MCAT Biology Review
Chapter 4: The Nervous System
The nervous system is one of the most fascinating and complex of the human body; millions upon millions of cells allow for our appropriate interactions in the everyday world. It is the seat of personality and, ultimately, the system that makes you you. In medical school, your courses on neuroscience will go into astounding detail about the nervous system, including the circuits that govern sensations such as pain and temperature, and circuits that allow your body to move and function.
In this chapter, we explored the nervous system at both a cellular and organizational level. Neurons are the primary cells of the nervous system, propagating impulses through both electric and chemical means—action potentials and synaptic transmission, respectively. Neurons can be grouped together to form nerves, which are the primary organizational structures in one major branch of the nervous system, the peripheral nervous system. This is in contrast to the central nervous system, which consists of the brain and spinal cord. The peripheral nervous system can be subdivided into the somatic and autonomic nervous systems, the latter of which can be further subdivided into the sympathetic and parasympathetic nervous systems.
The nervous system is heavily tested on the MCAT because it plays a role in the function of almost every other major organ system. Neurons cause muscles to move and digestive structures to carry food along through peristalsis, and they regulate breathing rate, heart rate, and glandular secretions. The nervous system is not the only system that has such a profound effect throughout the body, however. The endocrine system, which we will explore in the next chapter, serves a similar role—but through chemical messengers carried in the blood called hormones.
Cells of the Nervous System
· Neurons are highly specialized cells responsible for the conduction of impulses.
· Neurons communicate using both electrical and chemical forms of communication.
o Electrical communication occurs via ion exchange and the generation of membrane potentials down the length of the axon.
o Chemical communication occurs via neurotransmitter release from the presynaptic cell and the binding of these neurotransmitters to the postsynaptic cell.
· Neurons consist of many different parts.
o Dendrites are appendages that receive signals from other cells.
o The cell body or soma is the location of the nucleus as well as organelles such as the endoplasmic reticulum and ribosomes.
o The axon hillock is where the cell body transitions to the axon, and where action potentials are initiated.
o The axon is a long appendage down which an action potential travels.
o The nerve terminal or synaptic bouton is the end of the axon from which neurotransmitters are released.
o Nodes of Ranvier are exposed areas of myelinated axons that permit saltatory conduction.
o The synapse consists of the nerve terminal of the presynaptic neuron, the membrane of the postsynaptic cell, and the space between the two, called the synaptic cleft.
· Many axons are coated in myelin, an insulating substance that prevents signal loss.
o Myelin is created by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system.
o Myelin prevents dissipation of the neural impulse and crossing of neural impulses from adjacent neurons.
· Individual axons are bundled into nerves or tracts.
o A single nerve may carry multiple types of information, including sensory, motor, or both. Tracts contain only one type of information.
o Cell bodies of neurons of the same type within a nerve cluster in ganglia in the peripheral nervous system.
o Cell bodies of the individual neurons with a tract cluster in nuclei in the central nervous system.
· Neuroglia or glial cells are other cells within the nervous system in addition to neurons.
o Astrocytes nourish neurons and form the blood–brain barrier, which controls the transmission of solutes from the bloodstream into nervous tissue.
o Ependymal cells line the ventricles of the brain and produce cerebrospinal fluid, which physically supports the brain and serves as a shock absorber.
o Microglia are phagocytic cells that ingest and break down waste products and pathogens in the central nervous system.
o Oligodendrocytes (CNS) and Schwann cells (PNS) produce myelin around axons.
Transmission of Neural Impulses
· All neurons exhibit a resting membrane potential of approximately –70 mV.
o Resting potential is maintained using selective permeability of ions as well as the Na+/K+ ATPase.
o The Na+/K+ ATPase pumps three sodium ions out of the cell for every two potassium ions pumped in.
· Incoming signals can be either excitatory or inhibitory.
o Excitatory signals cause depolarization of the neuron.
o Inhibitory signals cause hyperpolarization of the neuron.
o Temporal summation refers to the addition of multiple signals near each other in time.
o Spatial summation refers to the addition of multiple signals near each other in space.
· An action potential is used to propagate signals down the axon.
o When enough excitatory stimulation occurs, the cell is depolarized to the threshold voltage and voltage-gated sodium channels open.
o Sodium flows into the neuron due to its strong electrochemical gradient. This continues depolarizing the neuron.
o At the peak of the action potential (approximately +35 mV), sodium channels are inactivated and potassium channels open.
o Potassium flows out of the neuron due to its strong electrochemical gradient, repolarizing the cell. Potassium channels stay open long enough to overshoot the action potential, resulting in a hyperpolarized neuron; then, the potassium channels close.
o The Na+/K+ ATPase brings the neuron back to the resting potential and restores the sodium and potassium gradients.
o While the axon is hyperpolarized, it is in its refractory period. During the absolute refractory period, the cell is unable to fire another action potential. During the relative refractory period, the cell requires a larger than normal stimulus to fire an action potential.
o The impulse propagates down the length of the axon because the influx of sodium in one segment of the axon brings the subsequent segment of the axon to threshold. The fact that the preceding segment of the axon is in its refractory period means that the action potential can only travel in one direction.
· At the nerve terminal, neurotransmitters are released into the synapse.
o When the action potential arrives at the nerve terminal, voltage-gated calcium channels open.
o The influx of calcium causes fusion of vesicles filled with neurotransmitter with the presynaptic membrane, resulting in exocytosis of neurotransmitter into the synaptic cleft.
o The neurotransmitters bind to receptors on the postsynaptic cell, which may be ligand-gated ion channels or G protein-coupled receptors.
· Neurotransmitters must be cleared from the postsynaptic receptors to stop the propagation of the signal.
o The neurotransmitter can be enzymatically broken down.
o The neurotransmitter can be absorbed back into the presynaptic cell by reuptake channels.
o The neurotransmitter can diffuse out of the synaptic cleft.
Organization of the Human Nervous System
· There are three types of neurons in the nervous system: motor (efferent) neurons, interneurons, and sensory (afferent) neurons.
· The nervous system is made up of the central nervous system (CNS; brain and spinal cord) and peripheral nervous system (PNS; cranial and spinal nerves).
o In the CNS, white matter consists of myelinated axons, and grey matter consists of unmyelinated cell bodies and dendrites. In the brain, white matter is deeper than grey matter. In the spinal cord, grey matter is deeper than white matter.
o The PNS is divided into the somatic (voluntary) and autonomic (automatic) nervous systems.
o The autonomic nervous system is further divided into the parasympathetic (rest-and-digest) and sympathetic (fight-or-flight) branches.
· Reflex arcs use the ability of interneurons in the spinal cord to relay information to the source of stimuli while simultaneously routing it to the brain.
o In a monosynaptic reflex arc, the sensory (afferent, presynaptic) neuron fires directly onto the motor (efferent, postsynaptic) neuron.
o In a polysynaptic reflex arc, the sensory neuron may fire onto a motor neuron as well as interneurons that fire onto other motor neurons.
Answers to Concept Checks
1. The axon transmits an electrical signal (the action potential) from the soma to the synaptic knob. The axon hillock integrates excitatory and inhibitory signals from the dendrites and fires an action potential if the excitatory signals are strong enough to reach threshold. Dendrites receive incoming signals and carry them to the soma. The myelin sheath acts as insulation around the axon and speeds conduction. The soma is the cell body and contains the nucleus, endoplasmic reticulum, and ribosomes. The synaptic bouton lies at the end of the axon and releases neurotransmitters.
2. A collection of cell bodies in the central nervous system is called a nucleus. In the peripheral nervous system, it is called a ganglion.
3. Astrocytes nourish neurons and form the blood–brain barrier. Ependymal cells produce cerebrospinal fluid. Microglia ingest and break down waste products and pathogens. Oligodendrocytes produce myelin in the central nervous system. Schwann cells produce myelin in the peripheral nervous system.
1. The action potential is initiated at the axon hillock.
2. The resting membrane potential is maintained by the Na+/K+ ATPase at approximately –70 mV.
3. Temporal summation is the integration of multiple signals close to each other in time. Spatial summation is the integration of multiple signals close to each other in space.
4. The sodium channel opens first at threshold (around –50 mV). It is regulated by inactivation, which occurs around +35 mV. Inactivation can only be reversed by repolarizing the cell. The opening of the sodium channel causes depolarization.
5. The potassium channel opens second at approximately +35 mV. It is regulated by closing at low potentials (slightly below –70 mV). The opening of the potassium channel causes repolarization and—eventually—hyperpolarization.
6. During the absolute refractory period, the cell is unable to fire an action potential regardless of the intensity of a stimulus. During the relative refractory period, the cell can fire an action potential only with a stimulus that is stronger than normal.
7. Calcium is responsible for fusion of neurotransmitter vesicles with the nerve terminal membrane.
8. A neurotransmitter’s action can be stopped by enzymatic degradation, reuptake, or diffusion.
1. The central nervous system includes the brain and spinal cord. The peripheral nervous system includes cranial and spinal nerves and sensors.
2. Afferent (sensory) neurons bring signals from a sensor to the central nervous system. Efferent (motor) neurons bring signals from the central nervous system to an effector.
3. The somatic nervous system is responsible for voluntary actions—most notably, moving muscles. The autonomic nervous system is responsible for involuntary actions, like heart rate, bronchial dilation, dilation of the pupils, exocrine gland function, and peristalsis.
4. The sympathetic nervous system promotes a “fight-or-flight” response, with increased heart rate and bronchial dilation, redistribution of blood to locomotor muscles, dilation of the pupils, and slowing of digestive and urinary function. The parasympathetic nervous system promotes “rest-and-digest” functions, slowing heart rate and constricting the bronchi, redistributing blood to the gut, promoting exocrine secretions, constricting the pupils, and promoting peristalsis and urinary function.
5. In a monosynaptic reflex, a sensory (afferent, presynaptic) neuron fires directly onto a motor (efferent, postsynaptic) neuron. In a polysynaptic reflex, a sensory neuron may fire directly onto a motor neuron, but interneurons are used as well. These interneurons fire onto other motor neurons.
· Behavioral Sciences Chapter 1
o Biology and Behavior
· Behavioral Sciences Chapter 2
o Sensation and Perception
· Biochemistry Chapter 3
o Nonenzymatic Protein Function and Protein Analysis
· Biochemistry Chapter 8
o Biological Membranes
· Biology Chapter 11
o The Musculoskeletal System
· General Chemistry Chapter 12