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Nervous System Structure & Function

MCAT Biology - Chapter 7 - Section 1.1 - Organ Systems - Nervous System Structure & Function
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Sample MCAT Question - Nervous System Structure & Function

Motor nerves that control skeletal muscle contractions fall under which part of the vertebrate nervous system?

a) The autonomic nervous system, which is part of the peripheral nervous system.

b) The somatic nervous system, which is part of the peripheral nervous system.

c) The enteric nervous system, which is part of the autonomic nervous system.

d) The sympathetic nervous system, which is part of the autonomic nervous system.

B is correct. The somatic nervous system, which is part of the peripheral nervous system. The vertebrate nervous system is made up of both the central nervous system and the peripheral nervous system. Within the peripheral nervous system is the autonomic nervous system, which controls involuntary functions, and the somatic nervous system, which controls the voluntary contraction of the skeletal muscle. Note: While the cell bodies of motor neurons are located in the central nervous system, motor nerves, which are bundles of axons from motor neurons, are part of the peripheral nervous system, and the motor nerves that control the voluntary contraction of skeletal muscle are located in the somatic nervous system specifically.

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Nervous System

This MCAT post will cover the structure and various functions of the vertebrate nervous system.

Nervous System Functions

The vertebrate nervous system has three primary functions: high-level control over body systems, integration of sensory information, and adaptation to external influences. For the MCAT, it is important to understand these three functions in detail.

Function #1: High-Level Control

Nervous system control over the body includes not only voluntary control of limbs but also involuntary control over several critical physiological processes. For example, digestion is a complex process partially controlled by the nervous system. When food particles enter the GI tract, the GI system will release enzymes and gastric juices to digest the food. However, even before food is physically ingested, the sight, smell, or even thought of food will trigger the brain to stimulate the GI tract to start releasing digestive juices and enzymes. This process essentially primes the body for feeding.

 

To give another example, the nervous system also exerts control over the reproductive system. Oxytocin, for instance, which is a hormone that controls uterine contraction during childbirth as well as lactation, is released from the periventricular cells of the hypothalamus in the brain. The hypothalamus also controls endocrine functions such as the regulation of the thyroid gland and the secretion of stress hormones from the adrenal gland.

Function #2: Sensory Integration

The nervous system has many sensory neurons that detect both external and internal stimuli. Photoreceptors in the eye, for instance, receive visual stimulus from the outside world, and nociceptors located on the skin detect injuries and facilitate pain. Internally, nociceptors detect the state of your GI system, eliciting feelings of comfort or discomfort, and baroreceptors detect changes in blood pressure based on the stretch of carotid arteries.

 

In the brain and spinal cord, sensory information is integrated and processed, thereby allowing the organism to respond appropriately. This is not always conscious. For instance, a person who accidentally touches a hot stove will reflexively pull their hand away before they feel the sensation of pain. This is because the hand-withdrawing reaction is mediated entirely by neurons in the spinal cord, without input from the brain.

Function #3: Adaptation to the Environment

The nervous system also allows organisms to adapt to their external environment. For example, let’s say someone was asked to hold a marker for a long period of time. At first, the individual would notice that the marker is in their hand. Over time, however, they would adapt to its presence, and their awareness of the marker would slowly disappear. Another example: if a person gets sick after eating a particular food, they learn to avoid that food in the future.

Nervous System Structure & Organization

The vertebrate nervous system can be divided into the central nervous system and the peripheral nervous system (see Figure 1). The central nervous system, or the CNS, consists of the brain and the spinal cord. The peripheral nervous system, or the PNS, consists of neurons whose cell bodies are located outside the brain and spinal cord, as well as the nerves that exit the brain and spinal cord to various parts of the body.

Nervous System Structure - MCAT Biology
Figure 1. Nervous System Structure

Divisions of the Peripheral Nervous System

The PNS can be further divided into the somatic nervous system and the autonomic nervous system (see Figure 2).

 

The somatic nervous system is under voluntary control. It includes sensory neurons that receive information from the skin, muscle, and joints, as well as motor neurons that control skeletal muscle contractions. An example of a somatic nervous system function is the voluntary movement of an arm or a leg.

 

The autonomic nervous system, on the other hand, is under involuntary control. Like the somatic nervous system, it includes both sensory and motor neurons. Sensory neurons in the autonomic nervous system receive information from the viscera and can detect things like GI discomfort or even stomach distension when a person is full. Motor neurons in the autonomic nervous system control the contraction of smooth muscle (e.g. peristaltic contractions in the GI tract) as well as secretions from glands (e.g. gastric acid secretion in the stomach).

Divisions of the Autonomic Nervous System

The autonomic nervous system can be further subdivided into the parasympathetic, sympathetic, and enteric nervous systems (see Figure 2).

 

The parasympathetic and sympathetic nervous systems are antagonistic to each other. The parasympathetic nervous system drives “rest and digest” processes, which are especially prominent after eating. These processes include salivation, urination, digestion, defecation, lacrimation, and sexual arousal. The sympathetic nervous system, by contrast, drives “fight or flight” responses that are prominent during a perceived attack or threat to survival. Such responses can include pupillary dilation, increases in heart rate and stroke volume, reduction of GI function, and activation of sweat secretion.

 

Lastly, the enteric nervous system refers to the neurons within the GI tract that regulate GI functions such as peristalsis and the secretion of GI hormones.

Organization of the Peripheral Nervous System - MCAT Biology
Figure 2. Organization of the Peripheral Nervous System

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