Nervous System for the MCAT

Medistudents Team
May 3, 2023

As part of the Biological and Biochemical Foundations of Living Systems section of the exam, the MCAT assesses your knowledge of the structure and function of the nervous system, including:

  • The major functions
  • The organization of vertebrate nervous system
  • Sensor and effector neurons
  • Sympathetic and parasympathetic nervous systems: antagonistic control
  • Reflexes
  • Integration with endocrine system: feedback control

This guide covers all key features detailed in the MCAT syllabus to help you to prepare for the exam.

Major functions of the nervous system

The nervous system has three major functions:

  • Sensory input
  • Motor output
  • Integration

Sensory input involves many sensory receptors monitoring changes inside and outside the body. The nervous system then interprets sensory input and decides what effector organs to activate. Effector organs can include glands and muscles, and when activated the response is known as motor output.

During integration, the nervous system has a high level of control – integrating multiple body systems while also being able to adapt to external influences. It does this by sending the sensory input to the central nervous system (the brain and spinal cord) and allowing this system to create an appropriate response and motor output.

Organization of the vertebrate nervous system

Neurons are the basic ‘unit’ of the nervous system, they use chemical signals and electrical impulses to transmit information to different parts of the nervous system. Synapses are found between neurons, allowing information to pass from one neuron to another.

The nervous system can be separated into many different groups. The central nervous system is made up of the brain and spinal cord. The peripheral nervous system is made up of the branching nerves that extend from the spinal cord to all parts of the body.

The autonomic system is crucial in maintaining homeostasis (state of steady internal conditions) and is known as an ‘involuntary’ nervous system, as it does not require conscious control. This system can be divided into three divisions – sympathetic (controls the ‘fight or flight’ response), parasympathetic (controls the ‘rest and digest’ response) and enteric (controls involuntary motor functions and the secretion of gastrointestinal enzymes).

Contrarily, the somatic nervous system is associated with the voluntary control of body movement as well as being responsible for the reflex arc.

Sensor and effector neurons

There are many subcategories of neurons. Sensory (afferent) neurons convey information from the sensory organs to the central nervous system. Effector (efferent) neurons convey information from the central nervous system to effector organs (necessary for feedback control).

Interneurons connect sensory and effector neurons allowing them to create a ‘circuit’ used for sending information to different parts of the nervous system.

Sympathetic and parasympathetic nervous systems: antagonist control

The two main divisions of the autonomic nervous system (the sympathetic and parasympathetic nervous system) usually have antagonistic (opposing) effects.

The sympathetic system controls the body’s response to danger (‘flight or fight’), preparing the body for strenuous physical activity by increasing heart rate, slowing digestion etc.

The parasympathetic system controls the body’s response when resting (no threat). This includes lowering the heart rate, stimulating digestion and lowering blood pressure – working in opposition to the sympathetic system.


A reflex action is an action that happens automatically in reaction to a stimulus. For example, if someone was to move their hand towards you as if to hit you, you would close your eyes (without you even thinking about closing your eyes).

The reflex arc

The reflex arc is a specific type of neural pathway that begins with a sensory neuron and ends with a motor neuron. To allow for faster reflex actions, the sensory neurons of a reflex arc bypass the brain and activate spinal motor neurons instead.

Reflex arcs can be categorized into autonomic (affecting inner organs) and somatic (affecting muscles). As well as either being monosynaptic (no interneurons – only one synapse, between the sensory and motor neuron) or polysynaptic (having one or more interneurons and therefore more than one synapse). Most reflexes are polysynaptic.

It must be noted that although the spinal cord is where the reflex arc occurs (not the brain), the brain is still aware of the reflex occurring.

Feedback loops

Positive feedback loops are used to amplify a response. For example, oxytocin is released when the uterus contracts during labor, which causes more contractions.

Negative feedback loops are used to inhibit a response so that the body can maintain homeostasis (a steady set of conditions). Most reflex arcs are negative feedback loops.

The role of the spinal cord

The spinal cord is where the synapses for the reflex arc are situated (not the brain). However, the brain is still aware of the occurrence of the reflex even though no electrical stimulus reaches the brain.

Contrarily, a supraspinal (‘above a spine’) circuit requires input from the brain to process stimuli.

Integration with the endocrine system

The endocrine system involves glands and their chemical messengers, hormones. This system is important in growth, development, reproduction and metabolism, and relies on feedback loops to control hormone levels.

Most of the endocrine system is under a negative feedback control, this prevents high levels of hormones (that are damaging to the body) to accumulate. For example, the pancreas relies on negative feedback loops to maintain blood glucose levels. When blood sugar levels get too low, glucagon is secreted from the pancreas to get blood sugar levels back up to the normal range.

Hopefully you now have a clear overview of the structure and function of the nervous system for the MCAT exam. Revising the topics covered in the syllabus will support you to prepare effectively and achieve a high MCAT score. We’ve got you covered with revision materials for a wide range of MCAT topics, including nerve cells and lipids.