Physiology of Sensations and Reflexes

Neurology: Reflexes, Senses & Pain

• This presentation covers spinal reflexes, how they work, brain modulation, general senses, and pain.
• Three main topics will be discussed: spinal reflexes, how the brain controls them, and the general senses, including pain.

Reflexes

• Reflexes are rapid, automatic responses to stimuli.
• There are three types:
  • Reflex: a rapid, automatic response to a stimulus.
  • Somatic reflex: involves contraction of skeletal muscles (e.g., knee-jerk).
  • Autonomic reflex: involves smooth muscle, cardiac muscle, and endocrine glands (e.g., digestion).

The Reflex Arc

• A reflex arc describes the pathway a nerve impulse takes from stimulus to response.
• Sensory receptor → Sensory neuron → Interneuron → Motor neuron → Effector (muscle or gland).
• Interneurons integrate sensory input with other signals, creating the appropriate response.

Motor Neuron Target

• Somatic motor neurons control skeletal muscles.
• Autonomic motor neurons control organs, blood vessels, cardiac muscle, and endocrine glands.

Types of Somatic Reflexes

• Stretch reflex: maintains muscle tone.
• Withdrawal reflex: protects from harmful stimuli.
• Extensor reflex: maintains balance during withdrawal.

Stretch Reflex

• A simple, monosynaptic or two-neuron reflex arc.
• It has one synapse between sensory and motor neurons.
• The stretch receptor is a muscle spindle, sending signals to the spinal cord, then back to the muscle.
• Prevents overstretching and maintains muscle tone.

Patellar Reflex

• Commonly in neuro exams, it helps diagnose spinal cord trauma, peripheral nerve damage, or muscle disease.
• The reflex hammer taps the patellar ligament, stretching the quadriceps femoris; triggering a contraction and the leg extending.

Stretch + Relaxation

• The stretch reflex also causes opposing muscles to relax.
• Afferent sensory neurons synapse with inhibitory interneurons in the spinal cord.
• Inhibitory neurotransmitters are released.
• This prevents opposing muscles from working against the other, allowing one to contract freely.

Withdrawal Reflex

• Occurs when a harmful stimulus is received.
• The hand is pulled away from the source of heat in a classic example.
• The reflex involves several interneurons and synapses with motor neurons that contract muscles and opposing muscles that release inhibitory neurotransmitters

Crossed Extensor Reflex

• If a painful stimulus is applied, the opposite leg extends to maintain balance and prevent a fall.
• Afferent sensory impulses cross to the opposite side of the spinal cord.

Brain's Role in Moderating Reflexes

• The brain can modify the intensity of reflex responses.
• The brain does not dampen the reflex if the spinal cord is injured above the reflex area.
• The response is stronger (hyperreflexive) in this instance.

Spinal Injury at the Level of the Reflex

• An injury at the level of the spinal reflex may cause absence or decreased intensity of reflex response (hyporeflexive).
• If the injury is above the reflex (L2, L3, L4) the reflex is hyperreflexive
• If the injury is below L2-L4, the reflex is normal

Other Reflexes (Palpebral, Pupillary Light Reflex)

• Palpebral reflex: a response to tapping the inner eyelid causing blinking.
• Pupillary light reflex (PLR): the eye constricting simultaneously in response to shining light into the eye.

Clinical Case: Intervertebral Disc Disease (IVDD)

• This is a neurologic condition resulting from spinal disc bulges or ruptures causing swelling around the spinal cord.
• Signs include ataxia (lack of coordination), paralysis, or decreased pain.
• Diagnoses are made through radiographs, MRI or CT to detect the exact segment with injured disc.
• Treatment for IVDD depends on the severity, but mostly involves surgical decompression or/and medical management.

General Senses

• General senses provide information about the body's environment (both internal and external).
• Includes visceral sensations (internal organs), touch, temperature, and proprioception (body position).

Visceral Sensations

• These inform the brain about the status of internal organs.
• Includes sensations like hunger, thirst, pain from stretch, gas in the GI tract, kidney stones and urinary bladder.
• Inflammation of peritoneum or pleura also causes pain.

Touch

• The sensation of something being in contact with the body.
• Includes light touch, deep pressure, vibration, and hair movement.

Temperature

• Temperature receptors detect changes in body temperature (hypothermia, hyperthermia).
• Superficial receptors (skin) detect skin temperature changes.
• Central receptors (hypothalamus) monitor core temperature.
• Responses to temperature variations include blood flow to the skin, sweating, piloerection, shivering, and thyroid hormone production

Pain (Nociception)

• Nociceptors are pain receptors found throughout the body (except the brain).
• They are simple, free nerve endings sensitive to mechanical force, temperature, and chemicals.
• Nociception is the complex process of perceiving pain.
• The steps involved in nociception include transduction, transmission, modulation, and perception.

Classifications of Pain

• Categorized as superficial (skin, SQ tissue), deep (muscles, joints), or visceral (organs).
• Pain can also be acute (sharp, intense) or chronic (dull, aching).

Wind-Up Pain

• Spinal cord amplifies pain signals resulting in increased pain even when under anesthesia.
• Multimodal pain control with medications can help to block pain perception at different parts of the pain pathway (transduction, transmission, modulation, and perception).

Challenges with Assessing Pain in Animals

• Hiding pain is a survival mechanism for animals.
• Animals less likely to complain about pain compared to people.

Pain Scales (canine and feline)

• Pain scales provide tools for measuring and evaluating animal pain. (Specific scales mentioned are not part of this broad summary bullet formation requirement without additional detail, however the concepts of their use is important and a part of this study summary).

Proprioception

• The sense of body position and movement in space.
• Involves stretch receptors in muscles, tendons, and ligaments.
• Used to evaluate neurological function.

Clinical Scenario: Old Dog Vestibular Disease

• Vestibular system involves brain and inner ears.
• Symptoms are rolling, nystagmus (eye movements), ataxia, vomiting, anorexia, and drooling.
• Diagnoses often use labs, radiographs, BP, ear exam, and cytology.
• Treatment generally involves hospitalization, IV fluids, and motion-sickness medications.

Test your knowledge on the physiological mechanisms of sensations and reflexes in the human body. This quiz covers key concepts such as temperature changes, reflex arcs, and the roles of nociceptors. Perfect for students in anatomy and physiology courses.