Sensory Receptors and Their Functions

• The sensory receptors are specialized structures at the end of afferent neurons.
• Detectors: detect both internal and external changes within the body's environment.
• Transducers: convert detected energy changes into electrical impulses.

Classification of Receptors by Stimulus

• Mechanoreceptors: detect touch, pressure, and vibration in the skin.
• Proprioceptors: found in muscles and tendons, relay information about length and tension.
• Baroreceptors: located in the aortic arch and carotid sinus.
• Stretch receptors: found in alveoli of the lungs and right atrium.
• Thermoreceptors: detect thermal energy and changes in temperature (cold and warm receptors).
• Nociceptors: pain receptors, stimulated by tissue damage.
• Chemoreceptors: stimulated by the chemical composition of the environment, including taste and smell receptors.
• Glucoreceptors and Osmoreceptors: located in the hypothalamus.
• Electromagnetic receptors: stimulated by electromagnetic waves of light, such as rods and cones in the retina.

Properties of Receptors

Specificity (Differential Sensitivity)

• Muller's Law: Receptors are most sensitive to one specific type of stimulus, called the adequate stimulus.
• Modality of sensation depends on the area in the brain ultimately activated, regardless of the method of stimulation.
• Receptors can be stimulated by non-adequate stimuli if the intensity is high enough, still giving their specific modality of sensation.

Excitability (Receptor or Generator Potential)

• Definition: the ability of the receptor to respond to stimuli.
• Resting condition: receptor is polarized (outer surface +, inner surface -), RMP is -70 mV.
• On stimulation:
  • Receptor or generator potential: a local, graded potential.
  • Cause: opening of Na+ channels in the receptor membrane leads to Na+ inflow.
  • When the receptor potential reaches the threshold, action potentials are generated at the first node of Ranvier.
  • Increased stimulus amplitude increases the action potential frequency in the afferent sensory fiber.

Properties of Receptor Potential vs Action Potential

• Receptor Potential: local, unpropagated, graded (does not obey all-or-none), no refractory period, activated by mechanically or chemically gated channels, not blocked by local anesthetics.
• Action Potential: propagated, obeys all-or-none, has a refractory period, activated by voltage gated channels, blocked by local anesthetics.

Adaptation

• Definition: decrease in the number of nerve impulses due to a decline in the receptor potential amplitude.
• Rapidly adapting (phasic) receptors: adapt quickly, detect changes in stimulus intensity, e.g., olfactory and touch receptors.
• Slowly adapting (tonic) receptors: adapt slowly or not at all, signal the intensity of prolonged stimuli, e.g., muscle stretch and pain receptors.
• Mechanisms of adaptation involve loss of stimulus energy, decreased excitability of the first node of Ranvier, and inactivation of sodium channels.

Coding of Sensory Information

• Definition: ability of the brain to discriminate modality, locality, and intensity of different stimuli.
• Modality Discrimination: depends on differential receptor sensitivity and specialized adequate stimulus.
• Locality Discrimination (Projection): relies on each receptor having a specific pathway to a representational area in the sensory cortex.
• Intensity Discrimination: depends on action potential frequency and the number of stimulated receptors.
  • Stronger stimuli lead to increased action potential frequency and stimulation of more receptors.

Types of Mechanoreceptive Sensation

Tactile Mechanoreceptive Sensation

• Touch sensation: ability to localize physical contact with skin while eyes are closed.
  • Light touch: not sharply localized, results from stimulation of superficial tactile receptors.
  • Tactile discrimination (2-point discrimination): ability to perceive 2 distinct stimuli with eyes closed, dependent on variable thresholds in different areas of the body.
• Pressure sensation: ability to discriminate different weights, results from tactile receptor stimulation in deeper layers of the skin.
• Vibration sensation: results from rapidly repetitive sensory signals from tactile receptors.

Proprioceptive (Kinaesthetic) Sensations

• Conscious proprioception: provides awareness of joint position and movement via receptors in the joints.
• Unconscious proprioception: includes senses of muscle length and tension, with impulses relayed to the cerebellum.

Thermoreceptive Sensation

• Cutaneous thermoreceptors: specialized free nerve endings found with highest density in the skin of the hands and face.
• Function: monitor skin temperature (not body temperature) to detect external environmental temperature changes.
• Info reaches the hypothalamus, which is the heat regulating center to initiate nervous and humoral mechanisms for maintaining constant body temperature.
• Cold receptors (cool receptors): respond to temperatures between 10-35°C (silent above 40°C), on dendritic endings of Ad and C fibres.
• Warmth receptors: respond to temperatures between 35-45°C (silent above 40°C), on dendritic endings of C fibres.
• Cold receptors may give a brisk discharge at 45°C producing a false cold sensation called paradoxical cold.
• Cold-pain: respond to temperatures between 0°C and 10°C.
• Hot-pain: respond to temperatures above 45°C.
• At 0°C all receptors stop discharging, a fact made use of cold for local anaesthesia.

Common Features Within Each Somatosensory Pathway

• Sensory fibers enter the CNS via spinal dorsal nerve roots or the trigeminal root.
• Nervous pathways conducting impulses to higher CNS levels are referred to as ascending tracts.
• First-order neuron cell bodies are in the dorsal root ganglia or the trigeminal ganglion.

Dorsal Column Lemniscal Pathway (Gracile & Cuneate Tracts)

• Transmits light touch, two-point discrimination, pressure and vibration sense, and conscious proprioceptive sensations.
  • First order neuron (dorsal root ganglion).
    • Aa and Aß fibres mediating the sensations enter the spinal cord via the dorsal nerve roots and pass to the dorsal white column on the same side : -
      • Fibres from the lower half of the body (below T6) ascend ipsilaterally in the most medial part of the dorsal column till relaying in the gracile nucleus in the medulla oblongata forming the fasciculus gracilis .
      • fibres from the upper part of the body (above T6) ascend ipsilaterally in the lateral part of the dorsal column till relaying in the cuneate nucleus in the medulla oblongata forming the fasciculus cuneatus.
    • Second order neuron (gracile and cuneate nuclei).
      • Axons of the gracile and cuneate nuclei cross to the opposite side as the internal arcuate fibers.
    • then ascend as the medial lemniscus to relay in the ventral posterolateral nucleus of the thalamus.
    • Third order neuron (VPL nucleus of the thalamus).
    • Axons of these cells ascend as the sensory radiation and terminate in the sensory area of cerebral cortex in postcentral gyrus of the parietal lobe.
• Deficits after lesions in the dorsal column pathway may lead to :sensory ataxia, paresthesia, Loss of tow-point discrimination, vibration and pressure sensations and astereognosis

Ventrolateral (Anterolateral) System

• Transmits pain and temperature, itch.
• The second order neuron of the anterolateral system forms the following pathway : -The spinothalamic tract & spinoreticular tract . -The spinomesencephalic tract. -Pathway of spinothalamic tract & spinoreticular tract :-
  • First order neuron (dorsal root ganglion). - Second order neuron (dorsal horn cells)
    • spinoreticular tract (paleospinothalamic): Fibres carrying slow pain & temperature pass to thereticular formation then to the intralaminar thalamic nuclei in directly.
    • spinothalamic tract (neospinothalamic) : fibers carrying fast pain pass to the ventral posterolateral nucleus of the thalamus directly.
  • Third order neuron. -fibers from the intralaminar thalamic nuclei are associated with arousal they project to insular cortex which keeps memory of the stimulus that resulted in pain.
    • the fibers from the ventral posterolateral nucleus of the thalamus project to the somatic sensory area in the post central gyrus which is responsible for the discriminative aspect of pain.

Trigeminal Sensory Pathway

• First order neuron (trigeminal ganglion): carries touch, proprioception, pain, and temperature from the face.
• Second order neuron (trigeminal sensory nuclei): these fibers relay in the 3 trigeminal sensory nuclei in the brainstem.
• Third order neuron (thalamus): then terminate in the sensory area of the cerebral cortex in postcentral gyrus of parietal lobe.

Spinocerebellar Tracts

• Transmit unconscious proprioceptive sensations.
  • Dorsal spinocerebellar tract :This fiber relay in Clarke's nucleus at the base of the dorsal horn from T1 through L3 spinal segments. Ventral spinocerebellar tract
  • Cuneocerebellar tract .Fibers ascend from Clarke's column neurons on the same side of the spinal cord in the posterolateralcolumn of white matter up to relays in the accessory cuneate nucleus in the medulla.

Pain

• Definition: unpleasant sensory and emotional experience associated with actual or potential tissue damage
• Pain receptors (Nociceptors) -Definition: Detect signals from damaged tissue, sensory receptors in the free nerve endings . Type of fiber are transmit the impulse from pain receptor : Thin myelinated A8 Thin myelinated A8 ( ( (30%) and Unmyelinated C fibres (70%)
• Types of pain receptors Mechanosensitive pain receptors , Thermosensitive pain receptors ,Chemically sensitive pain receptors and Polymodal pain receptors

Types of pain

• Neuropathic pain , Nociceptor pain , Visceral pain And Somatic pain .

Fast And Slow Pain

• Fast pain (epicritic pain)short duration, cutaneous only.. Transmitted by myelinatedAd fibers : (thin ,2-5µm in diameter) that rapidly conduct. well localized
• Slow pain (protopathic pain )long duration, can be cutaneous or deep Transmitted by unmyelinated C fibers: (0.4-1.2 µm in diameter ) that slowly conduct. Poorly localized [dull and diffuse].

Somatic Pain Reactions (effects) :

Somatic (motor) reflexes : fast pain initiates the withdrawal reflex. Autonomic effects :Cutaneous pain is associated with sympathetic effects ,Severe pain: parasympathetic effects occur. Emotional effects Emotional effects due to stimulation of the limbic system Cutaneous hyperalgesia (tenderness)- hyperalgesia In the area of red skin In this area, the pain threshold is lowered The diffusion of pain producing Substances (e.g. histamine, proteolyticenzymes, kinins) in the healthy skin area surrounding the area of flare In this area, the pain threshold is not lowered Increased excitability of the dorsal horn cells following tissue injury

Visceral Pain

• Characters of visceral pain associated with autonomic effects contraction of muscles of the anterior abdominal wall, resulting from stimulation of pain receptors in the viscera itself.It is dull aching and poorly localized afferents Transmitted through autonomic visceral afferents.

Referred Pain

-Definition: frequent Irritation of the visceral organs frequently pain but in some structure that may be a considerable distance away. Mechanism of referred pain: dermatomal rule And convergence-facilitation theory

Neuropathic pain

-Definition arises from functional changes occurring in the CNS secondary to peripheral nerve injury Causes Diabetes mellitus Herpes zoster infection Side effect of chemotherapy and Diseases of the central nervous system

Control system at the level of the brain

 - .Opiate analgesia (OA) :The most effective used drugs for producing temporary analgesia are the opioid family.

Somatosensory Areas

Divided into Somatosensory area I (S1) ,Somatosensory area II and Somatosensory association area. Lesion can result in impaired position sense and impaired tactile sensation or result in abnormal perception of objects

Motor areas of cerebral cortex

Primary motor area (area 4) , Premotor area 6 , Supplementary motor area (SMA)

• Prefrontal association area (control the personality) is divided into :the dorsolateral prefrontal cortex and the ventromedial prefrontal cortex (orbitofrontal cortex)

Cortical areas involved in language

The non-language areas for the spoken words and written words

The language areas

The Wernicke's area , the inferior parietal lobule , The Broca's area

Function of the spinal cord

ascending tracts, descending tracts, performs reflexive actions and contains preganglionic neurons of the autonomic nervous system.

Motor Descending Tract

Pyramidal tracts and Extra- Pyramidal tracts

The Reflexes

Are Several ways of classifications According to the center, Location, Number of synapses.

• Spinal cord reflexes

General properties

Forward direction, Summation, Reflex delay, Reflex fatigue and block, Irradiation, Reverberating circuits.

Stretch reflex (myotatic reflex)

Disynaptic protective reflex which Functions of the muscle tone: Maintains erect posture, helps both venous return and lymph flow Spinal Cord Transection Causes: tumor, trauma, ischemia.

Reticular formation

Definition: Brainstem area with multiple functions. Functions: Muscle tone, gaze control, autonomic functions, pain modulation, consciousness.

The limbic system

Anatomical structure of the limbic system (cortical structures And subcortical structures that is connected to the oribitofrontal cortex). Functions of the limbic system such as the thalamus, the anterior cingulate ,the Hypothalamus and controls emotions, memories etc.. Brain areas involved in arousal (areas And neurotransmitters ): The brainstem ,Hypothalamus and neurotransmitters ( The Cholinergic etc)

EEG And Sleep

The electroencephalogram (EEG) is recording of the electrical activity of the brain Types of normal EEG waves: alpha, beta, theta and delta in awake and sleep states Sleep is defined as physiological loss of consciousness has rapid eye movement sleep (REM ) & non-rapid eye movement sleep (non-REM Slow wave sleep (non-REM sleep) and Rapid eye movement sleep (REM sleep) have differences in wave form and depth phases.

The Thalamus

Definition: The collection of nuclei that process sensory inputs in the cortex Type of thalamic nuclei specific 1 Non specific nuclei And specific nuclei have functions in the :Somato , Visual ,Motor functions And Emotions .

The Hypothalamus

Functions: Food intake, Regulate of water balance, ANS, Controls Endocrine functions and sleep

BASAL NUCLEI

The basal Nuclei function not by themselves in body and has parts in its motor and visual areas.

The Cerebellum

Functional divisions :vestibulo (archi)-cerebellum Spino (paleo)-cerebellum and Cerebro (neo)- cerebellum with each having distinct functions Body functions: has functions ,the motor functions that aids in comparison of intent and action and generation of corrective signals and has effects : helps maintaining of balance, Equilibrium .

The Eye

the refractive media and it has main part which are the cornea. Light travel after reaching the retina