Principles of the nervous system II (Anatomy)

Question 1

Outline the characteristics and functions of autonomic (parasympathetic and sympathetic) and somatic nervous system

Answer 1

Autonomic Nervous System (ANS)

Controls involuntary functions of the body, regulating internal organs, glands and smooth muscles to maintain homeostasis, divided into sympathetic and parasympathetic nervous system

Sympathetic Nervous System:

• Part of the fight or flight response, preparing the body for stressful or emergency situations
• Originates from the thoracolumbar region of the spinal cord (thoracic and lumbar segments)
• Sympathetic nerves located close to vertebral column
• Plentiful nerve branching so multiple organs innervated simultaneously
• Uses short preganglionic fibers that synapse in ganglia close to the spinal cord (sympathetic trunk), and long postganglionic fibers to reach target organs
• Neurotransmitter: Preganglionic neurons release acetylcholine, while postganglionic neurons release norepinephrine to activate target organs
• Increases heart rate and blood pressure by causing the heart to beat more forcefully and quickly
• Dilates pupils to enhance vision in low light
• Dilates bronchioles to increase airflow to the lungs
• Inhibits digestion and urination by diverting blood flow away from the gastrointestinal (GI) tract and kidneys
• Constricts blood vessels in non-essential organs (e.g., GI tract) to increase blood flow to skeletal muscles, heart, and brain
• Mobilizes energy stores by stimulating the breakdown of glycogen to glucose and lipids to fatty acids for use by muscles and other tissues
• Prepares the body for rapid, energetic responses to perceived threats

Parasympathetic Nervous System:

• Part of the rest and digest response, which conserves energy and promotes routine maintenance activities when the body is at rest
• Originates from the craniosacral regions: cranial nerves (especially the vagus nerve) and sacral segments of the spinal cord
• Comparatively, very little nerve branching so unable to reduce the impact of multiple organs rapidly
• Uses long preganglionic fibers that synapse in ganglia close to or within the target organs, and short postganglionic fibers
• Neurotransmitter: Both preganglionic and postganglionic neurons release acetylcholine
• Decreases heart rate and blood pressure, promoting a relaxed state
• Constricts pupils to reduce light intake
• Stimulates digestion by increasing secretion of digestive enzymes, enhancing motility in the GI tract, and promoting nutrient absorption
• Promotes urination and defecation by increasing activity in the bladder and bowels
• Reduces energy use by slowing metabolic activities and conserving resources.

Somatic Nervous System (SNS)

Controls voluntary movements by innervating skeletal muscles. Conscious control over body movement and the transmission of sensory information from external stimuli to the CNS

• Composed of sensory (afferent) neurons that relay information from receptors (e.g. in the skin, muscles and joints) to the CNS
• Contains motor (efferent) neurons that transmit commands from the CNS to skeletal muscles to produce voluntary movements
• No ganglia are involved—motor neurons have direct connections to the skeletal muscle fibers
• Voluntary control of skeletal muscles: this control allows for coordinated actions, such as walking, lifting objects, and facial expressions
• Sensory Feedback of pain, temperature, touch and proprioception (awareness of body position)
• Reflex action, without conscious involvement

(What is a ganglion)

Question 2

Demonstrate an understanding of the function of the spinal cord

Answer 2

Function

1) Signal Conduction:

• Ascending (sensory) pathways: carry sensory information from receptors (e.g., touch, pain, temperature) in the body to the brain for processing
• Sensory neurons transmit signals via the dorsal root of the spinal nerve
• Descending (motor) pathways: carry motor commands from the brain to muscles and glands, initiating voluntary movement and regulating autonomic functions.
• Motor neurons transmit signals via the ventral root of the spinal nerve

2) Reflex action:

• Sensory neurons
• Interneurons
• Motor neurons
• Monosynaptic Reflex: Simple reflex arc involving just one synapse between the sensory and motor neurons, such as the knee-jerk reflex
• Polysynaptic Reflex: Involving interneurons and multiple synapses, such as the withdrawal reflex

3) Motor Function:

• Motor commands from the brain are transmitted through descending pathways to motor neurons in the spinal cord, which then activate specific muscles
• The corticospinal tract is a major motor pathway that transmits motor signals from the brain to the limbs and trunk
• Somatic motor neurons located in the ventral horn of the spinal cord are sponsible for stimulating skeletal muscles

4) Autonomic Functions:

• Part of the ANS, controlling smooth muscles, cardiac muscles and glandular activity
• Sympathetic and parasympathetic neurons in the spinal cord contribute to the regulation of heart rate, digestion, respiration, etc

5) Segmental Organisation

• The spinal cord has a segmental organization that corresponds to the spinal nerves that arise from it
• Each segment of the spinal cord gives rise to a pair of spinal nerves, and each spinal nerve provides motor and sensory innervation to specific areas of the body
• The dermatome is the area of skin innervated by the sensory fibers of a single spinal nerve
• The myotome is the group of muscles innervated by the motor fibers of a single spinal nerve

Anatomy

White matter: contains myelinated axons that transmit signals up and down the spinal cord, forming ascending and descending tracts

Grey Matter: composed of neuron cell bodies, dendrites and unmyelinated axons, organised into:

• Dorsal Horn: Contains sensory neurons that receive input from the body
• Ventral Horn: contains motor neurons that send output to skeletal muscles
• Lateral Horn (present in the thoracic and upper lumbar regions): Contains neurons involved in autonomic functions, especially for the sympathetic nervous system

Dorsal and Ventral roots:

• Dorsal Root: Carries sensory information from the body to the spinal cord
• Ventral Root: Carries motor information from the spinal cord to the muscles and glands

(What is the difference between the d/v horns to d/v roots?)

Clinical Significance

• Spinal Cord Injury (SCI): Damage to the spinal cord can disrupt the transmission of sensory and motor signals, leading to paralysis or loss of sensation below the site of injury
• Reflex Testing: Reflex tests (such as the knee-jerk reflex) can help assess the integrity of the spinal cord and peripheral nerves. Abnormal reflexes may indicate damage to the spinal cord or specific nerve roots
• Referred Pain: The spinal cord plays a role in referred pain, where pain from internal organs is perceived as occurring in specific dermatomes on the skin. For example, pain from the heart during a heart attack can be felt in the left arm or jaw due to shared nerve pathways

Question 3

Describe the anatomy of a peripheral nerve and formation of a typical spinal nerve

Answer 3

Anatomy of a Peripheral Nerve

part of the Peripheral Nervous System (PNS), transmit sensory and motor information between the CNS and the rest of the body. These nerves contain bundles of axons (nerve fibers) and are classified as mixed nerves because they carry both sensory (afferent) and motor (efferent) signals.

Structure:

Axons (Nerve Fibers):

• Each axon is a long, thread-like extension of a neuron that transmits electrical impulses
• Axons may be myelinated or unmyelinated

Connective Tissue Layer:

• Endoneurium: A delicate layer of connective tissue that surrounds individual axons, providing support and insulation
• Perineurium: This layer encases groups of axons, forming bundles known as fascicles. It acts as a protective barrier against external substances
• Epineurium: The outermost layer that surrounds the entire peripheral nerve, bundling multiple fascicles together. It also contains blood vessels that nourish the nerve

Blood Supply (Vasa Nervorum):

• Peripheral nerves have a rich blood supply through small blood vessels that penetrate the epineurium to provide oxygen and nutrients to nerve fibers

Sensory and Motor Components:

• Peripheral nerves carry both Afferent (sensor to CNS) and Efferent fibers (Motor from CNS to target)

Formation of a typical Spinal Nerve

Spinal nerves are mixed nerves that emerge from the spinal cord, connecting the CNS to the body. Both sensory and motor fibers and arises from the spinal cord through specific, organised structures

Roots of the Spinal Nerve:

• A typical spinal nerve is formed by the union of two roots:
• Dorsal Root (Posterior Root): contains sensory (afferent) fibers. The dorsal root ganglion is a cluster of sensory neuron cell bodies located near the spinal cord. These sensory neurons are pseudo-unipolar (having a single process that splits into two branches)
• Ventral Root (Anterior Root): Contains motor (efferent) fibers that transmit motor commands from the spinal cord to muscles and glands. The ventral root lacks a ganglion because the motor neuron cell bodies are located within the ventral horn of the spinal cord’s grey matter

When the dorsal and ventral roots merge, they form a spinal nerve that exits the spinal column through the intervertebral foramen

After leaving the spinal cord, the spinal nerve quickly divides into two primary branches (rami):

• Dorsal (posterior) Ramus: innervates the muscle and skin of the back, providing both sensory and motor supply to the paravertebral muscles and skin overlying the vertebrae, typically smaller than ventral ramus due to its localised function
• Ventral (Anterior) Ramus:
• Ventral ramus innervates the anterior and lateral parts of the trunk as well as the limbs. It forms plexuses (such as the brachial plexus and lumbosacral plexus) which supply sensory and motor fibers to the limb and parts of the torso
• Meningeal branch: small branch that re-entres the vertebral canal to supply the vertebrae, meninges and blood vessels
• Rami Communicantes: These branches connect the spinal nerve to the sympathetic chain of the autonomic nervous system, facilitating autonomic (involuntary) functions such as heart rate and digestion

Question 4

Outline differences between spinal and cranial nerves

Question 5

Describe the concept of dermatomes and myotomes in the human body

Answer 5

Dermatome:

• An individual area of skin innervated by a single spinal nerve [primary ramus]; this being the area of skin responsible for sensory input to the dorsal root.

Myotome:

• An individual set of muscles innervated by a single spinal nerve [primary ramus]
• The term is also used in embryology to describe that part of thesomite which develops into themuscles.
• More complicated in limbs- due to >1 spinal nerve innervating large muscle groups

Clinical importance of Dermatones

• Loss/altered sensation and/or pain from dermatome indicates possible injury to spinal cord, nerve root, or the spinal nerve.
• i.e. loss of sensation from skin over umbilicus may indicate damage to…… T10
• Also, visceral [organ] pain can be referred to skin [dermatomes] as cutaneous pain; i.e. diaphragmatic irritation referred to shoulder [C3-C5 dermatomes]

Question 6

Outline the basic mechanism of referred pain