Chapter 7: The Nervous SystemLecture Notes taken predominantly from:

Marieb,E.N. 2009. Essentials of Human Anatomy and Physiology. PBC The Nervous System performs 3 major functions: 1. Sensory – monitors changes inside and out of the body, a.k.a. senses by being irritated by stimuli (like touch, heat, light, pain, and chemicals like neurotransmitters); 2. Integration – processes the sensory input and determines if action is needed; and 3. Motor Ouput – sends signals to muscles and glands to respond to stimuli in various ways.

The Nervous System is categorized structurally (anatomically) into 2 major systems: The Central Nervous System (CNS) = the brain and spinal cord and serves the main function of integration.

The Peripheral Nervous System (PNS) = all nerves outside of the brain and spinal cord (the spinal nerves and cranial nerves). The PNS serves primarily the Sensory (afferent) and Motor (efferent) functions. The Sensory division of the PNS monitors the body and its surroundings; the Motor division is further divided into the Somatic (voluntary) and the Autonomic (involuntary) subdivisions. Somatic nerves are devoted to conscious control while Autonomic nerves are devoted to involuntary (or reflex) actions. The Autonomic subdivision is divided further still into the Parasympathetic (relaxed, ‘rest and digest’) and the Sympathetic (stressed, ‘fight or flight’) nerves. Often, sympathetic and parasympathetic nerves stimulate the same organs but in opposite manners (ex. Sympathetic = divert blood from digestive tract; Parasympathetic = divert blood to digestive tract)

Support Cells (a.k.a. neuroglia = ‘nerve glue’) of the Central nervous system include the 4 following types of cells:1. Astrocytes – control the chemical environment of the brain by absorbing excess electrolytes from the cerebrospinal fluid (the nourishing and waste removing blood plasma derivative of the brain) and link neurons to blood vessels; 2. Microglial – consume and digest cellular debris, consume damaged neurons; 3. Ependymal – line CNS and use cilia to circulate cerebrospinal fluid; and 4. Oligodendrocytes – insulate nerve fibers (form Myelin Sheath in CNS).

Support Cells of the PNS include Schwann cells that insulate and form the Myelin Sheath in the PNS nerve fibers and Satellite cells that protect the cell bodies (metabolic centers) of the neurons in the PNS.

Neurons are the money makers of the nervous system, i.e. the functional cells of both the CNS and PNS. They are delicate cells with similar features that include a cell body (site of nucleus and metabolic maintenance) and long extensions of the cell membrane known as dendrites (deliver electric signals toward the cell body) and axons (deliver electrical impulses to other cells). Be able to identify the axons, cell, body, dendrites, Schwann cells, and Nodes of Ranvier, axon terminals on a nerve of the PNS like the one shown below.

Axons end in axon terminals that synapse (connect, in a way) with other cells. The Myelin Sheath (of Schwann cells) insulates the long axons and helps electrical messages travel much faster than they would otherwise.

Because cell bodies of neurons in the CNS are not insulated and usually cluster together, they form grey areas known as grey matter. The insulated axons also cluster together in areas of white matter. Cell bodies of nerves in the PNS also tend to cluster together. These areas of cell bodies outside the CNS are known as ganglia.

How do neurons work? i.e. transmit messages?Like muscle cells, neurons maintain a voltage across their membranes by constantly pumping out Na+ ions ‘at rest’.

1. When a stimulus is present (i.e. heat, light, touch, damaged cells, or even neurotransmitters) Na+ channels open on the cell membrane of the nerve. 2. If enough Na+ is allowed to enter the neuron near the stimulus, Na+ channels begin to open like a series of dominoes and produce an action potential. An action potential cannot be stopped once it starts: Na+ channels open all along the axon and allow Na+ to flood into the cell all the way to the membranes of the axon terminals. 3. At the axon terminals, the flood of Na+ stimulates the nerve cell to release neurotransmitters (chemicals like Acetyl choline) into the extracellular space; the neurotransmitters quickly diffuse across the synaptic cleft (tiny gap between the 2 cells) and bind with sodium channels on the membrane of the next cell. 4. If enough neurotransmitters bind to the next cell, the next cell initiates its own action potential and the message is forwarded to the next cell in line. 5. When action potentials reach the end of motor neurons, the neurotransmitters stimulate the muscle to contract or gland cells to secrete hormones.

In the above manner, changes in the environment are sensed by sensory nerves and the message is passed to interneurons that process the sensory messages. If merited, the interneurons initiate an action potential that then passes the message to motor neurons (via neurotranmitters) that eventually stimulate the muscles or glands (also via neurtransmitters). Know the pathway indicated above.

Reflex (or involuntary) pathways make up much of the nervous system. Reflexes are rapid, predictable (always follow the same rules), and involuntary. Simple reflex pathways involve 3 neurons (sensory to interneuron (integration) to motor); 2 neuron pathways (sensory synapses directly with motor neuron) exist as well – knee jerk reflex for ex. Somatic reflexes activate skeletal muscle (like the knee-jerk). Autonomic reflexes control smooth muscle contraction, heart rates, digestion processes, and glandular activity among other things without any conscious thought or effort.

Nervous System gross anatomy:The brain: I. Cerebrum – biggest region of the brain divided into right and left hemispheres. The hemispheres are

connected to and communicate with one another through the corpus callosum. Each hemisphere is further divided by deep fissures into 4 major lobes (frontal, parietal, occipital, and temporal lobes). The cerebrum is the sight of higher brain functioning and includes areas for speech, memory, logic, voluntary movement, and sense interpretation. Much of the brain has been mapped out according to function, including the homunculus (map of conscious sensations and voluntary movement).

II. Diencephalon – completely surrounded by cerebrum, perched atop the inferior brain stem. Includes the Thalmus (relay station from senses to cerebrum) and Hypothalmus (regulator of water balance, body temp, metabolism – thirst, appetite, sex, pain, pleasure).

III. Brain Stem – Connects brain to spinal cord. Includes midbrain (vision/hearing reflex), pons (mainly nerve axons, breathing), and medulla oblongata (control of heart rate, blood pressure, breathing, swallowing, vomiting reflexes).

IV. Cerebellum – 2 lobes like cerebrum, sits inferior and posterior to cerebrum; connects sight and balance senses to skeletal muscle control for coordination of movement with intentions.

The CNS is protected by skin, bone, the meninges (three protective membranes that surround the brain and spinal cord), cerebrospinal fluid (derived from blood plasma, cushions brain from blows to the head), and the blood-brain barrier (impermeable capillary system that prevents brain infections from blood-born bacteria; however, it does allow fat soluble substances and drugs like alcohol and nicotine to pass into the brain and effect neurons).

The 3 meninges are known as the dura mater (most superficial), arachnoid mater (web-like middle membrane), and the pia mater (deepest membrane) see Figure (a) below). The meninges are continuous with the spinal cord and can be sites of dangerous infections known as meningitis (can quickly cause brain damage due to swelling around brain).

(a) View of meninges and brain relationship View of nerve bundle

The Spinal Cord extends from the base of the skull (foramen magnum of occipital bone) to the 1st or 2nd lumbar vertebra. 31-paired PNS nerves extend outward from the spinal cord from between each vertebra and from openings in the sacrum. The end of the spinal cord is diverted into several independent nerves that control and sense the lower parts of the body; these smaller nerves fill the lumbar and sacral regions of the spinal column and form the cauda equina (horse tail). The lumbar/cauda equina region is commonly used to take cerebrospinal fluid in a diagnostic procedure commonly known as a spinal tap.

The PNS:Each of the paired nerves that exit through the vertebrae originates from the spinal cord as 2 separate nerve bundles, the ventral root and the dorsal root. The ventral root is usually composed of the axons of motor neurons; the cell bodies of these motor neurons are typically housed inside the spinal cord and help form the grey matter of the spinal cord. The cell bodies of sensory nerves bringing messages to the CNS are often housed in dorsal root ganglia found on the dorsal root (see diagram above).

The large nerves of the PNS that extend from the spine and cranium are packaged very similarly to skeletal muscle. Each individual nerve cell is wrapped in a layer of connective tissue known as the endoneurium; several individual nerve cells are further bundled into fascicles by perineurium connective tissue; the entire set of fascicles as well nourishing blood vessels are wrapped together by an epineurium to form a nerve (see diagram in top left of this page).