Understanding the Nervous System

Perception

• The awareness of sensory input is called perception.

Analyzing and Storing Information

• Perception is a complex process that involves analyzing and storing information to help us lead to appropriate responses.

Motor Activity (Output)

• Efferent nerves deliver signals to muscles and glands (effectors).
• These signals are responsible for motor activity.

Nervous Systems

• The nervous system is divided into the somatic nervous system (SNS) and the autonomic nervous system (ANS).

Somatic Nervous System (SNS)

• Sensory neurons originate from the head, body wall, limbs, and special sense organs.
• Motor neurons connect to skeletal muscle, allowing for voluntary movements.

Autonomic Nervous System (ANS)

• Sensory neurons originate from viscera.
• Motor neurons connect to viscera, including cardiac muscle, smooth muscle, and glands, providing involuntary control.

Division of the Peripheral Nervous System (PNS)

• The PNS is further divided into the autonomic nervous system (ANS) and the enteric nervous system (ENS).

Autonomic Nervous System (ANS)

• The ANS is subdivided into the sympathetic and parasympathetic divisions.

Sympathetic Division

• Known as "fight-or-flight" or "fight-fright-flight" response.
• Activated in emergency situations.

Parasympathetic Division

• Known as "rest-and-digest" response.
• Maintains normal bodily functions and promotes relaxation.

Enteric Nervous System (ENS)

• Often referred to as the "brain of the gut."
• Controls the digestive tract.
• Regulates smooth muscle contractions, secretions, and endocrine cell activity.

Histology of Nervous Tissue

• Nervous tissue consists of two main cell types: neurons and neuroglia.

Neurons

• Responsible for generating and transmitting electrical signals (nerve impulses or action potentials).
• Function in response to stimuli.

Neuroglia

• Do not conduct nerve impulses.
• Support, nourish, and protect neurons.

Neuronal Structure

• Dendrites: Highly branched structures that receive input signals from other neurons.
• Axon: Conducts signals away from the cell body towards other neurons, muscles, or glands.
• Axon terminals: Contain synaptic vesicles that release neurotransmitters.

Structural Classes of Neuron

• Multipolar: Have multiple dendrites and one axon.
• The most common type found in the brain and spinal cord.

Neuroglia of the CNS

• Astrocytes: Provide structural support, regulate the blood-brain barrier, and play a role in learning and memory.
• Oligodendrocytes: Produce myelin in the CNS.
• Microglia: Protect the CNS from disease by acting as phagocytes.
• Ependymal cells: Form the lining of the ventricles and central canal of the spinal cord, responsible for producing cerebrospinal fluid (CSF).

Neuroglia of the PNS

• Schwann cells: Produce myelin in the PNS and aid in axon regeneration.
• Satellite cells: Support neurons in PNS ganglia.

Myelination

• The process of wrapping an axon with a myelin sheath.
• Myelin is a lipid-rich material that insulates neurons and increases the speed of nerve conduction.
• Myelinated axons appear white, contributing to the "white matter" of the brain.

Sensory (Afferent) Neurons

• Conduct impulses towards the CNS.
• Typically unipolar in structure.
• Responsible for conveying sensory information.

Motor (Efferent) Neurons

• Conduct impulses away from the CNS to effectors.
• Multipolar in structure.
• Responsible for controlling muscle contractions and gland secretions.

Parasympathetic (P) Division

• Preganglionic neurons have cell bodies located in the brainstem and the spinal cord segments S2-S4.
• The P division is therefore referred to as "craniosacral."
• Axons travel in cranial nerves III, VII, IX, and X, as well as in pelvic nerves from S2-S4.
• The vagus nerve (cranial nerve X) carries approximately 80% of all P nerve impulses, innervating various thoracic and abdominal visceral organs.
• P preganglionic axons do not branch or pass through sympathetic trunk ganglia, instead, they travel directly to terminal ganglia located near or within the target organ.
• P postganglionic neurons have cell bodies within terminal ganglia, resulting in precise and localized effects on target organs.

Sympathetic (S) Division

• Preganglionic neurons have cell bodies located in the lateral horns of the spinal cord segments T1-L2.
• The S division is therefore referred to as "thoracolumbar."
• Axons travel in spinal nerves to the sympathetic trunk ganglia.
• Within the sympathetic trunk, axons may synapse with postganglionic neurons, ascend or descend the trunk, or pass through without synapsing to reach prevertebral ganglia.
• Prevertebral ganglia lie anterior to the three large abdominal arteries: celiac, superior, and inferior mesenteric ganglia.
• These ganglia innervate abdominal viscera, including the stomach, intestines, kidneys, liver, and spleen.
• Axons exit the ganglia and travel to visceral targets via sympathetic nerves.

Sympathetic Effects (Fight-or-Flight)

• Increased heart rate and contraction force.
• Increased blood pressure.
• Pupil dilation (mydriasis).
• Bronchodilation (dilation of airways).
• Vasodilation (dilation of blood vessels) to skeletal muscles, heart, liver, and adipose tissue.
• Vasoconstriction (constriction of blood vessels) to non-essential organs, including skin, GI tract, and kidneys.
• Mobilization of nutrients for energy, including glucose and fats.

Parasympathetic Effects (Rest-and-Digest)

• SLUDD:
• Salivation
• Lacrimation
• Urination
• Digestion
• Defecation
• Decreased heart rate.
• Constriction of airways.
• Pupil constriction.

Cardiovascular System: The Heart

• Located in the thoracic cavity between the two lungs.
• Positioned approximately two-thirds to the left of the midline.
• Enclosed by the pericardium, which consists of two layers: the fibrous pericardium and the serous pericardium.

Fibrous Pericardium

• Outer, inelastic layer that anchors the heart in place.

Serous Pericardium

• Double layer surrounding the heart.
• Parietal layer: Fused to the fibrous pericardium.
• Visceral layer: Adheres tightly to the heart surface.
• Pericardial fluid fills the space between the layers, reducing friction during heartbeats.

Heart Wall

• Epicardium: The outer layer.
• Myocardium: Composed of cardiac muscle, responsible for the heart's contractile function.
• Endocardium: The inner layer, lining the chambers of the heart.

Chambers of the Heart

• The heart has four chambers:
• Right atrium: Receives deoxygenated blood from the body.
• Right ventricle: Pumps deoxygenated blood to the lungs.
• Left atrium: Receives oxygenated blood from the lungs.
• Left ventricle: Pumps oxygenated blood to the body.

Myocarditis

• Inflammation of the myocardium, often due to viral infection, rheumatic fever, or exposure to radiation or certain chemicals.

Endocarditis

• Inflammation of the endocardium, typically affecting the heart valves.
• Most cases are caused by bacteria (bacterial endocarditis).
• Treatment involves intravenous antibiotics.

This quiz covers the essential concepts of the nervous system, including perception, the somatic and autonomic nervous systems, and motor activity. Test your knowledge of how sensory input is processed and how different types of neurons function in both voluntary and involuntary actions.