrum and the diencephalon. The latter contains the thalamus and hypothalamus. The hindbrain includes the cerebellum and the medulla oblongata. The latter (along with the pons and midbrain) forms the BRAINSTEM.
In addition to these more-or-less discrete anatomical divisions, we find collections of neurons, such as the RETICULAR FORMATION. These neurons run through the brainstem and beyond (through much of the spinal cord and most regions of the brain). A great deal of input is received, integrated, and processed by these neurons.
Clusters of reticular formation neurons form important control centers (for circulation, respiration, etc.) in the brainstem. Wakefulness and attentiveness are also markedly influenced by the reticular formation.
CEREBRUM
This consists primarily of right and left CEREBRAL HEMISPHERES, which are largely separated yet connected by a bundle of nerve fibers known as the CORPUS CALLOSUM.
The outer shell (the CEREBRAL CORTEX) is highly folded and divisible into four more-or-less discrete lobes.
[see Figure] The cerebrum is the highest center of integration with many important areas for motor and sensory association.
[see Figure]
[Fig. 7-20]
This is a 'homunculus' depicting the somatosensory cortex of a human. The left half of the body is represented on the right hemisphere, the left hemisphere being essentially a mirror image.
A 'homunculus' for the slightly more anterior somatomotor cortex would be quite similar in appearance.
[see Figure] The diencephalon (the second component of the forebrain) consists of the THALAMUS and the HYPOTHALAMUS.
The thalamus serves as a synaptic relay station (i.e. as sort of a "port of entry" to higher brain centers).
The hypothalamus contains many important control centers and also forms a critical link between the nervous system and the endocrine system.
CEREBELLUM
This large derivative of the hindbrain receives sensory input from many parts of the body. The cerebellum is important for balance and the coordination of muscle movements.
MEDULLA OBLONGATA
This hindmost portion of the hindbrain and major component of the brainstem contains some essential control centers (especially for the respiratory and cardiovascular systems).
[see Figure] As we noted earlier, there are many important control centers in the brainstem (i.e. medulla oblongata and pons) and many other important control centers in the hypothalamus.
[see Figure] A = fish, B = frog, C = bird, D = human. Note the increase in the relative size of the cerebrum during phylogeny.
[see Figure] Note that the relative amount of association cortex increases among mammals from rat to human.
PERIPHERAL NERVOUS SYSTEM
The sensory neurons of the AFFERENT DIVISION carry information to the spinal cord or brain. All those entering the spinal cord do so at the dorsal root. These are typically bipolar neurons and they may have very long (i.e. more than one meter in large vertebrates) axons.
EFFERENT DIVISION
The cell bodies of the neurons of the SOMATIC NERVOUS SYSTEM are located in the CNS. The axons of those leaving the spinal cord do so via the VENTRAL ROOT.
SOMATIC NERVOUS SYSTEM
These neurons are typically under voluntary (or reflex) control and they directly innervate skeletal muscle. The transmitter released by these axons is ACh, and they are (in vertebrates) always excitatory. The axons may be very long in the case of large animals.
AUTONOMIC NERVOUS SYSTEM
There are two major divisions of the ANS: The SYMPATHETIC DIVISION and the PARASYMPATHETIC DIVISION. The effector organs are glands, cardiac muscle, and smooth muscle. The effects can be either inhibitory or excitatory. Most effectors are innervatd by both sympathetic and parasympathetic fibers, the effects of which tend to be opposite (DUAL INNERVATION).
The neurons of the ANS synapse outside the CNS forming GANGLIA (sing. = GANGLION) which are clusters of nerve cell bodies. The first neuron is called PREGANGLIONIC and the second POSTGANGLIONIC.
The two divisions of the ANS differ anatomically, pharmacologically, and physiologically.
Anatomical differences: The sympathetic system has SHORT preganglionic fibers and LONG postganglionic fibers. The neurons exit the spinal cord from the Thoracic and Lumbar regions. Also, sympathetic preganglionics innervate the ADRENAL MEDULLA (which is essentially a modified ganglion) and control the release of hormones by this endocrine gland.
The parasympathetic system has LONG preganglionic fibers and SHORT postganglionic fibers. The neurons exit from the Cranial and Sacral regions of the CNS.
Pharmacologically, the two divisions of the ANS differ in their responses to stimulatory and inhibitory drugs, which largely reflects the different neurotransmitters they rely on.
For both divisions, ACh is the major neurotransmitter released by the preganglionic fibers. ACh is also the major neurotransmitter for the postganglionic fibers of the parasympathetic division. In contrast, Norepinephrine (NE) is the primary neurotransmitter associated with the postganglionic fibers of the sympathetic division.
NOTE: These distinctions are not absolute. Some postganglionic fibers release nitric oxide or other transmitters, some sympathetic postganglionics release ACh, and many preganglionics release cotransmitters.
Neurons secreting ACh (and their receptors) are called CHOLINERGIC. There are two main types: NICOTINIC (because nicotine is an agonist), which are found in autonomic ganglia and neuromuscular junctions, and MUSCARINIC (because a fungal compound is an agonist) found at parasympathetic neuroeffector junctions.
NOTE: An AGONIST is a drug which can combine with and activate a receptor, whereas an ANTAGONIST opposes or impedes receptor action.
Neurons secreting Norepinephine (and their receptors) are called ADRENERGIC. There are two main types: ALPHA (found in most sympathetic target tissues), which are relatively insensitive to Epinephrine, and BETA (two main subtypes — found in heart, etc.), which are sensitive to Epinephrine.
Physiologically, the parasympathetic division is primarily responsible for regulating maintenance activities. In contrast, the sympathetic division is more of an emergency system, activated in times of real (or imagined) danger or stress.
The sympathetic responses are FIGHT OR FLIGHT RESPONSES. The body is activated for maximal physical response. In addition to direct action by the postganglionic sympathetic nerves, there can be a massive release of Epinephrine into the blood stream from the adrenal medulla.
[see Figure] Sympathetic neurons are colored in red, and parasympathetic in blue.
Note the dual innervation to most organs and the importance of the tenth cranial (vagus) nerve. Cranial nerves III, VII, and IX also contribute to the parasympathetic division.
[see Fig. 6-44]
[see Figure] Cranial nerve X (the vagus — from a Latin word meaning "wanderer") innervates most of the major internal organs. It is, arguably, the most important single nerve in the vertebrate body.
[see Figure] This slide illustrates the neurons of the Efferent Division of the Peripheral Nervous System and their various Cholinergic or Adrenergic receptors.
[Fig. 6-46]