Ch. 12 (Somatic Sensation) - Behavioral Neuroscience

Mechanoreceptors

specialized sensory neurons that convert mechanical stimuli—such as pressure, touch, stretching, sound waves, and motion—into electrical signals sent to the central nervous system. Located in the skin, joints, and muscles, they are crucial for tactile sensation, proprioception (body position), and detecting vibrations

Merkel’s disks

• Location: Superficial layer of skin (epidermis), highly concentrated in fingertips, lips, and external genitalia.

• Function: Detect light pressure, edges, shapes, and textures

Meissner’s corpuscles

• Location: Superficial layer of skin (dermal papillae), common in hairless (glabrous) skin.

• Function: Detect fine detail, low-frequency vibration, flutter, and slip (important for grip).

Pacinian corpuscles

• Location: Deep in the dermis or hypodermis (subcutaneous tissue).

• Function: Detect high-frequency vibration and deep pressure.

Ruffini’s endings

• Location: Deep in the dermis, also found in ligaments and tendons.

• Function: Detect skin stretch, sustained pressure, and joint movement, contributing to proprioception.

Adaptation rate

Touch adaptation is the process where sensory receptors decrease their response to a constant, prolonged stimulus, with rates varying from rapid (milliseconds) to slow (seconds/minutes) depending on the receptor type. Rapidly adapting receptors (e.g., Meissner/Pacinian corpuscles) signal changes like movement, while slowly adapting receptors (e.g., Merkel cells) signal persistent pressure.

receptive field size

Receptive field (RF) size refers to the specific region of input data (such as pixels in an image or sensory receptors on skin) that influences a particular neuron's response. Larger RFs provide a wider, less precise view, while smaller RFs allow for fine-detail detection.

Primary afferent axons

sensory nerve fibers that transmit information from receptors in skin, muscle, and internal organs to the central nervous system (CNS).

Dermatome and spinal segments

A dermatome is a specific area of skin that receives sensory innervation from the dorsal root of a single spinal nerve segment. There are 31 pairs of spinal nerves, resulting in 30 dermatomes (the C1 spinal nerve usually lacks a corresponding dermatome). These dermatomes are organized in a distinct map on the body

Dorsal column-medial lemniscal pathway

a major sensory pathway that transmits fine touch, vibration, and conscious proprioception (position sense) from the skin and joints to the brain. It is a fast, three-neuron system that ascends ipsilaterally in the spinal cord, decussates in the medulla, and relays in the thalamus before reaching the primary somatosensory cortex

Dorsal Column

First-order neurons (primary afferents) ascend ipsilaterally through the spinal cord via the fasciculus gracilis (lower body) or fasciculus cuneatus (upper body).

Dorsal Column Nuclei

The first-order neurons synapse in the medulla at the nucleus gracilis and nucleus cuneatus.

Medial Lemniscus

Second-order neurons decussate (cross the midline) as internal arcuate fibers and ascend through the brainstem as the medial lemniscus to the thalamus.

Ventral Posterior Nucleus

Third-order neurons synapse in the ventral posterolateral (VPL) nucleus of the thalamus.

Primary Somatosensory Cortex

The pathway terminates in the postcentral gyrus (Brodmann areas 3, 1, 2) of the parietal lobe for conscious perception

Trigeminal touch pathway

The trigeminal touch pathway (ventral trigeminothalamic tract) transmits discriminative fine touch and pressure from the face to the brain

Trigeminal Nerves

Trigeminal Nerves (CN V): The main sensory nerve for the face, scalp, mouth, and nasal cavity. It splits into three branches: V1 (Ophthalmic), V2 (Maxillary), and V3 (Mandibular).

Trigeminal Nucleus (Brainstem Complex)

Located in the brainstem, this complex processes incoming sensory data.

Principal (Main) Sensory Nucleus

Processes light touch and proprioception.

Spinal Trigeminal Nucleus

Processes pain and temperature.

Ventral Posterior Nucleus (VPM)

Specifically, the ventral posterior medial (VPM) nucleus of the thalamus acts as the relay station for sensory information from the face.

Primary Somatosensory Cortex (S1)

Located in the postcentral gyrus of the parietal lobe, this is the destination for facial sensation, where it is mapped topographically

Somatotopy

the point-to-point mapping of body areas to specific, corresponding locations in the central nervous system, particularly the motor and sensory cortices

Cortical plasticity

the brain's ability to reorganize its structure, functions, and connections throughout life in response to experience, learning, or injury

Nociceptors

specialized peripheral sensory neurons (free nerve endings) that act as the body’s warning system by detecting damaging or potentially damaging stimuli—mechanical, thermal, or chemical—and transmitting pain signals to the brain. Located in skin, muscle, joints, and viscera, they initiate protective reflexes by releasing neurotransmitters like glutamate

spinothalamic tract

primary ascending pain pathway, serving as part of the anterolateral system that transmits sensations of pain, temperature, and crude touch/pressure from the periphery to the brain. It is a three-neuron system, with the pathway characterized by early decussation (crossing) at the spinal cord level and projection to the thalamus

ascending pain pathway

a neural network that transmits noxious stimuli from the body to the brain for perception. It starts with nociceptors sending signals via spinal neurons (primarily in the anterolateral system) to the brainstem, thalamus, and somatosensory cortex. The primary path is the spinothalamic tract, which crosses to the opposite side of the spinal cord and carries sensory data for pain and temperature

Gate theory of pain

a "gate" in the spinal cord's dorsal horn can block or allow pain signals to reach the brain. Non-painful, large-fiber stimulation (e.g., rubbing a bump) closes this gate, inhibiting smaller pain-carrying fibers, effectively reducing pain perception

Periaqueductal grey matter

critical grey matter region surrounding the cerebral aqueduct in the midbrain, acting as a key interface between the forebrain and lower brainstem. It plays a major role in pain modulation, defensive behavior (fight/flight/freeze), autonomic function (cardiovascular/respiratory), and vocalization