Hindbrain and Midbrain Structures

The dog's nervous system is divided into two major parts: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS includes the brain proper and the spinal cord (Fig. 3.1). The PNS encompasses all nervous processes extending beyond the spine and skull, including a subsystem called the autonomic nervous system (ANS). The ANS is composed of two antithetical but complementary branches: the sympathetic and parasympathetic. The ANS is intimately involved in regulating basic bodily processes and in the mediation of the physiological expression of emotion and distress. Later in this chapter, autonomic functions are discussed in detail, since they appear to play a very significant role in the elaboration of disruptive stress and maladaptive behavior.

Cingulate Area Corpus Callosum

Frontal Lobe

Cingulate Area Corpus Callosum

Frontal Lobe

Corpus Callosum Dog

Cerebellum

Olfactory Bulb

Brain Stem

F ig. 3.1. Medial view of the dog's brain. Note the comparatively large olfactory bulb, providing the dog with the neural means to detect and analyze olfactory information.

Olfactory Bulb

Cerebellum

Brain Stem

F ig. 3.1. Medial view of the dog's brain. Note the comparatively large olfactory bulb, providing the dog with the neural means to detect and analyze olfactory information.

Medulla Oblongata, Pons, and Cerebellum

The hindbrain consist of the medulla, pons, and cerebellum. The medulla is a primitive brain structure located just above the spinal cord and regulates many vital biological functions, such as the control of heart rate, respiration, gastrointestinal functions, salivation, coughing, and sneezing. Together with the pons, the medulla is an important relay site for auditory and vestibular information, gustatory sensations and associated motor reactions, and information about various visceral states. The central portion of the hindbrain contains the reticular formation, a network of interconnected neurons that is associated with wakefulness and generalized sensory arousal.

An important function of the hindbrain is the synthesis of monoamine neurotransmit-ters. Serotonin-producing cells are located in the raphe bodies, a narrow strip of specialized neurons in the hindbrain, extending from the medulla to the midbrain. Norepinephrine (NE) is made by a group cells in the pons called the locus coeruleus, an area with highly pigmented blue neurons (Ranson and Clark, 1959). Whereas NE is associated with wake-fulness and learning, serotonin appears to play an important role in the activation of sleep and the modulation of various inhibitory processes.

The cerebellum, a brain structure associated with "automatic" coordinated movement and sensory processing, is interconnected via thalamic relays with the sensory-motor areas of the cerebral cortex. These interconnections form a complex loop of ascending projections from the cerebellum to the motor cortex and, subsequently, from the motor cortex descending back to the cerebellum via pontine nuclei. Cerebellar lesioning results in uncoordinated and awkward movement. Although the cerebellum plays a minimal role in higher conscious functions, the lateral portion of the cerebellum appears to be involved in certain cognitive and memory functions, especially in the mediation of skilled motor performances and aversive conditioned responses (Lavond et al., 1993). Interestingly, the only output from the cerebellar cortex is inhibitory, which is mediated via axons of specialized Purkinje cells. The Purkinje cells are large GABAergic neurons [the primary neural effect of GABA (gamma-aminobutyric acid) is inhibitory]. The inhibitory Purkinje cells project to highly active and excitatory subcortical nuclei, exerting a modulatory and regulatory effect on these neurons whose fibers ultimately project to higher motor brain centers.

Reticular Formation

The reticular formation is a brain stem structure extending from the medulla to the thalamus. The primary function of the reticular formation is the maintenance of a state of generalized neural arousal and alertness. Many projections leave the reticular formation and extend throughout the limbic system and cerebral cortex. This system of diffuse reticular fibers is referred to as the ascending reticular activating system (ARAS). In addition to its arousal functions, the ARAS is also believed to mediate an integra-tive effect on the nervous system. Electrical stimulation of the reticular formation of a sleeping dog results in the dog's arousal and awakening. On the other hand, lesioning of the reticular formation results in a permanent comatose or sleeplike state. Besides the arousal and attentional functions of the ARAS, the reticular formation also receives and gates sensory inputs, apparently mediating increased excitement and arousal resulting from peripheral sensory transmissions relayed through it. Unlike the corticothalamic relays where more specific sensory sorting, routing, and information processing takes place, the reticular formation is concerned with the general enhancement of alertness and excitability caused by these sensory inputs and the subsequent elaboration of a nonspecific orienting response to them. The auditory tract sends collateral axonal fibers directly into the reticular formation, perhaps accounting for the rapid and intense orienting responsiveness dogs exhibit toward novel sounds.

Gray (1971) has speculated that the ARAS

is especially well connected with sensory tracts associated with pain. He reports studies carried out by James Olds in which direct electrical stimulation of various portions of the ARAS (especially the periventricular areas) resulted in the evocation of escape and other pronounced behavioral expressions evidencing pain and discomfort. According to Gray (1971), the midbrain ARAS may play an important role in the arousal and activation associated with punishment or frustra-tive nonreward. Electrical stimulation of the midbrain reticular formation results in a direct potentiation or strengthening of ongoing behavior in a manner similar to that observed during punishment or frustrative nonreward.

Arousal resulting from activation of the reticular formation probably depends on the neurotransmitter NE. Inescapable trauma and prolonged stress result in the depletion of NE, and NE depletion is associated with learned helplessness (Seligman, 1975) (see Chapter 9). Seligman reviews some of the relevant physiological literature indicating that learned helplessness and collateral symptoms of depression may be linked to adrenergic depletion. Some disagreement in the literature exists with regard to the importance of NE and dopamine in the production of pleasure and reward. Thompson (1993) and many others attribute the brain's reward-and-plea-sure system to dopaminergic activity. An important area for the elaboration of pleasure is the medial forebrain bundle (MFB)—a major ascending pathway of various neurotransmit-ters, including serotonergic, adrenergic, and dopaminergic fibers. Siegel and Edinger (1981) emphasize the importance of the MFB as a conduit for adrenergic fibers originating in the locus coeruleus and projecting into the lateral hypothalamus and amygdala. Animals stimulated with electrodes inserted into the MFB act as though they were actually eating, drinking, and copulating, that is, appearing to be rewarded by the consumption of the corresponding but absent reward item (viz., food, water, and sex). However, drugs that block dopaminergic activity also apparently inhibit the pleasure resulting from electrical stimulation of these areas (White and Milner, 1992). Animals operantly trained to perform a bar-press response for intracra-

nial stimulation of MFB sites quit when dopamine levels are reduced. According to this theory, a dopaminergic pathway exists between the MFB and the ventral tegmen-tum and terminates in the nucleus accumbens. Precise stimulation of the nucleus accumbens (located in the forebrain, anterior to the hypothalamus) produces all the effects observed during electrical stimulation of the MFB, suggesting that earlier studies may have confounded dopamine and NE pathways. Most authorities currently believe that reward is most likely mediated by dopamin-ergic systems. However, modulating NE pathways may play a significant role in the experience of pleasure and reward via enhanced alertness, mood, and feelings of well-being affected by NE activity. Low levels of dopamine in the brain result in a loss of affect and positive feelings, whereas low levels of NE result in depressed mood and a sense of helplessness (Seligman, 1975).

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Responses

  • kidane robel
    Where is the olfactory bulb located in a dog's brain?
    6 years ago
  • ciaran boyle
    Where is a dog's brainstem?
    6 years ago
  • miranda
    What is the function of the reticullar formation of the medulla oblongata in the dog?
    6 years ago
  • Ambretta Dellucci
    Do dogs have reticular formations?
    6 years ago
  • jennifer
    What is the network of neurons in the hindbrain and midbrain?
    6 years ago
  • BRIGITTE
    How do dogs hindbrain function?
    6 years ago
  • Eetu
    Where is the corpus olfactory located?
    5 years ago
  • fearne dickson
    Do dogs have a reticular formation?
    3 years ago
  • alice
    Where is the dogs brain llocated?
    2 years ago
  • MUHAMMAD
    Which structure is NOT part of the hindbrain?
    2 years ago
  • Toini
    What is a dogs forebrain function?
    2 years ago
  • Christopher Davidson
    Where is a dogs brain located?
    2 months ago

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