XIII. Neural Control

Brain

The brain is divided into the forebrain (telencephalon), the between brain (diencephalon), the midbrain (mesencephalon), cerebellum (metencephalon), and the medulla oblongata (myelencephalon) [ha]. The most anterior portion of a fishes brain, the forebrain, contains the olfactory lobes and its activity includes, but is not limited to, recognition of odors. The between brain connects with the pineal gland at the top of the skull. Through the lower section of the between brain known as the hypothalmus the between brain connects with the pituitary gland, as well. The between brain integrates incoming and outgoing neural and hormonal signals. The midbrain of fishes of includes the optic lobes, and besides vision, it is integral to learning. The primary function of the cerebellum is muscle coordination and swimming. The medulla oblongata serves as a relaying center between the spinal cord and the higher brain areas. The brain and spinal cord together are termed the central nervous system (CNS).

Innervation

Types of Vertebrate Innervation
I. Sensory neurons - input to the CNS; nerves leading to the brain from optic, auditory, tactile, and other receptors
II. Motor neurons - outflow from the CNS; after the CNS has processed the sensory information, voluntary or involuntary reactions are sent out along the motor neurons.
	A. Somatic motor neurons - neurons used in potentially voluntary control of skeletal muscles. They have only one neuron outside of the CNS.
	B. Autonomic motor neurons - neurons used in involuntary control of internal organs. They have two neurons with a synapse outside of the CNS.
		1. Sympathetic (spinal autonomic nerves) - tend to be used in involuntary actions that expend energy (e.g. increase of heart rate). These arise from the spinal cord and the synapse is in a ganglion close to the cord.
		2. Parasympathetic (cranial autonomic nerves) - tend to be used in involuntary actions that conserve energy (e.g. decrease of heart rate). They arise from the brain and the synapse tends to be close to, or on, the innervated organ.

The brain receives information from the senses and sends commands back to the body through neurons. Neurons connect with each other at synapses. Nerve signals are propagated along neurons by electrochemical pulses along the membrane. Signals travel from neuron to neuron or from neuron to muscle across the synapse by diffusion of neurotransmitters.

Skeletal muscles are controlled by somatic motor neurons in an ever-changing pattern in response to the sensory inputs of the moment. This is termed voluntary. Smooth and cardiac muscle are controlled in a more rhythmic, reflexive way by autonomic motor neurons that is termed involuntary. Though the terms voluntary and involuntary may be more applicable to humans than fish. Nearly all of the involuntary regulation is achieved by a balance between parasympathetic and sympathetic neurons.

Research into whether control is sympathetic or parasympathetic can be done by using blockers that are specific for the transmitters in each case, i.e. atropine blocks acetylcholine (ACh), the transmitter between the parasympathetic neuron and the muscle. The transmitter on the sympathetic side is typically adrenaline or its analog, noradrenalin, which is unaffected by atropine. So atropine can be used locally, say in the pericardium, to block the parasympathetic innervation without blocking sympathetic innervation of the heart and thus determine method of cardiac control. ACh is also the typical neurotransmitter in the somatic motor neuron system. The effects of many drugs are mediated through their action on neurotransmitters [an example].

Adrenaline in larger quantities in general circulation acts as hormone. The tissues that produce adrenaline (the adrenal medulla in mammals, the chromaffin tissue in fish) are derived from the endings of sympathetic nerves. That makes them one of the few endocrine tissues under direct neural control. It is not surprising, therefore, that the adrenal response is the fastest hormone action.

Hagfish have no innervation of the heart, so their heart beats steadily unless stimulated by adrenaline in their blood. Lampreys and elasomobranchs have only parasympethetic innervation, so they can only slow their heart by nerve impulses.

Besides heart rate, another example of autonomic neural regulation is gas bladder pressure.There is both sympathetic and parasympathetic innervation of the resorptive area and the gas gland. When there is a predominant parasympathetic outflow, the gas bladder gains pressure. These impulses tend to constrict the muscles and reduce the size of the resorptive area. In the gas gland, they increase lactic acid activity and increase dilation of capillaries. Sympathetic outflow would have the opposite effect.