Central Nervous System (CNS) PDF

**Central Nervous System (CNS)** The CNS is responsible for processing information from our environment and enabling us to interact with it. It includes key regions and structures that integrate sensory input with motor output: - **Somatosensory Cortex**: Processes sensory information from the...

**Central Nervous System (CNS)** The CNS is responsible for processing information from our environment and enabling us to interact with it. It includes key regions and structures that integrate sensory input with motor output: - **Somatosensory Cortex**: Processes sensory information from the body, such as touch, pain, and temperature. - **Motor Cortex**: Controls voluntary muscle movements. - **Thalamus**: Acts as a relay station, filtering and transmitting sensory and motor signals to the cerebral cortex. - **Basal Ganglia (BG) and Cerebellum**: Involved in coordinating movement and motor control. - **Brainstem**: Controls essential functions like heart rate, breathing, and arousal. - **Spinal Cord**: Transmits information between the brain and body; also plays a role in reflexes and motor control. The brain also includes areas crucial for higher mental processes and emotional regulation: - **Limbic System**: Regulates emotions and memory. - **Amygdala**: Key for processing emotions, especially fear. - **Prefrontal Cortex**: Involved in decision-making, social behavior, and personality. - **Broca\'s and Wernicke\'s Areas**: Important for speech production and comprehension, respectively. - **Reticular System**: Plays a role in regulating wakefulness and sleep-wake transitions. **Spinal Cord** The spinal cord is the caudal extension of the CNS, serving as a critical pathway for transmitting signals between the brain and the rest of the body. - **Meninges**: Protective layers that cover the brain and spinal cord. They help cushion and protect the CNS from injury. - **Cerebrospinal Fluid (CSF)**: Circulates in the subarachnoid space and central canal of the spinal cord, providing further protection and removing waste. - **Spinal Nerves**: There are 31 pairs of spinal nerves that emerge from the spinal cord, and they carry both motor (efferent) and sensory (afferent) signals. They are classified based on their exit points: cervical, thoracic, lumbar, sacral, and coccygeal regions. **Dermatomes and Myotomes** - **Dermatomes**: These are areas of the skin innervated by sensory fibers from a single spinal nerve root. They are crucial for diagnosing the level of nerve or spinal cord injury. - **Myotomes**: These are groups of muscles innervated by a single spinal nerve root, important for evaluating motor function and identifying specific areas of spinal injury. **Bell-Magendie Law** - This law describes the principle that the dorsal roots of the spinal cord are responsible for sensory functions, while the ventral roots control motor functions. **Spinal Reflexes** Reflexes are automatic, rapid, and predictable responses to stimuli. They involve neural circuits that integrate sensory and motor signals within the CNS. 1. **Stretch Reflex (e.g., Knee Jerk)** - A monosynaptic reflex involving only one synapse between the sensory and motor neurons. - Example: Tapping the patellar tendon stretches the quadriceps muscle, activating stretch receptors (muscle spindles). This leads to the contraction of the quadriceps, maintaining posture and preventing muscle overstretching. - The characteristic delay of \~ 40 msec! - Components of the reflex arc include: - **Receptor**: Muscle spindle in the quadriceps. - **Sensory Neuron**: Carries information to the spinal cord. - **Motor Neuron**: Carries the signal back to the quadriceps, causing contraction. - Clinically, this reflex is used to assess the integrity of the L3-L4 spinal segments. 2. **Withdrawal Reflex (e.g., Flexor Reflex)** - A polysynaptic reflex involving multiple synapses and neurons. It triggers the withdrawal of a limb from a painful stimulus. - more complex reflex is contralateral, prepotent, receptor? - - It involves: - **Sensory Neurons**: Detect painful stimuli. - **Interneurons**: In the spinal cord that process the input. - **Motor Neurons**: Cause flexion of the affected limb and extension of the opposite limb (crossed extensor reflex). - i.e Polysynaptic-many segments - Flexion of affected limb & inhibition of antagonistic muscles - Crossed extensor response i.e. - extension of opposite limb - Demonstrates a degree of autonomy @ a segment in spinal cord - - This reflex demonstrates a degree of autonomy in the spinal cord, as it can occur without direct involvement of the brain. - **Muscle Spindles** Muscle spindles are specialized sensory receptors located within skeletal muscles that monitor muscle stretch and contribute to the regulation of muscle tone. - **Structure**: Muscle spindles consist of intrafusal fibers, which are sensitive to changes in muscle length. - **Innervation**: They are innervated by two types of neurons: - **Sensory Neurons (Type 1a and II fibers)**: Detect both the speed and magnitude of muscle stretch. - **Motor Neurons (γ motor neurons)**: Control the sensitivity of the spindle by adjusting the tension of intrafusal fibers. - With a rapid stretch ONLY type 1a responds i.e. sensitive to length + velocity of stretch (transducers?). - **Function**: When a muscle is stretched, the spindles generate action potentials that help maintain muscle tone and posture. They also play a crucial role in reflexes, such as the stretch reflex. **Reinforcement (Jendrassik Maneuver)** - This technique involves voluntarily increasing the activity of γ motor neurons, which enhances the sensitivity of muscle spindles. It is used clinically to strengthen reflex responses. **Spinal Shock** Spinal shock occurs after spinal cord injury and is characterized by a temporary reduction or loss of reflexes and motor activity below the site of injury. Tissue around spinal cord often causes damage due to forces imparted in accidents. Injury highly variable Post trauma (\~ min.) a transient ↓ sensation, loss or ↓ reflexes, ↓ mobility & change in organ function is often observed below point of injury. - **Symptoms**: Loss of sensation, decreased reflexes, paralysis, and impaired organ function. - **Duration**: This state lasts for 2-4 weeks, after which reflexes typically begin to return. However, reflexes may be exaggerated upon return (hyperreflexia). - **Causes**: Spinal shock is thought to be caused by a combination of factors, including reduced inhibitory input from higher brain centers and changes in receptor expression. - Theories: 1) ↓ Tonic inhibitory input from higher centers (CNS remodeled) - 2\) ↑ expression of receptors - ∆ incomplete & complete injury. - \* **ASIA=American spinal injury association** **Clinical Significance of Reflexes** Reflex testing is a valuable tool for assessing the function of the nervous system, especially in unconscious patients. Reflexes can provide insight into: - The **location of a lesion**: Whether the damage is in the spinal cord or brain. - The **side of the lesion**: Reflexes can help determine whether the damage is on the same side (ipsilateral) or opposite side (contralateral) as the symptoms. - **Types of lesions**: Reflex activity helps distinguish between supraspinal (above the spinal cord) and spinal cord injuries. - **Diminished reflexes**: Can indicate nervous system damage, while reinforcement of reflexes may suggest compensatory neural changes. **Central Pattern Generators (CPGs)** **What are pattern generators?** - Networks of neurons found in spinal cord & reticular formation. Spinal animals - Responsible for → stereotyped action involving several muscles or groups of muscles (eg, breathing, coughing, walking) CPGs are networks of neurons in the spinal cord and brainstem responsible for producing rhythmic, repetitive movements without sensory input or higher brain control. They are essential for activities such as: - Breathing - Walking - Swallowing - Coughing