All You Need to Know: MCAT Nervous System

From facilitating your perception of the world around you to cherishing the most unforgettable moments of your life, the nervous system showcases incredible abilities among organ systems. A comprehensive grasp of the nervous system is vital for achieving a high MCAT score, as it serves as the cornerstone for various aspects of human physiology and behavior that are assessed in the exam. This article aims to provide you with all the essential knowledge about the nervous system, ensuring you are well-prepared to excel in your MCAT.

To learn how to master the biological bases of behavior for  the MCAT, visit Jack Westin’s MCAT Content Guide.

 

Structure of the Nervous System

The nervous system serves three key roles: receiving sensory input, processing information, and producing motor output. This flexible system is composed of two main components:

Central Nervous System (CNS):

The CNS, comprising the brain and spinal cord, is responsible for processing sensory input. Information collected by the Peripheral Nervous System (PNS) is sent to the CNS for analysis. Subsequently, the CNS instructs the PNS to execute diverse biological functions, primarily through muscle control.

Peripheral Nervous System (PNS):

The PNS gathers sensory input and transfers it to the CNS for further processing. The CNS then guides the PNS in coordinating various biological functions, with a primary emphasis on muscle control.

The Peripheral Nervous System (PNS) undergoes further subdivision into two distinct subsystems:

Somatic Nervous System:

The somatic nervous system is responsible for voluntary muscle control and sensory perception. It enables conscious control over skeletal muscles and facilitates the reception of sensory information from the external environment.

Autonomic Nervous System (ANS):

The autonomic nervous system regulates involuntary bodily functions, maintaining internal balance and responding to stress. It is subdivided into the sympathetic and parasympathetic divisions, orchestrating automatic processes such as heart rate, digestion, and respiratory rate.

Microstructure of the Nervous System

The nervous system exhibits a microanatomical structure consisting of two primary types of cells:

Nerve Cells:

Nerve cells, also known as neurons, are fundamental units responsible for transmitting signals throughout the nervous system. They play a crucial role in conveying information through electrical impulses, facilitating communication within the neural network.

Key features of neuron structure and function include:

• Cell Body:

The cell body contains the nucleus and other organelles necessary for the neuron’s overall function.

• Dendrites:

Branched projections extending from the cell body that receive signals from other neurons or sensory receptors.

• Axon:

A long, slender extension of the neuron that carries electrical signals, known as action potentials, away from the cell body.

• Myelin Sheath:

A fatty, insulating layer covering some axons, enhancing the speed of nerve impulse transmission.

• Nodes of Ranvier:

Gaps in the myelin sheath that enable the action potential to jump from one node to the next, accelerating the speed of conduction.

 

Glial Cells:

Glial cells, a category of non-neuronal cells, fulfill essential roles by providing structural and functional support to neurons within the nervous system. Their contributions are vital for maintaining neuron health, functionality, and the modulation and regulation of neuronal activity.

The three types of glial cells:

• Astrocytes:

Astrocytes offer structural support and contribute to maintaining the appropriate chemical environment necessary for efficient neuron signaling.

• Oligodendrocytes (CNS) and Schwann Cells (PNS):

Oligodendrocytes in the Central Nervous System (CNS) and Schwann cells in the Peripheral Nervous System (PNS) play a crucial role in producing myelin. This fatty substance is essential for facilitating the transmission of nerve impulses, contributing to efficient communication between neurons.

• Microglia:

Microglia function as immune cells within the nervous system, participating in processes related to immune response and defense.

 

Signaling in the Nervous System: Action Potential and Synaptic Transmission

Action potentials and synaptic transmission play crucial roles in facilitating the transmission and processing of information within the nervous system. These processes encompass the exchange of electrical and chemical signals among neurons, enabling the coordination of adaptive responses to various stimuli. The effectiveness of action potentials and synaptic transmission is intricately linked to the functioning of gated ion channels.

Gated Ion Channels

Gated ion channels are integral transmembrane proteins forming pores in the cell membrane, selectively permitting specific ions to traverse. They exhibit the ability to open or close in response to diverse stimuli, including alterations in voltage, ligand binding, temperature, or mechanical force. These channels can be broadly categorized into two types:

• Voltage-Gated Ion Channels:

Responding to changes in membrane potential, voltage-gated ion channels selectively allow ions like sodium, potassium, and calcium to traverse the cell membrane. These channels are pivotal in generating and transmitting electrical signals along neurons.

• Ligand-Gated Ion Channels:

Activated by the binding of specific signaling molecules, such as neurotransmitters, ligand-gated ion channels can either depolarize or hyperpolarize the neuron’s membrane potential, initiating a postsynaptic response.

Examples of Voltage-gated Ion channels In Neurons:

• Sodium-Potassium (Na+/K+ ATPase) Pump:

An ion pump utilizing energy from ATP hydrolysis to transport Na+ ions out of the cell and K+ ions into the cell. It is crucial for maintaining the resting membrane potential and normal neuronal function.

• Calcium (Ca2+) Pumps:

Responsible for controlling intracellular Ca2+ concentration, these pumps, including plasma membrane Ca2+ ATPase and sarcoplasmic reticulum Ca2+ ATPase, are significant for neurotransmitter release and synaptic plasticity.

It’s important to note that the sodium-potassium pump operates differently from voltage-gated sodium (Na+) and potassium (K+) channels. While the pump actively transports ions, using ATP as an energy source, voltage-gated channels permit passive ion flow in response to membrane potential changes.

• Leak Channels:

These ion channels remain consistently open, allowing a continuous flow of ions through the cell membrane. They play a crucial role in maintaining the resting membrane potential and are not regulated by changes in membrane potential like voltage-gated ion channels.

 

The Dynamics of Action Potentials in Neurons

Neurons employ a specialized electrical signal known as an action potential to accurately transmit information over long distances. This signal is initiated by the activation of voltage-gated ion channels, leading to a sudden influx of positively charged Na+ into the cell and the subsequent propagation of the action potential along the neuron’s axon.

Distinct phases define the various stages of electrical activity during an action potential:

• Resting Phase:

The neuron is at rest, exhibiting a negative membrane potential. Both Na+ and K+ channels are closed, while leak channels are open.

• Depolarization Phase:

A stimulus induces membrane depolarization, opening sodium channels and allowing a rapid influx of Na+ into the neuron. This causes swift depolarization of the membrane potential, while K+ channels remain closed.

• Threshold Phase:

Reaching a specific threshold initiates a positive feedback loop, intensifying depolarization in an all-or-nothing response. If the threshold potential is not attained, the neuron does not generate an action potential. More Na+ channels open, and K+ channels stay closed.

• Repolarization Phase:

As the membrane potential approaches its peak, Na+ channels begin to close, and K+ channels open. This enables K+ to exit the neuron, leading to rapid repolarization toward the resting potential.

• Hyperpolarization Phase:

The membrane potential becomes more negative than the resting potential as K+ channels remain open, causing a brief hyperpolarization. Na+/K+ ATPase pumps and leak channels then restore the resting membrane potential.

• Refractory Period:

During this period, the neuron cannot generate another action potential. Na+ channels are inactivated, and the membrane potential returns to its resting state. This ensures discrete action potential events, allowing for precise temporal coding of neural information.

Synaptic Transmission

Synaptic transmission is the intricate process through which neurons communicate by releasing neurotransmitters at specialized junctions known as synapses. These synapses consist of two key components:

• Presynaptic Neuron:

The presynaptic neuron releases neurotransmitters in response to an action potential.

• Postsynaptic Neuron:

The postsynaptic neuron receives these neurotransmitters, initiating a new action potential in response.

There are two types of synapses facilitating communication between neurons:

• Chemical Synapses:

In chemical synapses, neurotransmitters released by the presynaptic neuron trigger an action potential in the postsynaptic neuron. This process involves the binding of neurotransmitters to receptors on the postsynaptic neuron.

• Electrical Synapses:

Electrical synapses convey electrical signals directly through gap junctions between neurons.

Neurotransmitters

Neurotransmitters, acting as chemical messengers, facilitate signal transmission across synapses by binding to specific receptors on the postsynaptic neuron. This binding induces depolarization or hyperpolarization of the membrane potential, and the nature of the response is determined by the particular neurotransmitter and receptor involved. Notably, some neurotransmitters exert excitatory effects, while others produce inhibitory effects.

There are two primary types of receptors involved in this process:

• Ionotropic Receptors:

These receptors directly open ion channels in the membrane upon binding with neurotransmitters.

• Metabotropic Receptors:

Metabotropic receptors activate intracellular signaling pathways, indirectly influencing ion channels. The impact of neurotransmitters can be either excitatory or inhibitory, depending on whether they induce depolarization or hyperpolarization of the membrane potential.

Reflex Responses and Neural Pathways

Reflexes represent automatic responses to stimuli, serving a vital role in maintaining homeostasis and safeguarding the body from potential harm. The neural pathway through which a nerve impulse travels during a reflex is termed a reflex arc. This process initiates with a stimulus activating a sensory receptor, followed by signal transmission through sensory neurons to the spinal cord. Subsequently, the signal is processed and relayed via motor neurons to the effector, typically a muscle or gland.

The spinal cord assumes a pivotal role in reflexes as the primary processing center for the reflex arc. However, supraspinal circuits, encompassing the brain and brainstem, can also modulate reflexes, either enhancing or inhibiting them based on the situation.

Specialized sensory receptors contributing to reflex regulation include muscle spindles and Golgi tendon organs. Muscle spindles detect alterations in muscle length and velocity, crucial for regulating muscle tone and averting injury. Golgi tendon organs, located in tendons, respond to changes in muscle tension, preventing excessive force generation and safeguarding muscles and tendons.

Two main types of reflexes are identified:

• Monosynaptic Reflexes:

Involving only one synapse between the sensory neuron and the motor neuron.

• Polysynaptic Reflexes:

Involving two or more synapses, typically with the inclusion of at least one interneuron.

Complex Cognitive Processes: Higher Brain Functions

Nervous and Endocrine Integration

The nervous system and the endocrine system work in close harmony, jointly regulating bodily functions through the transmission of signals and the release of hormones. This integration ensures precise adjustments in response to changing internal and external conditions.

Key interactions between the nervous and endocrine systems include:

• Neurotransmitter and Hormone Release:

The nervous system utilizes neurotransmitters to transmit signals between neurons, while the endocrine system releases hormones into the bloodstream to impact target cells throughout the body.

• Hypothalamus as a Link:

The hypothalamus acts as a crucial link, regulating the release of hormones from the pituitary gland. This communication influences distant target cells and controls various bodily functions such as temperature, hunger, and thirst.

• Stress Response Regulation:

The nervous and endocrine systems collaborate to manage the stress response. The hypothalamus triggers the release of hormones like cortisol and adrenaline in response to stress. In the endocrine system, feedback mechanisms play a crucial role in maintaining hormone levels:

• Negative Feedback:

Sustains stability by detecting and reversing changes in conditions to uphold homeostasis. For instance, thermoreceptors in the skin and hypothalamus respond to a rise in body temperature by initiating reactions such as sweating and blood vessel dilation to cool the body.

• Positive Feedback:

Amplifies physiological responses, resulting in self-perpetuating events. Though less common, an example is the hypothalamus triggering oxytocin release during childbirth. This stimulates uterine contractions, leading to further oxytocin release and stronger contractions until childbirth is complete.

To read more on this topic, check out Jack Westin’s guide on The Nervous System.

Conclusion

In conclusion, a profound understanding of the nervous system is indispensable for achieving success in the MCAT examination. Mastering these concepts not only ensures success in the MCAT but also lays the groundwork for a comprehensive understanding of the complexities of the human body, a fundamental requirement for any aspiring medical professional.

Make sure you don’t miss out on Jack Westin’s MCAT Podcast available on YouTube, Spotify, and Apple Podcasts. Dive into expert insights, elevate your prep, master key topics, and approach the MCAT with unwavering confidence!

To learn more about what’s tested on the MCAT, you can go to  Jack Westin’s MCAT Content or check out our admissions services and choose a package that best suits your needs.