Somatic Nervous System Lecture Flashcards
Describe the basic structure and classification of neurones.
A typical neuron consists of three main parts:
Cell Body (Soma)
Function: Contains the nucleus and most of the organelles. It is the metabolic center of the neuron, where protein synthesis and other cellular activities occur.
Structure: The soma is generally spherical or oval in shape, with dendrites branching off it. Inside, you’ll find the nucleus, mitochondria, ribosomes, and other organelles necessary for the neuron’s function.
Dendrites
Function: Receive incoming signals from other neurons or sensory receptors. Dendrites are highly branched, which increases the surface area for receiving signals.
Structure: They are short, tree-like extensions that project from the cell body. Dendrites have tiny protrusions called dendritic spines that help in synaptic connections.
Axon
Function: Transmits electrical signals away from the cell body to other neurons, muscles, or glands.
Structure: The axon is a long, slender projection that can vary in length from a fraction of a millimeter to over a meter in some cases (e.g., in motor neurons). The axon may be myelinated (covered with a fatty sheath called myelin) to speed up signal transmission.
Axon Terminals: At the end of the axon are small branches called axon terminals, which contain synaptic vesicles filled with neurotransmitters. These are released into the synaptic cleft to communicate with other neurons or target cells.
Myelin Sheath
Function: Myelin is a fatty substance that wraps around the axon in segments, forming an insulating layer. This insulation speeds up the transmission of electrical signals by allowing the impulses to jump between gaps in the myelin called Nodes of Ranvier.
Structure: Myelin is produced by Schwann cells in the peripheral nervous system (PNS) and oligodendrocytes in the central nervous system (CNS).
Synapses
Function: The synapse is the junction where one neuron communicates with another neuron or with an effector cell (e.g., muscle or gland). Communication occurs via neurotransmitters, which cross the synaptic cleft.
Structure: The presynaptic terminal of the axon terminal, the synaptic cleft (the gap between neurons), and the postsynaptic membrane on the next neuron or target cell.
Classification of Neurons
Neurons can be classified based on structure, function, and direction of signal transmission.
- By Structure
Neurons are classified based on the number of processes (projections) extending from the cell body:
Unipolar Neurons (Pseudounipolar):
Structure: Have a single process that branches into two: one branch acts as a dendrite, and the other acts as an axon.
Function: Common in sensory neurons, such as those that carry information about touch, pain, and temperature from the periphery to the spinal cord.
Bipolar Neurons:
Structure: Have two processes—one dendrite and one axon—extending from opposite ends of the cell body.
Function: Found in sensory organs, such as the retina of the eye and the olfactory system.
Multipolar Neurons:
Structure: Have one axon and multiple dendrites extending from the cell body.
Function: The most common type of neuron in the body. They are typically motor neurons (e.g., those that control muscles) and interneurons in the brain and spinal cord.
2. By Function
Neurons are also classified by their role in the nervous system:
Sensory (Afferent) Neurons:
Function: Carry sensory information from sensory receptors (e.g., in skin, eyes, ears) to the CNS (brain and spinal cord).
Structure: Often unipolar or bipolar neurons.
Motor (Efferent) Neurons:
Function: Transmit signals from the CNS to muscles or glands (effectors) to cause a response.
Structure: Typically multipolar neurons.
Interneurons (Association Neurons):
Function: Found only in the CNS, these neurons connect sensory neurons to motor neurons and play a key role in processing and integrating information.
Structure: Usually multipolar neurons.
3. By Direction of Signal Transmission
Afferent Neurons: Carry information towards the CNS (sensory input).
Efferent Neurons: Carry information away from the CNS to muscles or glands (motor output).
Understand the anatomical and functional divisions of the nervous system.
Anatomical division:
~ Central Nervous System (CNS) = Controls and processes sensory information, thought, and higher-order functions. Brain (cerebrum, cerebellum, brainstem) & spinal cord
~ Peripheral Nervous System (PNS) = Connects the CNS to limbs and organs, carries sensory and motor signals. Cranial nerves, spinal nerves, ganglia.
Functional Divisions:
~ Somatic Nervous System (SNS) = Controls voluntary movements, relays sensory information. Sensory neurons, motor neurons, skeletal muscles
~ Autonomic Nervous System (ANS) = Regulates involuntary body functions (e.g., heart rate, digestion). Sympathetic, parasympathetic, enteric systems
Understand the components and function of the somatic nervous system.
Sensory Division - Carries information from sensory receptors (touch, pain, temperature) to the CNS.
Motor Division - Sends voluntary commands from the CNS to skeletal muscles to initiate movement.
Somatic Reflexes - Involuntary, rapid responses to stimuli, mediated by the spinal cord.
~ The Somatic Nervous System (SNS) controls voluntary movements and processes sensory information from the external environment.
~ It consists of sensory neurons (afferent) that carry information from the body to the CNS, and motor neurons (efferent) that transmit commands from the CNS to the muscles.
~ The SNS allows for conscious perception of sensory stimuli (like touch, pain, and temperature) and the voluntary control of skeletal muscles.
~ Reflexes help the body respond quickly to stimuli, bypassing higher brain centers for faster reactions.
Differentiate between dermatome versus myotome and understand the relevance of
dermatome and myotome in clinical practice.
- Dermatomes refer to specific areas of skin that are supplied by the sensory fibers of a particular spinal nerve, and are useful in diagnosing sensory loss or dysfunction.
- Myotomes refer to muscle groups that are innervated by a specific spinal nerve, and they help in assessing motor weakness or paralysis.
- Both concepts are essential in clinical practice to identify nerve root injuries, spinal cord lesions, neuropathy, and other neurological disorders, guiding diagnosis, treatment, and rehabilitation strategies.
Describe the structural and functional components of a spinal nerve
Structural:
1. Roots of the Spinal Nerve
Dorsal Root (Sensory):
Structure: The dorsal root contains sensory fibers that carry information from sensory receptors (e.g., skin, joints, muscles) to the spinal cord. The cell bodies of these sensory neurons are located in the dorsal root ganglion (a cluster of sensory neuron cell bodies just outside the spinal cord).
Function: Transmits sensory input such as pain, temperature, touch, and proprioception to the CNS for processing.
Ventral Root (Motor):
Structure: The ventral root contains motor fibers that carry motor commands from the spinal cord to the muscles and glands. The cell bodies of these motor neurons are located in the ventral horn of the spinal cord gray matter.
Function: Carries motor commands to skeletal muscles for voluntary movements (somatic motor) and to glands for secretions.
2. Spinal Nerve Proper
Once the dorsal and ventral roots merge, they form the spinal nerve proper, which is a short segment (typically around 1-2 cm long) that exits the vertebral column through the intervertebral foramen.
This mixed nerve then divides into rami (branches) to further distribute its fibers.
3. Rami (Branches) of the Spinal Nerve
Dorsal Ramus: The dorsal ramus carries both sensory and motor fibers that innervate the deep muscles of the back and the skin overlying the back. It has a smaller distribution compared to the ventral ramus.
Ventral Ramus: The ventral ramus carries both sensory and motor fibers that supply the muscles and skin of the lateral and anterior parts of the body. It forms plexuses (e.g., brachial, lumbar, sacral plexuses) that innervate the limbs and other areas.
4. Meningeal Branches: These branches re-enter the vertebral canal to innervate the meninges and vertebrae, providing sensory feedback (e.g., pain) from the structures of the spine.
Functional:
1. Sensory Function (Afferent):
Sensory fibers in the dorsal root transmit information from the periphery (skin, muscles, joints) to the CNS. This allows us to feel sensations such as touch, pressure, temperature, pain, and proprioception.
Sensory Receptors: These include mechanoreceptors (for touch), nociceptors (for pain), and thermoreceptors (for temperature), all of which send signals to the CNS via spinal nerves.
Dermatomes: Sensory input from each spinal nerve corresponds to a specific area of the skin known as a dermatome.
2. Motor Function (Efferent):
Motor fibers in the ventral root transmit commands from the CNS to muscles and glands. This enables voluntary muscle contraction and glandular secretion.
Myotomes: Motor neurons from the spinal nerve innervate specific muscle groups, called myotomes, responsible for movement.
3. Autonomic Function (in some spinal nerves):
Some spinal nerves also contribute to the autonomic nervous system (ANS) by carrying sympathetic fibers (e.g., from the thoracolumbar region) that control involuntary functions such as heart rate, digestion, and blood vessel constriction.
Understand the implications of damage to the structural/functional components of a spinal nerve.
- Dorsal Root (Sensory) = Sensory loss (numbness, tingling), pain (neuropathic)
- Ventral Root (Motor) = Muscle weakness, paralysis, atrophy
- Spinal Nerve Proper = Combined motor and sensory deficits (weakness and sensory loss)
- Rami/Plexuses = Widespread sensory and motor deficits, affecting multiple regions
Clinical Relevance
- Spinal Nerve Injuries: Spinal nerve injuries, whether from trauma, herniated discs, infections, or tumors, can manifest as pain, sensory loss, muscle weakness, or autonomic dysfunction.
- Diagnosis: A careful examination of dermatomes (sensory) and myotomes (motor) can help localize nerve root involvement. This allows clinicians to determine the level of spinal nerve damage and tailor treatment (such as physical therapy, surgery, or pain management) accordingly.
Understand the structure and components of the somatic nerve arc.
Receptor = Detects stimulus (e.g., muscle spindle or skin receptor).
Sensory Neuron = Carries afferent signal from receptor to spinal cord (dorsal root).
Integration Center = Processes input in the spinal cord (may involve one or more interneurons).
Motor Neuron = Carries efferent signal from spinal cord to the effector (skeletal muscle).
Effector = The muscle or gland that responds to the motor signal (e.g., muscle contraction).
- The somatic reflex arc is responsible for rapid, involuntary reactions to stimuli.
- It consists of five key components: receptor, sensory neuron, integration center, motor neuron, and effector.
- Reflex testing helps evaluate the integrity of the nervous system, and abnormal reflexes can indicate nerve damage, spinal cord injury, or neuromuscular disorders.
Understand clinical anatomy relevant to the somatic nervous system.
- Spinal Nerves and Plexuses: Serve as the primary pathways for sensory and motor information. Damage to these nerves can result in sensory loss (dermatomal) or motor weakness (myotomal).
- Dermatomes and Myotomes: Useful for localizing nerve root damage or dysfunction.
- Neuromuscular Junction: Key site for muscle activation; disorders here (e.g., myasthenia gravis) lead to muscle weakness.
- Sensory Pathways: Include the dorsal columns and spinothalamic tract; damage to these can cause sensory loss.
- Common Disorders: Include radiculopathy, peripheral neuropathy, GBS, ALS, and myasthenia grav
