The nervous system is a complex network of cells and tissues that coordinates and regulates the body's voluntary and involuntary actions. It utilizes sensory receptors to sense information from internal and external stimuli, processes them in the brain, and subsequently sends signals to muscles, organs, or glands to produce a response and maintain proper cellular function. A solid understanding of the nervous system is essential for getting a good as it forms the foundation for many aspects of human physiology and behavior topics tested in the exam. In this article, you will learn everything you need to know about the nervous system to ace your MCAT and practice with to test your knowledge of the topic!
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The nervous system has three primary functions: sensory input, integration, and motor output. It is an adaptable system that consists of the
- central nervous system (CNS)
- peripheral nervous system (PNS).
The PNS collects sensory input and sends it to the CNS for processing, which then directs the PNS to perform various biological functions through muscle control.
Central nervous system (CNS)
The CNS receives and processes information from the PNS to generate motor responses. It consists of the brain and spinal cord:
Brain: the control center for the entire organism, regulating many involuntary processes such as breathing and heartbeat, as well as conscious thought, emotion, and memory. More about brain structure and functions are discussed in Section 7 of this module.
Spinal cord: a tubular structure that transmits sensory and motor signals between the brain and the rest of the body (PNS). It also contains reflex circuits for rapid responses to stimuli.
Peripheral nervous system (PNS)
The PNS consists of all the nerves and ganglia (clusters of nerve cell bodies) outside of the brain and spinal cord. It detects internal/external changes and transmits information to the CNS via sensory neurons that reach all parts of the body. In the PNS, motor neurons carry information from the CNS to the muscles, glands, and organs.
PNS is further divided into two subsystems:
The ANS is further divided into the sympathetic and parasympathetic nervous systems that have opposing effects on target organs to maintain homeostasis:
The nervous system is composed of two main types of cells:
- nerve cells
- glial cells.
Nerve cells, also known as neurons, are specialized cells that serve as the fundamental building blocks of the nervous system. They process and transmit information between different parts of the body, including the brain, spinal cord, and organs.
The structure and function of neurons:
There are three different types of neurons, each with its specific function in the nervous system.
Glial cells are a type of non-neuronal cells that provide structural and functional support to neurons in the nervous system. They play crucial roles in maintaining the health and function of neurons, as well as modulating and regulating neuronal activity.
There are three types of glial cells, including:
- Astrocytes provide structural support and help maintain the proper chemical environment for neuron signaling.
- Oligodendrocytes (CNS) and Schwann cells (PNS) produce myelin, which is important for the transmission of nerve impulses.
- Microglia act as immune cells in the nervous system.
Action potentials and synaptic transmission are essential for the nervous system to transmit and process information throughout the body. These processes involve the transfer of electrical and chemical signals between neurons, allowing for coordinated and adaptive responses to stimuli. The proper functioning of action potentials and synaptic transmission relies heavily on gated ion channels.
Gated ion channels
Gated ion channels are transmembrane proteins that form a pore through the cell membrane and can selectively allow specific ions to pass through. They can be opened or closed in response to various stimuli—such as changes in voltage, ligand binding, temperature, or mechanical force—for the generation and propagation of electrical signals. They can be classified into two major types:
- Voltage-gated ion channels open/close in response to changes in membrane potential, allowing specific ions such as sodium, potassium, and calcium to pass through. They play a crucial role in generating and propagating electrical signals along the neuron.
- Ligand-gated ion channels open/close in response to the binding of specific signaling molecules, such as neurotransmitters. These channels can either depolarize or hyperpolarize the neuron's membrane potential, leading to the generation of a postsynaptic response.
Examples of voltage-gated ion channels in the neurons include:
Sodium-potassium (Na+/K+ ATPase) pump, which is an ion pump that utilizes energy from ATP hydrolysis to move Na+ ions out of the cell and K+ ions into the cell. It is crucial for maintaining the resting membrane potential of neurons and normal neuronal function.
Calcium (Ca2+) pumps help to control the Ca2+ ion concentration inside the cell. Several Ca2+ pumps, including the plasma membrane Ca2+ ATPase and sarcoplasmic reticulum Ca2+ ATPase, are significant for neurotransmitter release and synaptic plasticity.
The sodium-potassium pump is a different mechanism from voltage-gated sodium (Na+) and potassium (K+) channels. The pump actively transports sodium ions out of the cell and potassium ions into the cell, using ATP as an energy source. On the other hand, voltage-gated Na+ and K+ channels are ion channels that allow the passive flow of ions across the membrane in response to changes in membrane potential.
Leak channels are ion channels that are always open and allow a constant flow of ions through the cell membrane. They play a vital role in maintaining the resting membrane potential and are not regulated by changes in membrane potential like voltage-gated ion channels.
Neurons generate an electrical signal called an action potential, which enables them to transmit information accurately over long distances. This signal is triggered by the opening of voltage-gated ion channels, resulting in a sudden influx of positively charged Na+ into the cell. This influx triggers the propagation of the action potential along the axon of the neuron.
These phases classify the various stages of electrical activity that occur during an action potential:
- Resting phase: the neuron is at rest with negative membrane potential, both Na+ and K+ channels are closed and leak channels are open.
- Depolarization phase: a stimulus depolarizes the membrane potential, causing sodium channels to open and allowing Na+ to enter the neuron rapidly, causing rapid depolarization of the membrane potential. K+ channels are closed
- Threshold phase: reaching a certain threshold triggers a positive feedback loop that increases depolarization, resulting in an all-or-nothing response. If the threshold potential is not reached, the neuron won't fire an action potential. More Na+ channels open and K+ channels remain closed
- Repolarization phase: as the membrane potential approaches its peak, Na+ channels begin to close and the K+ channels open, allowing K+ to leave the neuron and causing the membrane potential to rapidly repolarize towards its resting potential.
- Hyperpolarization phase: the membrane potential becomes more negative than the resting potential because K+ channels remain open, causing an efflux of K+ ions, which briefly hyperpolarizes the membrane potential. Na+/K+ ATPase pumps and leak channels then restore the resting membrane potential.
- Refractory period: during this time, the neuron is unable to generate another action potential as the Na+ channels are inactivated and the membrane potential is returning to its resting state. This ensures that action potentials are discrete events and allows for proper temporal coding of neural information.
Synaptic transmission is the process by which neurons communicate with each other through the release of neurotransmitters at specialized junctions called synapses. These junctions consist of:
There are two types of synapses between neurons. Chemical synapses use neurotransmitters released from the presynaptic neuron to trigger an action potential in the postsynaptic neuron, while electrical synapses transfer electrical signals directly through gap junctions between neurons.
The arrival of an action potential at the presynaptic terminal triggers the release of neurotransmitters that bind to receptors on the postsynaptic neuron, generating a new action potential. This release is caused by the influx of calcium ions (Ca2+) through voltage-gated channels, leading to the fusion of vesicles and the release of neurotransmitters into the synaptic cleft.
Neurotransmitters are chemical messengers that transmit signals across the synapse by binding to specific receptors on the postsynaptic neuron, causing depolarization or hyperpolarization of the membrane potential. The type of response depends on the type of neurotransmitter and receptor involved. Some neurotransmitters can have excitatory effects, while others can have inhibitory effects.
There are two main types of receptors:
- Ionotropic receptors, which directly open ion channels in the membrane when they bind to neurotransmitters
- Metabotropic receptors, activate intracellular signaling pathways that indirectly affect ion channels. The effects of neurotransmitters can be excitatory or inhibitory, depending on whether they cause depolarization or hyperpolarization of the membrane potential.
Neurotransmitter–gated receptors and their associated functions in the neurons include:
Reflexes are automatic responses to stimuli that are important for maintaining homeostasis and protecting the body from harm. The pathway that a nerve impulse follows during a reflex is called a reflex arc. It begins with a stimulus that activates a sensory receptor. The signal is then transmitted through sensory neurons to the spinal cord, where it is processed and sent back via motor neurons to the effector, usually a muscle or gland.
The spinal cord plays a critical role in reflexes because it acts as the primary processing center for the reflex arc. However, supraspinal circuits, which include the brain and brainstem, can also modulate reflexes. These circuits can either enhance or inhibit reflexes depending on the situation.
Muscle spindles are specialized sensory receptors that detect changes in muscle length and velocity. They play a crucial role in regulating muscle tone and preventing injury. Golgi tendon organs are located in the tendons and respond to changes in muscle tension. They help prevent excessive force generation and protect the muscle and tendon from damage.
There are two main types of reflexes:
- Monosynaptic reflexes: involve only one synapse between the sensory neuron and the motor neuron.
- Polysynaptic reflexes: involve two or more synapses and at least one interneuron.
Sensory systems are responsible for the perception of the world around us, allowing us to detect various types of stimuli through different sensory receptors. Different sensory receptors detect various types of stimuli:
Motor systems are responsible for movement and muscle control:
- The pyramidal system, also known as the corticospinal tract, is responsible for voluntary movements such as reaching, grasping, and walking.
- The extrapyramidal system, which includes the basal ganglia and cerebellum, is responsible for regulating and coordinating movements, posture, and balance.
Higher brain function refers to the complex cognitive processes and behaviors that are unique to humans and other advanced animals, such as the integration of sensory information, memory, attention, perception, language, decision-making, problem-solving, creativity, and consciousness. They are essential for adapting to the environment, learning, social interactions, and overall human behavior.
The cerebral cortex is the outer layer of the brain and plays a critical role in conscious perception and higher cognitive functions. It is divided into four lobes, each with distinct functions:
The limbic system is a group of brain structures that are involved in emotion, motivation, and memory. It includes the amygdala, hippocampus, thalamus, hypothalamus, and basal ganglia. The amygdala plays a critical role in processing emotional information, while the hippocampus is essential in memory formation and spatial navigation.
Memory is the cognitive process to store, retain, and retrieve information. There are several types of memory, including:
- Sensory memory: a brief memory of sensory information such as visual or auditory stimuli.
- Short-term memory: also known as working memory, is a temporary storage of information for immediate use.
- Long-term memory: is the retention of information for an extended period, from hours to a lifetime.
Memory formation involves the consolidation of information from short-term to long-term memory. This process requires the interaction of various brain regions, including the hippocampus and prefrontal cortex.
The nervous system is closely integrated with the endocrine system, which regulates bodily functions through the release of hormones. This coordination enables precise regulation of bodily functions in response to changing internal and external conditions.
Interactions between nervous and endocrine systems in the body:
The endocrine system uses feedback mechanisms to regulate hormone levels in the body:
● Negative feedback maintains stability by detecting and reversing changes in conditions to maintain homeostasis. One example is the regulation of body temperature. When the body's temperature rises, thermoreceptors in the skin and hypothalamus detect the change and trigger responses such as sweating and dilation of blood vessels to cool the body down.
● Positive feedback amplifies physiological responses, leading to self-perpetuating events and is less common. One example is the hypothalamus triggering oxytocin release during childbirth, which in turn stimulates uterine contractions that further stimulate oxytocin release, resulting in stronger contractions until the baby is born.
This section provides a set of sample questions and answers designed to challenge your understanding of the nervous system and test your ability to apply your knowledge of the different components of the nervous system.
1. Which type of ion channels are important for regulating ion homeostasis in non-excitable cells?
a. Voltage-gated ion channels
b. Ligand-gated ion channels
c. Mechanically-gated ion channels
d. Leak channels
Answer: d. Leak channels
2. Which of the following types of neurons is responsible for carrying sensory information from the periphery to the central nervous system?
a. Motor neurons
c. Sensory neurons
d.. Pyramidal neurons
Answer: c. Sensory neurons
3. What is the role of voltage-gated ion channels in generating an action potential?
a. They allow the influx of Na+ ions into the cell, leading to depolarization
b. They allow the efflux of K+ ions out of the cell, leading to hyperpolarization
c. They open in response to the binding of neurotransmitters to the postsynaptic membrane
d. They regulate ion homeostasis in non-excitable cells
Answer: a. They allow the influx of Na+ ions into the cell, leading to depolarization
4. Which of the following structures is responsible for the integration of sensory information and the initiation of motor responses in reflexes?
b. Spinal cord
c. Cerebral cortex
Answer: b. Spinal cord
5. Which type of memory involves the retention of information for an extended period, from hours to a lifetime?
a. Sensory memory
b. Short-term memory
c. Long-term memory
d. Working memory
Answer: c. Long-term memory
6. What is the function of the prefrontal cortex in memory formation?
a. Consolidation of information from short-term to long-term memory
b. Processing of sensory information
c. Control of voluntary movement
d. Regulation of autonomic functions
Answer: a. Consolidation of information from short-term to long-term memory
7. Which of the following conditions is characterized by the degeneration of dopamine-producing neurons in the substantia nigra?
a. Alzheimer's disease
b. Parkinson's disease
c. Multiple sclerosis
Answer: b. Parkinson's disease
8. Which of the following sensory systems is responsible for the perception of touch, pressure, and temperature?
Answer: a. Somatosensation
9. Which part of the limbic system is involved in the processing of emotional information?
Answer: a. Amygdala
10. Which of the following disorders is characterized by abnormal electrical activity in the brain?
a. Alzheimer's disease
b. Parkinson's disease
c. Multiple sclerosis
Answer: d. Epilepsy