Chapter 9 Nervous system

9-4 What are GABA and glycine?

Gamma-aminobutyric acid (GABA) and glycine are inhibitory neurotransmitters. GABA is found in the brain, and glycine is found in the spinal cord.

9-3 Explain Why waves of depolarization are conducted faster in myelinated axons than in unmyelinated ones

. The myelin sheath prevents sodium ions from flowing across the neuronal cell membrane. Therefore, depolarization in myelinated axons can only take place at the gaps in the myelin sheath that occur at the nodes of Ranvier. When the sodium influx at one node is sufficient to open adjacent sodium channels, the next available sodium channel is at the next node of Ranvier. The depolarization wave in the myelinated axon skips from one node of Ranvier to the next, greatly accelerating the rate at which the depolarization wave moves from the neuron cell body to the other end of the axon.

9-8 What is the result of a normal PLR?

A normal response to shining a light into the eye of an animal (pupillary light reflex [PLR]) is for the iris in both eyes to constrict, thus making both pupils smaller.

9-8 What roles do the sensory receptor, sensory neuron, interneuron, and motor neuron play in the reflex arc?

A sensory receptor detects a change either in the external environment or within the body itself. Once stimulated to threshold, the sensory receptor sends an action potential (nerve impulse) along the sensory neuron to the gray matter of the spinal cord or brain stem. In the CNS gray matter, the sensory neuron synapses with other interneurons, which serve to integrate the incoming sensory impulse with other impulses from other sensory neurons. Finally, the integrated response of the reflex is sent out from the spinal cord or brain stem by the motor neuron, which ends at the target organ (muscle or endocrine gland).

9-8 What is the sensory receptor in the stretch reflex? What results from the stretch reflex arc stimulation? Is this an ipsilateral or contralateral reflex?

A specialized structure within the muscle called the muscle spindle. Muscle contraction results. Ipsilateral.

9-8 What happens in a withdrawal reflex? Is this reflex more or less complex than the stretch reflex?

A strong stimulus to a receptor causes the sensory somatic neuron to send impulses to the spinal cord. This reflex involves synapsing with several interneurons. Some of these interneurons will synapse with motor neurons that will cause contraction of a specific set of muscles responsible for pulling the limb away from the painful stimulus. Other interneurons will inhibit those opposing muscle groups so that the withdrawal of the limb is rapid and complete. This reflex is more complex than a stretch reflex.

9-4 How is acetylcholine different from acetylcholinesterase?

Acetylcholine is a neurotransmitter broken down quickly by an enzyme called acetylcholinesterase found on the postsynaptic membrane. The broken down components of acetylcholine are reabsorbed by the synaptic knob, reassembled into new acetylcholine molecules, and repackaged into vesicles for release with the next wave of depolarization.

9-5 Which part of the brain controls many autonomic functions related to cardiovascular, respiratory, and gastrointestinal functions?

Brain stem

9-4 What are catecholamines?

Catecholamines are the neurotransmitters norepinephrine, epinephrine, and dopamine.

9-5 What part of the brain is critical for coordination, posture, and fine motor control? How does this part of the brain accomplish these responsibilities?

Cerebellum. The cerebellum compares the movement the body intends with the actual position of muscles and joints to determine if the intentions of the cerebral cortex are actually being carried out. If the movements are not being carried out accurately, the cerebellum will stimulate or inhibit muscles to fine-tune the movements.

9-6 What is the fluid called that bathes, cushions, and aids in transport of materials to and from the CNS?

Cerebrospinal fluid

9-5 What part of the brain is responsible for conscious thought and perception of sensations?

Cerebrum

9-1 Name the parts of a typical neuron

Dendrite, axon, and cell body

9-1 How are the dendrites and axons different in structure and function?

Dendrites receive stimuli, or impulses, from other neurons and conduct this stimulation to the cell body. Dendrites also may be modified into sensory receptors that receive, or sense, stimuli, such as heat, cold, touch, pressure, stretch, or other physical changes from inside or outside the body. Dendrites tend to be short, numerous, multibranched projections extending from the cell body. The axon conducts nerve impulses away from the cell body toward another neuron or an effector cell (a cell that does something when stimulated, such as a muscle or gland cell). In contrast to the short, numerous, branched dendrites, the axon is a single structure that can be very long. For example, a single axon in the horse may extend several feet from the spinal cord all the way to the lower leg. Axons are often covered by a fatty substance called myelin.

9-5 What part of the brain serves as a relay station for impulses going to and from the cerebrum?

Diencephalon

9-4 What is the difference between an excitatory and an inhibitory neurotransmitter?

Excitatory neurotransmitters have an excitatory effect on the postsynaptic membrane when they combine with their specific receptors. Specifically, excitatory neurotransmitters usually cause an influx of sodium so that the postsynaptic membrane moves toward threshold. If the postsynaptic membrane is stimulated sufficiently by enough excitatory neurotransmitters, threshold will be attained and depolarization of the postsynaptic membrane will occur. In contrast, inhibitory neurotransmitters tend to hyperpolarize the postsynaptic membrane, making the inside of the cell more negative instead of positive and moving the charge within the postsynaptic cell farther away from threshold. When inhibitory neurotransmitters combine with their specific receptors on the postsynaptic side, they may cause chloride channels or potassium channels to open up on the postsynaptic membrane. This allows the negatively charged chloride ions (Cl) to enter the postsynaptic cell and potassium (K+) ions to leave the cell, making the inside of the cell more negatively charged (a change in charge that is opposite from that needed to reach threshold).

9-8 If trauma occurs in the segment of the spinal cord through which a particular reflex arc passes, will the reflex arc be hyperreflexive or hyporeflexive?

Hyporeflexive

9-2 Which are afferent nerve fibers: motor nerve fibers or sensory nerve fibers? Which are efferent?

Motor nerves are efferent and sensory nerves are afferent.

9-1 2.How are the functions of neurons and neuroglia different from each other?

Neurons are the basic functional units of the nervous system. That is, they are the smallest pieces of the system that show basic nervous system functions, such as responding to stimuli and conducting impulses from one part of the cell to another. The neuroglia, or glial cells (from the Greek glia, meaning "glue"), structurally and functionally support and protect the neurons. They outnumber neurons about 10 to 1, but they are not directly involved in the transmission of information or impulses through the nervous system. Rather, they are important parts of the infrastructure necessary for the neurons to do their jobs.

9-7 With which branch of the autonomic nervous system are muscarinic and nicotinic receptors associated?

Nicotinic acetylcholine receptors are found primarily on the postganglionic neurons of both the sympathetic and parasympathetic nervous systems. Muscarinic cholinergic receptors are found on the target organs and tissues supplied by the postganglionic neuron of the parasympathetic nervous system.

9-1 What is the relationship between the myelin sheath and the nodes of Ranvier?

Nodes of Ranvier are small gaps in the myelin sheaths. The nodes of Ranvier and the myelin work together to enhance the conduction of stimuli.

9-6 What are the 12 cranial nerves? Which nerves are motor, which are sensory, and which are both?

Olfactory: sensory Optic: sensory Oculomotor: motor Trochlear: motor Trigeminal: both sensory and motor Abducent: motor Facial: both sensory and motor Vestibulocochlear: sensory Glossopharyngeal: both sensory and motor Vagus: both sensory and motor Accessory: motor Hypoglossal: motor

9-8 What are the differences between an autonomic reflex and a somatic reflex?

Somatic reflexes involve contraction of skeletal muscles. Autonomic reflexes regulate smooth muscle, cardiac muscle, and endocrine glands.

9-7 With which branch of the autonomic nervous system are the alpha1, beta1, and beta2 receptors associated? What happens to the body when these particular receptors are stimulated?

Sympathetic nervous system. Alpha1-adrenergic receptors typically are found on blood vessels and cause the vasoconstriction of the skin, GI tract, and kidney associated with sympathetic stimulation. The increase in heart rate and force of contraction are the result of stimulation of beta1-adrenergic receptors by catecholamines, and the bronchodilation associated with sympathetic stimulation results from beta2-adrenergic receptor stimulation.

9-6 What helps keep dangerous poisons and certain drugs from leaving the blood and entering the brain? Describe this structure

The blood-brain barrier consists of capillary-wall cells in the brain that are aligned tightly together without openings, or fenestrations. In addition, the capillaries in the brain are covered by the cell membranes of glial cells.

9-2 What are the anatomic differences between the CNS and the PNS?

The central nervous system (CNS) is composed of the brain and spinal cord, which are found associated within the central axis of the animal's body. Peripheral means "to the side" or "away from the center"; therefore, the peripheral nervous system (PNS) is made up of those components of the nervous system that extend away from the central axis outward toward the periphery of the body.

9-5 What are the correct names for the bumps and fissures that make the cerebral cortex appear wrinkled?

The folds, called gyri (plural of gyrus), are separated by deep grooves called fissures and more shallow grooves called sulci (plural of sulcus). The most prominent groove is the longitudinal fissure, which divides the cerebrum into right and left cerebral hemispheres.

9-6 What are the protective membranes that surround, support, and protect the CNS?

The meninges. These are a set of connective tissue layers that surround the brain and spinal cord. The three layers of the meninges, from outside to innermost layer, are the tough, fibrous dura mater, the delicate, spider web-like arachnoid, and the very thin pia mater, which lies directly on the surface of the brain and spinal cord.

9-1 1. How do basic communication and control functions differ between the nervous system and the endocrine system?

The nervous system's chemical messengers, neurotransmitters, are contained in nerve endings. They are released across the small synaptic space when nerve impulses reach the nerve ending. This allows the nervous system to react quickly to changes, but neurons (nerve cells) cannot sustain individual activities for long periods of time. The endocrine system's chemical messengers, hormones, are released into the bloodstream, where they travel to their target cells. This causes the endocrine system to react more slowly to changes, but it can sustain individual activities for long periods of time.

9-3 What is the difference between the absolute and the relative refractory periods?

The neuron is said to be in a refractory period because it is refractory or insensitive to new stimuli until it recovers from the previous nerve impulse. During this period, the neuron cannot respond to cause a second depolarization. The absolute refractory period is the period of sodium influx and early potassium outflow when no stimulus of any intensity can cause the cell to depolarize again. The cell absolutely cannot respond. The relative refractory period is the period toward the end of repolarization when it may be possible to stimulate depolarization if a stimulus is very large relative to a "normal" stimulus.

9-4 What is the functional relationship between a neurotransmitter and a receptor? Will any neurotransmitter stimulate any receptor?

The neurotransmitter molecules released by the synaptic knob bind with receptors and trigger a change in the postsynaptic cell. However, the postsynaptic membrane receptors are very specific about which neurotransmitters they will bind. If the neurotransmitter and receptor are not matched, they will not bind to each other, and no change will be triggered in the postsynaptic cell. Thus synaptic transmission is only effective if receptors to the neurotransmitter exist on the postsynaptic cell's membrane.

9-8 What is the palpebral reflex? How is it used in veterinary medicine?

The palpebral reflex arc originates from receptors on the eyelid margins, travels via sensory neurons in CN V to the pons (brain stem), synapses with neurons in the pons, and then travels via CN VII to the muscles that blink the eyelids. If the reflex is active, a light tap on the medial canthus of the eye (the medial corner of the eye where the top and bottom eyelids meet) produces a blink of the eyelids. When an animal is anesthetized, the neurons in the pons become less responsive. Therefore, as anesthesia deepens, the palpebral reflex (also called the palpebral blink reflex) becomes less responsive and provides an indication of the animal's depth of anesthesia.

9-3 During depolarization, What ion channels open and what ion moves? Where does it move?

The potassium channels open and potassium moves out of the cell.

9-3 What normally maintains the resting membrane potential of a neuron during the resting state?

The resting membrane potential is maintained by the distribution of positive and negative charges from sodium, potassium, proteins, and other charged ions on either side of the neuronal membrane, creating a difference in electrical charge across the membrane, with the inside of the neuron being more negatively charged than the outside. By selectively pumping sodium out and potassium in, the sodium-potassium pump maintains this negatively charged resting membrane potential.

9-3 During depolarization, what ion channels open and what ion moves? Where does it move?

The sodium channels open and sodium ions move into the cell.

9-7 Compare and contrast sympathetic and parasympathetic nervous systems. Include in your comparison the preganglionic and postganglionic neurons neurons; the origin of preganglionic neurons; the neurotransmitters; and each system's impact on the heart, GI tract, blood vessels, bronchioles diameters, and the size of the pupil of the eye

The sympathetic nervous system emerges from the thoracolumbar area. The parasympathetic system emerges from the brain and the sacral vertebral regions and therefore is called the cranial-sacral system. Outside the thoracolumbar area of the spinal column are a series of autonomic ganglia (many ganglions) that form a chain called the sympathetic ganglion chain. The sympathetic preganglionic neuron extends out from the spinal cord and either synapses with a neuron within the ganglion chain or passes through the ganglionic chain and synapses with a neuron located beyond the sympathetic chain. Each sympathetic preganglionic neuron usually synapses with many postganglionic neurons in a wide variety of locations in the sympathetic chain or in ganglions outside the sympathetic chain. The sympathetic postganglionic neuron extends the remaining distance to the target organ. The parasympathetic preganglionic neuron travels directly from the CNS to its target organ, where it synapses with a short postganglionic neuron in the target organ. Thus the parasympathetic preganglionic neuron is relatively long compared with the very short postganglionic neuron. The sympathetic nervous system primarily uses norepinephrine as its key neurotransmitter. The neurons associated with the parasympathetic nervous system secrete acetylcholine as their neurotransmitter. Heart rate: increased by sympathetic and decreased by parasympathetic system Force of heart contraction: increased by sympathetic; no significant effect from the parasympathetic system Diameter of bronchioles: increased (dilated) by sympathetic and decreased (constricted) by parasympathetic system Diameter of pupil: increased (dilated) by sympathetic and decreased (constricted) by parasympathetic system Gastrointestinal motility, secretions, and blood flow: decreased by sympathetic and increased by parasympathetic system Diameter of skin blood vessels: decreased by sympathetic system; no significant effect from parasympathetic system Diameter of muscle blood vessels: increased by sympathetic system; no significant effect from parasympathetic system Diameter of blood vessels to kidney: decreased by sympathetic system; no significant effect from parasympathetic system

9-7 Which part of the autonomic nervous system is responsible for the fight-or-flight response and which is responsible for the rest-and-restore system?

The sympathetic nervous system is responsible for the fight-or-flight system and the parasympathetic nervous system is responsible for the rest-and-restore system.

9-4 What role do synaptic cleft, presynaptic neuron, neurotransmitter, and postsynaptic neuron play in the continuation of depolarization wave from one nerve to another?

The synapse is the junction between two neurons or a neuron and a target cell. The synapse consists of a physical gap between the two cells called the synaptic cleft. The neuron bringing the depolarization wave to the synapse and releasing the chemical to stimulate the next cell is called the presynaptic neuron. The chemical released by the presynaptic neuron is called the neurotransmitter, and the neuron that contains the receptors that receive the neurotransmitter is the postsynaptic neuron. In this way, depolarization continues from one cell to the next.

9-8 What is the role of the upper CNS in the reflex arc? If the CNS influence is removed or blocked, do reflexes become hyporeflexive or hyperreflexive?

The upper CNS normally produces a dampening or inhibitory effect on the reflex arc. If it is removed, the reflex becomes hyperreflexive.

9-3 What is threshold? What role does threshold play in the all-or-none principle?

Threshold is the number of sodium channels that must be opened in order for depolarization to occur. A stimulus must be strong enough for this threshold to be reached. According to the all-or-nothing theory, either the neuron depolarizes to its maximum strength or it does not depolarize at all (that is, the threshold is met or it isn't).

9-8 How is the crossed extensor reflex tied in with the withdrawal reflex? Is the crossed extensor reflex an ipsilateral or contralateral reflex?

When the withdrawal reflex arc is stimulated, the afferent somatic sensory neuron also synapses with another set of interneurons, causing extensor muscles in the opposite leg to contract and thus support the weight of the body when the other leg flexes. Thus the withdrawal and crossed extensor reflexes are related. The crossed extensor reflex is a contralateral reflex.

9-1 What is the difference between gray matter and white matter?

White matter refers to nervous tissue containing many myelinated axons. Conversely, nervous tissue that is made up largely of neuron cell bodies, which are not myelinated, appears darker and is called gray matter.

9-2 Identify each of the following as being controlled by the autonomic or somatic nervous system and as either sensory or motor: Conscious movement of the forelimb Slowing of the heart rate in response to an increased blood pressure Constriction of blood vessels in the skin in response to cold temperatures Perception of pain from an injection of antibiotics Perception of the amount of acidity present in the duodenum

• Conscious movement of the forelimb: somatic, motor • Slowing of the heart rate in response to an increased blood pressure: autonomic; sensory for sensing the increased blood pressure and motor for slowing the heart rate • Constriction of blood vessels in the skin in response to cold temperatures: autonomic; sensory for sensing the cold temperatures and motor for constricting the vessels • Perception of pain from an injection of antibiotics: somatic, sensory • Perception of the amount of acidity present in the duodenum: autonomic, sensory