Fundamentals of Nervous System and Tissue

1. Fundamentals of Nervous System and Tissue Chapter 12 Anatomy 32

2. Simplyfied schema of the nervous system

3. I. Basic Division of the Nervous System The nervous system is designed to gather information from the environment (sensory input), process it (integration), and stimulate the body to respond appropriately (motor output). All of this occurs through the interaction of the central nervous system (CNS) and the peripheral nervous system (PNS) A. Central Nervous System (CNS)- the brain and spinal cord make up this branch of the nervous system, both are protected by bone. B. Peripheral Nervous System (PNS)- all the nerves that exit the spinal cord and innervate the body make up the PNS. This branch has two subdivisions 1. Autonomic NS- this branch sends impulses to control involuntary actions 2. Somatic NS- this branch sends impulses to control voluntary actions

4. C. When a stimulus is detected by the NS the impulse is sent to the brain and is called afferent signal. Only the sensory organs are able to send this type of impulse. The brain receives the information and processes it; this is called integration. When it sends out a respond, the impulse sent to the body (ex. muscles) is called an efferent signal. This is possible only through motor neurons.

6. D. Sensory input and motor output is divided according to the region they intervate: 1. Somatic motor: efferent impulses that reach skeletal muscle (voluntary movement). 2. Branchial motor: The branch that innervates pharyngeal muscles (ex. those use to swallow) is called 3. Visceral motor: efferent impulses that reach the internal organs and regulates involuntary muscle contractions (makes up the autonomic NS). Thisi ncludes the heart and smooth muscle. 4. Somatic sensory: afferent impulses that originate from non-visceral areas such as skin and muscle. Proprioceptive senses sends sensory information from muscle, ligaments, tendons, and indicate body position. Special somatic senses are localized receptors such as those of the ear and eyes. 5. Visceral sensory: afferent impulses generated by internal organs, a specialized visceral sensory organ would be the taste buds.

8. II. Nervous Tissue- Nervous tissue develops from the embryonic neural tube and neuro crest. Two types of cells form: neurons and glial cells (supporting cells) A. The Neuron- these types of cells are excitable and can send an impulse (electrical signal). Neurons have three major parts: cell body, dendrites, axon. These cells live for many years, do not under mitosis, and are highly dependant on oxygen due to a high metabolic rate. 1. The cell body (soma)- contains a nucleus, cytoplasm, mitochondria, and a large number of rough ER (chromatiphilic bodies). They are usually found within the CNS and in the PNS they are called ganglia (ganglion) 2. Neuron processes- extensions of the cell body, may be termed dendrites or axons. Dendrites are receptive ends that allow signals to travel towards the cell body. Axons extend out of one side of the cell body called the axon hillock, it may be several feet in length and carry afferent signals. Collateral axons result when axons branch. At the end of an axon there are multiple branches called the terminal branches that end in knobs called terminal axons. The length of the axon is surrounded by schawnn cells that together make the myelin sheath. In between each schawnn cell is a gap called the node of Ranvier.

10. 3. Synapses- the connection that allows one neuron to communicate with the next. If the axon makes a synapse with a dendrite it’s called axodendritic synapse. If an axon connects with a cell body it’s called axosomatic synapse. If a synapse exists between two axons it’s called axoaxonic synapses. The cell conducting the impulse towards the synapse is called presynaptic neuron and the one that receives the impulse and sends it away from the synapse is called postsynaptic neuron.

12. Synaptic vesicles- membrane bound sacs containing neurotransmitter Neurotransmitter- chemical substance that allows an impulse to “jump” from neuron to neuron. Neurotransmitter vary depending on the type of signal that’s sent Synaptic cleft- the gap that separates the plasma membranes of the two neurons

13. 4. Impulse-an electrical signal is sent down the plasma membrane of a neuron. When an impulse reaches the synaptic end of a neuron the following occurs: a. The impulse signals the release of synaptic vesicles to fuse with the cell membrane with the presynaptic membrane at presynaptic density. b. Neurotransmitters that have been exocytosed enter the synaptic cleft and bind to the postsynaptic membrane at the postsynaptic density. c. The postsynaptic membrane changes from its polarized state (inside negative, outside positive) to being depolarized (charges switch sides). d. The depolarization continues down the neuron until the impulse reaches the next synaptic site.

14. 5. Types of potentials- the type of potential is defined by the activity stimulated a. Resting Potential- the neuron is not sending any type of impulse. The arrangement of ions makes the inside relatively more negative than the outside. b. Action Potential- also known as an impulse, the cells depolarizes to the extent that the inside is more positive than the outside. The impulse travels down the axons and as it leaves the axon returns to its resting potential.

16. c. Graded Potential- stimulus received by the dendrites or cell body determine whether an impulse will travel down the axon. A graded potential is a localized depolarization that travels towards the axon hillock and initiates the impulse. d. Synaptic potential- the potential changes at the synapse determine if the postsynaptic neuron sends an impulse or not. An excitatory synapse changes the potential so the postsynaptic neuron can send an impulse. An inhibitory synapse stops the travel of an impulse.

17. 6. Classification- neurons are classified based on the location of the cell body in relationship to the dendrites and axon. Different types of neurons carry out different types of functions; neurons are also classified by function. a. Multipolar neuron- about 99% of neurons are multipolar- meaning that the cell body has multiple extension/process b. Bipolar neuron- these are specialized sensory neurons found in the eye, ear, and nose. There are only two extensions/process on opposite sides of the cell body. c. Unipolar neuron-also known as sensory neurons. A short extension off the cell body connect the two process that leads to the axons and dendrites.

19. d. Sensory neuron (afferent)- These neurons have receptors in the PNS and send signals towards the CNS. The receptors are cells that capture a stimuli and transfer a signal onto the cendrite of the sensory neuron. The dendrite then connects tot eh cell body that sends an axon into the CNS (spinal cord). The bodies of sensory neurons are in ganglia outside the CNS. peripheral process going to receptors in the body (axon and dendrites). e. Motor neuron (efferent)- these are multipolar neurons and have cell bodies in CNS and connect in the body with effector cells (muscles/glands). f. Interneuron (association number)- multipolar neurons in the CNS that “link” sensory and motor neurons, relays messages to and from the brain and process information.

20. Neurons vary based on length of the axonand on branching of dendrites. The more complexthe brandhing, the greater the synaptic input.

21. B. Supporting Cells- these cells are responsible for insulating the neurons and preventing signal interference. 1. Supporting cell of Central Nervous system- Neurolia (glial cells) are smaller than neurons and are more numerous (in the brain) and have mitotic ability. a. astrocytes- star shaped, control ionic environment (prevents impulse interference), recapture and recycle neurotransmitters. b. microglia- small, least abundant, act as phagocytes c. epedymal- cells that make a simple epithelium lining the central cavity of spinal cord and brain and help to circulate the cerebrospinal fluid. d. oligodendrocytes- produce myelin sheaths of the CNS neurons. 2. Supporting cells of Peripheral Nervous system- the glial cells of the PNS are : a. satellite cells- surround cell bodies in the ganglion b. Schwann cells- wrap around axons to create the myelin sheath that assist in speeding up the rate at which the impulse travels.

23. 3. Myelin sheaths- these sheaths form an insulating layer to prevent leakage of electrical current. The membrane tightly wraps around the axon and the nucleus and cytoplasm together are called neurilemma. The impulses travel by jumping from one node of Ranvier to the other. Unmyelinated axons move signals slowly.

26. III. Nerves- these are bundles of axons (nerve fibers) that are myelinated or unmyelinated. Each axon is surrounded by an endoneurium, groups of nerves are bundles into nerve fascicles surrounded by perineurium and the whole nerve is surrounded by epineurium. *Know the difference between neuron, nerve fiber, and nerve.

27. IV. Basic Neural Organization of the Nervous System A. Reflex Arcs-Reflexes are involuntary unlearned actions that stimulate the somatic muscles or viscera to react quickly in response to a stimulus. There are five steps to a reflex: 1. The receptor in the PNS perceives a stimulus and sends a signal to the CNS (spinal cord) through the sensory neuron (afferent). 2. The impulse reaches the integration center in the CNS which may be simple called monosynaptic (no interneuron synapse betwee motor and sensory neurons), or very complex called polysynaptic (one or more interneuron synapses). 3. The motor neuron is stimulated to send an efferent impulse that reaches the effector. 4. The effector may be skeletal muscle (contracts), a gland (secretes), or visceral muscle (contraction).

29. B. Simplified design of the nervous system- The NS has areas called white matter which contain multiple myelinated axons, the areas of gray matter contain unmyelinated neuron parts such as cell bodies, dendrites, and unmyelinated axons. The NS is arranged so that motor neuron run out of the CNS on the ventral side and sensory neurons enter on the dorsal side. The cell bodies of sensory neuron are outside the CNS in ganglion while the motor and interneuron cell bodies are in the CNS gray matter. Interneurons process infor-mation received by sensory neurons and transmit signals to and from the brain.

31. V. Disorders of the nervous system- A. Multiple sclerosis- this is a disease in which small areas in the brain and spinal cord containing myelinated neurons are destroyed. It may be an autoimmune disease that causes repeating periods of disability and recovery. During disability the neurons are destroyed but during recovery the immune system may stop attacking the nervous system and the nerves regenerate. MS affects more women than men and it is influenced by genetic and environmental factors.