Parts of the Nervous System

There will be a refractory period after the cell has returned to its resting potential that means the cell can temporarily be unable to be activated again. The action potential is the electrical potential that moves down the cell axon or dendrite when the cell is activated. This is an all or none phenomena so the neuron is either activated or it is not.

PARTS OF THE NERVOUS SYSTEM

There are two major divisions of the nervous system. These are the central nervous system or CNS and the peripheral nervous system or PNS. The CNS is basically just the brain and the spinal cord, while the PNS is the rest of the nervous system—namely, the peripheral nerves.

Most of the peripheral nerves are axons that travel in bundles called nerves throughout the body. There are large and small nerves. There are also clusters of nerve cell bodies in the peripheral nervous system called ganglia. There are two subdivisions of the peripheral nervous system. These are the somatic nervous system and the autonomic nervous system.

The somatic nervous system consists of nerves that can be motor nerves or sensory nerves. Sensory nerves pick up signals from the environment and send the signal to the central nervous system to be processed. These are referred to as afferent nerves because they go into the body. Motor nerves are efferent nerves that send signals out of the central nervous system to the muscles that move in response to voluntary thought or in response to a sensory signal. There are interneurons within the central nervous system that can connect two different types of nerves together.

The autonomic nervous system is very important to psychology because its involuntary activities can be strongly influenced by stress and one s emotions. There are two subdivisions of the autonomic nervous system. These are the somewhat opposite systems of the sympathetic nervous system and the parasympathetic nervous system. Their function is to maintain the homeostasis or equilibrium of the body. Figure 8 shows the different activities of the autonomic nervous system branches:

Figure 8.

The sympathetic nervous system is the active system in the body s fight or flight response. This is what happens when the body is facing a real or imagined threat. There are several things that happen. The pupils dilate, the mouth dries up, the heart rate increases and the bronchial tree dilates opening the lungs. Digestion and the contraction of the bladder become inhibited.

The parasympathetic nervous system is the system related to rest and digestion. The pupils constrict and salivation increases. The heart rate slows down and the bronchi constrict. Digestion is stimulated and the bladder contracts.

As you can imagine, the sympathetic nervous system occurs during high-arousal situations, which used to be more adaptive among hunter-gatherer people who needed this system when facing game hunting or the threat of predators. It is less adaptive today and becomes activated when we are faced with increased stressors in our environment. It simply is less common today that we face a real-life threat and more common to activate this system when dealing with everyday stressful events. If activated too much, it can lead to negative health consequences.

The central nervous system consists of the brain and spinal cord. The brain consists of billions of neurons and glial cells. There are two halves or hemispheres in the brain,

which is also divided into several lobes. Each lobe has a specific function, although the lobes interact with one another.

The spinal cord is connected to the brain through the brainstem. It consists of many tracts that send signals up and down the spinal cord to and from the brain and peripheral nerves. There are some automatic reflexes that do not reach the conscious brain but are instead processed in the spinal cord itself. These include things like the knee jerk reflex and the automatic withdrawal when touching something hot or painful.

The surface of the brain is called the cerebral cortex. It consists of folds called gyri and grooves called sulci. These are basically landmarks that define the different parts of the brain. The deepest sulcus is called the longitudinal fissure, which is what separates the two hemispheres of the brain.

The brain is generally lateralized, which means that the different halves of the brain are responsible for different things. The left hemisphere mostly controls the right side of the body and vice versa with the right hemisphere. The corpus callosum is a thick bundle of nerves that connect the two hemispheres.

The largest part of the brain is referred to as the forebrain. It consists of the cerebrum, which is the biggest section of the brain, and subcortical structures, such as the thalamus, pituitary gland, structures of the limbic system, and the hypothalamus. The cerebral cortex is the thinking part of the brain. It is what gives us consciousness, reasoning, memory, and language. There are four lobes that make up the cerebrum. Figure 9 shows the different lobes of the brain:

Figure 9.

The different lobes of the brain are called the frontal lobe, the parietal lobe, the occipital lobe, and the temporal lobe. You should know the basic functions of each lobe. The frontal lobe is associated with emotions, motor control, language, and reasoning. It is where the motor cortex is located and it has the prefrontal cortex, which is concerned with higher-order cognitive processing. Broca s area is found in the frontal lobe; it is necessary for the production of intelligible speech.

The parietal lobe is found behind the frontal lobe. It processes sensory information. Part of the parietal lobe is the somatosensory cortex, which processes all sorts of sensory information, such as temperature, touch, and pain sensation.

The temporal lobe is on the side of the head. It is strongly associated with hearing, emotions, memory, and some parts of language. It contains Wernicke s area, which is essential for speech comprehension. People with defects in this area can produce speech but it is often unintelligible and there is little ability to understand speech. The auditory cortex is located in the temporal lobe and is concerned with hearing.

The occipital lobe is found at the back of the brain. It contains the primary visual cortex, which helps to interpret visual information. Defects in this part of the brain will lead to cortical blindness, even though the eyes themselves are normal.

The thalamus is located deeper within the brain than the cerebral cortex. It provides the sensory relay system for all of the senses except for the sense of smell. It can be thought of as a routing system for sensory input from the periphery of the body.

The limbic system is best thought of as the source of many of our emotions but it also deals with memory. The sense of smell does not go through the thalamus but instead goes through the limbic system, which explains why smell has such an emotional impact. The three structures of the limbic system are the amygdala, the hypothalamus, and the hippocampus. The hippocampus is crucial for memory and learning. The amygdala helps to make sense out of the emotions one experiences. The hypothalamus helps to regulate appetite, body temperature, and blood pressure. It also has endocrine function.

Deeper within the brain is the midbrain. These parts of the brain are largely out of conscious control. In the midbrain is the reticular formation that helps to control arousal, the sleep/wake cycle, motor activity, and alertness. Also, in the midbrain is the substantia nigra and the ventral tegmental area, which produce the neurotransmitter called dopamine, which is used to control certain aspects of movement. These area and dopamine are involved with reward, mood, and addiction.

The hindbrain is the back and deepest part of the brain. Within the hindbrain are the pons, medulla, and cerebellum. The medulla has centers for many unconscious processes, such as blood pressure, heart rate, and breathing. The pons connects the brain to the spinal cord and is active in regulating the activity of the brain during sleep.

The cerebellum is crucial for certain aspects of movement, such as coordination, balance, and adequate motor skills. Procedural memory, such as the ability to ride a bike and other tasks, are associated with the cerebellum.