The Nervous System: Anatomy, Function, and Physiological Processes, and Reflexes and Reflexes and Reflexes

Role and Communication Methods of the Brain

• The brain serves as the control center for all bodily functions, maintaining communication with different parts of the body continuously.

• There are two primary pathways through which the brain communicates:

  • Electrical signals and neurotransmitters transmitted via the nervous system.

  • Chemical messengers, known as hormones, transmitted via the endocrine system.

Fundamental Components of the Human Nervous System

• Neurons are the fundamental units of the nervous system.

• A neuron refers to a single individual nerve cell.

• The primary responsibility of neurons is to transmit signals between the brain and the body.

• The human nervous system is comprised of complex networks of neurons, totaling up to approximately $1 \times 10^{12}$ (one trillion) individual cells.

Divisions of the Nervous System

• The human nervous system is divided into two major components: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

• Central Nervous System (CNS):

  • Consists exclusively of the brain and the spinal cord.

• Peripheral Nervous System (PNS):

  • Consists of all neurons and nerve networks located throughout the body outside of the CNS.

  • The PNS is further subdivided into the Somatic and Autonomic systems.

• Somatic Nervous System:

  • Responsible for controlling voluntary movements of skeletal muscles.

  • Examples of its action include walking, writing, or picking up objects.

• Autonomic Nervous System:

  • Controls self-regulatory, involuntary bodily functions such as digestion, salivation, and lacrimation (the production of tears).

  • The Autonomic system is divided into the Sympathetic and Parasympathetic systems.

• Sympathetic System:

  • Exerts excitatory actions on the body.

  • Triggers the "fight or flight" response, which activates bodily functions to prepare for action.

• Parasympathetic System:

  • Exerts relaxing actions on the body.

  • Associated with the "rest and digest" state.

  • Maintains homeostasis and conserves the body's energy.

Neural Anatomy and Functional Types

• Types of Neurons:

  • Sensory Neurons: Transmit information related to the five senses from the PNS to the CNS.

  • Motor Neurons: Transmit information from the CNS to the PNS to initiate a physical response.

  • Interneurons (Relay Neurons): Found exclusively within the CNS (brain and spinal cord). They provide the connection between sensory and motor neurons.

• Structures of a Neuron:

  • Dendrite: Contains receptor sites designed to receive neurotransmitters from neighboring neurons.

  • Cell Body (Soma): The central part of the cell containing the nucleus and most organelles.

  • Nucleus: The control center of the cell which houses genetic material.

  • Axon: A long fiber that transfers electrical impulses from the cell body to the synapse. When axons are bundled together, they are referred to as "nerves."

  • Myelin Sheath: A fatty, insulating layer composed of Schwann cells that covers the axon. It keeps electrical signals contained within the cell and increases the speed of transmission while reducing interference from other neurons.

  • Schwann Cells: Specialized cells that produce and form the myelin sheath.

  • Axon Terminal: Located at the end of the axon; it contains vesicles that hold neurotransmitters until an electrical impulse triggers their release.

  • Synapse (Synaptic Cleft): A physical gap between neurons. Because neurons do not touch, messages must diffuse across this gap as chemical signals.

Physiological Mechanisms of Action Potentials

• Action Potential:

  • The term for the electrical or neural impulse that travels along the length of a neuron.

  • Neurons remain at rest until a specific threshold is reached.

  • All or None Principle: This principle states that a nerve must reach a specific threshold to trigger an action potential. If the threshold is not met, no signal is sent.

• Transmission Speed:

  • Speed varies between different neurons, ranging from $1\,\text{m/s}$ to more than $100\,\text{m/s}$.

  • Speed is determined by two main factors:

    • Diameter: The larger the diameter (width) of the axon, the faster the impulse travels.

    • Myelin Sheath: Axons with myelin insulation permit much faster travel of action potentials, functioning similarly to insulation on an electrical cable.

Synaptic Communication and Neurotransmitters

• The Transmission Process:

  • When an action potential reaches the axon terminal of the presynaptic neuron, it causes synaptic vesicles to release neurotransmitters into the synaptic cleft.

  • These chemical messengers migrate across the gap to the postsynaptic neuron.

  • Neurotransmitters bind to specific receptor sites on the dendrites of the postsynaptic neuron.

  • Receptor sites convert the chemical information back into electrical signals to be sent along the next axon.

• Specificity and Categorization:

  • Lock and Key Model: Each neurotransmitter has a specific molecular shape and only binds to specific, matching receptor sites.

  • Excitatory Messages: Increase the likelihood that the postsynaptic neuron will fire an action potential.

  • Inhibitory Messages: Decrease the likelihood that the postsynaptic neuron will fire an action potential.

Specific Neurotransmitters and Clinical Implications

• Dopamine:

  • Function: Acts on brain pathways associated with motor functions (movement), emotional arousal, and motivation.

  • Pathology: A deficiency in dopamine-producing cells can lead to Parkinson's disease, characterized by stiffness, tremors, and uncoordinated movement.

• Acetylcholine:

  • Function: Transmits messages from motor neuron axon terminals to skeletal muscles.

  • Pathology: The toxin Curare (derived from plants and used on blow-darts) prevents acetylcholine from binding to postsynaptic neurons, resulting in paralysis.

• Serotonin:

  • Function: Produced in the CNS and intestine; regulates behavior, memory, mood, and appetite.

  • Pathology: Low levels are theoretically linked to depression.

• Glutamate:

  • Function: Primary excitatory neurotransmitter in the CNS involved in learning and memory.

  • Pathology: High levels are linked to anxiety, depression, and ADHD (inability to concentrate). Low levels are linked to low energy, lack of concentration, and insomnia.

• GABA:

  • Function: An inhibitory neurotransmitter in the CNS that provides a calming effect.

  • Pathology: Low levels of GABA are associated with excess electrical activity in the brain, which can trigger seizures.

• Adenosine:

  • Function: Inhibits activity in the CNS to assist with sleep.

Reflex Actions and the Reflex Arc

• Reflex Action: A fast, automatic response designed to protect the body from immediate harm.

• Mechanism:

  • The spinal cord is primarily responsible for detecting the stimulus and initiating the response without waiting for the brain to process the information.

  • The brain is not immediately involved in the arc, which makes the response unconscious.

  • The brain registers the stimulus and the response only after a brief delay, once the action has already occurred.

• The Reflex Arc Model:

  • The arc involves a maximum of three neurons.

  • The pathway follows this sequence: stimulus $\rightarrow$ sensory receptors $\rightarrow$ sensory neuron $\rightarrow$ interneuron (in the spinal cord) $\rightarrow$ motor neuron $\rightarrow$ effector (muscle or gland) $\rightarrow$ response.

  • While the reflex arc does not require the brain for the response, the stimulus-response model of the nervous system differs because it includes the brain in the processing loop.

• Experimental Examples:

  • Pupillary Reflex: Observing the diameter of the pupil in a well-lit room, then observing the change after eyes are closed for 60 seconds and reopened.

  • Knee-jerk (Patellar) Reflex: Striking the patellar tendon just below the kneecap while the leg is crossed to observe the involuntary contraction.

Recent Advances in Connectomics

• A connectome is defined as a comprehensive map of neural connections within a brain.

• The Fruit Fly Mapping Study (March 2023):

  • An international team (MRC Laboratory of Molecular Biology at Cambridge University and Johns Hopkins University) successfully mapped the brain of a fruit fly.

  • This represents the most complex brain mapped to date.

  • The fruit fly brain shares structural similarities with the human brain, including two hemispheres, a structure similar to a brainstem, and an equivalent to a spinal cord.

Questions & Discussion

• State another name for an individual nerve cell.

  • Neuron.

• Recall the components that make up the CNS and the PNS.

  • CNS: Brain and spinal cord. PNS: Every neuron outside the CNS.

• Recall where interneurons are found in the body.

  • Only in the CNS (brain and spinal cord).

• Interneurons are sometimes called relay neurons. Why is this appropriate?

  • Because they provide the necessary connection between the sensory and motor components of the nervous system.

• State the function of the myelin sheath.

  • It acts as insulation for the neural impulse, facilitating fast electrochemical transmission and reducing interference from other neurons.

• A fly lands on someone’s face and they brush it off. Name the three types of neurons involved, in order.

  • Sensory neuron, interneuron, motor neuron.

• Recall what the 'all or none' principle means.

  • It means a nerve has a threshold that must be reached before an action potential is triggered.

• Describe the role of the postsynaptic neuron.

  • It contains receptor sites that receive neurotransmitters and initiate a new action potential.

• Is the statement 'The brain is not immediately involved in a reflex arc' true or false?

  • True. It is an unconscious response that does not involve immediate awareness by the brain.