Spinal Cord and Sensory Receptors PDF

Spinal cord and Sensory receptors Mona A Hussain Assistant Prof. In Physiology department- FOM- PSU Organization of the Nervous System  Three functional classes of neurons 1-afferent neurons, 2- efferent neurons, and 3- interneurons Structure and functions of CNS Structure an...

Spinal cord and Sensory receptors Mona A Hussain Assistant Prof. In Physiology department- FOM- PSU Organization of the Nervous System  Three functional classes of neurons 1-afferent neurons, 2- efferent neurons, and 3- interneurons Structure and functions of CNS Structure and functions of CNS  Spinal Cord The spinal cord is a long, slender cylinder of nerve tissue. extends from the brain stem. It is about 45 cm long and 2 cm in diameter. The spinal cord itself extends only to the level of the first or second lumbar vertebra why? Spinal nerves Spinal cord white matter and grey matter Spinal cord white matter Spinal cord grey matter  Spinal nerves Spinal nerves connect with each side of the spinal cord by a dorsal root and a ventral root 1- Afferent fibers carrying incoming signals from peripheral receptors enter the spinal cord through the dorsal root. 2- The cell bodies for the afferent neurons at each level are clustered in a dorsal root ganglion. 3- The cell bodies for the efferent neurons originate in the grey matter and send axons out through the ventral root. Therefore, efferent fibers carrying outgoing signals to muscles and glands exit through the ventral root. 4- The dorsal and ventral roots at each level join to form a spinal nerve that emerges from the vertebral column  Basic reflexes spinal cord to fulfill its two primary functions: (1) serving as a link for transmission of information between the brain and the remainder of the body, and (2) integrating reflex activity between afferent input and efferent output without involving the brain. This type of reflex activity is called a spinal reflex.  Basic reflexes A reflex is any response that occurs automatically without conscious effort and is part of a biological control system that links stimulus and response. There are two types of reflexes: (1) Simple (basic) reflexes, which are built-in, unlearned responses, such as pulling the hand away from a burning hot object; and (2) acquired (conditioned) reflexes, which are a result of practice and learning, such as a pianist striking a particular key upon seeing a certain note on a music staff.  Reflex arc The neural pathway involved in reflex activity is known as a reflex arc, which typically includes five basic components: 1. Receptor 2. Afferent pathway 3. Integrating centre 4. Efferent pathway 5. Effector  Basic reflexes: Stretch reflex When a skeletal muscle with an intact nerve supply is stretched, it contracts. This response is called the stretch reflex. The stimulus: passive stretch of the muscle. The sense organ is a small encapsulated spindlelike or fusiform shaped structure called the muscle spindle, located within the fleshy part of the muscle. Afferent fibers: fast sensory fibers ( A alpha and A beta ) Center : Anterior horn cells of the spinal cord Efferent fibers: alpha fibers70% +gamma efferent 30%. Effector organ: muscle fibers The response is contraction of muscle fibers. Type of reflex: Deep monosynaptic reflex. The neurotransmitter at the central synapse is glutamate. Basic reflexes: WITHDRAWAL REFLEX crossed extensor reflex  Basic reflexes Besides protective reflexes (such as the withdrawal reflex) and simple postural reflexes (such as the crossed extensor reflex), basic spinal reflexes mediate the emptying of pelvic organs (e.g., urination, defecation, and expulsion of semen). All spinal reflexes can be voluntarily overridden at least temporarily by higher brain centres.  sensory system A sensory system is a part of the nervous system that consists of: 1- Sensory receptors that receive stimuli from the external or internal environment, 2- the neural pathways that conduct information from the receptors to the brain or spinal cord, and 3- parts of the brain that deal primarily with processing the information.  Sensation and perception Regardless of whether the information reaches consciousness, it is called sensory information. If the information does reach consciousness, it can also be called a sensation. A person’s awareness of the sensation (and, typically, understanding of its meaning) is called perception.  1- Sensory receptors Sensory receptors at the peripheral ends of afferent neurons change sensory information into graded potentials that can initiate action potentials, which travel into the central nervous system. The sensitive membrane region that responds to a stimulus is either: (a) an ending of an afferent neuron or (b) on a separate cell adjacent to an afferent neuron  1- Sensory receptors Differential sensitivity & Adequate stimulus Visual receptors can respond to a single photon, the smallest quantity of light (the adequate stimulus). But they can be activated by an intense mechanical stimulus. For example, a poke in the eye can result in “seeing stars”—the sensation of light is still perceived even though the photoreceptors are stimulated by a mechanical stimulus. Regardless of how the receptor is stimulated, any given receptor gives rise to only one sensation.  Types of sensory receptors Five basic types of sensory receptors (according to type of stimulus to which they are sensitive): (1) mechanoreceptors, which detect mechanical compression or stretching of the receptor or of tissues adjacent to the receptor; (2) thermoreceptors, which detect changes in temperature, some receptors detecting cold and others warmth; (3) nociceptors (pain receptors), which detect damage occurring in the tissues, whether physical damage or chemical damage; (4) electromagnetic receptors, which detect light on the retina of the eye; (5) chemoreceptors, which detect taste in the mouth, smell in the nose, oxygen level in the arterial blood, osmolality of the body fluids, carbon dioxide concentration.  The Receptor Potential Regardless of the original form of the signal that activates sensory receptors, the information must be translated into the language of graded potentials or action potentials The process by which a stimulus is transformed into an electrical response is known as sensory transduction The transduction process in all sensory receptors involves that the stimulus causes the opening or closing of ion channels resulting in change in membrane electrical potential of the receptor (receptor potential) NB*** 1- the graded potential in separate cell receptors is called (receptor potential) 2- the graded potential in the peripheral end of the afferent nerve is called (generator potential) Pacinian corpuscle receptor potential The Receptor Potential  Relation of the Receptor Potential to Action Potential The more the receptor potential rises above the threshold level, the greater becomes the action potential frequency.  The Receptor Potential Factors that control the magnitude of the receptor potential include: Stimulus strength, Rate of change of stimulus strength, Temporal summation of successive receptor potentials and a process called adaptation  Receptor adaptation Adaptation is a decrease in receptor sensitivity, which results in a decrease in action potential frequency in an afferent neuron despite the continuous presence of a stimulus. Types of receptors according to their adaptation: 1- Tonic receptors (slowly adapting) 2- Phasic receptors (rapidly adapting ; on response and on-off response)  Mechanism of adaptation in mechanoreceptors 1- Readjustments in the structure of the receptor itself 2- Accommodation of the terminal nerve fibril due to sodium channels inactivation.  Primary Sensory Coding Coding is the conversion of stimulus energy into a signal that conveys the relevant sensory information to the central nervous system. Important characteristics of a stimulus include the type of input it represents, its intensity, the location of the body it affects and duration (adaptation) ***A single afferent neuron with all its receptor endings makes up a sensory unit  receptive field of a nerve Receptive fields of neighboring afferent neurons usually overlap so that stimulation of a single point activates several sensory units  law of specific nerve energies When the nerve from a particular sensory receptor is stimulated, the sensation evoked is that for which the receptor is specialized no matter how or where along the nerve the activity is initiated. This principle, first enunciated by Johannes Müller in 1835, has been called the law of specific nerve energies.  Primary Sensory Coding (Stimulus type or modality) The type of sensory receptor a stimulus activates plays the primary role in coding the stimulus modality. labeled line principle: each nerve tract terminates at a specific point in the central nervous system, and the type of sensation felt when a nerve fiber is stimulated is determined by the point in the nervous system to which the fiber leads.  Primary Sensory Coding(Stimulus Intensity) The frequency of action potentials in a single afferent neuron is one way, because increased stimulus strength means a larger receptor potential, and this in turn leads to more frequent action potentials stronger stimuli usually affect a larger area and activate similar receptors on the endings of other afferent neurons.  Primary Sensory Coding(Stimulus Location) Size of receptive field The frequency of action potential from central part vs. the peripheral part of stimulated receptive field Lateral inhibition  Dermatome The area of skin supplied with afferent nerve fibers by a single dorsal root is referred to as a dermatome. A single afferent neuron with all its receptor endings makes up a sensory unit Dermatomes