Sensory System: Nervous System (Chapter 1) PDF

# Chapter 1: Sensations ## Unit I: Sensory System - Nervous System The major functions of the nervous system are to detect, analyze, and transmit information. Information is gathered by sensory systems, integrated by the brain, and used to generate signals to motor and autonomic pathways for contr...

# Chapter 1: Sensations ## Unit I: Sensory System - Nervous System The major functions of the nervous system are to detect, analyze, and transmit information. Information is gathered by sensory systems, integrated by the brain, and used to generate signals to motor and autonomic pathways for control of movement and of visceral and endocrine functions. ### I) Anatomical organization of the nervous system: 1. **Central nervous system**, which includes: - **a) Brain** - **b) Spinal cord** 2. **Peripheral nervous system** which includes: - 12 pairs of cranial nerves and 31 pairs of spinal nerves. **Image Description:** A labelled diagram of the brain with various parts. On the left side of the brain, the following parts are labelled: - Cingulate Gyrus - Frontal Lobe - Corpus Callosum - Tissue Dividing Lateral Ventricles - Nucleus Accumbens - Hypothalamus - Pituitary Gland - Pons - Medulla - Spinal Cord On the right side of the brain, the following parts are labelled: - Parietal Lobe - Thalamus - Occipital Lobe - Superior and inferior colliculi - Midbrain - Cerebellum At the bottom, the diagram has the following labels: - Central canal of spinal cord - Figure 1: Human Brain ### II- Functional organization of the nervous system: 1. **Sensory nervous system**: - Detects changes in internal and external environments and informs the CNS about them. 2. **Motor nervous system**: - Initiates and controls the motor activities of skeletal muscles, plain muscles, cardiac muscles and even glands. 3. **Intellectual nervous system**: - Provides basis for consciousness, memory, learning, thoughts, emotions and behavior. ## Sensory System It is the division of the nervous system concerned with the detection of the stimuli (as regard nature, locality and intensity) and informs the specialized centers in CNS to adopt adequate responses. ### Components of the sensory nervous system: - **a) Sensory receptors** - **b) Afferent nerve fibers**, that carry impulses to, - **c) Specific areas in the brain & spinal cord (sensory centers).** **Image Description:** A labelled diagram of different parts of the sensory system: - **Receptor** - **Afferent nerve** - **CNS** - **Center** - Figure 2: Sensory System ## 2) According to nature of the stimulus: - They are classified into 5 major classes: ### 1) Mechanoreceptors: - They are stimulated by the mechanical forms of energy. - They include: - **a-Touch receptors** in the skin. - **b-Baroreceptors** "in the aortic arch & carotid sinus" that detect the changes in arterial blood pressure. - **c-Proprioceptors** "in the muscles" e.g. muscle spindle & golgi tendon organs. - **d-Pressure receptors** e.g. pacinian corpuscles. - **e-Sound receptors**. - **f- Vestibular receptors** that detect the changes in the equilibrium. - **g-Stretch receptors** as in alveoli, urinary bladder & right atrium. ### 2) Chemoreceptors: - They respond to the chemical stimuli e.g. - **a-Peripheral chemoreceptors** in the aortic and carotid bodies. - **b-Central chemoreceptors** in the brain stem. - **c-Taste & smell receptors**. - **d-Hypothalamic glucoreceptors & osmoreceptors.** ### 3) Thermal receptors: - They respond to changes in temperature. - They include **cold receptors** which respond to the drop of temperature & **warm receptors** which respond to the increase in temperature. ### 4) Pain receptors (nociceptors): - They respond to the injurious or painful stimuli. ### 5) Photorceptors or electromagneticreceptors: - They respond to the electromagnetic waves of light in the retina. ## Sensory Receptors ### Def: - Receptors are specialized microscopic structures located at the peripheral terminations of the afferent nerves. ### Functions of the receptors: - They detect the stimuli and convert these stimuli into nerve impulses which are then conducted along the afferent nerves to the sensory centers in central nervous system i.e. the receptors act as "detectors and transducers". **Image Description:** A labelled diagram of tactile receptors. In the **epidermis** the following are labelled. - **Meissner corpuscles (touch)** - **Pacinian corpuscles (pressure)** - **Ruffini endings (pressure)** In the **dermis**, the following are labelled: - **Free nerve endings (pain, heat, cold)** - **Merkel disks (touch)** - **Krause end bulbs (touch)** - **Root hair plexus (touch)** - Figure 3: Tactile receptors ## Physiological classification of receptors: ### 1) According to the location of the receptor: - They are classified into: - **(i) External receptors**: on the body surface mainly in skin. - **(ii) Internal receptors**: in deeper structures & viscera. ## Properties of Receptors 1. **Specificity (Adequate stimulus)** 2. **Excitability (Receptor potential)** 3. **Rate of discharge from the receptors (detection of the stimulus intensity)** 4. **Adaptation of the receptors** ### 1) Specificity - The receptors show a high degree of specificity as Muller's law applies: **Muller's law:** "Each type of receptors is highly sensitive to one type of stimuli called the adequate stimulus and its stimulation gives rise to one type of sensation whatever the way of stimulation”. **Example:** - Retinal receptors are highly sensitive to the light waves. - Auditory receptors are highly sensitive to the sound waves...etc. **However:** The receptors may respond to stimuli other than their specific or adequate ones, provided that these stimuli are very strong; but still the response is the same modality to which the receptor is specialized *e.g* retinal receptors are normally stimulated by the light waves and give rise to sense of vision. But if heavy mechanical stimuli applied to the eye as in heavy blow, the retinal receptors can be stimulated and eye sees flashes of light. ### 3) Rate of Discharge from Receptors - The relation between the intensity of the stimulus and the rate of discharge from the receptor is explained by, Weber- Fechner 's law: **Weber- Fechner 's Law:** The frequency of impulses discharged from the receptors through afferent nerves is directly proportionate with the logarithm intensity of the stimulus. **Logarithmic relation means:** - To increase the rate of discharge twice, you must increase the intensity of the stimulus 100 times, for example 100 times increase in intensity of the stimulus is expressed by increasing the rate of discharge twice only. Thus, the receptors compress wide range of stimuli into narrow range of discharge. - This is called “compression function of receptors”.